Reduced TrkB and BACE1 Elevation: Potential Link in Alzheimer's Pathogenesis

描述（由申请方提供）：脑源性神经营养因子（BDNF）及其受体原肌球蛋白相关激酶B（Trk B）在神经元可塑性和存活中起关键作用。越来越多的证据表明，BDNF-TrkB信号转导缺陷可能介导淀粉样蛋白（？）在阿尔茨海默病（AD）中诱导神经毒性、突触功能障碍和记忆缺陷。然而，还没有研究是否BDNF-TrkB功能障碍也可能有助于致病机制上游到A？积累有趣的是，我们最近的研究表明，选择性TrkB激动剂7，8-二羟基黄酮不仅恢复受损的记忆和TrkB信号，但也降低A？通过降低？AD小鼠模型中的分泌酶BACE1。在R03申请中，我们将测试我们的重点假设，即在早期AD或其先兆轻度认知障碍（MCI）中发现的TrkB信号传导减少可能导致BACE1升高，从而引发和/或加速疾病进程。 具体而言，我们将使用（i）药理学（ANA-12，一种选择性和血脑屏障渗透性TrkB拮抗剂）和（ii）遗传学（TrkB单倍不足）方法，以实验方式重现与MCI或早期AD相关的TrkB信号传导受损。在目标1 - 1中，这些方法将应用于年轻的5XFAD转基因小鼠，其发展很少或仅微弱的淀粉样蛋白病理，并且记忆功能仍然正常。我们将首先比较ANA-12处理的5XFAD或TrkB +/-.5XFAD双基因小鼠（3月龄）与年龄匹配的5XFAD对照小鼠中的BACE1表达水平。然后，我们将探索TrkB信号传导的药理学和遗传学减少可能导致BACE1升高的分子机制（例如，转录、翻译、蛋白质稳定性的变化）并加重AD样表型，包括这些5XFAD小鼠中的记忆缺陷。在目标1 - 2中，我们将进一步测试ANA-12或TrkB单倍不足的施用是否足以上调野生型小鼠中的BACE1。鉴于A？42可能对AD进展过程中淀粉样斑块周围神经元中BACE1表达的增加很重要，确定TrkB信号传导减少和BACE1升高在完全缺乏毒性人A？物种如果数据是阳性的，我们将检查在野生型小鼠中TrkB信号转导受损后，与5XFAD小鼠中发现的机制是否相同，是否可以介导BACE1升高，无论是否存在人A？。 总之，拟议的研究预计将解决一个关键问题，是否缺陷的BDNF-TrkB信号可能不仅是记忆障碍的介导者作为A？这种作用不仅与BACE1的积累有关（如先前的研究所示），而且还通过BACE1升高在启动或加速AD过程中起关键作用。

DESCRIPTION (provided by applicant): Brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) play a crucial role in neuronal plasticity and survival. Increasing evidence suggests that deficient BDNF-TrkB signaling may mediate amyloid- (?)-induced neurotoxicity, synaptic dysfunction and memory deficits in Alzheimer's disease (AD). However, it has not been examined whether BDNF-TrkB dysfunction may also contribute to pathogenic mechanisms upstream to A? accumulation. Interestingly, our recent study demonstrated that the selective TrkB agonist 7,8-dihydroxyflavone not only restores impaired memory and TrkB signaling but also lowers A? levels by reducing the ?-secretase enzyme BACE1 in a mouse model of AD. In this R03 application, we will test our focused hypothesis that reduction in TrkB signaling, which is found in early AD or its harbinger mild cognitive impairment (MCI), may be responsible for inducing BACE1 elevation and consequently initiating and/or accelerating disease processes. Specifically, we will use (i) pharmacological (ANA-12, a selective and blood-brain barrier permeable TrkB antagonist) and (ii) genetic (TrkB haploinsufficiency) approaches to experimentally reproduce the impairment of TrkB signaling associated with MCI or incipient AD. In Aim 1-1, these approaches will be applied to young 5XFAD transgenic mice, which develop little or only faint amyloid pathology and are still normal in memory function. We will first compare the levels of BACE1 expression in ANA-12-treated 5XFAD or TrkB+/-.5XFAD bigenic mice (3-month old) with those of age-matched 5XFAD control mice. We will then explore the molecular mechanisms by which pharmacological and genetic reduction of TrkB signaling may cause BACE1 elevation (e.g., changes in transcription, translation, protein stability) and exacerbate AD-like phenotypes including memory deficits in these 5XFAD mice. In Aim 1-2, we will further test whether administration of ANA-12 or TrkB haploinsufficiency may be sufficient to upregulate BACE1 in wild-type mice. Given that A?42 may be important for increasing BACE1 expression in neurons around amyloid plaques during the progression of AD, it will be critical to determine a causal relationship between reduced TrkB signaling and BACE1 elevation in wild- type controls that completely lack toxic human A? species. If the data is positive, we will examine whether the same mechanisms as found in 5XFAD mice may mediate BACE1 elevation following impaired TrkB signaling in wild-type mice, regardless of the presence of human A?. Together, the proposed research is expected to address a key question whether deficient BDNF-TrkB signaling may not only be a mediator of memory impairments as a consequence of A? accumulation (as shown by previous studies) but also have a crucial role in initiating or accelerating the AD process via BACE1 elevation.