锂对Aβ引起的星形胶质细胞谷氨酸释放的抑制作用及意义

Synaptic loss is the cardinal feature linking neuropathology to cognitive decline in Alzheimer’s disease (AD), however, the mechanism of synaptic damage remains incompletely understood, inhibition of Glutamate-induced neurotoxicity has potential therapeutic benefit for the treatment or prevention of early AD, but there is still a lack of effective treatments and drug targets. Emerging evidence suggests that this injurious effects of amyloid β peptide (Aβ)may be mediated, at least in part, by excessive activation of extrasynaptic NMDARs. Recently, study has found that Aβ can induce astrocytic glutamate release, and eNMDAR stimulation is followed rapidly by a decrease in mEPSC frequency with accompanying generation of nitric oxide (NO), hyperphosphorylation of tau, and activation of caspase-3, these results suggest a glutamate-mediated cascade triggered by Aβ in which early eNMDAR activation may contribute to subsequent synaptic damage and consequent cognitive decline in AD. Microdose lithium treatment can stabilize cognitive impairment in patients with AD, however, its therapeutic mechanism of action remains unclear, Our previous datas have shown that pretreatment with lithium carbonate chronic 2 weeks could both significant suppressed AdA(an agonist of IP3 receptors) and 4-CMC(an agonist of ryanodine receptors)-evoked the increase of intracellular Ca2+ concentration, our results also have shown that lithium carbonate chronic 2 weeks could significant suppressed 45mM K+-evoked the increase of intracellular Ca2+ concentration, therefore, we hypothesized that lithium may work by inhibiting Aβ-induced Ca2+-dependent astrocyte glutamate release through decreasing intracellular Ca2+ concentration, this effect could be important in the treatment or prevention of AD, to confirm this hypothesis, we will use the molecular biology techniques, HPLC and microdialysis techniques, to reveals the mechanism of lithium in the treatment of AD, which may provide a theoretical basis for the development of new anti-AD therapies.

突触的丢失是导致阿尔茨海默病(AD)患者认知功能障碍的主要原因，抑制谷氨酸对突触的兴奋性损伤在AD早期防治中具有重要意义，但目前尚缺乏有效治疗药物及作用靶点。最近研究发现Aβ引起的星形胶质细胞钙离子依赖的谷氨酸释放，是早期突触外NMDA受体活化，进而导致神经元损伤的主要因素。锂抗AD作用机制并未阐明，本课题组首次证明锂可抑制星形胶质细胞内质网上IP3R或RyR激动剂引起的细胞内钙离子的增加，也可抑制细胞外高钾去极化引起的细胞内钙离子增加，因此我们推测，抑制Aβ引起的星形胶质细胞钙离子依赖的谷氨酸释放是其保护神经元的潜在机制。为证实该假说，我们将通过整体和细胞两个层次，采用分子生物学、HPLC和微透析等技术揭示锂对Aβ引起的星形胶质细胞内钙离子、钙通道相关蛋白以及钙离子依赖的谷氨酸释放的抑制作用，并探讨该作用对改善AD模型小鼠学习记忆能力和对抗突触损伤中的意义，为疾病早期防治提供新的靶点。

抑制谷氨酸的兴奋性损伤在阿尔茨海默病中具有治疗意义，但目前尚缺乏有效的治疗药物及作用靶点，星形胶质细胞是脑内数量最多的细胞，是谷氨酸代谢的关键部位，本项目首次以星形胶质细胞为药物作用靶点，从离体和在体水平研究锂对星形胶质细胞对谷氨酸释放的抑制作用，并探讨其机制。该研究结果表明250 nM Aβ能引起星形胶质细胞内Ca2+浓度持久的增加，该作用一方面通过电压依赖性L-channel 钙通道引起外钙内流，另一方面也通过促进细胞内质网中储存的内钙释放。不同浓度的锂作用2周后，可抑制Aβ引起的星形胶质细胞内Ca2+浓度的增加，该作用与其抑制星形胶质细胞内Ca2+再充功能有关。Aβ在抑制Aβ引起的星形胶质细胞内Ca2+浓度的增加的同时，也抑制了Ca2+依赖的谷氨酸的释放。研究表明细胞内Ca2+释放后TRPC是Ca2+再充的主要通道，星形胶质细胞上主要表达TRPC1, TRPC4, 和TRPC5三种亚型，有研究显示TRPC1是星形胶质细胞Ca2+内流的主要通道，能够通过调节维持细胞内Ca2+稳态浓度，RT-PCR研究结果表明锂慢性作用2周可抑制TRPC1mRNA和蛋白表达，但对TRPC4, 和TRPC5无影响。Western blot结果也显示该作用与下调TRPC1有关，而与Na+/Ca2+ exchanger 1(NCX1)无关。有报道长期小剂量锂可改善APP/PS1转基因小鼠的学习记忆，因此，本研究采用APP/PS1转基因小鼠进一步观察并探讨锂的保护作用及机制，微透析检测结果初步证明锂也可抑制脑室内谷氨酸含量，morris水迷宫结果显示给予锂5周，可改善小鼠的学习记忆功能、减轻神经元的损伤，该机制可能与其抑制脑内谷氨酸的释放有关。该研究结果为开发和探寻新的治疗药物提供新思路。