Synaptic protein dyshomeostasis and functional loss is an early invariant feature of Alzheimer’s disease (AD), yet the unifying etiological pathway remains largely unknown. Knowing that cyclin-dependent kinase 5 (CDK5) plays critical roles in synaptic formation and degeneration, its phosphorylation targets were reexamined in search of candidates with direct global impacts on synaptic protein dynamics, and the associated regulatory network was also analyzed. Quantitative phosphoproteomics and bioinformatics analyses were performed to identify top-ranked candidates. A series of biochemical assays was used to investigate the associated regulatory signaling networks. Histological, electrochemical, and behavioral assays were performed in conditional knockout, small hairpin RNA–mediated knockdown, and AD-related mice models to evaluate the relevance of CDK5 to synaptic homeostasis and functions. Among candidates with known implications in synaptic modulations, BAG3 ranked the highest. CDK5-mediated phosphorylation on S297/S291 (mouse/human) destabilized BAG3. Loss of BAG3 unleashed the selective protein degradative function of the HSP70 machinery. In neurons, this resulted in enhanced degradation of a number of glutamatergic synaptic proteins. Conditional neuronal knockout of in vivo led to impairment of learning and memory functions. In human AD and related mouse models, aberrant CDK5-mediated loss of BAG3 yielded similar effects on synaptic homeostasis. Detrimental effects of BAG3 loss on learning and memory functions were confirmed in these mice, and such effects were reversed by ectopic BAG3 reexpression. Our results highlight that the neuronal CDK5-BAG3-HSP70 signaling axis plays a critical role in modulating synaptic homeostasis. Dysregulation of the signaling pathway directly contributes to synaptic dysfunction and AD pathogenesis.

中文翻译：

---

细胞周期蛋白依赖性激酶 5 依赖性 BAG3 降解调节突触蛋白周转

突触蛋白动态平衡失调和功能丧失是阿尔茨海默病 (AD) 的早期不变特征，但统一的病因途径仍然很大程度上未知。了解细胞周期蛋白依赖性激酶 5 (CDK5) 在突触形成和退化中发挥关键作用后，我们重新检查了其磷酸化靶点，以寻找对突触蛋白动态具有直接全局影响的候选者，并分析了相关的调控网络。进行定量磷酸蛋白质组学和生物信息学分析以确定排名靠前的候选者。使用一系列生化测定来研究相关的调节信号网络。在条件敲除、小发夹 RNA 介导的敲除和 AD 相关小鼠模型中进行组织学、电化学和行为测定，以评估 CDK5 与突触稳态和功能的相关性。在对突触调节具有已知影响的候选者中，BAG3 排名最高。 CDK5 介导的 S297/S291（小鼠/人）磷酸化使 BAG3 不稳定。 BAG3 的缺失释放了 HSP70 机制的选择性蛋白质降解功能。在神经元中，这导致许多谷氨酸突触蛋白的降解增强。体内条件性神经元敲除导致学习和记忆功能受损。在人类 AD 和相关小鼠模型中，异常 CDK5 介导的 BAG3 缺失对突触稳态产生类似的影响。在这些小鼠中证实了 BAG3 缺失对学习和记忆功能的不利影响，并且这种影响可以通过异位 BAG3 重新表达来逆转。我们的结果强调，神经元 CDK5-BAG3-HSP70 信号轴在调节突触稳态中发挥着关键作用。信号通路的失调直接导致突触功能障碍和 AD 发病机制。