Vestibular hair cell turnover in normal adult mammals

描述（由申请人提供）：许多美国人患有前庭功能障碍，这会降低生活质量，增加跌倒和过早死亡的风险。前庭功能障碍的一个主要原因是毛细胞（HCs）的丧失，毛细胞将头部运动转化为神经信号，使我们能够保持凝视、定向和控制身体运动。由于耳毒性药物治疗、内耳感染和与衰老相关的变化，前庭hc死亡。在哺乳动物中，前庭HC历来被认为是不可替代的，因为许多与HC丧失相关的症状是永久性的。然而，一些研究表明，成年啮齿动物在损伤后可以自发地将非感觉支持细胞（SCs）转化为hc，而无需介入细胞分裂。我们研究的长期目标是开发基于生物学的方法来替代丢失的hc，以提高人类患者的生活质量。要实现这一目标，需要了解正常条件下成年哺乳动物的HC和SC种群动态。一般认为，除非发生损伤，否则成熟哺乳动物的前庭上皮不会添加hc。然而，来自几个实验室的间接证据表明，成年啮齿动物的前庭hc在没有损伤的情况下会发生更新（移除和替换）。例如，在成年蝙蝠和啮齿动物的正常（未受损）前庭上皮中检测到分化和死亡的hc。如果成年哺乳动物的前庭HC不断被替换，这种可塑性将对HC稳态、衰老过程中的HC损失和损伤后的HC再生产生重大影响。本研究将直接验证正常成年小鼠前庭细胞发生转换的假设。在目标1中，我们将确定前庭细胞是否正在死亡，如果是，定义死亡的位置、数量和类型。我们还将评估SCs是否作为吞噬细胞清除HC碎片。在Aim 2中，我们将使用转基因和敲入小鼠来确定在正常条件下小鼠前庭上皮中是否添加了新的hc。通过追踪sc和hc的命运，我们将验证sc转分化为II型hc和II型hc随后转化为I型hc的假设。我们还将确定SCs是否通过细胞分裂得到更新。在目的3中，我们将确定正在进行的HC死亡率和添加率是否随着年龄的增长而改变。在目的4中，我们将评估在HC几乎完全破坏后sc到HC的转分化率是否会改变。该研究项目将有助于表征年轻和老年成年小鼠前庭上皮中HC替代的自然机制，这可以在不久的将来开发出更好的治疗平衡障碍的方法。

DESCRIPTION (provided by applicant): Many Americans suffer from vestibular dysfunction, which can reduce the quality of life and increase the risk for falls and premature death. A major cause of vestibular dysfunction is the loss of hair cells (HCs), which transduce head movements into neural signals and allow us to maintain gaze, orientation, and control of body movements. Vestibular HCs die as a result of ototoxic drug treatments, inner ear infection, and changes associated with aging. In mammals, vestibular HCs have historically been considered to be irreplaceable, since many symptoms associated with HC loss are permanent. However, several studies have shown that adult rodents can spontaneously replace a small number of HCs after damage, by conversion of non-sensory supporting cells (SCs) into HCs without an intervening cell division. The long-term goal of our research is to develop biologically based methods to replace lost HCs, in order to improve the quality of life of human patients. An understanding of HC and SC population dynamics in adult mammals under normal conditions is needed to achieve this goal. It is generally assumed that HCs are not added to the vestibular epithelia of mature mammals unless damage occurs. However, indirect evidence from several laboratories suggests vestibular HCs in adult rodents undergo turnover (removal and replacement) in the absence of damage. For example, both differentiating and dying HCs have been detected in the normal (undamaged) vestibular epithelia of adult bats and rodents. If vestibular HCs are continuously replaced in adult mammals, this plasticity would have major implications for HC homeostasis, HC loss during aging, and HC regeneration after damage. The proposed research will directly test the hypothesis that vestibular HCs undergo turnover in normal adult mice. In Aim 1, we will determine if vestibular HCs are dying and if so, define the position, number, and types that are dying. We will also assess if SCs act as phagocytes to clear HC debris. In Aim 2, we will use transgenic and knock-in mice to determine if new HCs are added to mouse vestibular epithelia under normal conditions. By tracing the fate of SCs and HCs, we will test the hypotheses that SCs transdifferentiate into type II HCs and type II HCs later convert into type I HCs. We will also determine if SCs are renewed by cell division. In Aim 3, we will determine if the rates of ongoing HC death and addition are altered with aging. In Aim 4, we will assess if the rate of SC-to-HC transdifferentiation is altered after near-complete HC destruction. This research project will help to characterize the natural mechanisms for HC replacement present in the vestibular epithelia of young and aged adult mice, which can be exploited to develop better therapies for balance disorders in the near future.