Vestibular hair cell turnover in normal adult mammals

描述(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，增加摔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的丧失，毛细胞将头部的运动转化为神经信号，使我们能够保持凝视、定向和对身体运动的控制。由于耳毒性药物治疗、内耳感染和与衰老相关的变化，前庭HCS死亡。在哺乳动物中，前庭HCS历来被认为是不可替代的，因为许多与HC丢失相关的症状是永久性的。然而，一些研究表明，成年啮齿动物在受损后，可以通过将非感觉支持细胞(SCs)转化为HCs而自发地替换少量HCs，而不需要干预细胞分裂。我们研究的长期目标是开发基于生物学的方法来取代丢失的HCS，以提高人类患者的生活质量。为了实现这一目标，需要了解成年哺乳动物在正常条件下的HC和SC种群动态。一般认为，在成熟哺乳动物的前庭上皮细胞中，除非发生损伤，否则不会添加HCs。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HCs在没有损伤的情况下经历了翻转(移除和替换)。例如，在成年蝙蝠和啮齿动物的正常(未受损)前庭上皮细胞中，已经检测到分化和死亡的HCS。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的动态平衡、衰老过程中的HC丢失以及损伤后的HC再生产生重大影响。这项拟议的研究将直接检验前庭HCs在正常成年小鼠中经历翻转的假设。在目标1中，我们将确定前庭Hcs是否正在死亡，如果是，定义正在死亡的位置、数量和类型。我们还将评估干细胞是否作为吞噬细胞来清除HC碎片。在目标2中，我们将使用转基因和敲入小鼠来确定在正常情况下是否有新的HCS加入到小鼠的前庭上皮细胞中。通过追踪SCs和Hcs的命运，我们将检验SCs转分化为II型Hcs和II型Hcs后来转换为I型Hcs的假设。我们还将确定干细胞是否通过细胞分裂来更新。在目标3中，我们将确定持续的HC死亡和增加的比率是否随着年龄的变化而改变。在目标4中，我们将评估在几乎完全破坏HC后，SC到HC转分化的比率是否发生了变化。这项研究将有助于揭示幼年和老年小鼠前庭上皮细胞中HC替换的自然机制，这将有助于在不久的将来开发更好的平衡障碍治疗方法。

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.