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）最强的环境危险因素。临床和实验的创伤性脑损伤与β -淀粉样蛋白（Abeta）沉积加速有关，这是AD病理的一个标志。β肽是由淀粉样蛋白前体蛋白（APP）通过n端β分泌酶和c端γ分泌酶的连续蛋白水解而产生的。β位点app切割酶（BACE）已被鉴定为β分泌酶。BACE水平在AD脑中升高，BACE作为应激相关蛋白酶在脑缺血和TBI后被诱导。我们最近报道由于BACE蛋白的翻译后稳定，BACE和β -分泌酶活性在体内脑缺血和体外caspase激活后增加。我们还发现BACE降解受损是由于半胱天酶介导的GGA3耗损，GGA3是一种参与BACE运输的衔接蛋白（Tesco et al. 2007）。在目前的建议中，我们报告了GGA3基因消融增加体内大脑中BACE水平。此外，我们发现脑外伤后GGA3被耗尽，而BACE蛋白水平升高，其模式与脑缺血后观察到的相似。这些新发现表明，由caspase裂解介导的GGA3缺失以及随之而来的BACE上调可能是A？脑缺血和TBI后的生产。由于β已被证明会损害突触传递，因此β水平的升高可能是脑外伤后功能受损的原因。这一机制也可以解释脑外伤如何随着时间的推移导致患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GGA3水平都有所下降，这表明GGA3水平较低的受试者患AD的风险更大（Tesco et al. 2007）。对于中风和TBI患者尤其如此，即使在损伤后的慢性时期也会发生caspase激活。这项提议的长期目标是确定新的治疗靶点，以防止急性学习和记忆缺陷以及TBI后AD的发展。我们提出以下具体目标：1)确定GGA3的缺失在多大程度上调节BACE的水平和活性，并导致小鼠的行为改变；2)确定GGA3水平降低对小鼠脑外伤后BACE水平、活性和功能结局的影响程度；3)确定GGA3缺乏或低水平在多大程度上加剧A？在AD病理小鼠模型中（Tg2576转基因小鼠，表达带有“瑞典”突变（KM670/671NL）的人APP），在正常情况下和TBI后沉积。公共卫生相关性：多年来人们已经知道，创伤性脑损伤（TBI）可增加阿尔茨海默病（AD）的风险，但风险增加的机制尚不清楚。我们的研究表明，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，这是一种积聚在AD患者大脑中的有毒肽。因此，我们建议使用小鼠模型来确定？-分泌酶是负责产生淀粉样蛋白的酶之一，它可以改善认知缺陷，减少头部外伤后的AD病理。此外，我们建议通过缺失GGA3基因的小鼠模型来确定低水平的转运分子GGA3（调节b分泌酶）是否可能是脑外伤后认知缺陷和AD发展的一个新的危险因素。总的来说，这项研究的结果将有助于确定新的治疗方法，以防止受创伤性脑损伤影响的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.