From spatiotemporal restriction of imprecise synaptic partner choice to a robust representation of navigational information in the fly brain

在成人大脑中，彼此相邻的密切相关的神经元亚型通常表现出高度特异的、发散的连接。果蝇视觉系统中特定形态的光感受器神经元和它们的突触伙伴就是这种现象的一个例子：当眼睛的主要部分和潜在的视神经毛发处理运动和颜色特定的视觉输入时，背缘区域(DRA)处理天光偏振，最终导致中央大脑中的导航信息在各种刺激条件下都是健壮的。P3的动机是我们最近的RobustCircuit PI团队的工作，他们发现颜色敏感的R7神经元在接触时很容易与不正确的伙伴形成突触连接，通过在时间和空间上分离的突触生成相互作用来确保发育中的野生型脑中的突触特异性。因此，在P3中，我们直接将DRA光感受器R7和R8的选择性靶向与其处理天光信号的特定下游神经元与其颜色敏感的对应神经元进行了比较。根据我们对DRA区神经元的初步数据，我们假设DRA神经元处理的信息的稳健表征是基于它们通过空间和时间上的突触相遇的不同调节而混杂地形成突触的能力。P3被设计用三种互补的方法来检验这一假说：(I)通过多光子体外成像定量研究突触前和突触后细胞类型的丝足动力学；(Ii)通过基于EM的连接组学来量化神经元形态和突触分布的可变性；(Iii)使用光学生理学(钙成像)来量化在单个细胞上以及在整体水平上产生强大的细胞反应的可变性和不精确性。(I)和(Iii)都将包括野生型大脑，以及扰乱纤毛动力学和神经元相遇的有针对性的操作。然后，我们将把这些研究扩展到连接视叶和中央大脑的神经元，从而在那里产生强大的天窗信息的地形图表示。当这些研究结束时，我们将建立一个完整的四维描述机制，以一个完整的大脑中的随机动力学为基础，发展一个健壮的导航信息表征。

Closely related neuronal subtypes neighboring each other in the adult brain often exhibit highly specific, divergent connectivity. Modality-specific photoreceptor neurons and their synaptic partners in the Drosophila visual system represent an instance of this phenomenon: While the main portion of the eye and underlying optic neuropils process motion- and color-specific visual input, the dorsal rim area (DRA) processes skylight polarization, ultimately leading to a representation of navigational information in the central brain that is robust over a variety of stimulus conditions. P3 is motivated by our recent team effort of RobustCircuit PIs which discovered that color-sensitive R7 neurons readily form synaptic connections with incorrect partners upon contact, ensuring synaptic specificity in the developing wild type brain by separated synaptogenic interactions in time and space. In P3, we therefore directly compare the selective targeting of DRA photoreceptors R7 and R8 to their specific downstream neurons which process skylight cues, to their color-sensitive counterparts. Based on our preliminary data on neurons in the DRA region, we hypothesize that the robust representation of information processed by DRA neurons are based on their ability to form synapses promiscuously through the differential regulation of synaptogenic encounters in space and time. P3 is devised to test this hypothesis using three complementary approaches: (i) the quantitative investigation of filopodial dynamics of both pre- and postsynaptic cell types via multi-photon ex vivo imaging; (ii) the quantification of variability of neuronal morphology and synaptic distribution by EM-based connectomics; (iii) the quantification of variability and imprecision for creating robust cellular responses, both on a single cell-, as well as on a bulk level, using optophysiology (calcium imaging). Both (i) and (iii) will include wild type brains, as well as targeted manipulations to disrupt filipodial dynamics and neuronal encounters. We will then expand these studies towards neurons that connect the optic lobes with the central brain leading to a robust topographic representation of skylight information there. When these studies are concluded, we will have established a complete 4-dimensional description of the mechanisms for the development of a robust representation of navigational information based on stochastic dynamics in an intact brain.