红景天苷通过抑制PDE2A调节海马神经元能量代谢和突触可塑性而改善学习记忆的机制研究

Cognitive impairment is one of the main characteristics of brain aging and neurodegenerative diseases such as Alzheimer's disease (AD). Phosphodiesterase 2A (PDE2A), a member of the PDE super family, densely distributed in the hippocampus, hydrolyzes both cAMP and cGMP, recognized as a novel target for discovery of drugs with cognitive improvement. Our previous studies suggested a potent efficacy of salidroside (SAL) on delaying aging and improving the learning and memory function, which is partly related to its anti-inflammatory and antioxidantive potency. However, detailed mechanism regarding its effect on cognitive improvement remains largely unknown. Both findings from our last NSFC work and others' imply that modulation of mitochondrial biogenesis and synaptic plasticity of neurons may contribute to the improvement of learning and memory of SAL. Our preliminary data indicated that SAL has a selective inhibitive role on expression of PDE2A, which is probably through a p53-dependent manner as p53 seems to be a novel transcription factor of PDE2A. This project intends to use the primary hippocampal neurons and natural aging mice model to investigate the following issues by using luciferase reporter gene expression system, inhibitors and other methods: 1) the molecular mechanism of SAL regulating PDE2A expression through p53; 2) the regulatory effect of SAL on the mitochondrial biogenesis in hippocampal neurons resulting from SIRT1 activation via cAMP-CaMKKβ-AMPK signaling pathway; 3) and the modulation of synaptic plasticity of hippocampal neurons by SAL through cAMP-PKA-CREB and / or cGMP-PKG-CREB activation. According to our research proposal, we will provide a novel insight of SAL regarding its promotive effect on learning and memory through inhibition on PDE2A, thus inspiring a potential interventional strategy for brain aging and neurodegenerative diseases by using SAL.

脑老化和神经退行性疾病如AD的主要特征之一是认知功能减退。在海马内密集分布的磷酸二酯酶2A（PDE2A）可水解cAMP和cGMP，是改善认知药物的新靶标。前期研究表明红景天苷（SAL）具有延缓衰老和改善学习记忆的功能，作用机制除抗炎、抗氧化外，可能与改善神经元能量代谢和突触可塑性相关，进一步研究表明SAL具有选择性抑制PDE2A表达的活性，并新发现p53可能是其转录调控因子。本课题拟利用原代海马神经元和自然衰老小鼠模型，采用荧光素酶报告基因系统、过表达和抑制剂等方法，研究1）SAL通过p53调控PDE2A表达的分子机制；2）SAL调控cAMP-CaMKKβ-AMPK信号通路激活SIRT1而改善海马神经元线粒体能量代谢的作用；3）以及通过cAMP-PKA-CREB和/或cGMP-PKG-CREB调节海马神经元突触可塑性的机制，从而揭示SAL通过下调p53抑制PDE2A而改善学习记忆的新机制。

脑老化及衰老相关的神经退行性疾病是危害老年人群健康的重要因素之一。中药及其活性成分是研究干预衰老药物的宝库，通过系统研究证实红景天主要成分之一红景天苷（salidroside，SAL）具有良好的改善脑老化及学习记忆的功能。改善线粒体能量代谢及其功能是延缓脑老化和延迟神经退行性疾病发生的另一有效途径。磷酸二酯酶2A（PDE2A）可水解cAMP和cGMP，进而调控cAMP-CaMKKβ-AMPK以及cAMP-PKA-CREB和/或cGMP-PKG-CREB信号通路调节线粒体能量代谢及神经生长。利用人胚肺成纤维细胞2BS和原代小鼠海马神经元细胞，发现了红景天苷可剂量依赖地抑制PDE2A的基因表达，并且这一作用部分依赖于SAL降调节p53蛋白表达。利用新生小鼠海马神经元发现，SAL 显著促进小鼠海马神经元的神经突起生长以及突起分叉，上调pCREB、BDNF及其受体TrKB的蛋白表达，PKA和CaMKKβ则介导了SAL体外促进神经生长的作用。在体内研究中，衰老的C57BL/6J 小鼠（18月龄）海马神经元中PDE2A的表达本身呈现增龄性下降，与预期相反，提示SAL可能通过其他机制改善学习记忆功能。根据前期发现SAL可以通过激活SIRT1发挥促进线粒体生物合成而调节能量代谢的作用，进一步利用原代小鼠海马神经元细胞和SH-SY5Y细胞发现，SAL具有上调Sirtuins家族的另一分子SIRT3蛋白表达的功能，SIRT3作为定位于线粒体的去乙酰化酶，在通过调节线粒体自噬而维持线粒体稳态中具有重要作用，与衰老相关退行性疾病具有重要联系。同时发现SAL可以通过SIRT3依赖的方式发挥保护神经元损伤及调节线粒体自噬的体外作用。进一步利用5XFAD转基因AD小鼠模型，从体内水平证实SIRT3是介导SAL改善认知功能的关键分子。综上，我们的研究丰富了SAL发挥神经保护功效的作用机制，为其进一步的临床应用提供了更为充分的科学依据。