Trophic Interactions in Developing and Adult Inner Ear

描述(由申请人提供)：内耳神经元长期存活的细胞和分子机制仍然知之甚少。组织病理学相关性表明，毛细胞是第七神经生存信号的关键来源。然而，我们的工作表明，耳蜗和前庭上皮细胞中的支持细胞通过神经调节蛋白1-erbB受体和神经营养因子-Trk信号通路在神经元维持中发挥关键作用。出生后支持细胞中erb B信号的丢失会导致神经元变性，在此之前，耳蜗中的NT3和前庭器官中的BDNF特异性地丢失，而这些神经营养因子(NTs)主要由支持细胞表达。由于erbB受体信号可诱导其他非神经细胞表达NT，而内耳感觉神经元表达NRG1，我们推测：NRG1由耳蜗神经元和前庭感觉神经元产生，可诱导支持细胞在Corti器表达NTS、NT3，在前庭上皮细胞表达BDNF。这些NTs作用于神经元，维持它们与毛细胞的突触接触，诱导它们存活，并保持它们的功能。我们将使用转基因小鼠来验证这一假设。在目标1中，可诱导的细胞特异性基因敲除将测试支持细胞或毛细胞中NT3或BDNF的消除是否会导致神经元变性和内耳功能障碍；条件性细胞特异性过度表达转基因将测试是否可以通过增加NT表达来挽救由于支持细胞中的erbB信号丢失而导致的神经元退化。在目标2中，我们使用缺乏高亲和力硫胺转运体的小鼠来评估内毛细胞对神经元长期维持的贡献，以使饮食中限制硫胺导致内毛细胞的广泛和选择性损失，而不损害支持细胞。随着时间的推移，内耳组织将通过毛细胞突触和感觉神经节细胞的共聚焦形态测量、诱发反应的功能测试以及使用RT-PCR和原位杂交评估关键营养因子及其受体的基因水平/表达模式来进行定量评估。比较不同出生后不同年龄毛细胞和支持细胞中NT3和BDNF的上调或下调所获得的表型，将有助于深入了解每种神经营养因子的特定作用，参与信号通路的细胞，以及神经元对营养因子剥夺的易感性的年龄相关性。这些实验将为感觉神经性听力损失和外周平衡障碍的发病机制和潜在的治疗方法提供重要的见解。内耳进行性功能障碍是一个重要的健康问题。尽管听力和平衡障碍的发生率很高，但有助于内耳结构和功能长期完整的细胞和分子机制仍然知之甚少。该项目将使用转基因小鼠模型来研究内耳感觉神经元长期存活和功能所涉及的细胞和分子机制。

DESCRIPTION (provided by applicant): The cellular and molecular mechanisms underlying long-term neuronal survival in the inner ear remain poorly understood. Histopathological correlations have suggested that hair cells are a critical source of survival signals for the VIIIth nerve. However, our work indicates that supporting cells in both cochlear and vestibular epithelia play key roles in neuronal maintenance via the neuregulin 1-erbB receptor and neurotrophin-Trk signaling pathways. Postnatal loss of erbB signaling in supporting cells leads to neuronal degeneration, preceded by a specific loss of NT3 in the cochlea and BDNF in the vestibular organs, at a time when these neurotrophins (NTs) are expressed primarily by supporting cells. Since erbB receptor signaling induces NT expression in other non-neuronal cells, and since inner ear sensory neurons express NRG1, we hypothesize that: NRG1, produced by cochlear and vestibular sensory neurons, induces supporting cells to express NTs, NT3 in the organ of Corti and BDNF in the vestibular epithelia. These NTs, acting back on the neurons, maintain their synaptic contacts with hair cells, induce their survival and preserve their function. We will test this hypothesis using genetically modified mice. In Aim 1, inducible cell-specific knockouts will test if elimination of either NT3 or BDNF in supporting cells or hair cells causes neuronal degeneration and inner ear dysfunction; and conditional cell-specific overexpression transgenics will test if the neuronal degeneration caused by loss of erbB signaling in supporting cells can be rescued by increasing NT expression. In Aim 2, we evaluate inner hair cell contributions to long-term neuronal maintenance using mice lacking the high-affinity thiamine transporter such that dietary thiamine restriction leads to widespread and selective loss of inner hair cells, without damage to supporting cells. Inner ear tissues will be quantitatively assessed over time by confocal morphometry of hair cell synapses and sensory ganglion cells, functional tests of evoked responses, and assessment of gene levels/patterns of expression of key trophic factors and their receptors using RT-PCR and in situ hybridization. Comparison of phenotypes obtained when controlling the up- or down-regulation of NT3 vs. BDNF in hair cells vs. supporting cells at different post-natal ages will provide insights into the specific roles of each neurotrophin, the cells involved in the signaling pathways, and the age-dependence of neuronal susceptibility to trophic factor deprivation. These experiments will provide important insights into the pathogenesis and potential treatment of sensorineural hearing loss and peripheral balance disorders. Progressive dysfunction of the inner ear is an important health issue. In spite of the high incidence of hearing and balance disorders, the cellular and molecular mechanisms that contribute to the long-term integrity of inner ear structure and function remain poorly understood. This project will investigate the cellular and molecular mechanisms involved in the long-term survival and function of inner ear sensory neurons using transgenic mouse models.