Disrupted Transport of NGF-TrKA Signaling in Mouse Models of Down Syndrome

描述（由申请人提供）：阿尔茨海默病（AD）的特征是基底前脑胆碱能神经元（BFCN）的功能障碍和丧失，其变性导致认知困难。该项目的长期目标是确定BFCN退化的细胞和分子基础。一个线索是AD的特征，包括BFCN变性，存在于患有唐氏综合征（DS）（即21三体）的老年人中，其中许多人还表现出进行性认知下降。为了将21号染色体上一个或多个pf基因的表达增加与BFCN变性联系起来，研究了DS的遗传模型Ts 65 Dn小鼠。我们发现BFCN的变性与神经生长因子（NGF）的逆行轴突运输失败有关。在最近的研究中，我们发现，失败的NGF运输和变性的BFCN是由增加的淀粉样前体蛋白（APR）的基因表达，目前在这些小鼠中的三个副本。转运缺陷在转野生型人APR或导致AD的突变型APR的转基因小鼠中重现。初步数据表明，增加的APR C-末端片段（CTF）内体破坏神经生长因子的运输。我们的假设是，在DS中，内体内全长APR和/或其跨膜C末端片段（CTF）的增加起到抑制NGF和NGF-TrkA信号传导的逆行转运的作用，从而导致神经元功能障碍和变性。使用Ts 65 Dn和转基因APR小鼠，我们将：1）进一步表征由APR表达增加导致的轴突结构和功能的缺陷; 2）确定APR表达增加是否降低BFCN的轴突和细胞体中的NGF-TrkA信号传导，并确定所涉及的细胞区室; 3）显示失败的NGF-TrkA信号传导是否是BFCN变性和异常海马学习的原因;和4）在体外定义APR过表达起作用的机制。使用一个培养系统，允许精确跟踪的NGF运输，并建立在初步研究表明，Ts 65 Dn DRG神经元也表现出显着的缺陷，在NGF运输，我们将确定哪些APR亚型负责中断运输和信号传导，并辨别所采用的机制。这些研究是阐明APR表达增加背景下BFCN神经变性发病机制的重要第一步，并可能激发新的治疗策略。

DESCRIPTION (provided by applicant): Alzheimer's disease (AD) features the dysfunction and loss of basal forebrain cholinergic neurons (BFCNs) whose degeneration contributes to cognitive difficulties. The long term goal of this project is to define the cellular and molecular basis for the degeneration of BFCNs. One clue is that the hallmarks of AD, including BFCN degeneration, are present in elderly people with Down syndrome (DS) (i.e. trisomy 21), many of whom also show progressive cognitive decline. To link increased expression of one or more pf the genes on chromosome 21 to BFCN degeneration examined the Ts65Dn mouse, a genetic model for DS. We showed that degeneration of BFCNs is linked to failed retrograde axonal transport of nerve growth factor (NGF). In recent studies, we showed that failed NGF transport and degeneration of BFCNs are caused by increased expression of the gene for the amyloid precursor protein (APR), present in three copies in these mice. The defect in transport was recapitulated in mice transgenic either for wild type human APR or for a mutant APR that causes AD. Preliminary data suggest that increased APR C-terminal fragments (CTFs) within endosomes disrupts NGF transport. Our hypothesis is that in DS an increase in full length APR, and/or its transmembrane C-terminal fragments (CTFs), within endosomes acts to inhibit retrograde transport of NGF and NGF-TrkA signaling leading to neuronal dysfunction and degeneration. Using Ts65Dn and transgenic APR mice we will: 1) characterize further the defects in axonal structure and function that result from increased expression of APR; 2) determine whether or not increased expression of APR decreases NGF- TrkA signaling in the axons and cell bodies of BFCNs and to define the cellular compartment involved; 3) show whether or not failed NGF-TrkA signaling is responsible for BFCN degeneration and abnormal hippocampal learning; and 4) define in vitro the mechanism by which APR overexpression acts. Using a culture system that allows for precise tracking of NGF transport, and building upon preliminary studies showing that Ts65Dn DRG neurons also show a marked deficit in NGF transport, we will determine which APR isoforms are responsible for disrupted transport and signaling and discern the mechanism(s) employed. These studies are an important first step in clarifying the pathogenesis of BFCN neurodegeneration in the setting of increased APR expression and may motivate novel treatment strategies.