APP剪切生成的Aβ聚集为老年斑块是AD发病的重要病理特征之一，但Aβ生成及其参与AD的具体机制尚不明确。申请者发现：E3泛素化连接酶CDT2在AD患者脑内显著升高；过表达CDT2致激酶CDKs活性上调、APP过度磷酸化，并促进APP剪切限速酶BACE1与APP结合，引起毒性Aβ产生增加及认知功能障碍；而抑制CDKs则逆转上述现象。推测：在AD进程中，CDT2上调CDKs活性，促进APP磷酸化，引起过度磷酸化的APP更易与BACE1结合且被剪切，导致Aβ产生增加并聚集形成老年斑，促发AD。本项目拟联合运用基因定点突变、生化分子生物学、免疫荧光和电生理等技术，在细胞和整体水平，确定CDT2上调CDKs活性所致Aβ聚集的关键作用，明确CDT2通过上调CDKs促进APP磷酸化的具体位点，阐明CDT2介导Aβ毒性参与AD的具体分子机制。研究结果将为AD发病机制提出新线索，为AD防治提供新分子靶标。

Aβ(β-amyloid) generated by APP aggregates into senile plaque is one of the important pathological features of AD (Alzheimer's disease), yet the key molecules causing the Aβ toxicity are still unclear. We have previously found that the level of E3-ubiquitin ligase CDT2 is elevated in AD human brain, CDT2 overexpression induces CDKs (cyclin-dependent kinases) activation, APP hyperphosphorylation, and increases the binding of APP to BACE1(β-secretase),which is the rate-limiting enzyme for APP cleavage. This causes an increased production of toxic Aβ and cognitive impairment, suggesting that CDT2 is an important risk factor for AD. CDKs have been found to phosphorylate APP and affect its cleavage. We therefore hypothesize a cascade mechanism of AD pathogenesis in which CDT2 upregulates CDKs activity resulting in hyperphosphorylated APP that is more easily to bind to and be cleaved by BACE1, leading to an increase in the generation of toxic Aβ which aggregates into senile plaque, triggering AD. We are proposing to continue our research to determine the key role of CDT2 in the upregulation of CDKs to induce toxic Aβ accumulation; to reveal the specific APP phosphorylation sites targeted by the CDT2-upregulated CDKs; and to elucidate the specific molecular mechanism of CDT2-mediated Aβ toxicity in AD. These will be investigated by using site-directed mutagenesis, biochemical molecular biology, immunofluorescence and electrophysiology in both cellular and animal models. The results of the above proposed research will raise new clues for the AD pathogenesis，and provide new targets for the prevention and treatment of AD.