Wiring direction-selective circuits in the mouse retina

 描述（由申请人提供）：星爆无长突细胞（SAC）是放射状对称的抑制性中间神经元，对于赋予哺乳动物视网膜中DS视觉回路的方向选择（DS）特性至关重要，其赋予图像运动检测，这是生物体生存的关键功能。由于缺乏轴突，SAC已经进化为从其远端树突状乔木的静脉曲张中释放神经递质。神经元缺乏明显可区分的轴突-树突区室，因此缺乏离散的专业化区域，其电路组装的原则定义不清。在小鼠视网膜中，通过引导信号semaphorin 6A（Sema 6A）及其受体的信号传导是正确的SAC电路组装所必需的。存在两种SAC镜像种群，它们响应于光的增加（ON）或减少（OFF）。SAC最初混合在发育中的视网膜中，但随后分层进入ON和OFF SAC回路。Sema 6A-PlexA 2信号不仅驱动ON和OFF SAC分层，而且还建立了各个ON SAC的适当径向对称性，这对于准确的DS视觉响应是必要的。Sema 6A也需要适当的光（即视觉提示）诱发抑制性突触后反应在ON SAC。Sema 6A-PlexA 2信号通路如何引起这种不同的细胞反应尚不清楚。该建议的目的是研究的机制，区分这些分支的指导线索信号在单一的SAC，并定义范围内的指导线索，有助于DS SAC布线。本研究的具体目的是：1）评估视网膜发育过程中Sema 6A-PlexA 2信号通路的方向性，涉及ON和OFF SAC回路的分离和SAC对称性的发展; 2）探索Sema 6A突变体中SAC抑制不当的潜在机制; 3）询问其他6类Sema和/或A类丛状蛋白是否有助于连接DS回路。我将通过以下方法来解决这些基本的生物学问题：1）在单个SAC中有条件地去除Sema 6A或PlexA 2以确定信号传导的方向性; 2）检查Sema 6A和PlexA 2突变体SAC中突触的定位，以询问不适当的突触形成是否有助于不适当的SAC抑制性输入;和3）6类Sema和A类双突变体组合的表型分析，以询问这些分子中是否有任何调节SAC分层或SAC对称性的阐述。这些问题的答案有可能揭示指导CNS电路组装的一般原则，特别是在神经元缺乏轴树突极性的情况下。由于这些分子也有助于在发育中的CNS中布线其他电路，除了在发育中的CNS中布线DS电路之外， 视网膜，理解它们的信号传导的方向性和它们指导视网膜中回路细化的不同方面的机制将对治疗其中这些和相关信号传导分子无功能的发育性神经元回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.