Development of synaptic inputs on spinal interneurons

描述(申请人提供)：运动的发展依赖于脊髓运动回路的成熟。胚胎的运动输出首先以自发活动为特征，导致胎儿的痉挛运动，后来变得更有节奏，并显示出肢体交替。在新生儿中，运动发育经历了一段反射和姿势成熟的时期，导致了协调的负重运动。所有这些变化都是脊髓突触网络不同成熟阶段的结果，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，脊髓中间神经元的多样性和复杂性阻碍了这一领域的发展。幸运的是，最近表达遗传编码报告以识别神经元间谱系的小鼠模型的发展现在允许通过发育来研究它们，并允许对支配脊髓间神经元发育的原理有新的理解。我们的工作一直集中在胚胎中间神经元的一个谱系上，称为V1，它为同侧运动神经元提供抑制控制。在此之前，我们发现该组神经元可分化为不同类型的成年中间神经元，包括Renshaw细胞和Ia抑制中间神经元(IaINs)，它们分别对同一运动神经元提供经常性抑制，以及在单个关节周围具有拮抗作用的运动神经元之间相互抑制。因此，这两个中间神经元在运动控制中扮演着关键但不同的角色。那么，一个基本的问题是，在成人中，是什么机制使单个胚胎学的中间神经元群体多样化，形成不同的功能类别。以前对Renshaw细胞的分析提供了一些重要的见解，包括这种指定可能发生在胚胎发育的早期，以及不同类型的细胞在脊髓发育的不同阶段获得不同的突触和功能。在这项建议中，我们的目标是研究大群V1中间神经元的发育，包括插入本体感觉反射的IaINs。基本的假设是，IaINs是一组由其早期出生(与Renshaw细胞同时)指定的V1中间神经元，以在胚胎中获得与成人截然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，以了解它们的出生日期(AIM1)、它们在胚胎中的主要突触输入和输出(AIM2)以及出生后相互抑制回路的成熟(AIM3)。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网络具有不成熟的成人连接，而更有可能是适合于较早发育点的不同连接的网络。因此，重要的是要了解这种早期连接的性质，以便更好地了解新生儿表现出的神经运动缺陷的多样性。这项工作将研究脊髓发育，重点放在调节协调运动和运动成熟的神经网络上。这项研究将使用强大的新小鼠遗传学方法来分析在发育过程中组织这个网络的细胞元素。因此，这项研究有助于了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