Assessing the contact-mediated role of netrin1 in axon guidance

建立神经回路需要神经元将轴突延伸到相当大的范围内。 利用胚胎环境中的分子线索来确定它们生长锥体的方向。这本教科书 引导因子的例子是netrin1，它是层粘连蛋白超家族的成员，最初在脊髓中表现出特征 电源线。经典研究表明，由底板(Fp)细胞产生的netrin1通过趋化、扩散 远距离引导DCC+连合轴突朝向FP。Netrin1随后被牵连到 整个神经系统的指导决策。然而，我们最近的研究表明， Netrin1的主要来源是脊髓脑室区域的神经前体细胞(NPC)。而不是扮演一个 远程扩散梯度，我们的数据支持一个模型，在该模型中，NPC衍生的netrin1在局部作用，形成一个 轴突的定向路径，也促进轴突分束。Npc的两极几何结构允许它们 在脊髓的基底(软膜)边缘沉积网织蛋白1。然后，这种netrin1+底物在本地作用于 通过触觉的直接束状、腹侧轴突延伸，细胞沿粘附物定向生长 浮出水面。然而，netrin1可能为轴突介导了一种更复杂的活动，而不仅仅是单纯的粘连： 然后沉积到DCC+轴突上，这些轴突精确地围绕VZ的边界生长，即Netrin1- 表达结构域。消融一小段netrin1表达会导致轴突偏离其轨迹 遵循异位的netrin1边界。我们建议将这一系列活动称为“健康”增长 边界，由支持常春藤(属：Hedera)生长的墙的类比而来，该墙本身不被穿透 在常春藤旁。Netrin1可形成局部生长底物，并促进周围轴突的束状生长 Netrin1表达结构域。这种对netrin1起作用的机制基础的重新解释解释了 为什么使用可溶性netrin1作为再生素进展甚微。为了实现这一目标，我们将 评估[1]脊髓神经前体细胞建立netrin1+Hapotatic底物促进 目标1中定向的、成束的轴突生长，[2]鼻咽癌和FP细胞是否有不同的作用来产生Netrins 在目标2和[3]中，Netrins在目标3中是否在前脑的轴突引导中具有突触活性。 目的1：确定脊髓npC建立netrin1“异体”边界的机制 假设：脊髓npC向硬膜表面运输netrin1，以建立一种突触底物。然后是Netrin1 转移到DCC+轴突以促进其在netrin1表达区域周围的束状轴突生长。 目的2：确定FP衍生的netrin1在脊髓中的范围作用 假设：FP来源的netrin1短距离作用于穿过中线的连合轴突 目的3：确定鼻咽癌来源的netrin1/3在前脑中的作用 假设：发育中的前脑中的神经前体利用Netrin家族来建立生长边界 它引导轴突在间脑和端脑之间。

The establishment of neural circuits requires neurons to extend axons over considerable distances, using molecular cues in the embryonic environment to orient their growth cones. The textbook example of a guidance factor is netrin1, a member of the laminin superfamily first characterized in the spinal cord. Classic studies suggested that netrin1, produced by floor plate (FP) cells, acts by chemotaxis, diffusing over long distances to guide Dcc+ commissural axons towards the FP. Netrin1 was then implicated in guidance decisions throughout the nervous system. However, our studies have recently demonstrated that the key source of netrin1 is neural progenitor cells (NPCs) in the spinal ventricular zone. Rather than acting as a long-range diffusible gradient, our data supports a model in which NPC-derived netrin1 acts locally, forming a directional path for axons that also promotes axonal fasciculation. The bipolar geometry of NPCs permits them to deposit netrin1 at the basal (pial) margins of the spinal cord. This netrin1+ substrate then acts locally to direct fasciculated, ventral axon extension by haptotaxis, the directed growth of cells along an adhesive surface. However, netrin1 may mediate a more complex activity for axons beyond pure adhesion: netrin1 is then deposited onto Dcc+ axons, and these axons then grow precisely around the border of VZ, i.e. a netrin1- expressing domain. Ablating a small region of netrin1 expression causes axons deviate from their trajectory to follow the ectopic netrin1 boundary. We propose to call this collection of activities a “hederal” growth boundary, from the analogy of a wall supporting the growth of ivy (genus: hedera) that is not itself penetrated by the ivy. Netrin1 can form a local growth substrate, and promote the fasciculated growth of axons around a netrin1-expressing domain. This reinterpretation of the mechanistic basis by which netrin1 functions, explains why little progress has been made using soluble netrin1 as a regenerative factor. To further this goal, we will assess [1] the mechanisms by which spinal NPCs establish a netrin1+ haptotactic substrate that promotes directed, fasciculated axonal growth in Aim 1, [2] whether NPC vs FP cells have distinct roles producing netrins in Aim 2 and [3] whether netrins have haptotactic activities in axon guidance in the forebrain in Aim 3. Aim 1: Determine the mechanism by which spinal NPCs establish netrin1 “hederal” boundaries Hypothesis: Spinal NPCs traffic netrin1 to the pial surface to establish a haptotactic substrate. Netrin1 then transfers to Dcc+ axons to promote their fasciculated axon growth around netrin1-expressing regions. Aim 2: Determine the range role of FP-derived netrin1 in the spinal cord Hypothesis: FP-derived netrin1 acts over short range to fasciculate commissural axons crossing the midline Aim 3: Determine the role of NPC-derived netrin1/3 in the forebrain Hypothesis: Neural progenitors in the developing forebrain use the netrin family to establish growth boundaries that guide axons between the diencephalon and telencephalon.