MOLECULAR MECHANISMS OF RETINAL CIRCUIT ASSEMBLY

轴突和树突的形态塑造神经元回路的连通性和功能； 轴突和树突畸形是神经发育障碍的常见特征。为了建立特定细胞类型的形态，发育中的轴突需要(1)向正确的位置生长并在正确的地方分支(即轴突 目标)，(2)具有不同分枝模式和几何形状的精致乔木(例如，轴突形状)，以及(3) 占据适当的区域(即轴突大小)。轴突和树突如何生长到准确的大小，树枝的大小如何 调节连接性，以及它如何影响特定的电路计算还不是很清楚。在预赛中 研究中，我们确定了四种细胞黏附分子(cAMs；amigo1、amigo2、netrin-g1和ngl1)，它们调节 视网膜两个回路中神经元的树突和轴突大小：方向选择(DS)回路，它提取 视网膜内的运动信息，以及视杆双极通路，从视网膜中传输微弱的光信号 从外到内的视网膜。恒星爆发单元具有径向对称的乔木，这些乔木在相邻区域之间广泛重叠 并表达Amigo2。每个刀杆的中央三分之二接受输入，外围三分之一提供输出。 来自星状爆发细胞的抑制性输入对神经节细胞的DS反应至关重要。恒星暴增细胞的轴突大小是 在Amigo2基因敲除(Amigo2-/-)小鼠中增加，同时保持功能区隔。在目标1中， 我们将分析Amigo2的S作用的分子机制，测试其对神经突起形态和功能的影响。 连通性、DS电路功能以及图像稳定头部和眼睛运动。在杆子的第一阶段 双极通路，水平细胞轴突介导杆之间的侧向抑制，为杆双极提供输入 树枝状结构。水平细胞表达Amigo1。在Amigo1-/-小鼠中，水平细胞轴突和杆状双极树突 两者都缩小了尺寸。在目标2中，我们将描述Amigo1的信号机制，探索领域 突触伙伴之间的匹配，分析对连接性的影响，并测量沿杆的光敏感度 双极通路，以及行为反应。在杆状双极通路的第二阶段，表达Netrin-G1的杆状双极轴突突触到表达NGL1的AII细胞上。杆状双极轴突的大小在Netrin- G1-/-和NGL1-/-小鼠，提示反式突触Netrin-G1/NGL1复合体的逆行信号调节 轴突生长。在目标3中，我们将探索前向信号是否控制所有树枝的大小。我们将决定如何 Netrin-G1/NGL1复合体对视杆双极神经元突触数目、超微结构和功能的影响 AII细胞，并评估它们对视杆双极通路光反应和对小鼠能力的影响 来探测微弱的闪光。总之，这些研究将为我们提供对控制 视网膜中轴突和树突的大小，揭示了轴突大小如何调节连接性，以及它是如何形成特定的 电路计算和影响视觉引导行为。

The morphology of axons and dendrites shapes the connectivity and function of neuronal circuits; and dysmorphic axons and dendrites are a common feature of neurodevelopmental disorders. To establish cell-type-specific morphologies, developing neurites need to (1) grow towards and branch in the right places (i.e. neurite targeting), (2) elaborate arbors with distinct branching patterns and geometries (i.e. neurite shape), and (3) occupy appropriate territories (i.e. neurite size). How axons and dendrites grow to an exact size, how arbor size regulates connectivity, and how it influences specific circuit computations is not well understood. In preliminary studies, we identified four cell adhesion molecules (CAMs; Amigo1, Amigo2, netrin-G1, and NGL1) that regulate dendrite and axon size of neurons in two circuits of the retina: the direction selective (DS) circuit, which extracts motion information in the inner retina, and the rod bipolar pathway, which transmits dim-light-signals from the outer to the inner retina. Starburst cells have radially symmetric arbors that overlap extensively among neighbors and express Amigo2. The central two thirds of each arbor receive input and the peripheral third provides output. Inhibitory input from starburst cells is critical for DS responses of ganglion cells. Neurite size of starburst cells is increased in Amigo2 knockout (Amigo2-/-) mice, while functional compartmentalization is maintained. In Aim 1, we will analyze the molecular mechanisms of Amigo2’s actions, test its influence on neurite morphology and connectivity, DS circuit function, and image stabilizing head and eye movements. At the first stage of the rod bipolar pathway, horizontal cell axons mediate lateral inhibition among rods, which provide input to rod bipolar dendrites. Horizontal cells express Amigo1. In Amigo1-/- mice, horizontal cell axons and rod bipolar dendrites are both reduced in size. In Aim 2, we will characterize the signaling mechanism of Amigo1, explore territory matching between synaptic partners, analyze effects on connectivity and measure light sensitivity along the rod bipolar pathway, and in behavioral responses. At the second stage of the rod bipolar pathway, netrin-G1-expressing rod bipolar axons synapse onto NGL1-expressing AII cells. Rod bipolar axon size is reduced in netrin- G1-/- and NGL1-/- mice, suggesting that retrograde signals of trans-synaptic netrin-G1/NGL1 complexes regulates axon growth. In Aim 3, we will explore whether forward signals control AII arbor size. We will determine how netrin-G1/NGL1 complexes affect the number, ultrastructure and function of synapses between rod bipolar and AII cells, and assess their influences on light responses along the rod bipolar pathway and on the ability of mice to detect dim light flashes. Together these studies will provide insights into the molecular mechanisms that control axon and dendrite size in the retina, reveal how neurite size regulates connectivity, and how it shapes specific circuit computations and influences visually guided behaviors.