GSK-3β上调c-Jun的机制及其在帕金森病多巴胺能神经元死亡中的作用

Our previous work has demonstrated for the first time that GSK-3β plays a critical role in dopaminergic neuronal death in Parkinsonian mouse model by using GSK-3β inhibitors and conditional knockout mice (Gsk3b cKO). We have observed that Gsk3b cKO inhibits upregulation of the protein levels but not the mRNA levels of c-Jun during dopaminergic neurodegeneration, suggesting that GSK-3β increases c-Jun protein stability. It is known that E3 ligase Fbw7 ubiquitinates c-Jun to promote its degradation. Here we find loss of Fbw7 protein expression within the nuclei of nigra dopaminergic neurons in Parkinsonian mice, and Gsk3b cKO attenuates this loss. We thus raise the question that whether GSK-3β upregulates c-Jun by regulation involving subcellular localization of Fbw7. There have been reported Fbw7 exemplifies this type of control. Fbw7 recognises a conserved phosphodegron motif which is often phosphorylated by GSK-3β. Via further analysis, we propose that during dopaminergic neurodegeneration, GSK-3β phosphorylates cytosolic substrates, such as tau. Phosphorylated tau might act like pseudosubstrate directly recruiting Fbw7 and exert competitive inhibition of c-Jun degradation, leading to dopaminergic neuronal death. In this project, we plan to clarify the mechanism(s) underlying GSK-3β-mediated upregulation of c-Jun by using genetically engineered mice and the methods of molecular neurobiology, and to provide scientific evidence to establish GSK-3β as a new therapeutic target of neuroprotection for Parkinson’s disease.

我们前期研究首次在帕金森病(PD)小鼠中借助GSK-3β抑制剂和多巴胺能神经元(DAN)特异敲除鼠(Gsk3b cKO)证实GSK-3β是介导DAN死亡的关键激酶。最近发现Gsk3b cKO明显抑制PD小鼠DAN促死亡转录因子c-Jun蛋白上调而不抑制其mRNA，提示GSK-3β介导c-Jun蛋白稳定。Fbw7是促c-Jun降解的E3连接酶。我们发现PD鼠DAN核内Fbw7减少，可被Gsk3b cKO逆转，且GSK-3β定位于胞浆，是否GSK-3β通过抑制Fbw7核定位稳定c-Jun？文献报道Fbw7可被假底物竞争招募，而其假底物模序符合GSK-3β底物序列，磷酸化促进招募。通过分析胞浆GSK-3β底物，我们提出科学假设：PD时DAN中GSK-3β磷酸化底物如tau，后者作为Fbw7假底物竞争抑制c-Jun降解，导致DAN死亡。本项目拟验证该假设，为以GSK-3β为靶治疗PD提供科学依据。

帕金森病(Parkinson's disease, PD)的病理本质是中脑黑质多巴胺能神经元(dopaminergic neurons, DAN)死亡，但其机制不清。糖原合成激酶-3(GSK-3)在PD DAN死亡发挥重要作用，但机制未完全阐明。我们在MPTP诱导的PD小鼠模型首次发现，GSK-3介导DAN重要的自噬受体p62从胞浆转位入核，使p62胞浆定位减少，后者会加重DAN死亡。此外，我们还首次在 PD 模型上证明了最具临床转化潜力的GSK-3抑制剂tideglusib主要通过抑制GSK-3活性对DA 神经元死亡有保护性干预作用，而且改善了PD小鼠的运动症状。这些发现为确立“以GSK-3为靶”治疗PD的应用前景提供科学依据；为以后开展tideglusib治疗PD的临床试验提供临床前证据支持；有望为PD治疗提供具转化前景的候选药物；也为研发治疗PD的有转化潜力的GSK-3抑制剂提供动力和方向。