Role of MEF2 and neural activity in cortical synaptic weakening and elimination

项目概述：皮质结构（海马和新皮质）对认知和 认知能力和它们的不正常功能与智力残疾和自闭症有关。建立 正常的皮层回路需要神经活动和控制大脑活动的遗传程序之间复杂的相互作用。 特定突触连接的形成和消除。结构兴奋性突触的研究，如 spines发现，在出生后发育的早期有一个快速的突触发生期， 消除（或修剪）--在人类和小鼠中。重要的是，感官体验和回路活动 在体内修剪刺。脊柱消除也是由成人的学习触发的， 维持记忆的电路的精细化。然而，棘是突触的间接测量， 数目，并提供很少的信息，关于修剪如何调节突触功能和连接的具体 皮层通路此外，实际上对活性的细胞和分子机制一无所知 和皮层神经元中感觉经验依赖性突触消除。利用突触功能的分析 在孤立的皮层通路中，我们已经积累了证据，表明活动依赖性突触 修剪受转录因子的肌细胞增强因子2（MEF 2）家族的激活调节。 我们发现，MEF 2 - 1需要RNA结合蛋白，脆性X智力迟钝蛋白（FMRP）。 通过调节MEF 2产生的转录本的翻译触发突触消除-包括 Protocadherin 10（Pcdh 10）和Arc/Arg3.1。我们发现Arc和Pcdh 10介导突触的消除 通过不同的机制。重要的是，MEF 2C、FMRP和Pcdh 10中的功能缺失突变与 智力残疾、自闭症和电路过度兴奋。对生理和体内的 导致皮层神经元上功能性突触连接消除的条件，以及是否或如何 涉及MEF 2c、FMRP、Pcdh 10和Arc。在目标1中，我们将使用光遗传学来诱导生理模式 的CA 1神经元放电和突触消除，以确定MEF 2亚型，Fmr 1和Pcdh 10的作用， 生理活性依赖性突触消除。在目标2中，我们将确定内源性MEF 2 同种型有助于发育修剪功能性兴奋性突触连接到皮层 体内神经元我们也有新的数据表明，新的经验激活MEF 2依赖性Arc 转录启动CA 1神经元的长期突触抑制后激活代谢 谷氨酸受体（mGluR-LTD）。mGluR-LTD的新奇启动可能是突触消除的前兆 并有助于形成稀疏的皮层网络表征的记忆。在目标3中，我们提出 确定MEF 2是否有助于新奇诱导的基因表达、mGluR-LTD的引发和新奇性 习惯化在目标4中，我们将使用光遗传学和电刺激来建立一个体外模型， 新颖性诱导的LTD启动，以揭示细胞机制。

Project Summary: Cortical structures (the hippocampus and neocortex) are critical for cognition and perception, and their improper function is implicated in intellectual disability and autism. The establishment of proper cortical circuits requires a complex interaction of neural activity and genetic programs that control the formation and elimination of specific synaptic connections. Studies of structural excitatory synapses, such as spines, find there is a rapid period of synaptogenesis early in postnatal development followed by a later period of elimination (or pruning) - in both humans and mice. Importantly, sensory experience and circuit activity drives the pruning of spines in vivo. Spine elimination is also triggered by learning in adults and may mediate the refinement of circuits that maintain memories. However, spines are an indirect measure of synaptic number and provide little information about how pruning regulates synaptic function and connectivity of specific cortical pathways. Furthermore, virtually nothing is known of the cellular and molecular mechanisms of activity and sensory experience-dependent synapse elimination in cortical neurons. Using assays of synaptic function in isolated cortical pathways, we have accumulated evidence indicating that activity-dependent synaptic pruning is regulated by the activation of the Myocyte-Enhancer Factor 2 (MEF2) family of transcription factors. We find that the RNA binding protein, Fragile X Mental Retardation Protein (FMRP) is required for MEF2- triggered synapse elimination by regulating the translation of MEF2-generated transcripts - including Protocadherin10 (Pcdh10) and Arc/Arg3.1. We find that Arc and Pcdh10 mediate elimination of synapses through distinct mechanisms. Importantly, loss of function mutations in MEF2C, FMRP and Pcdh10 are linked to intellectual disability (ID), autism and circuit hyperexcitability. Little is known of the physiological and in vivo conditions that lead to elimination of functional synaptic connections on cortical neurons and if or how this involves MEF2c, FMRP, Pcdh10, and Arc. In Aim 1 we will use optogenetics to induce physiological patterns of CA1 neuron firing and synapse elimination to determine the role of MEF2 isoforms, Fmr1 and Pcdh10 in physiological activity-dependent synapse elimination. In Aim 2 we will determine if endogenous MEF2 isoforms contribute to developmental pruning of functional excitatory synaptic connections onto cortical neurons in vivo. We also have new data suggesting that a novel experience activates MEF2-dependent Arc transcription which primes CA1 neurons for long-term synaptic depression upon activation of metabotropic glutamate receptors (mGluR-LTD). Novelty-priming of mGluR-LTD may be a precursor to synapse elimination and contribute to the formation of sparse cortical network representations of memories. In Aim 3 we propose to determine if MEF2 contributes to novelty-induced gene expression, priming of mGluR-LTD, and novelty habituation. In Aim 4 we will use optogenetics and electrical stimulation to establish an in vitro model of novelty-induced priming of LTD to reveal cellular mechanisms.