Stem Cell Derived Motoneurons to study motor neuron development

移动能力是所有动物共有的最基本的特征之一，也是生存所必需的。即使是最简单的运动形式，比如爬行，也需要神经系统对不同的肌肉群进行精细的控制，以确保屈肌和伸肌的适当交替收缩。神经系统不仅需要激活和关闭肌肉群以进行精确的运动，还必须确定需要多少激活才能产生适当的力量。在发育过程中，精确的运动控制是如何建立起来的，人们仍然知之甚少。从神经肌肉的角度来看，精确肌肉群的激活始于对特定运动神经元亚群的选择性引导，使其到达正确的肌肉目标。精确的激活水平开始于运动神经元特性的发展，这允许一个系统的、平滑的力量梯度。研究表明，运动轴突是通过一系列的引导分子被引导到肌肉目标的。虽然许多相关分子的身份已经确定，但控制其表达和下游途径激活的细胞机制尚不清楚。本研究将探讨LIM转录因子Lhx1如何调节LMCl运动神经元的选择性运动轴突引导，以支配后肢背侧肌肉。因此，我们建议使用来自胚胎干细胞的运动神经元和小鼠遗传学作为补充模型系统来研究Lhx1是否调节轴突靶向。我们还将研究当轴突面对已知参与其选择性靶向的引导分子时，哪些细胞内信号分子被激活。电机控制中最成熟的原理之一被称为“尺寸原理”。尺寸原则能够在需要更大的力时实现平滑的收缩梯度，并确保基于尺寸和疲劳性的连续方式招募运动单元。尽管大小原则是运动控制中最基本的原则之一，但对于不同运动单元在发育过程中如何形成不同的解剖特性（如细胞体大小和神经支配比例），甚至何时形成，我们所知甚少。基于不同大小的运动神经元之间的蛋白质组学筛选，我们假设大小原则至少在一定程度上是通过Lats2/Yap信号通路调节的。我们将通过培养有丝分裂后发育中的运动神经元特异性缺乏Lats2/Yap信号的小鼠来验证这一假设。利用形态学和电生理学技术的结合，我们将测试运动神经元中Lats2/Yap信号的丢失如何影响大小原理的出现。这些研究是新颖的，因为它们将检查精确电机控制的重要特性；神经回路是如何形成的，以及回路的内在特性如何发展以确保其正常功能。

The ability to move is one of the most fundamental traits shared by all animals and is essential for survival. Even the simplest forms of movement, such as those required for crawling, require exquisite control of different muscle groups by the nervous system to ensure proper alternating contractions of flexor and extensor muscles. The nervous does not only have to activate and de-activate muscle groups for precise movements, it must also determine how much activation is required to generate the proper amount of force. How precise control of movement is established during development remains poorly understood. From a neuromuscular point of view, activation of precise muscle groups begins with selective guidance of specific motoneuron subgroups to their correct muscle targets. Precise level of activation begins with the development of motoneuron characteristics that allow for a systematic, smooth gradation in force. Studies have shown that motor axons are guided to the muscles target(s) using a series of guidance molecules. While the identities of many of the molecules involved have been identified, the cellular mechanisms controlling their expression and downstream pathways activated are less well understood. This proposal will examine how the LIM transcription factor Lhx1 regulates selective motor axon guidance of LMCl motoneurons to innervate dorsal hindlimb muscles. Thus, we propose to use motoneurons derived from embryonic stem cells and mouse genetics as complementary model systems to examine whether Lhx1 regulates axon targeting. We will also examine what intracellular signaling molecules are activated when the axons confront guidance molecules known to be involved in their selective targeting. One of the most established principle in motor control is known as the "size principle". The size principle enables the smooth gradation in contraction as more force is required and ensures that motor units are recruited in a sequential manner based on size and fatigability. Even though the size principle is one of the most fundamental principles in motor control, very little is known about how, or even when, the distinct anatomical properties (e.g. cell body size and innervation ratio) of different motor units form during development. Based on a proteomic screen between motoneurons differing in size we hypothesis that the size principle is regulated, at least in part, by signaling through the Lats2/Yap signaling pathway. We will test this hypothesis by generating mice specifically lacking Lats2/Yap signaling in post-mitotic developing motoneurons. Using a combination of morphological and electrophysiological techniques we will test how the loss of Lats2/Yap signaling in motoneurons affects the emergence of the size principle. These studies are novel as they will examine to important properties for precise motor control; how the neural circuit is formed and how intrinsic properties of the circuit development to ensure its proper function.