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

描述(申请人提供)：创伤性脑损伤(TBI)是阿尔茨海默病(AD)最强烈的环境风险因素。临床和实验性脑损伤与加速的β-淀粉样蛋白(Abeta)沉积有关，这是AD病理的一个标志。Abeta多肽是由淀粉样前体蛋白(APP)在N端的β-分泌酶和C-端的伽马分泌酶的一系列蛋白分解而来的。β-位点APP裂解酶(BACE)被鉴定为β-分泌酶。BACE在AD脑中的水平升高，在啮齿类动物脑缺血和脑损伤后，BACE作为一种应激相关的蛋白被诱导。我们最近报道，由于BACE蛋白翻译后的稳定，体内脑缺血后BACE和β-分泌酶活性增加，体外Caspase激活。我们还发现，BACE的受损降解是由于caspase介导的GGA3的耗尽，GGA3是一种参与BACE运输的接头蛋白(Tesco等人。2007)。在目前的提案中，我们报告了在活体中基因消融GGA3会增加大脑中BACE的水平。此外，我们还发现，脑损伤后GGA3被耗尽，而BACE蛋白水平上升，其模式与脑缺血后观察到的模式相似。这些新的发现表明，由caspase裂解介导的GGA3耗竭以及由此导致的BACE上调可能是A？脑缺血和脑损伤后的产物。由于Abeta已被证明会损害突触传递，Abeta水平的升高可能与脑损伤后的功能结果受损有关。这一机制也可以解释脑外伤是如何随着时间的推移导致AD风险增加的。为了支持我们的假设，我们发现AD患者颞叶皮质和小脑中的GGA3水平都降低，这表明GGA3水平较低的患者患AD的风险更大(Tesco等人。2007)。这可能对中风和脑外伤患者尤其如此，在这些患者中，caspase即使在损伤后的慢性期也会被激活。这项建议的长期目标是确定新的治疗目标，以预防急性学习和记忆障碍以及脑外伤后AD的发展。我们提出了以下具体目标：1)确定GGA3缺乏在多大程度上调节BACE的水平和活性并导致小鼠行为改变；2)确定GGA3水平降低对小鼠脑创伤后BACE水平、活动和功能结局的影响程度；3)确定缺乏或低水平GGA3加重A？阿尔茨海默病小鼠模型(Tg2576转基因小鼠表达带有“瑞典”突变(KM670/671NL)的人APP)在正常条件下和脑损伤后的沉积。公共卫生相关性：多年来，众所周知，创伤性脑损伤会增加患阿尔茨海默氏病(AD)的风险，但增加风险的机制尚不清楚。我们的工作表明，头部创伤可以触发一系列生化事件，增加β-淀粉样蛋白的产生，β-淀粉样蛋白是积累在AD患者大脑中的有毒多肽。因此，我们建议使用小鼠模型来确定抑制β-分泌酶是否改善了头部创伤后的认知障碍和AD病理。此外，我们建议使用GGA3基因缺失的小鼠模型来确定调节b-分泌酶的运输分子GGA3的低水平是否可能是脑损伤后认知障碍和AD发展的新的危险因素。总体而言，这项研究的结果将有助于确定新的治疗方法，以预防受脑外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.