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

描述(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(BFCNs)的功能障碍和丢失，其退化导致认知困难。该项目的长期目标是确定BFCNs退化的细胞和分子基础。一条线索是，包括BFCN变性在内的AD的特征存在于患有唐氏综合症(即21三体)的老年人身上，他们中的许多人也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PF基因的表达增加与BFCN退行性变联系起来，研究了DS的遗传模型Ts65Dn小鼠。我们发现BFCNs的退变与神经生长因子(NGF)的逆行轴突运输失败有关。在最近的研究中，我们发现NGF转运失败和BFCNs变性是由淀粉样前体蛋白(APR)基因表达增加引起的，APR基因在这些小鼠中以三个副本存在。无论是野生型人APR转基因小鼠还是导致AD的突变型APR转基因小鼠，运输缺陷都得到了再现。初步数据表明，内体内APR C末端片段(CTF)的增加扰乱了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末端片段(CTF)的增加抑制了NGF和NGF-TrkA信号的逆行运输，导致神经元功能障碍和变性。使用Ts65Dn和转基因APR小鼠，我们将：1)进一步鉴定因APR表达增加而导致的轴突结构和功能缺陷；2)确定APR表达增加是否会减少BFCNs轴突和细胞体中的NGF-TrkA信号，并确定相关的细胞室；3)证明NGF-TrkA信号失效是否导致BFCN变性和海马区学习异常；以及4)在体外确定APR过表达的作用机制。使用能够精确跟踪神经生长因子运输的培养系统，以及初步研究表明Ts65Dn背根节神经元在神经生长因子运输方面也显示明显缺陷的基础上，我们将确定哪些APR亚型负责中断运输和信号传递，并弄清所采用的机制(S)。这些研究是在APR表达增加的背景下阐明BFCN神经变性的发病机制的重要的第一步，并可能激励新的治疗策略。

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.