Visualizing Remodeling at the Retinogeniculate Synapse

描述（由申请人提供）：在发育中的中枢神经系统（CNS）中形成精确的突触连接对神经功能至关重要。在连接视网膜和丘脑外侧膝状核（LGN）的视网膜原状突触（retinogeniculate synapse）上，几个发育阶段有助于突触回路的形成、完善和成熟。在神经元初始映射到其目标后，轴突乔木的总体形态重排，因为视网膜神经节细胞（RGC）轴突分离成特定于眼睛的层。在小鼠中，我们发现在RGC轴突分离到LGN的适当区域后很长一段时间（出生后第8天，p8），存在两个强健的突触可塑性和重塑期。突触可塑性的第一阶段发生在眼睛睁开时（p12-14），此时视网膜输入到给定LGN中继神经元的某些输入增强，而其他输入则被修剪。第二个未被发现的可塑性阶段发生在p20之后，此时视网膜回突触的强度和连通性对感官体验变得敏感。在此，我们拟研究视网膜轴突树突的形态变化与突触可塑性的两个时期相对应。为了做到这一点，我们将利用现有的转基因小鼠，并产生新的小鼠品系，在这些品系中，它们的RGCs的稀疏子集共同表达标签，这些标签使用不同的荧光颜色标记轴突树枝和突触前标记物突触体素。使用这些小鼠，我们将检查选择视网膜神经节细胞轴突的形态学变化和轴突乔木区域内突触接触的相对分布。这些变化将被量化为正常发育期间的连接重塑，以及在可塑性第二阶段对视觉剥夺的反应。我们将测试一个假设，即RGC轴突树突结构比功能上需要的更宽，并且在睁眼时变得稳定。我们还将研究突触强大可塑性的时期是否代表固定轴突乔木支架内突触释放位点的重排。一个广泛的结构支架的发现，其中突触接触可以形成，断裂和重新排列，可能代表了一种相对新颖的神经可塑性。通过将结构与功能联系起来，我们希望对突触发育的结构机制有更清晰的了解。这个项目的目标是了解神经元如何形成相互之间的连接，称为突触。我们建议可视化突触前神经元的轴突末端结构在发育过程中如何变化以及轴突形式如何与突触功能相对应。了解神经系统的正常连接是如何完成的，将有助于深入了解由异常连接引起的神经系统疾病，例如某些形式的智力迟钝、认知障碍和癫痫。

DESCRIPTION (provided by applicant): The formation of precise synaptic connections in the developing central nervous system (CNS) is critical for neurological function. At the retinogeniculate synapse, the connection between the retina and the lateral geniculate nucleus (LGN) of the thalamus, several developmental phases contribute to the formation, refinement and maturation of synatic circuits. After the initial mapping of a neuron to its target, there is gross morphological rearrangement of the axon arbors, as retinal ganglion cell (RGC) axons segregate into eye-specific layers. In the mouse, we have found that long after RGC axons segregate into the proper region of the LGN (postnatal day 8, p8), there are two periods of robust synaptic plasticity and remodeling. The first phase of synaptic plasticity occurs around the time of eye opening (p12-14) when some of the retinal inputs to a given LGN relay neuron strengthened while other inputs are pruned. A second, previously undetected, phase of plasticity occurs after p20, when the strength and connectivity of the retinogeniculate synapse becomes sensitive to sensory experience. Here we propose to study the morphological changes of retinal axon arbors that correspond to the two periods of synaptic plasticity. To do this, we will take advantage of available transgenic mice, and also generate new mouse lines in which a sparse subset of their RGCs co-express labels that tag axon arbors and the presynaptic marker, synaptophysin, using different fluorescent colors. Using these mice, we will examine changes in the morphology of select retinal ganglion cell axons and the relative distribution of the synaptic contacts within an axon arbor territory. These changes will be quantified as the connection remodels during normal development, and in response to visual deprivation during the second phase of plasticity. We will test the hypothesis that the RGC axon arbor structure is broader that functionally necessary and becomes stable around the time of eye opening. We will also examine whether the periods of robust synaptic plasticity represent rearrangements of synaptic release sites within a fixed axon arbor scaffold. A finding of a broad structural scaffold in which synaptic contacts can form, break and rearrange may represent a relatively novel type of neural plasticity. By relating structure to function, we hope to gain clearer understanding of the structural mechanisms that underlie synaptic development. The goal of this project is to understand the how neurons form connections, called synapses, with each other. We propose to visualize how the structure of the axon terminals of presynaptic neurons change during development and how axonal form corresponds with synaptic function. Understanding how normal wiring of the nervous system is accomplished will provide insight into neurological disorders that result from aberrant connections, such as some forms of mental retardation, cognitive disorders and epilepsy.