Mechanisms of Growth Factor Responsiveness in the Aging Auditory System

摘要： 在听力发育期间，听觉神经元在质量和数量上都被正确地连接在一起， 特殊类型的螺旋神经节神经元(SGN；听觉传入)和耳蜗核(CN)神经纤维。 显然，听神经通路的成熟是沿着功能纯正频率轴进行的。 神经营养因子(NTS)与时间和空间/位置依赖梯度的相关性。此外，SGN 发展对声音的耳蜗性反应，实现向耳蜗核(CN)的耳蜗性投射。这个 SGN的活动维持着CN中细胞的数量、大小和功能。之前的研究表明这一点 这一过程在一定程度上受神经营养因子(如脑源性神经营养因子)的调节。表达式 数据显示，BDNF的表达经历了发育和年龄依赖性的细胞和 在听觉通路中表达的纵向模式。这一模式被提出用来指示不同的尖峰- 听神经元电特性的基本功能，进而要求耳蜗区和中枢性听觉 神经元微调。尽管NT梯度和年龄相关的听神经假说很有吸引力 财产，这一想法建立在相关证据的基础上，一些人对此有争议。我们试图毫不含糊地测试和澄清 NT梯度预测，并了解BDNF介导的听觉功能可塑性及其如何塑造 年龄相关性听力损失(ARHL)。 我们推测BDNF信号的逐渐减弱是ARHL的常见原因之一。 我们将使用描述良好的Cre系列来揭示BDNF在听觉神经元可塑性中的功能(例如 Rosa26-Creer；Fgf8-cre，Atoh1-cre)选择性地减少或消除花系中的BDNF，以研究长的 脑源性神经营养因子水平对衰老小鼠听觉信号处理的长期影响。在目标1中，我们将量化BDNF 在听觉系统中的信号表达，确定来源/S和随后的年龄相关变化 听觉神经通路。单分子荧光原位杂交与免疫细胞化学 将使用技术来量化BDNF的mRNA和蛋白的表达以及与年龄相关的变化。 此外，BDNF受体表达与年龄相关的变化将被量化。在目标2中，我们将确定 脑源性神经营养因子介导的部分或延迟性缺失的听觉可塑性。这些目标将通过以下方式实现 可诱导的cre系(例如，rosa26-creer)，可在~3周至2年的不同老化阶段消除所有BDNF 老老鼠。我们将确定听觉神经元(如SGN)与年龄相关的细胞特性。最后，在AIM中 3、我们将通过BDNF的部分和延迟性丢失来鉴定BDNF介导的神经和突触可塑性。我们会 使用Aim 2中概述的动物模型来识别由于以下原因引起的Hold氏囊的突触功能的变化 BDNF的损失/下降。这种中枢听觉突触起源于耳蜗腹核(VCN)，并投射 与斜方体的内侧核(MNTB)相对，并被发现经历了形态和 衰老过程中的分子变化。我们还将研究CN功能的变化。 因此，我们将解决BDNF的表达在衰老过程中如何影响SGN/VCN/MNTB功能， 为BDNF信号是听力下降的常见原因提供了证据。这一知识将使我们了解 将脑源性神经营养因子用于ARHL后保护听神经元活性和功能的治疗策略的努力。

Abstract: During hearing development, auditory neurons are wired correctly, both qualitatively and quantitatively, with specific types of spiral ganglion neurons (SGN; auditory afferents) and cochlear nucleus (CN) nerve fibers. Maturation of the auditory neural pathway ensues along the functional tonotopic frequency axis, apparently correlating with time and space/location-dependent gradients in neurotrophins (NTs). In addition, the SGNs develop cochleotopic responses to sound and achieve cochleotopic projections to the cochlear nuclei (CN). The activity of SGNs maintains the number, size and functions of cells in the CN. Previous studies suggest that this process is regulated in part by neurotrophic factors (e.g. brain-derived neurotrophic factor (BDNF)). Expression data show that BDNF expression undergo developmental and age-dependent shifts in their cellular and longitudinal patterns of expression in the auditory pathway. This pattern was proposed to dictate distinct apico- basal function of auditory neuron electrical properties, in turn requirements for cochleotopic and central auditory neuron fine tuning. Despite the appeal of the NT-gradient and age-dependent hypothesis for auditory neural properties, this idea rests on correlative evidence, disputed by some. We seek to unequivocally test and clarify the NT-gradient predictions, and to understand BDNF-mediated auditory functional plasticity and how it sculpts age-related hearing loss (ARHL). We hypothesize that gradual decline in BDNF signaling is one of the common cause for ARHL. We will unravel the function of BDNF in auditory neuronal plasticity using well-characterized cre lines (e.g. Rosa26-creER; Fgf8-cre, Atoh1-cre) to selectively reduce or eliminate BDNF in floxed lines, to study the long- term influence of BDNF levels on auditory signal processing in aging mice. In Aim 1, we will quantify BDNF signaling expression in the auditory system, determine the source/s and the ensuing age-related changes in the auditory neural pathway. Single molecule fluorescent in situ hybridization (SmFISH) and immunocytochemical techniques will be used to quantify mRNA and protein expression and the age-related changes of BDNF. Additionally, age-related changes in BDNF-receptors expression will be quantified. In Aim 2, we will determine BDNF-mediated auditory plasticity with partial or delayed loss of BDNF. These goals will be accomplished using inducible cre lines (e.g. Rosa26-creER) to eliminate all BDNF at various stages of aging from ~3-week to 2-year old mice. We will determine the age-related cellular properties of auditory neurons (e.g. SGNs). Finally, in Aim 3, we will identify BDNF-mediated neural and synaptic plasticity with partial and delayed loss of BDNF. We will use the animal models outlined in Aim 2 to identify changes in synaptic function at the calyx of Held, due to BDNF loss/decline. This central auditory synapse, originates in the ventral cochlear nucleus (VCN), and project contralaterally to the medial nucleus of the trapezoid body (MNTB), and is found to undergo morphological and molecular alterations during aging. We will also examine CN functional changes. Thus, we will resolve how the expression of BDNF impacts SGN/VCN/MNTB functions during aging, providing evidence for BDNF signaling as a common cause for auditory decline. This knowledge will inform efforts to use BDNF in therapeutic strategies to preserve auditory neuron viability and function after ARHL.