Mechanisms of Robo1/2-mediated motor neuron cell body and axon positioning relative to the spinal cord midline

项目总结： 神经元在发育过程中通过吸引和排斥的分子线索的作用而连接起来 通过激活前导通路上的受体引导迁移的神经元胞体和轴突到达目的地 神经元的突起，生长锥体。生长锥体经常暴露在诱人和令人厌恶的环境中。 对这些信号的轴突反应由两种主要线索模式中的一种来决定 集成：并行或分层信令。在并行信令中，信令之间的交互很少 路径和生长锥的行为由来自不同来源的总的吸引和排斥信号的总和决定 方向。然而，分层串扰在很大程度上依赖于信号级联的交集，并且 对选定线索的反应可以通过沉默来自其他线索的信号来主导生长锥行为。 受体Robo1和Robo2调节Sit家族分泌的指导线索的排斥力，而DCC是 具有吸引力的配体Netrin-1的受体。脊髓连合轴突引导的研究进展 支持Sit-Robo1/2信号可以抑制DCC介导的对Netrin-1的吸引力的观点。因此，Robo 在连合神经元中，信号可能主导于DCC信号，但这一层次的Robo 而DCC通路在其他类型的细胞中的作用仍然难以捉摸。 最近的研究发现，在脊髓运动神经元迁移和运动轴突引导方面存在几个缺陷。 Robo1/2双突变小鼠。在缺乏Robo1和Robo2的小鼠中，运动神经元胞体异常迁移 从腹角进入连合，运动神经轴突穿过脊髓中线。楼面上的地板 腹中线同时表达Netrin-1和Sits，目前尚不清楚运动神经元是否受到侵袭 Robo1/2突变小鼠的中线缺失是由狭缝排斥力的丧失或Netrin-1吸引力的获得引起的 由于缺乏Robo1/2介导的DCC沉默。在这里，我们通过结合鼠标来解决这个问题 遗传学，体内表型分析，以及体外运动神经元迁移和轴突引导分析。我们的 结果表明，至少部分Robo1/2突变的运动神经元表型是由丢失引起的 缝隙介导的中线斥力，而不是Netrin-1吸引力的增加。因此，Sit-Robo1/2和Netrin-1-DCC可以 运动神经元中的信号平行。

PROJECT SUMMARY: Neurons are wired up during development through the action of attractive and repulsive molecular cues that guide migrating neuronal cell bodies and axons to their destinations by activating receptors on the leading process of the neuron, the growth cone. Growth cones are frequently exposed to both attractive and repulsive cues at the same time, and axonal responses to these signals are dictated by one of two main modes of cue integration: parallel or hierarchical signaling. In parallel signaling, there are few interactions between signaling pathways, and growth cone behavior is dictated by a sum of total attractive and repulsive signals from different directions. Hierarchical cross-talk, however, relies heavily on the intersection of signaling cascades, and the response to select cues can dominate growth cone behavior by silencing signaling from other cues. The receptors Robo1 and Robo2 mediate repulsion from secreted guidance cues of the Slit family, and DCC is a receptor for the attractive ligand Netrin-1. Previous research on the guidance of spinal commissural axons supports the idea that Slit-Robo1/2 signaling can silence DCC-mediated attraction to Netrin-1. Thus, Robo signaling might dominate over DCC signaling in commissural neurons, but whether this hierarchy of the Robo and DCC pathways is at work in other cell types has remained elusive. Recent studies have identified several defects in spinal motor neuron migration and motor axon guidance of Robo1/2 double mutant mice. In mice lacking Robo1 and Robo2, motor neuron cell bodies aberrantly migrate from the ventral horn into the commissure, and motor axons cross the spinal cord midline. The floor plate at the ventral midline expresses both Netrin-1 and Slits, and it has remained unclear whether motor neuron invasion of the midline in Robo1/2 mutant mice is caused by a loss of Slit repulsion or a gain of Netrin-1 attraction due to absence of Robo1/2-mediated DCC silencing. Here, we address this question by combining mouse genetics, in vivo phenotype analysis, and in vitro motor neuron migration and axon guidance assays. Our results demonstrate that at least some of the Robo1/2 mutant motor neuron phenotypes are caused by a loss of Slit-mediated midline repulsion, not a gain of Netrin-1 attraction. Hence, Slit-Robo1/2 and Netrin-1-DCC can signal in parallel in motor neurons.