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

描述（由申请人提供）：皮质结构（海马和新皮质）对认知和感知至关重要，其功能不正常与智力残疾和自闭症有关。正确的皮层回路的建立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。对结构兴奋性突触（如棘）的研究发现，在人类和小鼠出生后发育的早期，突触发生有一个快速的时期，随后是一个较晚的消除（或修剪）时期。重要的是，感觉经验和回路活动驱动体内脊椎的修剪。脊髓消除也是由成人的学习触发的，并可能介导维持记忆的回路的完善。然而，棘是突触数量的间接测量，并提供修剪如何调节突触功能和特定皮层通路的连通性的信息很少。此外，几乎没有什么是已知的细胞和分子机制的活动和感觉经验依赖性突触消除皮层神经元。使用孤立的皮质通路中的突触功能测定，我们已经积累了证据表明，活性依赖性突触修剪是由肌细胞增强因子2（MEF 2）家族的转录因子的激活调节。我们发现，RNA结合蛋白，脆性X智力迟钝蛋白（FMRP）是MEF 2触发的突触消除所必需的，通过调节MEF 2产生的转录本的翻译-包括Protocadherin 10（Pcdh 10）和Arc/Arg3.1。我们发现，弧和Pcdh 10介导消除突触通过不同的机制。重要的是，MEF 2C、FMRP和Pcdh 10的功能缺失突变与智力残疾（ID）、自闭症和回路过度兴奋有关。很少有人知道的生理和体内条件，导致皮层神经元上的功能性突触连接的消除，如果或如何涉及MEF 2c，FMRP，Pcdh 10，和弧。在目的1中，我们将使用光遗传学诱导CA 1神经元放电和突触消除的生理模式，以确定MEF 2亚型，Fmr 1和Pcdh 10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MEF 2亚型是否有助于体内皮质神经元上功能性兴奋性突触连接的发育修剪。我们也有新的数据表明，一种新的经验激活MEF 2依赖的Arc转录，启动CA 1神经元的长期突触抑制后，激活代谢型谷氨酸受体（mGluR-LTD）。新奇启动mGluR-LTD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，引发mGluR-LTD和新奇习惯化。目的4：利用光遗传学和电刺激技术建立新颖性诱发LTD启动的体外模型，揭示LTD启动的细胞机制。

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.