Role of BACE in the pathogenesis of Alzheimer's disease after head trauma

描述（由申请人提供）：创伤性脑损伤（TBI）是阿尔茨海默病（AD）最强的环境风险因素。临床和实验性TBI与加速的β-淀粉样蛋白（Abeta）沉积相关，这是AD病理学的标志。A β肽是通过淀粉样前体蛋白（APP）的连续蛋白水解而衍生的，N-末端的β-分泌酶随后是C-末端的γ-分泌酶。β-位点APP裂解酶（BACE）已被鉴定为β-分泌酶。BACE水平在AD脑中升高，并且BACE在啮齿动物脑缺血和TBI后作为应激相关蛋白酶被诱导。我们最近报道，由于BACE蛋白的翻译后稳定化，在体内脑缺血后BACE和β-分泌酶活性增加，在体外caspase活化。我们还发现，BACE降解受损是由于胱天蛋白酶介导的GGA 3消耗所致，GGA 3是一种参与BACE运输的衔接蛋白（Tesco et al. 2007）。在目前的提案中，我们报告说，GGA 3的基因消融增加了体内大脑中BACE的水平。此外，我们发现GGA 3在TBI后耗尽，而BACE蛋白水平以与脑缺血后观察到的模式相似的模式增加。这些新的研究结果表明，GGA 3耗尽，介导的半胱天冬酶裂解，以及随之而来的BACE上调可能是共同的潜在机制增加A？在脑缺血和TBI后产生。由于Abeta已被证明会损害突触传递，因此Abeta水平升高可能是TBI后功能结果受损的原因。这种机制也可以解释TBI如何导致随着时间的推移发展AD的风险增加。为了支持我们的假设，我们发现AD受试者的颞叶皮层和小脑中的GGA 3水平均降低，这表明GGA 3水平较低的受试者发生AD的风险可能更大（Tesco et al. 2007）。这对于中风和TBI患者尤其如此，其中半胱天冬酶激活甚至发生在损伤后的慢性期。该提案的长期目标是确定新治疗的目标，以预防急性学习和记忆缺陷以及TBI后AD的发展。我们提出了以下具体目标：1）确定GGA 3的耗竭调节BACE水平和活性并导致小鼠行为变化的程度; 2）确定GGA 3水平降低对小鼠TBI后BACE水平和活性以及功能结果的影响程度; 3）确定GGA 3缺乏或低水平加重A？在正常条件下和TBI后，在AD病理学的小鼠模型（表达具有“Swedish”突变（KM 670/671 NL）的人APP的Tg 2576转基因小鼠）中的沉积。公共卫生相关性：创伤性脑损伤（TBI）会增加阿尔茨海默病（AD）的风险，这一点已被人们了解多年，但其潜在的机制尚不清楚。我们的工作表明，头部创伤可以引发一系列生化事件，增加β-淀粉样蛋白的产生，β-淀粉样蛋白是一种在AD患者大脑中积累的有毒肽。因此，我们建议使用小鼠模型，以确定是否抑制？分泌酶是负责产生淀粉样蛋白-β的酶之一，改善认知缺陷并减少头部创伤后的AD病理。此外，我们建议使用GGA 3基因已被删除的小鼠模型，以确定是否低水平的贩运分子GGA 3，调节β-分泌酶，可能是一种新的风险因素，为发展认知障碍和AD TBI后。总的来说，这项研究的结果将有助于确定新的治疗方法，以防止TBI受试者的短期认知缺陷和AD的发展。

DESCRIPTION (provided by applicant): Traumatic brain injury (TBI) is the strongest environmental risk factor for Alzheimer's disease (AD). Clinical and experimental TBI is associated with accelerated beta-amyloid (Abeta) deposition, a hallmark of AD pathology. The Abeta peptide is derived by serial proteolysis of amyloid precursor protein (APP) by beta-secretase at the N-terminus followed by gamma-secretase at the C-terminus. Beta-site APP-cleaving enzyme (BACE) has been identified as beta-secretase. BACE levels are elevated in AD brain, and BACE is induced as a stress-related protease following cerebral ischemia and TBI in rodents. We recently reported that BACE and beta-secretase activity increase following cerebral ischemia in vivo and caspase activation in vitro due to post-translational stabilization of BACE protein. We also found that the impaired degradation of BACE is due to caspase- mediated depletion of GGA3, an adaptor protein involved in BACE trafficking (Tesco et al. 2007). In the current proposal, we report that genetic ablation of GGA3 increases levels of BACE in the brain in vivo. Furthermore, we have found that GGA3 is depleted following TBI while BACE protein levels increase with a pattern similar to the one observed following cerebral ischemia. These new findings indicate that GGA3 depletion, mediated by caspase cleavage, and consequent BACE upregulation may be the common underlying mechanism of increased A? production following cerebral ischemia and TBI. Since Abeta has been shown to impair synaptic transmission, increased Abeta levels may be responsible for impaired functional outcome after TBI. This mechanism may also explain how TBI leads to increased risk of developing AD over time. In support of our hypothesis, we have found that GGA3 levels are decreased in both temporal cortex and cerebellum from AD subjects, suggesting that subjects with lower levels of GGA3 could be at greater risk of developing AD (Tesco et al. 2007). This may be particularly true for patients with stroke and TBI in which caspase activation occurs even in the chronic period after injury. The long-term goal of this proposal is to identify targets for novel treatments to prevent acute learning and memory deficits as well as development of AD following TBI. We propose the following specific aims: 1) determine the extent to which depletion of GGA3 regulates levels and activity of BACE and causes behavioral changes in mice; 2) determine the extent to which decreased levels of GGA3 affect BACE levels and activity and functional outcome following TBI in mice; 3) determine the extent to which lack or low levels of GGA3 exacerbate A? deposition in a mouse model of AD pathology (Tg2576 transgenic mice expressing human APP with the "Swedish" mutation (KM670/671NL)) in normal conditions and following TBI. PUBLIC HEALTH RELEVANCE: It has been known for several years that traumatic brain injury (TBI) can increase the risk of Alzheimer' s disease (AD), but the mechanism underlying that increased risk has not been understood. Our work shows that head trauma, can trigger a series of biochemical events that increase the production of beta-amyloid, the toxic peptide that accumulates in the brain of AD patients. Thus, we propose to use mouse models to determine whether the inhibition of ?-secretase, one of the enzymes responsible for the production of amyloid-beta, ameliorates the cognitive deficits and reduces AD pathology following head trauma. Furthermore, we propose to determine whether low levels of the trafficking molecule GGA3, which regulates b-secretase, may represent a novel risk factor for the development of cognitive deficits and AD following TBI using a mouse model in which the GGA3 gene has been deleted. Overall, the outcome of this study will help to identify novel therapies to prevent both short-term cognitive deficits and the development of AD in subjects affected by TBI.