Astrocyte regulation of synapse maturation

在发育中的大脑中，含有钙离子通透性AMPA谷氨酸受体的第一型兴奋性突触 (AMPAR)，当突触稳定并被含有钙不通透性AMPAR的GluA2取代 成熟。从钙通透性AMPAR到钙不通透性AMPAR的转换对突触起作用 通过限制钙离子进入突触后细胞，成熟和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种星形胶质细胞的表面和突触水平 神经元中的Ampar亚单位(GluA1-4)，并发现Glypicans 4和6是星形胶质细胞分泌的信号， 特异性地将GluA1钙渗透性AMPAR招募到突触，而对GluA2没有影响。这引发了 星形胶质细胞可以通过释放不同的信号来调节突触上AMPAR亚单位的组成 信号，这有可能改变突触的成熟和可塑性。进行了筛查以确定 GluA2募集因子，并鉴定了星形胶质细胞分泌的BMP拮抗剂Chordin like 1(Chrdl1)。 在体外培养的神经元中加入chrdl1足以增加GluA2的突触聚集，并诱导 功能成熟的突触形成。在体内，chrdl1由小鼠脑内的星形胶质细胞特异性表达 而在神经元中不存在。Chrdl1的表达仅限于皮质星形胶质细胞和皮质内。 皮质上层星形胶质细胞的表达高于深层星形胶质细胞，并在突触时达到高峰 GluA2的成熟和突触募集。全球范围内缺乏chrdl1的小鼠显示出突触延迟的证据 成熟，突触传递动力学改变，兴奋性时GluA2水平下降 突触。该提案调查了与以下相关的悬而未决的问题：1)Chrdl1如何与 神经元将GluA2招募到突触，2)是什么调节了chrdl1在脑内的区域和时间表达 星形胶质细胞，3)体内缺乏chrdl1是否增强突触可塑性。AIM 1调查Chrdl1是否 发挥其作为BMP信号的分泌性拮抗剂的已知作用，通过典型的BMP途径 或者是在非规范的机制中。目的2问是否chrdl1的体内表达模式是由于特异性 神经神经支配的模式，并询问来自神经元的信号是什么，诱导星形胶质细胞中的chrdl1。在……里面 目的3，是否观察到chrdl1突触成熟受损和突触不成熟的问题 KO小鼠导致突触可塑性增强的研究是在视觉系统中，通过观察 在临界可塑性时期。这些实验将提供重要的机制洞察突触是如何 成熟在发育中的大脑中受到控制，而星形胶质细胞在这一过程中扮演的角色。

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.