Prions activate a p38 MAPK synaptotoxic signaling pathway

Synaptic degeneration is one of the earliest pathological correlates of prion disease, and it is a major determinant of the progression of clinical symptoms. However, the cellular and molecular mechanisms underlying prion synaptotoxicity are poorly understood. Previously, we described an experimental system in which treatment of cultured hippocampal neurons with purified PrPSc, the infectious form of the prion protein, induces rapid retraction of dendritic spines, an effect that is entirely dependent on expression of endogenous PrPC by the target neurons. Here, we use this system to dissect pharmacologically the underlying cellular and molecular mechanisms. We show that PrPSc initiates a stepwise synaptotoxic signaling cascade that includes activation of NMDA receptors, calcium influx, stimulation of p38 MAPK and several downstream kinases, and collapse of the actin cytoskeleton within dendritic spines. Synaptic degeneration is restricted to excitatory synapses, spares presynaptic structures, and results in decrements in functional synaptic transmission. Pharmacological inhibition of any one of the steps in the signaling cascade, as well as expression of a dominant-negative form of p38 MAPK, block PrPSc-induced spine degeneration. Moreover, p38 MAPK inhibitors actually reverse the degenerative process after it has already begun. We also show that, while PrPC mediates the synaptotoxic effects of both PrPSc and the Alzheimer’s Aβ peptide in this system, the two species activate distinct signaling pathways. Taken together, our results provide powerful insights into the biology of prion neurotoxicity, they identify new, druggable therapeutic targets, and they allow comparison of prion synaptotoxic pathways with those involved in other neurodegenerative diseases.

突触变性是朊病毒病（prion disease）最早出现的病理相关特征之一，同时也是临床症状进展的核心决定因素。然而，朊病毒突触毒性（prion synaptotoxicity）背后的细胞与分子机制仍鲜为人知。
此前本课题组曾报道一种实验体系：使用纯化的感染型朊蛋白（PrPSc），即朊蛋白（prion protein）的感染型，处理培养的海马神经元（cultured hippocampal neurons），可诱导树突棘（dendritic spines）快速回缩，该效应完全依赖靶神经元表达内源性细胞型朊蛋白（PrPC）。本研究在此体系基础上，通过药理学手段解析其背后的细胞与分子机制。
研究发现，PrPSc可启动逐步级联的突触毒性信号通路，该通路涵盖NMDA受体（NMDA receptors）激活、钙离子内流、p38丝裂原活化蛋白激酶（p38 MAPK）及多种下游激酶的活化，以及树突棘内肌动蛋白细胞骨架（actin cytoskeleton）的解聚。该突触变性仅局限于兴奋性突触，不会累及突触前结构，并可导致功能性突触传递功能受损。
对该信号通路任一环节进行药理学抑制，或是表达p38 MAPK的显性负性突变体（dominant-negative form），均可阻断PrPSc诱导的树突棘变性。此外，在变性进程启动后，p38 MAPK抑制剂甚至可逆转该退行性过程。
本研究同时证实，尽管在该体系中PrPC可介导PrPSc与阿尔茨海默病Aβ肽（Alzheimer’s Aβ peptide）的突触毒性效应，但二者所激活的信号通路并不相同。
综上，本研究结果为朊病毒神经毒性的生物学特性提供了关键见解，鉴定出全新的可靶向治疗的药物靶点，同时也为对比朊病毒突触毒性通路与其他神经退行性疾病的相关通路提供了重要研究基础。