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

描述(由申请人提供)：皮质结构(海马体和新皮质)对认知和感知至关重要，其不适当的功能与智力残疾和自闭症有关。建立适当的大脑皮层回路需要复杂的相互作用 控制特定突触连接的形成和消除的神经活动和遗传程序。对结构性兴奋性突触(如棘突)的研究发现，在人类和小鼠中，在出生后发育的早期有一个突触形成的快速时期，随后是一个消除(或修剪)的时期。重要的是，感觉体验和回路活动推动了体内脊椎的修剪。脊椎消除也是由成年人的学习触发的，可能会调节维持记忆的回路的完善。然而，棘突是突触数量的间接测量，几乎没有提供关于修剪如何调节突触功能和特定皮质路径的连接性的信息。此外，对皮层神经元的活动和感觉经验依赖性突触消除的细胞和分子机制几乎一无所知。利用分离的皮质通路中的突触功能分析，我们积累了证据表明，依赖活性的突触修剪受肌细胞增强因子2(MEF2)家族转录因子的激活调节。我们发现，RNA结合蛋白，脆性X智力低下蛋白(FMRP)是MEF2触发的突触消除所必需的，它通过调节MEF2产生的转录本的翻译-包括Protocadherin10(Pcdh10)和Arc/Arg3.1。我们发现Arc和Pcdh10通过不同的机制介导突触的消除。重要的是，MEF2C、FMRP和Pcdh10功能突变的丧失与智能障碍(ID)、自闭症和回路过度兴奋有关。目前尚不清楚导致皮层神经元上功能性突触连接消失的生理和体内条件，以及这是否或如何涉及MEF2C、FMRP、Pcdh10和Arc。在目标1中，我们将利用光遗传学来诱导CA1神经元放电和突触消除的生理模式，以确定MEF2亚型、Fmr1和Pcdh10在生理活性依赖性突触消除中的作用。在目标2中，我们将确定内源性MEF2亚型是否有助于体内大脑皮质神经元上功能性兴奋性突触连接的发育修剪。我们还有新的数据表明，一种新的经验激活了MEF2依赖的Arc转录，从而使CA1神经元在代谢性谷氨酸受体(mGluR-Ltd)激活时，为长期的突触抑制做好准备。MGluR-LTD的新颖性启动可能是突触消除的先兆，有助于形成稀疏的皮层记忆网络表征。在目标3中，我们建议确定MEF2是否有助于新颖性诱导的基因表达、mGluR-LTD的启动和新颖性习惯化。在目标4中，我们将利用光遗传学和电刺激建立LTD新奇诱导启动的体外模型，以揭示细胞机制。

DESCRIPTION (provided by applicant): Cortical structures (the hippocampus and neocortex) are critical for cognition and perception, and their improper function is implicated in intellectua 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.