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

描述（由申请人提供）：记忆障碍仍然是美国主要的卫生保健负担。随着人口老龄化，与年龄相关的认知能力下降的患病率预计将会增加。记忆障碍的有效治疗是缺乏的，新的药理方法是必要的。尽管有一个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，但记忆的分子调节背后的细节仍然难以捉摸。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可遗传的分子机制，稳定地影响基因表达；这些报告表明，在很大程度上，有丝分裂后的大脑已经选择了这些过程来进行动态和持续的转录调节。对神经元活性依赖的胞嘧啶碱基DNA甲基化和组蛋白（核心染色质成分与DNA密切相反）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。DNA甲基转移酶催化活性DNA甲基化，但实际上没有一致的证据表明存在逆反应的分子调节剂。然而，最近的两项研究揭示了生长阻滞和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亚型的表达对海马体中的神经元去极化有反应，海马体是大脑中对记忆处理至关重要的区域。Gadd45b作为DNA去甲基化的中介和直接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。本课题旨在研究Gadd45b在海马体依赖性记忆形成中的作用。使用基因工程缺失gadd45b位点的小鼠的遗传方法将被用来解决这个问题。为了研究Gadd45b在记忆形成中的功能，野生型和突变型小鼠将在行为电池中进行评估，以研究记忆和基线行为。长期记忆的巩固是通过突触效能的持续变化来实现的，这种现象被称为突触可塑性。为了研究Gadd45b在海马长期增强中的功能，我们将采集两种基因型的海马记录。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去甲基化的药理学诱导，以测量假定的目标。最后，将采用体内方法；动物将接受联想记忆任务的训练，并研究海马体中基因表达的表观遗传调控。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而且在行为中发挥相反的作用。本研究旨在进一步研究DNA去甲基化的行为学、生理学和分子作用的广度。

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.