M1胆碱受体对AMPA受体GluA1亚基的调控及其在突触长时程增强和学习记忆中的作用及机制

The accurate and complex interactions between neurotransmitters and receptors form the mechanistic bases for learning and memory. It has been widely demonstrated that both M1 muscarinic acetylcholine receptor and AMPA receptor (an ionotropic glutamate receptor) play important roles in the regulation of the synaptic strength and learning and memory. However, the relationship between them has not been fully clarified. Moreover, M1 agonists are considered to be potential treatment for Alzheimer's disease (AD). However, AMPA receptor has been reported to be reduced in AD brains. Therefore, it is necessary to explore the modulation of AMPA receptor by M1 receptor, in order to further elucidate the molecular mechanisms of synaptic plasticity and cognitive function and to examine the efficiency of M1 agonist in the treatment of AD. In our preliminary study, we demonstrated that activation of M1 receptor could increase the number of GluA1 subunit of AMPA receptor on cell surface of cultured primary hippocampal neurons. Further, activation of M1 receptor modulated the phosphorylation of GluA1, which has been shown to be critical for the function of the receptor. Based on our preliminary results, we will study the details of the modulation of GluA1 by activation of M1 receptor and the underlying mechanism through live cell imaging, immunoflurescence of postsynaptic densities and site-directed mutagenesis. We will investigate the role of such modulation in long-term potentiation and in learning and memory by employing the phosphomutant mice of GluA1. Furthermore, we will investigate the regulation of GluA1 by M1 recptor in AD animal models and elucidate its role in synaptic plasticity and learning and memory in AD. The study may provide new insights of the mechanisms underlying function of M1 receptor and it may give a chance to the development of novel methods to combat the impairment of cognitive function in Alzheimer's disease.

学习记忆等高级认知活动是神经递质、受体之间精细、复杂的调控结果。大量研究表明M1胆碱受体与离子型谷氨酸受体AMPA受体都对突触长时程增强（LTP）和学习记忆有密切关系，然而两者之间的相互调控尚未明了。并且M1受体激动剂被认为是治疗阿尔茨海默病（AD）的有效药物，但AD患者脑内AMPA受体下调。因此有必要深入研究M1受体对AMPA受体的调控作用，将有助于阐明学习记忆的生理机制，并进一步探明M1受体激动剂治疗AD的有效性。我们前期研究发现激动M1受体增加AMPA受体GluA1亚基在海马神经元表面的数量并增加其磷酸化程度。本课题拟深入研究M1受体调控GluA1亚基的作用及机制；研究该作用在M1受体增强LTP和整体学习记忆中的作用；明确AD病理情况下M1受体对GluA1亚基的调控及其在M1受体增强LTP和学习记忆中的作用。本课题是对M1受体调控学习记忆作用机制的新探索，并可能为AD治疗提供新思路。

M1毒蕈碱型胆碱受体（M1受体）在学习和记忆等高级认知活动中起重要作用，其功能的改变与缺失和阿尔茨海默病（Alzheimer’s disease, AD）的学习记忆减退之间存在密切关系。离子型谷氨酸受体AMPA受体是哺乳动物脑内介导快速兴奋性突触传递的最主要的受体，调控突触可塑性和学习记忆。但M1受体和AMPA受体在学习记忆和突触可塑性方面的相互作用尚不清楚。通过本项目，我们发现海马组织M1受体基础活性有助于维持AMPA受体GluA1亚基的正常表达并介导水迷宫空间参考记忆所诱导的GluA1亚基转位至突触后膜。M1受体激动促进GluA1亚基转位至神经元细胞膜表面和突触后膜以及增强长时程增强的作用与其磷酸化GluA1 Ser845有关。进一步研究表明M1受体通过cAMP-PKA-PI3K-Akt-mTOR信号通路增加GluA1亚基Ser845的磷酸化。在体实验表明激动M1受体可提升正常小鼠的水迷宫空间参考记忆和工作记忆，并能够逆转β-淀粉样蛋白聚合物所造成的学习记忆障碍，这些作用都与M1受体调控GluA1 Ser845磷酸化有关，GluA1 Ser845突变小鼠中激动M1受体促进学习记忆的作用缺失。重复激动M1受体还可增加β-淀粉样蛋白聚合物所致学习记忆障碍小鼠海马内GluA1的表达，但这一作用在9月龄APP/PS1 转基因AD模型小鼠中缺失，并且不能改善APP/PS1小鼠学习记忆。另外我们还发现激动M1受体可以增强空间反向学习，该作用与M1受体介导的长时程抑制依赖于AMPA受体GluA2亚基，并且这些作用在缺失GluA2 Ser880位点被PKC磷酸化的突变小鼠中消失，这些结果表明M1受体通过调控GluA2亚基增强认知灵活性。上述研究成果揭示了M1受体通过调控AMPA受体GluA1和GluA2亚基提升和改善学习记忆以及增强认知灵活性，并且GluA1的表达和维持与M1受体活性有关，有助于深入理解学习记忆的生理机制，对认识M1受体和AMPA受体协同调控在学习记忆中的作用具有重要的理论价值。并且由于认知灵活性的损害在AD中先于记忆损害发生，并且激动M1受体促进学习记忆中的作用在9月龄AD转基因模型鼠中效果不佳，因此本项目也提示以M1受体为靶标的药物更适用于AD前阶段和AD早期，为临床药物开发和指导临床用药提供了理论依据。