The Role of DNA Demethylation by Gadd45b in Memory and Synaptic Plasticity

描述(由申请人提供)：记忆障碍在美国仍然是一个主要的医疗负担。随着人口老龄化，与老龄化相关的认知能力下降的流行率预计会增加。记忆障碍缺乏有效的治疗方法，需要新的药理学方法。尽管新兴的研究机构致力于研究记忆及其在疾病状态下的功能障碍，但记忆分子调控背后的细节仍然扑朔迷离。最近的研究指出了表观遗传机制在记忆形成中的一种新功能。表观遗传学传统上被描述为一套稳定影响基因表达的可遗传分子机制；这些报告表明，主要是有丝分裂后的大脑已经选择了这些过程来动态和持久地调节转录。对胞嘧啶碱基DNA的神经元活性依赖性甲基化和组蛋白的共价修饰的研究证实了这些机制在记忆形成中的功能作用。组蛋白是与DNA密切相反的核心染色质成分。DNA甲基转移酶催化活性DNA甲基化，但几乎没有一致的证据表明反向反应的分子调节因素。然而，最近的两项研究揭示了生长停滞和DNA损伤诱导蛋白45(GADD45)家族在DNA去甲基化中的作用。值得注意的是，β亚型的表达对海马区的神经元去极化做出反应，海马区是大脑中对记忆处理至关重要的区域。Gadd45b作为DNA去甲基化的中介和即刻早期基因的功能表明，它可能在记忆形成过程中诱导DNA去甲基化。本研究的目的是研究Gadd45b在海马区依赖记忆形成中的作用。一种利用基因工程缺失gadd45b基因的小鼠的遗传方法将被用来解决这个问题。为了研究Gadd45b在记忆形成中的作用，野生型和突变小鼠将在一个旨在研究记忆和基线行为的行为电池中进行评估。长期记忆巩固被突触效力的持久变化所取代，这种现象被称为突触可塑性。为了研究Gadd45b在海马长时程增强中的作用，我们将对这两种基因的海马区进行记录。进一步的研究旨在研究Gadd45b介导的去甲基化的遗传靶点。将在切片中进行DNA去甲基化的药物诱导，以确定可能的靶点。最后，将采取活体方法；动物将接受联想记忆任务的训练，并将研究海马体基因表达的表观遗传调节。初步数据表明，Gadd45b在记忆巩固中具有抑制作用。这表明DNA甲基化和去甲基化不仅在分子过程中发挥着相反的功能，而且在行为上也是如此。这项拟议的研究旨在更详细地研究DNA去甲基化在行为学、生理学和分子方面的广泛作用。 与公共卫生相关：越来越多的证据表明，在一些神经退行性疾病和认知疾病状态(21、45、48、49、67)中抑制组蛋白脱乙酰酶具有治疗益处。然而，目前还不知道DNA甲基化机制中的平行靶点。这项研究的初步结果以及对记忆中DNA去甲基化抑制功能的进一步深入可能会发现Gadd45b和这一分子事件的其他介体是第二类神经表观遗传药物靶点。

DESCRIPTION (provided by applicant): Disorders of memory remain a major health care burden in the United States. As the population ages, the prevalence of aging-associated cognitive decline is expected to grow. Effective treatment of memory disorders is lacking, and novel pharmacological approaches are necessary. Despite a burgeoning body of research dedicated to the study of memory and its dysfunction in disease states, details behind the molecular regulation of memory remains elusive. Recent studies point to a novel function of epigenetic mechanisms in memory formation. Epigenetics has traditionally been described as a set of heritable molecular mechanisms that stably influence gene expression; these reports indicate the largely postmitotic brain has co-opted these processes for both dynamic and persistent regulation of transcription. Investigations into neuronal activity- dependent methylation of DNA at cytosine bases and covalent modification of histones, core chromatin constituents closely opposed to DNA, confirm functional roles of these mechanisms in memory formation. DNA methyltransferases catalyze active DNA methylation, but virtually no consistent evidence existed for a molecular regulator of the reverse reaction. Two recent studies, however, uncover the role of the Growth arrest and DNA damage-inducible protein 45 (Gadd45) family in DNA demethylation. Notably, the expression of the beta isoform responds to neuronal depolarization in the hippocampus, a brain region critical to memory processing. The function of Gadd45b as a mediator of DNA demethylation and as an immediate early gene suggests that it potentially induces DNA demethylation in memory formation. The purpose of this research proposal is to study the contribution of Gadd45b in hippocampus-dependent memory formation. A genetic approach using mice with engineered deletion of the gadd45b locus will be employed to address this question. To study the function of Gadd45b in memory formation, wildtype and mutant mice will be assessed in a behavioral battery designed to study memory and baseline behavior. Long-term memory consolidation is subserved by lasting changes in synaptic efficacy, a phenomenon known as synaptic plasticity. To study the function of Gadd45b in hippocampal long-term potentiation, hippocampal recordings from both genotypes will be taken. Further studies are designed to study the genetic targets of Gadd45b-mediated demethylation. Pharmacological induction of DNA demethylation in slices will be performed to gauge putative targets. Finally, an in vivo approach will be taken; animals will be trained in an associative memory task, and epigenetic regulation of gene expression in the hippocampus will be studied. Preliminary data indicate a suppressive function of Gadd45b in memory consolidation. This suggests DNA methylation and demethylation perform opposing functions not only in molecular processes but also in behavior. The proposed study is designed to investigate the breadth of the ethological, physiological and molecular role of DNA demethylation in further detail. PUBLIC HEALTH RELEVANCE: Burgeoning evidence points to the therapeutic benefits of inhibiting histone deacetylases in a number of neurodegenerative and cognitive disease states (21, 45, 48, 49, 67). No parallel target in the DNA methylation machinery, however, is currently known. Preliminary results from this study and further insights into the suppressive function of DNA demethylation in memory could uncover Gadd45b and other mediators of this molecular event as a second class of neuroepigenetic drug targets.