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的适当发育和重塑，这是大脑中兴奋性突触的主要位置。Rho家族GTP酶在调节这些过程中起着关键作用。特别是，Rho GTPase Rac促进树突的树枝形成和脊椎的形成和维持。对Rac活性的精确时空调控对其功能至关重要，因为Rac信号的异常会导致树突和棘突的异常以及包括智力低下在内的认知障碍。尽管它很重要，但调节神经元中RAC信号的机制仍然知之甚少。我们以前发现RAC特异性激活剂Tiam1是树突、脊椎和突触发育的关键调节因子。我们证明，Tiam1通过将NMDA受体和EphB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信号通路来调节NMDA受体和EphB受体依赖的脊柱发育。最近，我们还发现RAC特异性抑制剂BCR是一种与Tiam1相互作用的蛋白，可以阻断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限制了棘突和树突的形成和生长。Tiam1和BCR之间的复合体可能作为一个“开-关开关”，精确地调节神经元中的RAC信号，这对于棘突、突触和树突的正确形成和重塑是必不可少的。为了验证这一假说，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Tiam1-BCR复合体的机制，并确定其对Rac激活和突触发育的影响；以及3)阐明Tiam1-BCR复合体在调节N-钙粘附素介导的突触黏附中的作用。为了解决这些问题，我们将使用多方面的方法，使用分子、细胞、生化和高分辨率成像技术的组合。这项研究的结果将为调节RAC激活和RAC依赖的突触和树突发育的基本机制提供重要的见解，并有助于阐明RAC GTPase信号的中断如何导致认知障碍，如智力低下。与公共健康相关：我们建议调查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，当人类发生突变时，这种信号通路会导致智力低下。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的中断如何导致智力低下等大脑疾病的理解。

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.