DEVELOPMENT OF SYNAPTIC INPUTS ON SPINAL INTERNEURONS

描述(申请人提供)：神经生物学的一个基本问题是，成熟的突触回路是如何在出生后从复杂的未分化突触和胚胎神经元阵列中出现的。特别是，尽管中间神经元对神经回路的形成、功能和功能障碍非常重要，但对它们的成熟知之甚少。成体中间神经元以特定的突触输入和输出为特征，在这些不同突触结构的发育过程中，遗传因素和活动依赖机制之间的相互关系是一个尚未解决的主要问题。最近对脊髓中几种主要的胚胎中间神经元亚型的鉴定，由不同的遗传背景定义，以及表达这些群体谱系标记的转基因小鼠的发展，为研究成人型中间神经元及其突触输入的发育提供了可能性，这些中间神经元是由胚胎中产生的几组遗传决定的“前体”中间神经元组成的。我们的长期目标是了解在形成脊髓运动突触回路的大量脊髓中间神经元上，突触输入是如何被差异化地选择和成熟的。这种突触网络的功能障碍，包括发育过程中的错误，会导致成年人和新生儿的运动异常。其中许多症状的病因目前尚不清楚。我们建议研究来自胚胎中间神经元的一个亚类，即V1组的中间神经元上的成体突触组织的出现。使用在Vl来源的中间神经元的胞体或轴突中表达LacZ或GAP43-EGFP的转基因小鼠，我们将能够跟踪它们从出生到成年的位置、结构和发育。我们的初步数据表明，Vl来源的中间神经元产生了几个成年脊髓末级抑制中间神经元，其中包括Renshaw细胞和La抑制中间神经元(LAIN)。有趣的是，每种细胞类型都有不同的兴奋性突触输入：谷氨酸/肌肉传入优先靶向LALN，而胆碱能/运动轴突触位于Renshaw细胞。我们假设，这种独特的突触结构在出生后通过活动依赖机制在V1遗传背景上稳定和成熟，因此V1中间神经元多样化成几个成体亚型。为了验证这一假设，我们提出了以下假设：1)鉴定成年V1来源神经元的特征；2)研究每个突触输入到V1神经元亚群的正常发育，产生Renshaw细胞和LALN；3)研究这两个输入的超微结构和分子成熟度；4)测试在出生后发育过程中偏向谷氨酸能传递或胆碱能运动轴活性对V1神经元间突触组织的影响。我们还将研究神经营养因子和Trk受体在V1中间神经元中表达的时间模式，这可能有助于这些过程。

DESCRIPTION (provided by applicant): A fundamental problem in neurobiology is how mature synaptic circuits emerge postnatally from a complex array of undifferentiated synapses and embryonic neurons. In particular, little is known about the maturation of interneurons, despite their great importance for neural circuit formation, function and dysfunction. Adult interneurons are characterized by specific synaptic inputs and outputs and a major unresolved question is the interrelationship between genetic factors and activity dependent mechanisms in the development of these different synaptic architectures. The recent characterization of a few cardinal embryonic interneuron subtypes in the spinal cord, defined by different genetic backgrounds and the development of transgenic mice expressing lineage markers for these populations, opens the possibility of investigating the development of adult-type interneurons and their synaptic inputs, from a few groups of genetically determined "predecessor" interneurons generated in embryo. Our long-term objective is to understand how synaptic inputs are differentially selected and mature on the large diversity of spinal interneurons that form the spinal cord motor synaptic circuits. Dysfunction of this synaptic network, including errors in its development, leads to motor abnormalities in adults and newborns. The etiologies of many of these syndromes are currently unknown. We propose to investigate the emergence of adult synaptic organization on interneurons derived from one subclass of embryonic interneurons, the V1 group. Using transgenic mice that express either lacZ or GAP43-EGFP in the soma or axons of Vl-derived interneurons we will be able to follow their location, structure and development from birth to adulthood. Our preliminary data suggests that Vl-derived interneurons give rise to several adult spinal cord lastorder inhibitory interneurons and, among others, Renshaw cells and la Inhibitory Interneurons (laIN). Interestingly each cell type is characterized by different excitatory synaptic inputs: glutamatergic/muscle afferents preferentially target lalNs while cholinergic/motor axons synapse on Renshaw cells. We hypothesize that this distinctive synaptic structure is stabilized and matured postnatally over the V1 genetic background through activity-dependent mechanisms, and as a consequence V1- interneurons diversify into several adult subtypes. To test the hypothesis we propose to 1) identify the characteristics of adult Vl-derived neurons, 2) study the normal development of each synaptic input onto Vl-neuron subgroups giving rise to Renshaw cells and lalNs, 3) study the ultrastructural and molecular maturation of both inputs, and 4) test the effect of experimentally biasing either glutamatergic transmission or cholinergic motor axon activity during postnatal development on Vl-interneuron synaptic organization. We will also examine temporal patterns of expression of neurotrophins and trk receptors in V1- interneurons that could contribute to these processes.