Synapse Remodeling in Mecp2 Mouse Models

描述(申请人提供)：Rett综合征(RTT)是一种遗传性神经发育障碍，在女孩中每10,000名新生儿中就有1人受到影响，其特征是智力低下、癫痫发作、重复行为和社交异常。甲基CpG结合蛋白2基因(MECP2)的突变被发现是大多数RTT病例的原因。这种障碍的一个显着特征是最初明显的正常发育，随后是交际能力和运动能力的退化。越来越多的证据表明，RTT的突触连接是异常的。然而，突触发育被破坏的阶段仍然不清楚。突触从一开始就形成错误吗？或者，突触形成正常，但随后不能适当地加强？消除多余连接的精细化过程是否不正常？或者，在正常发育后，大脑是否无法正常维持突触？为了解决这些问题，我们建议研究MeCP2突变小鼠视觉丘脑发育过程中的突触功能，这是研究RTT的小鼠模型。一系列功能发育阶段被很好地描述的模型系统是视网膜原性突触，它是眼睛中的视网膜神经节细胞和视觉丘脑中的中继神经元之间的连接，使其成为突触发育的一个很好的测试方法。利用电生理学技术，我们先前已经证明，这种突触的发展经历了三个不同的阶段。在发育的第一阶段之后，当突触最初形成时，有两个随后的突触剧烈重塑时期。发育的第二阶段大约在睁开眼睛的时候发生，此时特定中继神经元的一些视网膜输入得到加强，而其他输入被消除。第三个阶段发生在发育后期，感觉体验的变化可以激活突触连接的重塑，这一过程被认为是突触电路适应感觉体验所必需的。在这里，我们将研究RTT的两个MeCP2小鼠模型的突触发育，一个是整个MeCP2基因被破坏(MeCP2-/y)，另一个是内源性MeCP2蛋白的Ser421残基被丙氨酸残基取代(Mecp2S421A/y)。在体外，这种丝氨酸的磷酸化与树突状细胞和棘状细胞的形态调节有关。我们将检验两个假设：1)对感觉体验的正常反应的破坏是在RTT中观察到的发育退化的基础，2)突触发育的这一阶段是由MeCP2蛋白Ser421残基的磷酸化调节的。因此，我们将评估突触强度和连接性对发育和对感觉体验变化的反应。这些信息反过来将指导我们未来为RTT儿童设计治疗干预措施。公共卫生相关性：在这项资助中，我们建议研究雷特综合症和其他自闭症谱系障碍的MeCP2小鼠模型中突触电路的形成。通过确定MeCP2小鼠突触发育被破坏的阶段，我们可以开始阐明在正常突触发育中重要的机制。此外，通过表征MeCP2小鼠这些突触的潜在可塑性，我们将测试是否可以重新连接这些小鼠模型中的突触电路，以纠正异常突触连接。这些研究的结果可能有助于指导设计未来治疗Rett综合征和自闭症谱系障碍的方法。

DESCRIPTION (provided by applicant): Rett Syndrome (RTT) is a genetic neurodevelopmental disorder in girls affecting 1 in 10,000 births that is characterized by mental retardation, seizures, repetitive behaviors and abnormalities in social interactions. Mutations in the methyl-CpG-binding protein 2 gene (MECP2) have been found to be responsible for the majority of cases of RTT. A striking feature of this disorder is the initial apparent normal development that is followed by a regression in communicative and locomotive abilities. There is growing evidence that synaptic connections are abnormal in RTT. However, the phase at which synapse development that is disrupted is still unclear. Are synapses formed incorrectly from the start? Or do synapses form normally, but then fail to strengthen appropriately? Is the refinement process by which excess connections are eliminated abnormal? Or after normal development, does the brain fail to maintain synapses properly? To address these questions, we propose to study synaptic function during development of the visual thalamus in Mecp2 mutant mice, mouse models for the study of RTT. One model system in which a series of functional developmental phases is well characterized is the retinogeniculate synapse, the connection between retinal ganglion cells in the eye and relay neurons in the visual thalamus, making this a good assay for synapse development. Using electrophysiological techniques, we have previously shown that development of this synapse involves three distinct phases. After the first phase in development, when synapses are initially formed, there are two subsequent periods of intense synaptic remodeling. The second phase of development occurs around the time of eye opening when some retinal inputs to a given relay neuron are strengthened while other inputs are eliminated. A third phase occurs later in development when changes in sensory experience can activate the remodeling of synaptic connections, a process thought to be necessary for the adaptation of synaptic circuits to sensory experience. Here, we will examine synapse development in two Mecp2 mouse models for RTT, one in which the entire Mecp2 gene is disrupted (Mecp2-/y), and the other in which the Ser421 residue of the endogenous MeCP2 protein, a site of neuronal activity-dependent modification, has been replaced with an alanine residue (Mecp2S421A/y). Phosphorylation of this serine has previously been implicated in the regulation of dendritic and spine morphology in vitro. We will test two hypotheses: 1) Disruption of the normal response to sensory experience underlies the developmental regression observed in RTT, and 2) that this phase in synapse development is regulated by phosphorylation of the Ser421 residue of the MeCP2 protein. Thus we will evaluate synaptic strength and connectivity over development and in response to changes in sensory experience. This information, in turn, will guide our future design of therapeutic interventions for children with RTT. PUBLIC HEALTH RELEVANCE: In this grant we propose to study the formation of synaptic circuits in Mecp2 mouse models for Rett Syndrome and other Autism Spectrum Disorders. By identifying the stage of synapse development that is disrupted in Mecp2 mice, we can begin to elucidate the mechanisms that are important in normal synapse development. Moreover, by characterizing the potential plasticity of these synapses in Mecp2 mice, we will test whether synaptic circuits in these mouse models can be rewired to correct for abnormal synaptic connections. The results from these studies may help guide the design future therapies for Rett Syndrome and Autism Spectrum Disorders.