Precise assembly of retinal circuitry through rejection of inappropriate synaptic partners

在发育过程中，视网膜神经元与特定的突触伙伴进行精确的连接。 这些突触的选择影响视网膜回路的计算能力，从而影响视觉感知。 第一个问题。细胞表面识别分子通过编码两种跨细胞的 信号：1)连接神经元和它们的电路伙伴的吸引信号；2)避开非神经元的排斥信号 目标单元格。这两种类型的线索都是精确的视网膜连接所必需的，但其背后的分子机制 对不适当的突触伙伴的排斥是未知的。这里的目标是识别识别机制- 防止不适当的突触伙伴之间的连接的神经质。我们的中心假设是FLRT 细胞-表面分子UC5家族介导排斥的受体-配体相互作用，从而防止交叉- 环路突触形成。这项工作的基本原理是，它将揭示一种新的突触选择识别- 通过排斥力机制起作用的分子。了解如何避免错误的突触 是最终破译突触伴侣选择背后的分子逻辑的必要步骤。至 为此，提出了以下具体目标：1)确定阻止视网膜神经元Se-1的配体。 选择不合适的突触伙伴。视网膜环路占据内丛内的平行亚层 层(IPL)神经纤维层。这种排列通过将回路部分的树枝聚集在一起来促进突触专一性。 NER位于定义的位置，其中它们与非目标小区隔离。在初步研究中使用 以小鼠方向选择(DS)环路为模型，我们获得了初步证据表明，该细胞对小鼠骨骼肌动蛋白有抑制作用。 Face蛋白是一种排斥配体，它将DS电路支架限制在其合适的亚层上。这一假设 将使用Unc5c突变小鼠和Unc5c体内错误表达进行测试。2)身份受体介导的分子生物学效应。 强化突触专一性的选择性机制。初步研究使我们假设 细胞表面蛋白Flrt2由DS环路神经元表达，是一种重要的神经递质受体。 将DS电路板罚款至其相应的子层。这一假说将通过生物化学和 活体遗传方法。3)确定视网膜神经元避开不适当的细胞机制 突触伙伴。在枝晶生长过程中，许多探索性分支被消除。我们的初步数据 提示在Flrt2和Unc5c突变体中，错配的乔木的消除受到损害。因此，我们假设- Fuc5C-Flrt2排斥消除错误分支以防止神经元接触不适当的大小 突触伙伴。这一想法将通过对新生的DS回路树突和突触进行延时成像来检验 在Flrt2和Unc5c突变体中。这些目标的完成有望定义细胞和分子机制-- 神经元避免错误的突触选择的突触。这一贡献将是重大的，因为一旦 突触特异性的排斥机制是已知的，它将可能理解排斥是如何 和吸引力共同作用，产生了突触伴侣选择的首要分子逻辑。

During development, retinal neurons make exquisitely precise connections with specific synaptic partners. These synaptic choices impact the computational capacity of retinal circuits, and thereby influence visual per- ception. Cell-surface recognition molecules mediate synaptic choices by encoding two kinds of trans-cellular signals: 1) attractive signals that connect neurons with their circuit partners; 2) repulsive signals that shun non- target cells. Both types of cues are needed for precise retinal wiring, but the molecular mechanisms underlying rejection of inappropriate synaptic partners are unknown. The objective here is to identify recognition mecha- nisms that prevent connections between inappropriate synaptic partners. Our central hypothesis is that FLRT and UNC5 families of cell-surface molecules mediate repulsive receptor-ligand interactions that prevent cross- circuit synapse formation. The rationale for this work is that it will reveal a new class of synaptic choice recog- nition molecules that act through repulsive mechanisms. Understanding how the wrong synapses are avoided is a necessary step towards ultimately deciphering the molecular logic underlying synaptic partner choice. To this end, the following Specific Aims are proposed: 1) Identify ligands that prevent retinal neurons from se- lecting inappropriate synaptic partners. Retinal circuits occupy parallel sublayers within the inner plexiform layer (IPL) neuropil. This arrangement facilitates synapse specificity by bringing together arbors of circuit part- ners in a defined location where they are segregated from non-target cells. In preliminary studies using the mouse direction-selective (DS) circuit as a model, we obtained preliminary evidence that the UNC5C cell sur- face protein is a repulsive ligand that confines DS circuit arbors to their appropriate sublayers. This hypothesis will be tested using Unc5c mutant mice and Unc5c misexpression in vivo. 2) Identity receptor-mediated mo- lecular mechanisms that enforce synaptic specificity. Preliminary studies led us to hypothesize that the cell surface protein FLRT2, which is expressed by DS circuit neurons, serves as an UNC5C receptor that con- fines DS circuit arbors to their appropriate sublayers. This hypothesis will be tested using biochemical and in vivo genetic approaches. 3) Determine cellular mechanisms by which retinal neurons shun inappropriate synaptic partners. During dendrite growth, many exploratory branches are eliminated. Our preliminary data suggest that elimination of mistargeted arbors is impaired in Flrt2 and Unc5c mutants. We therefore hypothe- size that UNC5C-FLRT2 repulsion eliminates errant branches to prevent neurons from accessing inappropriate synaptic partners. This idea will be tested by time-lapse imaging of nascent DS circuit dendrites and synapses in Flrt2 and Unc5c mutants. Completion of these Aims is expected to define cellular and molecular mecha- nisms by which neurons avoid incorrect synaptic choices. This contribution will be significant because, once repulsive mechanisms for synapse specificity are known, it will become possible to comprehend how repulsion and attraction work together to produce the overarching molecular logic of synaptic partner choice.