MeCP2 Modulation of BDNF Signaling: Shared Mechanisms of Rett and Autism

描述(申请人提供)：Rett综合征(RTT)是一种自闭症谱系障碍，是一种破坏性的儿童障碍，其对个人(全球1：10,000-15,000出生)、他们的家庭和社会的影响。RTT是由编码甲基CpG结合蛋白2(MeCP2)的基因功能丧失突变引起的，MeCP2是一种转录调节因子，与DNA启动子区域的甲基化CpG位点结合。海马区兴奋性和抑制性突触功能的失衡与认知障碍和智力低下相关的神经发育障碍有关。缺乏MeCP2的小鼠皮质神经元表现出低水平的神经元活性，这是由于兴奋/抑制失衡有利于突触抑制，而MeCP2的表达水平调节了海马神经元之间兴奋性突触的形成。MeCP2转录调控的靶基因之一是脑源性神经营养因子(BDNF)，它是活性依赖型突触发育、功能和可塑性的有力调节器。考虑到BDNF对抑制性GABA能突触的成熟至关重要，根据我们的初步结果，我们的一般假设是，由于MeCP2缺乏的神经元释放的活性依赖性BDNF减少，导致抑制性GABA能突触的发育受损，导致了海马区兴奋性和抑制性突触功能的失衡。我们提出了以下四个具体目标：(1)检测神经元MeCP2基因敲除小鼠海马区的过度兴奋性是否由CA3区GABA能突触功能受损引起；(2)检测神经元MeCP2基因缺失小鼠海马齿状回颗粒细胞轴突苔藓纤维的活性依赖性BDNF释放是否减少；(3)建立一种新的RTT模型-齿状颗粒细胞特异性MeCP2基因敲除小鼠-并测试海马的过度兴奋性是否与颗粒细胞苔藓纤维活性依赖性BDNF释放受损有关；(4)检测增强BDNF表达或模拟BDNF/TrkB信号通路是否能预防MeCP2基因敲除小鼠和齿状突起细胞特异性MeCP2基因敲除小鼠的海马区过度兴奋性。我们预计，拟议的实验将产生关于MeCP2缺失对海马区兴奋/抑制平衡的影响的新信息，揭示RTT和自闭症谱系障碍的神经病理所涉及的基本大脑机制，并测试缓解患有相关神经发育障碍的儿童的认知障碍和智力低下的实验原理。 公共卫生相关性：Rett综合征(RTT)是一种与自闭症和智力低下相关的X连锁神经发育障碍，由MECP2突变引起，MECP2是一种DNA结合蛋白，调节包括BDNF在内的靶基因。我们将测试由于脑源性神经营养因子释放减少而导致的海马抑制性突触发育受损是否导致与认知障碍和自闭症有关的突触功能的兴奋性/抑制性失衡。

DESCRIPTION (provided by applicant): Rett syndrome (RTT), an autism spectrum disorder, is a devastating childhood disorder due to its impact on individuals (1:10,000-15,000 births worldwide), their families and society. RTT is caused by loss-of- function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcriptional regulator that binds to methylated CpG sites in promoter regions of DNA. An imbalance of excitatory and inhibitory synaptic function in the hippocampus has been implicated in neurodevelopmental disorders associated with cognitive impairments and mental retardation. Mouse cortical neurons lacking Mecp2 show low levels of neuronal activity caused by an excitation/inhibition imbalance that favors synaptic inhibition, and Mecp2 expression levels modulate excitatory synapse formation between hippocampal neurons. One of the target genes of Mecp2 transcriptional control is Brain-derived neurotrophic factor (Bdnf), a potent modulator of activity-dependent synaptic development, function and plasticity. Considering that BDNF is critical for the maturation of inhibitory GABAergic synapses, and based on our Preliminary Results, our general hypothesis is that impaired development of inhibitory GABAergic synapses due to reduced activity- dependent BDNF release from Mecp2-deficient neurons causes an imbalance of excitatory and inhibitory synaptic function in the hippocampus. We propose the following four Specific Aims: (1) test if the hyperexcitable hippocampal network of neuronal Mecp2 null mice is caused by impaired GABAergic synapse function in area CA3; (2) test whether activity-dependent BDNF release from mossy fibers, the axons of dentate gyrus granule cells, is reduced in neuronal Mecp2 null mice; (3) generate a novel RTT model - dentate granule cell-specific Mecp2 knockout mice - and test whether hippocampal hyperexcitability is associated with impaired activity-dependent BDNF release from granule cell mossy fibers; (4) test if enhancing BDNF expression or mimicking BDNF/TrkB signaling prevents hippocampal hyperexcitability in Mecp2 null mice and dentate granule cell-specific Mecp2 knockout mice. We anticipate that the proposed experiments will yield novel information regarding the consequences of Mecp2 deletion for the excitation/inhibition balance in the hippocampus, uncovering fundamental brain mechanisms involved in the neuropathology of RTT and Autism Spectrum Disorders, and testing an experimental rationale to relieve cognitive impairments and mental retardation in children with associated neurodevelopmental disorders. PUBLIC HEALTH RELEVANCE: Rett syndrome (RTT) is an X-linked neurodevelopmental disorder associated with autism and mental retardation, which is caused by mutations in MECP2, a DNA-binding protein that regulates target genes, including Bdnf. We will test whether impaired development of hippocampal inhibitory synapses due to reduced BDNF release contributes to the excitatory/inhibitory imbalance of synaptic function implicated in cognitive impairments and autism in RTT.