The role of Fat3 in amacrine cell dendrite development.

描述（由申请人提供）：大脑功能依赖于通过轴突和树突之间的精确连线连接的信息流。一个回路内的神经元在树突的数量和排列上可以有很大的不同，有些神经元只延伸一个初级树突成为一个明确的神经节，而另一些神经元则发展出多个树突，在细胞体周围对称地延伸。然而，与揭示轴突规范和引导机制的良好进展相比，对树突的初始规范和生长知之甚少。体外研究表明，神经元最初延伸多能性神经突，其中一个成为轴突，剩下的分化为树突。这些结果表明，树突分化的许多方面都受到内在调控。然而，在体内，树突的发育还必须与周围组织协调，这样树突才能正确定位以形成适当的突触连接。细胞外信号如何诱导驱动树突初始规格和随后形态发生的细胞内重排尚不清楚。在过去，由于缺乏合适的测定方法和缺乏任何明显的分子参与者，这个问题很难解决。我们通过建立一个研究视网膜无突细胞树突发育的系统来解决这些问题。无毛细胞是典型的单极细胞，将一个单一的顶端树突延伸成一个离散的突触层，称为内丛状层（IPL）。然而，在缺乏非典型钙粘蛋白Fat3的小鼠中，无毛细胞发育出第二个指向IPL的树突状乔木。由于Fat3是一种细胞表面受体，这些结果表明，Fat3通过诱导迁移前体收缩其尾随过程来响应IPL中遇到的信号。Fat3信号最终如何促进根尖树突的发育是一个谜，没有已知的效应物或配体。为了建立更详细的树突发育分析的基础知识，将进行两项探索性研究。首先，我们将开发一种实时成像分析方法，用于描述神经突行为和高尔基体定位的动态变化，这些变化发生在前导过程变成树突和后导过程缩回时。其次，为了在树突内部工作，我们将通过测试可能的候选蛋白质和对与Fat3细胞内结构域相互作用的蛋白质进行无偏筛选来寻找Fat3的下游效应物。总之，这些研究将定义树突规范的重要事件，并阐明发生在Fat3下游的信号事件。

DESCRIPTION (provided by applicant): Brain function depends on the flow of information through precisely wired connections between axons and dendrites. Neurons within a circuit can vary widely in the number and arrangement of their dendrites, with some neurons extending only one primary dendrite into a well-defined neuropil and others developing multiple dendrites that extend symmetrically about the cell body. However, in contrast to excellent progress in uncovering mechanisms of axon specification and guidance, relatively little is known about the initial specification and outgrowth of dendrites. In vitro studies suggest that neurons initially extend multipotent neurites, one of which becomes an axon, leaving the remainder to differentiate as dendrites. These results suggest that many aspects of dendrite differentiation are intrinsically regulated. However, in vivo, dendrite development must also be coordinated with the surrounding tissue, such that dendrites are properly positioned to form the appropriate synaptic connections. How extracellular signals induce the intracellular rearrangements that drive the initial specification and subsequent morphogenesis of dendrites is unknown. In the past, this issue has been hard to tackle due to the lack of a suitable assay and the absence of any obvious molecular players. We have been addressing these problems by establishing a system for studying dendrite development in the amacrine cells of the retina. Amacrine cells are typically unipolar, extending a single apical dendrite into a discrete synaptic layer called the inner plexiform layer (IPL). However, in mice lacking the atypical cadherin Fat3, amacrine cells develop a second dendritic arbor that points away from the IPL. Since Fat3 is a cell surface receptor, these results suggest that Fat3 acts by inducing migrating precursors to retract their trailing processes in response to a signal encountered in the IPL. How Fat3 signaling ultimately promotes development of the apical dendrite is a mystery, with no known effectors or ligands. To establish a baseline of knowledge for more detailed analysis of dendrite development, two exploratory studies will be performed. First, we will develop a live imaging assay that can be used to describe the dynamic changes in neurite behavior and Golgi localization that occur as the leading process becomes a dendrite and the trailing process is retracted. Second, to work our way inside the dendrite, we will search for downstream effectors for Fat3, both by testing likely candidate proteins and by performing an unbiased screen for proteins that interact with the Fat3 intracellular domain. Together, these studies will define the salient events of dendrite specification and elucidate the signaling events that occur downstream of Fat3. PUBLIC HEALTH RELEVANCE: Our sense of vision is mediated by precisely organized circuits in the retina, a sensory tissue located at the back of the eye. Millions of Americans are blind and unable to participate in daily activities such as reading and driving. Understanding how visual circuits are wired during development will improve efforts to identify the causes of blindness and eventually design neural prostheses for the restoration of sight.