Cytoskeletal Signaling and Axon Guidance

细胞形态和极性的建立和维持在多细胞生物的发育中起着至关重要的作用。在发育中的神经系统中，神经元将轴突发送到正确的目标，形成一个轴突支架，在这个支架上建立功能性的神经元连接。这一建议涉及到在新生神经系统中轴突寻径到目标的机制。细胞外信号提供引导信息，并被生长锥上的跨膜受体检测到，生长锥是延伸轴突远端的感觉运动结构。作为对这些信号的响应，生长锥的肌动蛋白细胞骨架，介导生长锥的运动，被改变以实现生长锥的定向迁移。我们对秀丽隐杆线虫的研究表明，在轴突寻路中，三种Rac GTP酶定义了三种冗余的细胞骨架信号通路，而GTP交换因子UNC-73 Trio控制着这三种通路。我们发现肌动蛋白结合蛋白UNC-115作用于三种Rac通路之一的下游，可能直接调节肌动蛋白结构以响应Rac信号传导。此外，我们还发现了一种新的Rac调节因子，即7-WD重复蛋白SWR-1，它直接结合UNC-115和Rac，并在神经元形态发生中抑制Rac活性。本文描述的实验旨在阐明Rac信号转导在轴突寻路中的作用，包括识别和表征Rac信号转导中的新分子，以及分析这些分子在轴突寻路中的相互关系。第一个目的是表征新的DAD结构域在下游Rac细胞骨架效应物UNC-115在轴突寻路中的作用。第二个目的是确定Rac负调节因子SWR-1抑制Rac的分子机制。第三个目标是通过利用Rac功能在轴突寻路中的冗余来识别和表征新分子及其与Rac信号的相互作用。了解细胞骨架信号传导的分子机制对于开发旨在减轻中枢神经系统创伤（如脊髓损伤和中风）以及肿瘤转移影响的疗法至关重要

The establishment and maintenance of cell shape and polarity play key roles in the development of multicellular organisms. In the developing nervous system, neurons send axons to their correct targets to form an axon scaffold upon which functional neuronal connections are made. This proposal involves mechanisms of axonal pathfinding to their targets in the nascent nervous system. Extracellular cues provide guidance information and are detected by transmembrane receptors present on the growth cone, the sensory-motile structure at the distal tip of an extending axon. In response to such cues, the actin cytoskeleton of the growth cone, which mediates growth cone movement, is altered to achieve directed migration of the growth cone. Our studies in Caenorhabditis elegans have revealed that three Rac GTPases define three redundant cytoskeletal signaling pathways in axon pathfinding, and that the GTP exchange factor UNC-73 Trio controls all three pathways. We have found that the actin-binding protein UNC-115 acts downstream of one of the three Rac pathways, possibly to directly modulate actin structure in response to Rac signaling. Further, we have identified a novel Rac regulator, the 7-WD repeat protein SWR-1, which directly binds to both UNC-115 and Racs and represses Rac activity in neuronal morphogenesis. Experiments described here aim to elucidate the role of Rac signal transduction in axon pathfinding, including identification and characterization of new molecules in Rac signaling and analyses of how these molecules relate to one another in axon pathfinding. The first aim is to characterize the role of the novel DAD domain in the function of the downstream Rac cytoskeletal effector UNC-115 in axon pathfinding. The second aim is to determine the molecular mechanisms of Rac repression by the Rac negative regulator SWR-1. The third aim is to identify and characterize new molecules and interactions with Rac signaling by exploiting the redundancy of Rac function in axon pathfinding. Understanding the molecular mechanisms of cytoskeletal signaling could prove critical for developing therapies aimed at mitigating the effects of central nervous system trauma (e.g. spinal cord injury and stroke) as well as tumor metastasis