Role of proteoglycan-mediated trans-axonal signaling in pre-target topographic sorting

项目总结 准确地将轴突投影组织成地形图对大脑功能至关重要，尤其是- 尤其是在感觉系统中。地形图形成的一个重要发展机制 是目标前轴突分类，根据轴突的身份，轴突在前往目标的途中被预先排序 和/或位置原点。例如，在视觉系统中，轴突沿着视束进行分类，其中Dor- SAL和视网膜腹侧轴突分别分离到视束的腹侧支和背侧支 达到他们的大脑目标。虽然靶前轴突分类在地形图绘制中具有指导作用，但它是如何 是在发展过程中建立的，人们对此仍然知之甚少。使用独特的透明度和可访问性 对于斑马鱼胚胎，我们的研究表明，沿视束的地形顺序尚未建立 在最初的轴突引导期间，而不是通过错误分类的背侧轴突的选择性退化来实现 它们错误地沿着背侧分支错位。硫酸乙酰肝素(HS)，一种糖胺聚糖 由硫酸肝素蛋白多糖(HSPGs)引起的，是这种选择性退化所需要的非细胞自主的。 然而，有几个问题仍然没有得到解决。HS是如何指示错位的背侧轴突退化的 是否正确地保留了那些沿着神经束腹侧分支伸展的部分？是否有特定的HSPG参与其中？ HS调节哪条分子途径(S)？分类错误和目标正确之间的关键区别 背侧轴突是指它们与腹侧先锋轴突的距离。当错误地沿着背侧分支导航时， 错位的背侧轴突似乎与已经延长的腹侧轴突紧密接触。而且，我们的 初步研究表明，HSPG GLYPICAN-3(GPC3)在成熟视网膜节细胞中选择性表达 视网膜。因此，我们假设GPC3介导的腹侧先锋和腹侧轴突之间的跨轴突信号转导。 错分的背侧跟随者视网膜轴突触发错位背侧轴突的退化，从而建立 LISH在视觉系统中对目标前地形进行排序。我们将通过刻画 GPC3在视网膜腹侧神经节细胞和相应轴突中的表达和定位(目标1)，并通过 以细胞特异性方式测试GPC3在视束分选中的功能(目标2)。在Pio表面的GPC3- 近轴突可以充当直接作用于错误分类的背侧轴突以触发其退化的指导信号， 或作为调节因子，通过调节跨轴突信号通路间接控制分选。为了获得 深入了解哪些信号因子可能受GPC3调节，我们将测试信号素-神经粘连蛋白- 已知在其他系统中调节轴突排序的丛状蛋白途径也有助于视束分类- ING(目标3)。总之，这些研究将是第一次确定先锋之间的跨轴突信号传递 而跟随者轴突在视觉系统中建立前目标地形分类，从而解决主要的 在我们对控制地形图形成的分子机制的理解上存在差距。

Project summary The precise organization of axonal projections into topographic maps is crucial for brain function, espe- cially in sensory systems. An important developmental mechanism contributing to topographic map formation is pre-target axon sorting, whereby axons are pre-ordered en route to their target according to their identity and/or positional origin. In the visual system, for instance, axon sorting occurs along the optic tract, where dor- sal and ventral retinal axons segregate respectively into the ventral and dorsal branches of the tract before reaching their brain target. While pre-target axon sorting has an instructive role in topographic mapping, how it is established during development remains poorly understood. Using the unique transparency and accessibility of the zebrafish embryo, our studies have shown that topographic order along the optic tract is not established during initial axon guidance but instead achieved through the selective degeneration of missorted dorsal axons that have erroneously misrouted along the dorsal branch. Heparan Sulfate (HS), a glycosaminoglycan carried by Heparan Sulfate Proteoglycans (HSPGs), is required non-cell autonomously for this selective degeneration. Yet, several questions remain unsolved. How does HS instruct missorted dorsal axons to degenerate while preserving those correctly elongating along the ventral branch of the tract? Is there a specific HSPG involved? Which molecular pathway(s) does HS regulate? A key difference between missorted and correctly targeted dorsal axons is their proximity to ventral pioneer axons. While erroneously navigating along the dorsal branch, missorted dorsal axons appear in close contact with ventral axons that have already elongated. Moreover, our preliminary data show that the HSPG glypican-3 (Gpc3) is selectively expressed in ventral RGCs in the mature retina. Thus, we hypothesize that Gpc3-mediated trans-axonal signaling between ventral pioneer and missorted dorsal follower retinal axons triggers the degeneration of missorted dorsal axons to estab- lish pre-target topographic sorting in the visual system. We will test that hypothesis by characterizing the expression and localization of Gpc3 in ventral retinal ganglion cells and corresponding axons (Aim 1), and by testing the function of Gpc3 in optic tract sorting in a cell specific manner (Aim 2). Gpc3 at the surface of pio- neer axons may act as a guidance cue acting directly on missorted dorsal axons to trigger their degeneration, or as a modulating factor controlling sorting indirectly by regulating a trans-axonal signaling pathway. To gain insight into which signaling factors might be regulated by Gpc3, we will test whether the semaphorin-neuropilin- plexin pathway, which is known to regulate axon ordering in other systems, also contributes to optic tract sort- ing (Aim 3). Altogether, these studies will be the first to determine how trans-axonal signaling between pioneer and follower axons establishes pre-target topographic sorting in the visual system, thus addressing a major gap in our understanding of the molecular mechanisms controlling topographic map formation.