Stability of synaptic memory engrams

记忆被认为是大脑中持久的生理变化。在小鼠模型中进行的几项研究发现，在记忆形成过程中，大脑各个区域的神经元亚群显示出神经元活动的增加，并表明对这些细胞的操纵可以诱导人工检索或丢失存储的记忆。这表明记忆存储和检索是由特定的神经元群体介导的，因此被认为是细胞印迹。然而，印迹可以在不同的空间尺度上被识别，范围从系统到突触水平。根据Hebbian假设，活动模式高度相关的神经元之间的连接得到加强，而活动模式弱相关的神经元之间的连接则被抑制甚至失去。因此，共同活动神经元之间突触强度的增加增加了在编码过程中发生的相同时空模式的神经活动在稍后的检索过程中再次发生的可能性。这一假说表明，协同激活的神经元之间的突触加强形成了记忆的神经基质，或突触印痕。最近，由于绿色荧光蛋白跨突触伙伴重构（mGRASP）技术，标记印迹神经元之间的突触并研究它们的特性成为可能。该技术允许用GFP表达标记突触前和突触后表达mGRASP成分的神经元之间的突触。将该系统与基于立即-早期基因的印迹神经元标记方法相结合，最终为在突触水平上实现印迹的可视化提供了具体的可能性。在这个项目中，我建议利用小鼠纵向双光子脑深部光学成像来研究CA1结构突触印痕的稳定性。首先，我们将使用mGRASP标记CA3和CA1神经元之间的突触，并使用即时早期基因Arc的启动子将mGRASP前后构建体靶向CA3和CA1神经元，从而标记CA1突触结构。其次，我们将利用光学成像来跟踪CA1突触恐惧印痕在基线、消退和增强下的长期稳定性。这将使我们能够首次研究持续学习对结构突触印痕的影响。

Memories are thought to be encoded as enduring physi¬cal changes in the brain. Several studies in murine models have identified subpopulations of neurons throughout various brain regions showing increased neuronal activity during memory formation and shown that manipulation of these cells can induce either artificial retrieval or loss of stored memories. This demonstrates that memory storage and retrieval are mediated by specific populations of neurons which are thus believed to be cellular engrams. Engrams, however, can be identified at different spatial scales, ranging from the systems to the synaptic levels. According to the Hebbian postulate connections between neurons with highly correlated activity patterns are strengthened while connections between neurons whose activity patterns are weakly correlated are depressed or even lost. Increase in synaptic strength between co-active neurons thus increases the likelihood that the same spatial-temporal pattern of neural activity that occurred dur¬ing encoding will occur again at a later time, during retrieval. This hypothesis suggests that synaptic strengthening between coactivated neurons forms the neural substrate of memory, or a synaptic engram. It has recently become possible to label synapses between engram neurons and study their properties, thanks to the Green fluorescent protein Reconstitution Across Synaptic Partners (mGRASP) technique. This technique allows to label with GFP expression synapses between neurons expressing the presynaptic and the postsynaptic mGRASP components. Combining this system with established methods to label engram neurons based on Immediate-Early genes, finally offers the concrete possibility to visualize the engram at the synaptic level. In this project I propose to investigate the stability of structural synaptic engrams in the CA1 by using longitudinal two-photon deep-brain optical imaging in mice. First, We will label structural CA1 synaptic engrams by using mGRASP to highlight synapses between CA3 and CA1 neurons and by using the promoter of the immediate early gene Arc to target the pre- and post-mGRASP constructs to CA3 and CA1 engram neurons. Second, we will employ optical imaging to track the long-term stability of CA1 synaptic fear engrams under baseline, extinction and reinforcement. This will enable us to investigate for the first time the effects of continued learning on structural synaptic engrams.