Control of synapse formation and maturation by astrocytes

描述(申请人提供)：突触是神经系统的基本功能单位，但调节它们建立的分子和细胞相互作用在很大程度上是未知的。使用纯化的视网膜神经节细胞神经元(RGC)的研究表明，星形胶质细胞分泌强烈诱导兴奋性突触形成的信号，如凝血酶敏感蛋白。在我们的初步研究中，我们发现了另一种星形胶质细胞分泌的突触生成蛋白，Hevin。在纯化的RGC培养中加入Hevin可强烈刺激兴奋性突触发生。此外，Hevin缺失的小鼠的兴奋性突触明显较少，这些突触存在显著的结构缺陷，这表明Hevin是体内突触形成和形态成熟所必需的。星形胶质细胞也表达与Hevin相近的同源物，称为SPARC。有趣的是，SPARC不是突触发生的，但特异性地抑制Hevin诱导的突触形成。这些数据首次表明，星形胶质细胞不仅通过刺激，还通过抑制突触发生来调节突触连接。我们的结果表明，星形胶质细胞通过调节Hevin和SPARC的相对水平，可以主动控制发育中和成年脑中突触网络的发育和功能，这是令人兴奋的可能性。Hevin如何诱导突触形成以及SPARC拮抗功能的性质尚不清楚。因此，我们的目标是揭示一种新的分子机制，通过Hevin/SPARC信号来调节星形胶质细胞突触的发育和维持。在这一应用中，我们将首先检验Hevin和SPARC调节突触形态(目标1)和体内树突棘突触形成(目标2)的假设。其次，我们将确定Hevin/SPARC信号在体内对突触功能的贡献(目标2)。第三，我们将检验这一假设，即Hevin通过与跨突触黏附分子Neurexins和Neurliins的相互作用来介导突触发生，而SPARC通过竞争Hevin与神经Ligins的结合来拮抗Hevin(目标3)。这些研究很重要，因为它们将为星形胶质细胞控制突触的形成、维持和功能提供新的见解。这些新的见解将通过促进我们对星形胶质细胞-神经元相互作用的分子和细胞理解而产生重大的积极影响，这些相互作用协调中枢神经系统的发育和功能。深入了解突触形成的机制以及星形胶质细胞如何参与这一过程将导致开发创新的方法来预防或治疗神经疾病，如自闭症、抑郁症和成瘾。 公共卫生相关性：这项拟议的研究旨在促进我们对星形胶质细胞-神经元相互作用的理解，这些相互作用协调中枢神经系统的发育和功能。了解突触发生是如何调控的，对于了解我们的大脑在发育过程中是如何塑造的，以及我们作为成年人是如何学习和记忆的，是至关重要的。此外，对于阿尔茨海默病、癫痫、自闭症和药物成瘾等突触形成失调引起的疾病，开发新的预防和治疗策略需要了解突触发生如何出错。

DESCRIPTION (provided by applicant): Synapses are the fundamental functional units of the nervous system, but the molecular and cellular interactions that regulate their establishment are largely unknown. Studies using purified retinal ganglion cell neurons (RGCs) showed that astrocytes secrete signals such as thrombospondins that strongly induce excitatory synapse formation. In our preliminary studies, we identified another astrocyte-secreted synaptogenic protein, hevin. Addition of hevin to purified RGC cultures robustly stimulates excitatory synaptogenesis. Moreover, Hevin-null mice have significantly less excitatory synapses that present striking structural defects suggesting that hevin is required for the formation and morphological maturation of synapses in vivo. Astrocytes also express a close homolog of hevin called SPARC. Intriguingly, SPARC is not synaptogenic but specifically inhibits hevin-induced synaptogenesis. These data show for the first time that astrocytes regulate synaptic connectivity not only by stimulating, but also by inhibiting synaptogenesis. Our results signify the exciting possibility that astrocytes, through the regulation of relative levels of hevin and SPARC, can actively control the development and function of synaptic networks in the developing and adult brain. How hevin induces synapse formation and the nature of SPARC's antagonistic function are unknown. Therefore, our objective here is to unravel a novel molecular mechanism of regulation of synaptic development and maintenance by astrocytes through hevin/SPARC signaling. In this application, we will first test the hypothesis that hevin and SPARC regulate synaptic morphology (Aim 1) and formation of dendritic spine synapses in vivo (Aim 2). Second, we will determine the contribution of hevin/SPARC signaling to synaptic function in vivo (Aim 2). Third, we will test the hypothesis that hevin mediates synaptogenesis through interactions with the trans-synaptic adhesion molecules neurexins and neuroligins, whereas SPARC antagonizes hevin by competing for hevin-binding to neuroligins (Aim 3). These studies are important since they will provide new insights into the control of formation, maintenance and function of synapses by astrocytes. These new insights will have a significant positive impact by advancing our molecular and cellular understanding of astrocyte-neuron interactions that orchestrate central nervous system development and function. A deeper mechanistic understanding of synapse formation and how astrocytes participate in this process will lead to the development of innovative approaches to prevent or cure neurological disorders such as autism, depression and addiction. PUBLIC HEALTH RELEVANCE: The proposed study aims to advance our understanding of astrocyte-neuron interactions that orchestrate central nervous system development and function. Understanding how synaptogenesis is regulated is crucial for understanding how our brains are sculpted during development, and how we learn and remember as adults. In addition, knowledge on how synaptogenesis can go awry is required for development of new prevention and treatment strategies against disorders that stem from dysregulated synapse formation such as Alzheimer's disease, epilepsy, autism and drug addiction.