Amyloid precursor protein (APP) is predominantly located in synapses of neurons and its mutations have been well recognized as the most important genetic causal factor for the familial Alzheimer’s disease (AD). While most disease-causal mutations of APP occur within the Aβ-coding region or immediately proximal, the pathological impacts of mutations in the N-terminus of APP protein, which remote from the Aβ sequence, on neuron and synapse are still largely unknown. It was recently reported a pathogenic APP N-terminal Val225Ala mutation (APP_V225A) with clinically featuring progressive dementia and typical AD pathologies in brain. In our present study, we further found that APP_V225A mutation alters the N-terminal structure of APP, which enhances its binding affinity to tau protein and significantly increases APP-mediated endocytosis. Consequently, APP_V225A promotes the uptake of extracellular tau into SH-SY5Y cells, further linking the structural change in APP to intracellular tau accumulation. In addition, APP_V225A also notably alters the liquid-liquid phase separation (LLPS) of intracellular tau and intensified tau phosphorylation and aggregation in SH-SY5Y cells. Moreover, APP_V225A promote AD-like tau pathology and synaptic damages in human induced pluripotent stem cells (hiPSCs)-derived neural progenitor cells and neurons, as well as in hiPSCs-derived human brain organoids and mouse brain, which can be ameliorated by tau knockdown. Proximity labeling identified several key APP_V225A-interacting proteins, including HS3ST3A1, which was shown to directly regulate tau LLPS and phosphorylation. These findings nicely build on our previous work on roles for APP in tau-related pathological phenotypes and further highlight the involvement of N-terminal APP as the key region for both amyloidopathy and tauopathy, two aspects of AD pathogenesis and progression. Our study may also provide a theoretical breakthrough for AD therapy and highlight the important hub roles of APP and making previously neglected N-terminal APP as a potential target for the discovery of novel disease-modifying therapeutic agents against AD, holding significant scientific values and clinical promise.

淀粉样蛋白前体蛋白 （APP） 主要位于神经元的突触中，其突变已被公认为家族性阿尔茨海默病 （AD） 最重要的遗传致病因素。虽然 APP 的大多数疾病致病突变发生在 Aβ 编码区或紧邻近端，但远离 Aβ 序列的 APP 蛋白 N 端突变对神经元和突触的病理影响在很大程度上仍然未知。最近报道了致病性 APP N 端 Val225Ala 突变 （APP_V225A），临床特征为进行性痴呆和典型的大脑 AD 病理。在我们目前的研究中，我们进一步发现 APP_V225A 突变改变了 APP 的 N 端结构，从而增强了其与 tau 蛋白的结合亲和力，并显着增加了 APP 介导的内吞作用。因此，APP_V225A 促进细胞外 tau 摄取到 SH-SY5Y 细胞中，进一步将 APP 的结构变化与细胞内 tau 积累联系起来。此外，APP_V225A 还显着改变了细胞内 tau 的液-液相分离 （LLPS），并增强了 SH-SY5Y 细胞中的 tau 磷酸化和聚集。此外，APP_V225A 在人诱导多能干细胞 （hiPSC） 衍生的神经祖细胞和神经元以及 hiPSC 衍生的人脑类器官和小鼠脑中促进 AD 样 tau 病理和突触损伤，这可以通过 tau 敲低来改善。邻近标记鉴定了几种关键的 APP_V225A 相互作用蛋白，包括 HS3ST3A1，其被证明可以直接调节 tau LLPS 和磷酸化。 这些发现很好地建立了我们之前关于 APP 在 tau 相关病理表型中的作用的工作，并进一步强调了 N 端 APP 作为淀粉样变和 tau 病的关键区域，这是 AD 发病机制和进展的两个方面。我们的研究也可能为 AD 治疗提供理论突破，突出 APP 的重要枢纽作用，并使以前被忽视的 N 端 APP 成为发现针对 AD 的新型疾病修饰治疗药物的潜在靶点，具有重要的科学价值和临床前景。