Trophic Interactions During Visual System Development

描述（由申请人提供）：Netrin-1是一种分泌分子，在轴突引导过程中可以作为引诱剂或驱避剂。在视觉系统中，神经网络与视觉轴突从眼睛向大脑目标的引导有关。我们的长期目标是阐明控制视网膜与其中心靶点之间适当连接发育的调节机制。在这里，我们提出验证netrin-1与视网膜发育的后期阶段有关的假设，这对视觉功能有重大影响。具体来说，我们的目标是证明；1) netrin-1通过在特定受体亚型上的独立信号传导，影响目标视网膜轴突及其脑突触后神经元；2)视网膜顶区的正常布线需要netrin和脑源性神经营养因子（BDNF）之间的功能相互作用，BDNF是一种调节突触发生的营养因子。我们基于以下观察得出这些假设；i) netrin在视网膜轴突寻路期后影响视网膜轴突，ii)结肠直肠癌（DCC）中缺失的两个netrin受体和UNC-5在视网膜直肠发育期间以互补模式表达，iii) BDNF和netrin具有共同的信号传导机制来影响轴突的引导。本文将在爪蟾蝌蚪中进行实时成像实验和网络信号的时空控制操作，以研究网络在视觉系统发育中的作用。需要检验的具体假设有：1。Netrin-1通过DCC受体的激活，指示视网膜轴突的终止位置，并形成视轴突的树突结构和突触连接。神经网络水平和视神经顶盖内DCC信号的调控将用于分析神经网络在轴突靶向、分支和突触前位点分化中的作用。2. Netrin-1通过UNC-5受体信号传导影响突触后顶叶神经元的形态和突触发育。体内成像实验将用于确定netrin是否参与早期树突发生，控制先锋树突过程的方向，以及netrin是否调节树突分支和突触后位点分化。3. Netrin和BDNF在功能上相互作用，形成视顶突触连通性。分子和体内成像研究将检查通过DCC受体的netrin信号是否在BDNF调节视网膜轴突分支和突触发生的上游。BDNF拯救受DCC信号改变影响的视网膜轴突的能力也将被测试。总的来说，拟议的研究将揭示netrin-1是视网膜回路上发育的关键角色，并将进一步我们对控制视觉通路发育的机制的理解，这对维持正常视觉功能至关重要。

DESCRIPTION (provided by applicant): Netrin-1 is a secreted molecule that can function as either an attractant or repellent during axon guidance. In the visual system, netrin has been implicated in the guidance of optic axons out of the eye towards their targets in the brain. Our long-term goal is to elucidate the regulatory mechanisms controlling the development of proper connections between the retina and its central targets. Here, we propose to test the hypothesis that netrin-1 is implicated in late aspects of retinotectal development that significantly impact visual function. Specifically, we aim to demonstrate that; 1) netrin-1, through independent signaling on specific receptor subtypes, influences retinal axons at the target and their postsynaptic neurons in the brain, and 2) proper retinotectal wiring requires functional interactions between netrin and brain-derived neurotrophic factor (BDNF), a trophic factor that modulates synaptogenesis. We base these hypotheses on the observations that; i) netrin influences retinal axons past the period of retinal axon pathfinding, ii) the two netrin receptors deleted-in-colorectal cancer (DCC) and UNC-5 are expressed in complementary patterns during retinotectal development and, iii) BDNF and netrin share common signaling mechanisms to influence axon guidance. Real-time imaging experiments and spatially and temporally controlled manipulations in netrin signaling will be performed in living Xenopus tadpoles to examine netrin's role in visual system development. The specific hypotheses to be tested are: 1. Netrin-1, through DCC receptor activation, instructs retinal axons where to terminate and shapes optic axon arbor structure and synaptic connectivity. Manipulations in netrin levels and DCC signaling within the optic tectum will be used to analyze the role of netrin in axon targeting, branching, and presynaptic site differentiation. 2. Netrin-1, through UNC-5 receptor signaling, influences the morphological and synaptic development of postsynaptic tectal neurons. In vivo imaging experiments will be used to establish whether netrin participates in early dendritogenesis, controlling the orientation of pioneer dendritic processes and whether netrin modulates dendritic branching and postsynaptic site differentiation. 3. Netrin and BDNF interact functionally to shape retinotectal synaptic connectivity. Molecular and in vivo imaging studies will examine if netrin signaling through DCC receptors is upstream of BDNF regulation of retinal axon branching and synaptogenesis. The ability of BDNF to rescue retinal axons affected by altered DCC signaling will also be tested. Collectively, the proposed studies will reveal that netrin-1 is a key player in retinotectal circuit development and will further our understanding of the mechanisms that control the development of visual pathways, that are critically important in the maintenance of normal visual function.