Synapse Remodeling in Mecp2 Mouse Models

描述（由申请人提供）：Rett综合征（RTT）是一种遗传性神经发育障碍，在女孩中发病率为万分之一，其特征是智力迟钝，癫痫发作，重复行为和社会互动异常。甲基cpg结合蛋白2基因（MECP2）的突变已被发现是大多数RTT病例的原因。这种障碍的一个显著特征是最初的明显正常发展，随后是交流和运动能力的退化。越来越多的证据表明RTT中突触连接异常。然而，突触发育中断的阶段尚不清楚。突触从一开始就形成错误吗？或者突触是正常形成的，但后来没有得到适当的加强？消除多余连接的精炼过程是否异常？还是在正常发育之后，大脑不能正常维持突触？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表征的一个模型系统是视网膜原突触，它是眼睛视网膜神经节细胞和视丘脑中继神经元之间的连接，这使得它成为突触发育的一个很好的分析方法。利用电生理技术，我们之前已经证明这种突触的发育涉及三个不同的阶段。在发育的第一阶段之后，当突触最初形成时，随后有两个强烈的突触重塑期。第二阶段的发育发生在眼睛睁开的时候，此时一些视网膜输入到一个给定的中继神经元被加强，而其他输入被消除。第三个阶段发生在发育后期，此时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回路适应感觉体验所必需的。在这里，我们将研究两种Mecp2小鼠RTT模型中的突触发育，其中一种模型中整个Mecp2基因被破坏（Mecp2-/y），另一种模型中内源性Mecp2蛋白的Ser421残基（神经元活性依赖性修饰位点）被丙氨酸残基（Mecp2S421A/y）所取代。该丝氨酸的磷酸化先前被认为与体外树突和脊柱形态的调节有关。我们将验证两个假设：1)在RTT中观察到的对感官体验的正常反应的破坏是发育倒退的基础；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，我们将评估突触的强度和连通性在发展和响应感官经验的变化。反过来，这些信息将指导我们未来设计RTT儿童的治疗干预措施。公共卫生相关性：在这项拨款中，我们建议研究Rett综合征和其他自闭症谱系障碍Mecp2小鼠模型中突触回路的形成。通过确定Mecp2小鼠突触发育中断的阶段，我们可以开始阐明正常突触发育中重要的机制。此外，通过表征Mecp2小鼠中这些突触的潜在可塑性，我们将测试这些小鼠模型中的突触回路是否可以重新连接以纠正异常的突触连接。这些研究的结果可能有助于指导设计Rett综合征和自闭症谱系障碍的未来治疗方法。

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.