Establishment of the circuit of the mushroom body calyx during development

对感觉信息的正确评估、对准确行为目标的定义、对运动的协调控制，都依赖于神经系统内精心设计的信息流--由神经元网络的结构来保证。在发育过程中，神经元迁移到确定的目标位置，并分化出复杂的和神经元类型特异性的轴突和树突。在到达适当的目标区域后，他们面临的主要任务是在众多伙伴中确定正确的伙伴，并建立适当数量的联系。随着电子显微镜重建神经元回路所提供的分辨率水平，局部微回路结构的复杂性最近变得更加清晰。在蘑菇体（MB）花萼内，感觉信息被处理以引起凯尼恩细胞（KCs）的稀疏反应代码。我们正在进行的工作，由FOR 2705资助，表明MB（以及进化过程中的许多电路）的这种基本特性依赖于MB花萼的主要结构单元微肾小球（MG）的结构。 我们专注于嗅觉信息的处理，并已描述了如何在每个MG内，嗅觉投射神经元的轴突终扣被许多KC的树突包围，以形成高度互连的元件的球形结构。根据我们的描述水平，MG似乎是一个理想的系统，可以解决在开发过程中如何形成特定的连接。然而，几乎没有人知道MG是如何产生的。我们建议在这里揭示关键的分子因素和这些复合物如何组装的基本逻辑，通过执行一个经典的遗传筛选。使用转录组数据，我们已经定义了一组表面分子表达的投射神经元或KC作为候选人支持神经元-神经元识别的花萼。我们将通过RNAi介导的敲低筛选MG、投射神经元终扣、KC树突或其突触接触未正确形成、维持或定位的条件。最好的分子候选人将通过遗传学，高分辨率成像和细胞生物学技术的组合进行彻底分析。为了支持这些研究并帮助澄清突变表型是如何出现的，我们将利用延时成像研究发育过程中单个轴突和树突的动态行为，并解决该过程中活动的影响。最后，除了由MG代表的局部微电路之外，我们还将通过生成感觉表征的地图来解决MB萼的全球组织的逻辑。我们将讨论这个地图是如何出现的，以及它在个体中的刻板印象。总而言之，我们将在完整回路的功能特性的背景下，为成年苍蝇大脑中关键回路的发育组装逻辑奠定基础。

The correct evaluation of sensory information, the definition of an accurate behavioral goal, the coordinated control of movement, all depend upon carefully designed flows of information within the nervous system – guaranteed by the architecture of neuronal networks. During development, neurons migrate to defined target locations and differentiate their complex and neuron-type specific axon and dendrites. Having reached their appropriate target region, they face the major tasks of identifying their correct partners among many and of generating connections in appropriate numbers. With the level of resolution provided by electron microscopy-based reconstructions of neuronal circuits, the complexity of local microcircuit architecture has recently emerged even more clearly. Within the mushroom body (MB) calyx, sensory information is processed to elicit a sparse code of response by the Kenyon cells (KCs). Our ongoing work, funded by the FOR2705, indicates that this fundamental property of the MB (and of many circuits across evolution) relies on the architecture of the main structural unit of the MB calyx, the microglomeruli (MGs). We concentrate on the processing of olfactory information and have described how, within each MG, the axonal bouton of an olfactory projection neuron is surrounded by the dendrites of many KCs to form a spherical structure of highly interconnected elements. With the level of description we produced, MGs seem an ideal system to address how specific connections are formed during development. However, virtually nothing is known about how MGs arise. We propose here to reveal key molecular factors and the fundamental logic of how these complexes assemble by performing a classic genetic screen. Using transcriptomic data, we have defined a set of surface molecules expressed by projection neurons or by KCs as candidates for supporting neuron-neuron recognition in the calyx. We will screen by RNAi- mediated knock-down for conditions in which the MGs, the projection neuron boutons, the KC dendrites or their synaptic contacts are not correctly formed, maintained or positioned. The best molecular candidates will be thoroughly analyzed with a combination of genetics, high-resolution imaging and cell biology techniques. To support these studies and help clarifying how mutant phenotypes emerged, we will investigate with time-lapse imaging the dynamic behavior of individual axons and dendrites during development and address the impact of activity during the process. Finally, in addition to the local microcircuits represented by the MGs, we will address also the logic of the global organization of the MB calyx, by generating a map of sensory representation. We will address how this map emerges and its stereotypy among individuals. Taken together, we will lay the ground of the logic of developmental assembly of a key circuit in the adult fly brain – in the context of the functional properties of the completed circuit.