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的适当发育和重塑，树突和树突棘是大脑中兴奋性突触的主要部位。Rho家族GTPases在调节这些过程中起关键作用。特别是，Rho GTPase Rac促进树枝状树杈化和棘的形成和维持。Rac活动的精确时空调节对其功能至关重要，因为异常的Rac信号会导致树突和脊柱异常以及包括智力迟钝在内的认知障碍。尽管它很重要，但调控Rac信号在神经元中的机制仍然知之甚少。我们之前发现rac特异性激活子Tiam1是树突、脊柱和突触发育的关键调节因子。我们证明Tiam1介导NMDA受体和EphB受体依赖的脊柱发育，通过将这些受体偶联到控制肌动蛋白细胞骨架重塑和蛋白质合成的Rac信号通路。最近，我们还发现Rac特异性抑制剂Bcr是一种tiam1相互作用蛋白，可阻断tiam1诱导的Rac激活和肌动蛋白重塑。过表达和敲除实验表明，Bcr限制了棘和树突的形成和生长。Tiam1和Bcr之间的复合物可能是精确调节神经元中Rac信号的“开关”，这对于脊椎、突触和树突的正确形成和重塑至关重要。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受体调控Tiam1-Bcr复合物的机制，并确定其对Rac激活和突触发育的影响；3)阐明Tiam1-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，结合分子、细胞、生化和高分辨率成像技术。这些研究的结果将对调控Rac激活和Rac依赖的突触和神经元树突发育的基本机制提供重要的见解，并有助于阐明Rac GTPase信号传导的中断如何引起认知障碍，如智力迟钝。

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.