Wiring direction-selective circuits in the mouse retina

 描述(申请人提供)：星状突起无长突细胞(SACs)是放射状对称的抑制性中间神经元，对哺乳动物视网膜中DS视觉回路的方向选择性(DS)特性至关重要，它提供图像运动检测，这是生物体生存的关键功能。由于缺少轴突，囊已进化为从其远端树突枝的曲张中释放神经递质。神经元缺乏清晰可区分的轴突-树突隔间，因此缺乏专门化的离散区域，因此，支配神经元电路组装的原理定义不清。在小鼠视网膜中，通过引导线索信号素6A(Sema6A)及其受体发出的信号是正确组装SAC电路所必需的。存在两个镜像囊群体，它们对光的增加(开)或减少(关)作出反应。SAC最初混杂在发育中的视网膜中，但随后分层进入开启和关闭SAC回路。Sema6A-PlexA2信号不仅驱动SAC分层的开启和关闭，而且还建立了SAC上个体的适当的径向对称性，这是准确的DS视觉反应所必需的。Sema6A也是适当的光(即视觉线索)在囊上引起抑制性突触后反应所必需的。Sema6A-PlexA2信号通路如何引起这种不同的细胞反应尚不清楚。这项建议旨在研究在单个囊内区分这些引导线索信号分支的机制，并定义有助于DS SAC连接的引导线索的范围。这项研究的具体目的是：1)评估视网膜发育过程中Sema6A-PlexA2信号通路的方向性，涉及开关SAC回路的分离和SAC对称性的发展；2)探讨Sema6A突变体中SAC抑制不当的机制；以及3)询问其他6类SemA和/或A类丛蛋白是否参与了DS电路的连接。我将通过1)有条件地移除单囊中的Sema6A或PlexA2来确定信号的方向性；2)检查Sema6A和PlexA2突变囊中突触的定位，以询问不适当的突触形成是否导致不正确的SAC抑制输入；以及3)对6类SEMA和A类双突变组合进行表型分析，以询问是否有这些分子调节SAC层积或SAC对称性的形成。这些问题的答案有可能揭示指导中枢神经系统电路组装的一般原则，特别是在神经元缺乏轴突极性的情况下。由于这些分子也有助于连接发育中的中枢神经系统中的其他电路，除了连接 了解它们信号的方向性以及它们指导视网膜电路精化的不同方面的机制，将对治疗这些信号分子和相关信号分子不起作用的发育神经元电路障碍具有重要意义。

 DESCRIPTION (provided by applicant): Starburst amacrine cells (SACs) are radially symmetric inhibitory interneurons essential to confer the direction- selective (DS) properties of DS visual circuits in the mammalian retina, which impart image motion detection, a critical function for organismal survival. SACs have evolved to release neurotransmitter from varicosities in their distal dendritic arbors, since they lack axons. The principles that govern circuit assembly of neurons that lack clearly distinguishable axonal-dendritic compartments, and therefore discrete regions of specialization, are poorly defined. In the mouse retina, signaling through the guidance cue semaphorin6A (Sema6A) and its receptor is required for proper SAC circuit assembly. Two mirror populations of SACs exist that respond to either increases (ON) or decreases (OFF) in light. SAC are initially intermingled in the developing retina, but then stratif into the ON and OFF SAC circuits. Sema6A-PlexA2 signaling drives not only ON and OFF SAC stratification, but also establishes appropriate radial symmetry of individual ON SACs, which is necessary for accurate DS visual responses. Sema6A is also required for proper light (i.e. visual cue) evoked inhibitory postsynaptic responses in ON SACs. How the Sema6A-PlexA2 signaling pathway elicits such disparate cellular responses is unclear. This proposal is designed to study the mechanisms that distinguish these branches of guidance cue signaling within single SACs, and to define the range of guidance cues that contribute to DS SAC wiring. The specific aims of this study are to 1) assess the directionality of Sema6A-PlexA2 signaling pathways during retinal development with respect to segregation of the ON and OFF SAC circuits and development of SAC symmetry; 2) probe the mechanism underlying improper SAC inhibition in Sema6A mutants; and 3) to ask if other class 6 Semas and/or class A Plexins contribute to wiring DS circuitry. I will address these basic biological questions by 1) conditional removal of either Sema6A or PlexA2 in single SACs to determine the directionality of signaling; 2) examining the localization of synapses in Sema6A and PlexA2 mutant SACs, to ask if inappropriate synapse formation contributes to improper SAC inhibitory input; and 3) phenotypic analysis of class 6 Sema and class A double mutant combinations, to ask if any of these molecules regulates SAC stratification or elaboration of SAC symmetry. The answers to these questions have the potential to reveal general principles that guide CNS circuit assembly, especially in contexts where neurons lack axodendritic polarity. Since these molecules also contribute to wiring other circuits in the developing CNS, in addition to wiring DS circuits in the retina, understanding the directionality of their signaling and the mechanisms by which they direct distinct aspects of circuit refinement in the retina will have significant implications for treating developmental neuronal circuitry disorders where these and related signaling molecules are non-functional.