Development of synaptic inputs on spinal interneurons

描述（由申请人提供）：运动的发展取决于脊髓运动回路的成熟。胚胎运动输出的特征首先是自发活动，导致胎儿抽搐运动，后来变得更有节奏，并表现出肢体交替。在新生儿中，运动发育经历了一段反射和姿势成熟的时期，导致协调的负重运动。所有这些变化都是脊髓突触网络成熟阶段不同的结果，更具体地说，是最终控制运动神经元放电模式和运动输出的局部中间神经元的成熟阶段不同的结果。我们的长期目标是了解这些局部脊髓回路的发展。过去，由于脊髓中间神经元的多样性和复杂性，这一领域的进展受到阻碍。幸运的是，最近小鼠模型的发展表达了基因编码的报告来识别神经间的谱系，现在允许他们通过发育进行研究，并允许对控制脊髓间神经元发育的原则有新的理解。我们的工作一直集中在一个谱系的胚胎中间神经元，称为V1，提供抑制控制同侧运动神经元。在此之前，我们发现这一群体分化为不同类型的成年中间神经元，包括Renshaw细胞和Ia抑制性中间神经元（IaINs），它们分别对同一运动神经元进行反复抑制，并在单个关节周围对运动神经元进行相互抑制。因此，这两个中间神经元在运动控制中发挥着关键但不同的作用。那么，一个基本的问题是，是什么机制使单个胚胎时期的中间神经元群在成人中分化成不同的功能类别。先前对Renshaw细胞的分析提供了一些重要的见解，包括这种规范可能发生在胚胎发育的早期，个体细胞类型通过脊髓发育的不同阶段获得不同的突触和功能。在这个提议中，我们的目标是研究包括IaINs在内的大群V1中间神经元的发育，这些神经元参与本体感觉反射。潜在的假设是，IaINs是一组v1 -中间神经元，由它们的早期出生（同时与Renshaw细胞）指定，在胚胎中获得与成人完全不同的瞬时运动轴突输入和功能。我们还假设，后来它们摆脱了这些胚胎输入并发展了连接，使它们能够调节相互抑制。因此，我们提出了三个目标，以找出他们的出生日期（aim1），他们的主要突触输入和输出在胚胎（aim2）和出生后相互抑制回路的成熟（aim 3）。这项工作的一个推论是，导致脊髓网络发育受阻的先天性缺陷并不一定意味着一个不成熟的成人连接网络，而更有可能是一个适合于早期发育点的不同连接网络。因此，了解这种早期连通性的本质对于更好地理解新生儿神经运动缺陷的多样性是很重要的。建议的工作将研究脊髓发育，重点是介导协调运动和运动成熟的神经元网络。这项研究将使用强大的新的小鼠遗传方法来分析在发育过程中组织这个网络的细胞元素。因此，这项研究有助于理解新生儿和婴儿的正常和异常运动发育

DESCRIPTION (provided by applicant): The development of movement depends on the maturation of the spinal cord motor circuits. Embryonic motor output is characterized first by spontaneous activity resulting in fetal jerk movements that later become more rhythmic and show limb alternations. In newborns motor development then undergoes a period of reflex and postural maturation leading into coordinated weigh-bearing locomotion. All these changes occur as a consequence of different maturation phases of the spinal synaptic networks and more specifically of local interneurons that ultimately control motor neuron firing patterns and motor output. Our long term objective is to understand the development of these local spinal circuits. Advances in this field were hampered in the past by the diversity and complexity of spinal interneurons. Fortunately, the recent development of mouse models expressing genetically-encoded reporters to identify interneuronal lineages now permits their study through development and allowed new understanding of the principles that govern spinal interneuron development. Our work has been focusing on one lineage of embryonic interneurons, named V1, that provide inhibitory control to ipsilateral motoneurons. Previously, we showed that this group diversifies into different types of adult interneurons, including Renshaw cells and Ia inhibitory interneurons (IaINs) that provide respectively recurrent inhibition to the same motoneurons and reciprocal inhibition between motoneurons with antagonistic actions around single joints. These two interneurons thus perform critical, but different roles in motor control. A basic question is then what mechanisms diversify single embryological groups of interneurons into distinct functional classes in adult. Previous analyses on Renshaw cells provided some important insights, including that this specification might occur early in embryological development and that individual cell types acquire different synaptology and functions through the different phases of spinal cord development. In this proposal we aim to investigate the development of the large group of V1 interneurons, including IaINs, that are interposed in proprioceptive reflexes. The underlying hypotheses are that IaINs are a group of V1-interneurons specified by their early birth (simultaneously with Renshaw cells) to acquire in the embryo transient motor axon inputs and functions that are quite distinct from those in adult. We also hypothesize that later they shed these embryonic inputs and develop connectivity that allows them to mediate reciprocal inhibition. Therefore we propose three aims to find out their birth dates (aim1), their major synaptic inputs and outputs in embryo (aim2) and the maturation of the reciprocal inhibitory circuit postnatally (aim 3). A corollary of this work is that congenital deficits that result in arrested spinal network development do not necessary imply a network with immature adult connectivity, but more likely networks of different connectivity appropriate to an earlier developmental point. Therefore it is important to understand the nature of this earlier connectivity to better appreciate the diversity of neurological motor deficits expressed in newborns. PUBLIC HEALTH RELEVENCE The work proposed will study spinal cord development focusing on the neuronal networks that mediate the maturation of coordinate movements and locomotion. The study will use powerful new mouse genetic methodologies to analyze the cellular elements that organize this network during development. The research is thus guided towards understanding normal and abnormal motor development in newborn and infants