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）以基底前脑胆碱能神经元（BFCNs）功能障碍和丧失为特征，其退化导致认知困难。该项目的长期目标是确定BFCNs变性的细胞和分子基础。一条线索是阿尔茨海默病的特征，包括BFCN变性，存在于患有唐氏综合症（DS）（即21三体）的老年人中，其中许多人还表现出进行性认知能力下降。为了将21号染色体上一个或多个基因的表达增加与BFCN变性联系起来，研究了DS的遗传模型Ts65Dn小鼠。我们发现BFCNs的变性与神经生长因子（NGF）逆行轴突运输失败有关。在最近的研究中，我们发现NGF运输失败和BFCNs变性是由淀粉样蛋白前体蛋白（APR）基因表达增加引起的，在这些小鼠中存在三个拷贝。在野生型人类APR或引起AD的突变型APR转基因小鼠中再现了运输缺陷。初步数据表明，核内体中APR c末端片段（CTFs）的增加会破坏NGF的运输。我们的假设是，在DS中，内体内全长APR和/或其跨膜c端片段（CTFs）的增加可抑制NGF和NGF- trka信号的逆行运输，从而导致神经元功能障碍和变性。利用Ts65Dn和转基因APR小鼠，我们将：1)进一步表征APR表达增加导致的轴突结构和功能缺陷；2)确定APR表达增加是否会降低BFCNs轴突和细胞体中NGF- TrkA信号，并确定所涉及的细胞区室；3)研究NGF-TrkA信号通路失败是否导致BFCN变性和海马学习异常；4)确定体外APR过表达的作用机制。利用能够精确跟踪NGF运输的培养系统，并在初步研究表明Ts65Dn DRG神经元也显示出NGF运输明显缺陷的基础上，我们将确定哪些APR异构体负责运输和信号传导中断，并辨别所采用的机制。这些研究是阐明在APR表达增加的情况下BFCN神经退行性变发病机制的重要的第一步，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.