CDK5 Is Essential for Soluble Amyloid β-Induced Degradation of GKAP and Remodeling of the Synaptic Actin Cytoskeleton

The early stages of Alzheimer's disease are marked by synaptic dysfunction and loss. This process results from the disassembly and degradation of synaptic components, in particular of scaffolding proteins that compose the post-synaptic density (PSD), namely PSD95, Homer and Shank. Here we investigated in rat frontal cortex dissociated culture the mechanisms involved in the downregulation of GKAP (SAPAP1), which links the PSD95 complex to the Shank complex and cytoskeletal structures within the PSD. We show that Aβ causes the rapid loss of GKAP from synapses through a pathway that critically requires cdk5 activity, and is set in motion by NMDAR activity and Ca2+ influx. We show that GKAP is a direct substrate of cdk5 and that its phosphorylation results in polyubiquitination and proteasomal degradation of GKAP and remodeling (collapse) of the synaptic actin cytoskeleton; the latter effect is abolished in neurons expressing GKAP mutants that are resistant to phosphorylation by cdk5. Given that cdk5 also regulates degradation of PSD95, these results underscore the central position of cdk5 in mediating Aβ-induced PSD disassembly and synapse loss.

阿尔茨海默病（Alzheimer's disease）的早期阶段以突触功能异常与突触丢失为特征。该过程源于突触组分的解离与降解，尤其是构成突触后致密区（postsynaptic density, PSD）的支架蛋白，即PSD95、Homer与Shank。本研究以大鼠额叶皮层解离培养物为模型，探究了GKAP（SAPAP1）的下调机制——GKAP可将PSD95复合物与Shank复合物及突触后致密区内的细胞骨架结构相连。研究结果显示，β淀粉样蛋白（Aβ）可通过一条严格依赖细胞周期蛋白依赖性激酶5（cdk5）活性的通路，快速引发突触内GKAP的丢失，该通路由N-甲基-D-天冬氨酸受体（NMDAR）活性与钙离子（Ca²+）内流启动。本研究还证实，GKAP是cdk5的直接底物，其磷酸化可导致GKAP发生多泛素化与蛋白酶体降解，并引发突触肌动蛋白细胞骨架的重塑（坍塌）；在表达对cdk5磷酸化具有抗性的GKAP突变体的神经元中，该后续效应被完全阻断。鉴于cdk5还可调控PSD95的降解，上述研究结果凸显了cdk5在介导Aβ诱导的突触后致密区解离与突触丢失过程中的核心地位。