Signaling Mechanisms Regulating Rac-dependent Synaptic and Dendritic Development

描述（由申请人提供）：功能性神经系统的形成需要树突和树突棘（大脑中兴奋性突触的主要部位）的适当发育和重塑。Rho家族GTP酶在调节这些过程中起关键作用。特别是，Rho GTdR Rac促进树突状分支以及棘的形成和维持。Rac活性的精确时空调节对于其功能是必不可少的，因为异常的Rac信号传导导致树突和棘异常以及包括智力迟钝的认知障碍。尽管其重要性，调节Rac信号在神经元中的机制仍然知之甚少。我们以前确定的种族特异性激活Tiam 1作为一个关键的调节树突，棘和突触的发展。我们证明，Tiam 1介导的NMDA受体和EphB受体依赖的脊柱发展耦合这些受体的Rac信号通路，控制肌动蛋白细胞骨架重塑和蛋白质合成。最近，我们还确定了Rac特异性抑制剂Bcr作为Tiam 1相互作用的蛋白，阻断Tiam 1诱导的Rac激活和肌动蛋白重塑。过表达和敲除实验表明，Bcr限制棘和树突的形成和生长。Tiam 1和Bcr之间的复合物可以作为精确调节神经元中Rac信号的“开关”，这对于棘、突触和树突的正确形成和重塑是必不可少的。为了验证这一假设，我们提出了以下具体目标：1）确定Bcr在限制突触发育和树突生长中的作用; 2）确定EphB和NMDA受体调节Tiam 1-Bcr复合物的机制，并确定Rac激活和突触发育的后果; 3）阐明Tiam 1-Bcr复合物在调节N-钙粘蛋白介导的突触粘附中的作用。为了解决这些问题，我们将采用多方面的方法，采用分子，细胞，生物化学和高分辨率成像技术的组合。从拟议的研究结果将提供关键的洞察调节Rac激活和Rac依赖的突触和树突发育的神经元的基本机制，并有助于阐明如何破坏Rac GTdR信号引起认知障碍，如智力低下。公共卫生相关性：我们建议研究在发育过程中调节大脑连接（突触）形成的机制，以及它们在学习和记忆等过程中如何重塑。我们正在研究一种特殊的信号通路，当它在人类中突变时会导致智力迟钝。我们的研究结果应该为大脑发育和记忆形成的基本机制提供新的见解，并应该加强我们对这些过程中的干扰如何引起大脑疾病（如智力迟钝）的理解。

DESCRIPTION (provided by applicant): Formation of a functional nervous system requires the proper development and remodeling of dendrites and dendritic spines, the primary sites of excitatory synapses in the brain. Rho family GTPases play critical roles in regulating these processes. In particular, the Rho GTPase Rac promotes dendritic arborization and the formation and maintenance of spines. Precise spatio-temporal regulation of Rac activity is essential for its function, since aberrant Rac signaling results in dendrite and spine abnormalities and cognitive disorders including mental retardation. Despite its importance, the mechanisms that regulate Rac signaling in neurons remain poorly understood. We previously identified the Rac-specific activator Tiam1 as a critical regulator of dendrite, spine, and synapse development. We demonstrated that Tiam1 mediates both NMDA receptor- and EphB receptor-dependent spine development by coupling these receptors to Rac signaling pathways that control actin cytoskeletal remodeling and protein synthesis. Recently, we have also identified the Rac-specific inhibitor Bcr as a Tiam1-interacting protein that blocks Tiam1-induced Rac activation and actin remodeling. Overexpression and knockout experiments indicate that Bcr restricts the formation and growth of spines and dendrites. The complex between Tiam1 and Bcr may serve as an "on-off switch" for precisely regulating Rac signaling in neurons, which is essential for the proper formation and remodeling of spines, synapses, and dendrites. To test this hypothesis, we propose the following specific aims: 1) to determine the role of Bcr in restricting synapse development and dendritic growth; 2) to identify the mechanisms by which EphB and NMDA receptors regulate the Tiam1-Bcr complex, and determine the consequences on Rac activation and synapse development; and 3) to elucidate the role of the Tiam1-Bcr complex in regulating N-cadherin-mediated synaptic adhesion. To address these questions, we will use a multifaceted approach employing a combination of molecular, cellular, biochemical, and high-resolution imaging techniques. Results from the proposed studies will provide critical insight into the fundamental mechanisms that regulate Rac activation and Rac-dependent synaptic and dendritic development in neurons, and help to elucidate how disruptions in Rac GTPase signaling give rise to cognitive disorders such as mental retardation. PUBLIC HEALTH RELEVANCE: We propose to investigate the mechanisms that regulate how connections in the brain (synapses) form during development and how they remodeling during processes like learning and memory. We are studying a particular signaling pathway that causes mental retardation when mutated in humans. Results from our studies should provide new insight into the fundamental mechanisms of brain development and memory formation, and should enhance our understanding of how disruptions in these processes give rise to brain disorders such as mental retardation.