Role of PIP2 metabolism imbalance in Down Syndrome

描述（由申请人提供）：唐氏综合症（DS）是导致智力迟钝的最常见原因。在DS中，由于人类21号染色体（HC21）的第三个拷贝的存在，整合基因表达发生了改变。尽管智力低下与HC21的非重叠区域有关，表明其病因具有多基因性，但单一基因对这种表型的相对贡献尚不清楚。在这里，我们提出Synj1是一个强有力的候选人贡献的精神发育迟缓的DS。Synj1编码synaptojanin 1，这是一种富含大脑的脂质磷酸酶，可使PIP2去磷酸化。这种质膜富集脂质调节许多过程，包括信号转导、膜运输和离子通道功能。我们之前的工作表明，PIP2调节突触囊泡（SV）运输，从而调节神经传递。这种特性很大程度上是由于这种脂质能够招募到质膜上的内吞机制的关键成分，这介导了sv的再循环。最近，我们的研究揭示了Ts65Dn小鼠大脑中PIP2代谢的改变，Ts65Dn是唐氏综合征的主要遗传模型。在Ts65Dn小鼠中，通过将Synj1恢复为二体，可以挽救这一缺陷，并且在基因组（BAC）构建体中过表达Synj1的转基因小鼠中也可以重现这一缺陷[Tg(Synj1)]。来自Tg（Synj1）小鼠的初级皮质神经元显示SV运输缺陷，这让人想起缺乏主要pip2合成酶的神经元中发现的缺陷。此外，Tg（Synj1）小鼠表现出Morris水迷宫功能缺陷，提示Synj1基因剂量失衡引起的PIP2失衡可能导致DS遗传模型中的脑功能障碍和认知障碍。基于我们的初步研究，本提案的主要目标将是：(i)进一步表征完整突触体和来自DS小鼠模型以及患有DS的人类胚胎皮层的培养神经元中的PIP2代谢缺陷；（ii）进一步表征Ts65Dn和Tg（Synj1）神经元突触的生理和超微结构缺陷；（iii）进一步探讨PIP2缺失对转基因小鼠在Morris水迷宫范式和其他行为任务中的学习能力的影响。特异表型的发现将促使我们评估在Ts65Dn背景下恢复Synj1是否能改善这些缺陷。我们期望我们的研究能够在退行性痴呆遗传模型中确定Synj1基因剂量失衡是脑功能障碍的关键因素，并为退行性痴呆神经生理缺陷和智力低下的分子基础提供更好的理解。公共卫生相关性：唐氏综合症是导致智力迟钝的最常见原因。我们提出的研究的主要目标是进一步表征基因SYNJ1在这种疾病的各种小鼠模型的神经生理和行为缺陷中的含义。该建议建立在我们自己的证据上，表明SYNJ1三体导致转基因小鼠模型中的生化缺陷，即称为PIP2的调节脂质缺乏和学习缺陷。

DESCRIPTION (provided by applicant): Down syndrome (DS) is the most common cause of mental retardation. In DS, integrated gene expression is altered due to the presence of a third copy of human chromosome 21 (HC21). Although mental retardation has been linked to non-overlapping regions of HC21, indicating the multigenicity of its etiology, the relative contribution of single genes to this phenotype is unknown. Here we propose that Synj1 is a strong candidate for contributions to mental retardation in DS. Synj1 encodes synaptojanin 1, a brain- enriched lipid phosphatase that dephosphorylates PIP2. This plasma membrane-enriched lipid regulates many processes, including signal transduction, membrane trafficking and ion channel function. Our previous work has shown that PIP2 regulates synaptic vesicle (SV) trafficking and thereby, neurotransmission. This property is largely due to the ability of this lipid to recruit to the plasma membrane key components of the endocytic machinery, which mediates the recycling of SVs. More recently, our studies have revealed that PIP2 metabolism is altered in the brain of Ts65Dn mice, the major genetic model of Down syndrome. This defect is rescued by restoring Synj1 to disomy in Ts65Dn mice and it is recapitulated in transgenic mice overexpressing Synj1 from a genomic (BAC) construct [Tg(Synj1)]. Primary cortical neurons derived from Tg(Synj1) mice show defects in SV trafficking that are reminiscent of those found in neurons lacking a major PIP2-synthesizing enzyme. Additionally, Tg(Synj1) mice exhibit deficits in performance of the Morris water maze, suggesting that PIP2 dyshomeostasis caused by gene dosage imbalance for Synj1 may contribute to brain dysfunction and cognitive disabilities in genetic models of DS. Based on our preliminary studies, the main goals of this proposal will be (i) to further characterize PIP2 metabolism defects in intact synaptosomes and in cultured neurons from the cortex of DS mouse models as well as of human embryos with DS; (ii) to further characterize physiologic and ultrastructural defects in synapses from Ts65Dn and Tg(Synj1) neurons; and (iii) to further explore the impact of PIP2 deficiency on the learning ability of transgenic mice in the Morris water maze paradigm as well as in other behavioral tasks. The discovery of specific phenotypes will prompt us to assess whether restoring Synj1 to disomy in the Ts65Dn background ameliorates these defects. We anticipate that our studies will identify gene dosage imbalance for Synj1 as a key factor in brain dysfunction occurring in genetic models of DS and provide a better understanding of the molecular basis underlying neurophysiological deficits and mental retardation in DS. PUBLIC HEALTH RELEVANCE: Down syndrome is the most common cause of mental retardation. The main goal of our proposed research is to further characterize the implication of a gene, SYNJ1, in neurophysiological and behavioral deficits in various mouse models of this disorder. This proposal builds on our own evidence showing that the trisomy of SYNJ1 causes a biochemical defect, i.e. the deficiency of a regulatory lipid called PIP2, and learning deficits in a transgenic mouse model.