Astrocyte regulation of synapse maturation

在发育中的大脑中，首先形成含有钙渗透性AMPA谷氨酸受体的兴奋性突触 （AMPAR），当突触稳定时，AMPAR被含GluA 2的钙不可渗透的AMPAR取代， 成熟AMPAR从钙可渗透到钙不可渗透的转换有助于突触 通过限制钙进入突触后细胞，可以抑制突触的成熟并限制随后的突触可塑性。 研究表明，星形胶质细胞的存在足以增加所有4种细胞的表面和突触水平。 AMPAR亚基（GluA 1 -4），并已确定磷脂酰肌醇蛋白聚糖4和6作为星形胶质细胞分泌的信号， 特异性募集GluA 1钙渗透AMPAR至突触，对GluA 2无影响。这就提出了一个 假设星形胶质细胞可以通过释放不同的AMPAR来调节突触处AMPAR的亚基组成， 信号，这有可能改变突触成熟和可塑性。进行筛选以确定 GluA 2募集因子，并鉴定了星形胶质细胞分泌的BMP拮抗剂chordin like 1（Chrdl 1）。 在体外向神经元中加入Chrdl 1足以增加GluA 2的突触簇集，并诱导 形成功能成熟的突触在体内，Chrdl 1由小鼠脑中的星形胶质细胞特异性表达 并且没有神经元。Chrdl 1的表达仅限于皮质星形胶质细胞， 与深层相比，在皮质上层的星形胶质细胞中表达更高，并且在突触形成时达到峰值。 GluA 2的成熟和突触募集。全球缺乏Chrdl 1的小鼠显示出延迟突触的证据 成熟，突触传递的动力学改变和兴奋性GluA 2水平降低 突触该提案调查了与以下相关的未回答的问题：1）Chrdl 1如何与 神经元募集GluA 2突触，2）是什么调节Chrdl 1的区域和时间表达， 星形胶质细胞，3）体内缺乏Chrdl 1是否增强突触可塑性。目的1研究Chrdl 1是否是 作为BMP信号传导的分泌拮抗剂，通过典型的BMP途径 或者在非规范机制中。目的2询问Chrdl 1的体内表达模式是否是由于特异性的 神经元神经支配的模式，并询问什么是神经元的信号，诱导星形胶质细胞中的Chrdl 1。在 目的3、探讨Chrdl 1中是否存在成熟受损和未成熟的突触 KO小鼠导致增强的突触可塑性在视觉系统中进行了研究，通过观察 关键期可塑性中的chrdl 1。这些实验将提供重要的机制洞察如何突触 成熟是在发育中的大脑中控制的，星形胶质细胞在这个过程中的作用。

In the developing brain excitatory synapses first form containing calcium permeable AMPA glutamate receptors (AMPARs), which are replaced by GluA2 containing calcium impermeable AMPARs as synapses stabilize and mature. The switch from calcium-permeable to calcium-impermeable AMPARs contributes to synapse maturation and limits subsequent synaptic plasticity, by limiting the entry of calcium into the postsynaptic cell. Studies have shown that the presence of astrocytes is sufficient to increase surface and synaptic levels of all 4 AMPAR subunits (GluA1-4) in neurons, and have identified glypicans 4 & 6 as astrocyte-secreted signals that specifically recruit GluA1 calcium-permeable AMPARs to synapses, with no effect on GluA2. This raised the hypothesis that astrocytes can regulate the subunit composition of AMPARs at synapses by releasing distinct signals, which has the potential to alter synapse maturation and plasticity. A screen was conducted to identify the GluA2 recruiting factor and identified the astrocyte-secreted BMP antagonist chordin like 1 (Chrdl1). Addition of Chrdl1 to neurons in vitro is sufficient to increase synaptic clustering of GluA2, and to induce functionally mature synapses to form. In vivo, Chrdl1 is specifically expressed by astrocytes in the mouse brain and absent from neurons. Expression of Chrdl1 is restricted to cortical astrocytes, and within the cortex expression is higher in astrocytes in upper cortical layers compared to deep, and peaks at the time of synapse maturation and synaptic recruitment of GluA2. Mice globally lacking Chrdl1 show evidence of delayed synapse maturation, with altered kinetics of synaptic transmission and decreased levels of GluA2 at excitatory synapses. This proposal investigates unanswered questions related to: 1) how Chrdl1 is interacting with neurons to recruit GluA2 to synapses, 2) what regulates the regional and temporal expression of Chrdl1 in astrocytes, 3) whether lack of Chrdl1 in vivo enhances synaptic plasticity. Aim 1 investigates whether Chrdl1 is acting in its known role as a secreted antagonist of BMP signaling, either through the canonical BMP pathway or in a non-canonical mechanism. Aim 2 asks if the in vivo expression pattern of Chrdl1 is due to specific patterns of neuronal innervation, and asks what is the signal from neurons that induces Chrdl1 in astrocytes. In Aim 3, the question of whether the impaired synapse maturation and immature synapses observed in Chrdl1 KO mice leads to enhanced synaptic plasticity is investigated in the visual system, by looking at the role of Chrdl1 in critical period plasticity. These experiments will give important mechanistic insight into how synapse maturation is controlled in the developing brain, and the role of astrocytes in this process.