Yates P A, Roskies A L, McLaughlin T, O'Leary D D
Molecular Neurobiology Laboratory, The Salk Institute, La Jolla, California 92037, USA.
J Neurosci. 2001 Nov 1;21(21):8548-63. doi: 10.1523/JNEUROSCI.21-21-08548.2001.
The retinotectal projection is the predominant model for studying molecular mechanisms controlling development of topographic axonal connections. Our analyses of topographic mapping of retinal ganglion cell (RGC) axons in chick optic tectum indicate that a primary role for guidance molecules is to regulate topographic branching along RGC axons, a process that imposes unique requirements on the molecular control of map development. We show that topographically appropriate connections are established exclusively by branches that form along the axon shaft. Initially, RGC axons overshoot their appropriate termination zone (TZ) along the anterior-posterior (A-P) tectal axis; temporal axons overshoot the greatest distance and nasal axons the least, which correlates with the nonlinear increasing A-P gradient of ephrin-A repellents. In contrast, branches form along the shaft of RGC axons with substantial A-P topographic specificity. Topography is enhanced through the preferential arborization of appropriately positioned branches and elimination of ectopic branches. Using a membrane stripe assay and time-lapse microscopy, we show that branches form de novo along retinal axons. Temporal axons preferentially branch on their topographically appropriate anterior tectal membranes. After the addition of soluble EphA3-Fc, which blocks ephrin-A function, temporal axons branch equally on anterior and posterior tectal membranes, indicating that the level of ephrin-As in posterior tectum is sufficient to inhibit temporal axon branching and generate branching specificity in vitro. Our findings indicate that topographic branch formation and arborization along RGC axons are critical events in retinotectal mapping. Ephrin-As inhibit branching along RGC axons posterior to their correct TZ, but alone cannot account for topographic branching and must cooperate with other molecular activities to generate appropriate mapping along the A-P tectal axis.
视网膜-顶盖投射是研究控制拓扑轴突连接发育的分子机制的主要模型。我们对鸡视顶盖中视网膜神经节细胞（RGC）轴突的拓扑映射分析表明，导向分子的主要作用是调节沿RGC轴突的拓扑分支，这一过程对图谱发育的分子控制提出了独特要求。我们发现，拓扑上合适的连接完全由沿轴突干形成的分支建立。最初，RGC轴突沿顶盖前后（A-P）轴越过其合适的终止区（TZ）；颞侧轴突越过的距离最大，鼻侧轴突越过的距离最小，这与 Ephrin-A 排斥分子的非线性增加的 A-P 梯度相关。相反，分支沿着具有显著 A-P 拓扑特异性的 RGC 轴突干形成。通过适当定位的分支的优先分支和异位分支的消除，拓扑结构得到增强。使用膜条测定法和延时显微镜，我们表明分支沿视网膜轴突从头形成。颞侧轴突优先在其拓扑上合适的前顶盖膜上分支。添加可溶性 EphA3-Fc 阻断 Ephrin-A 功能后，颞侧轴突在前顶盖膜和后顶盖膜上均等分支，这表明后顶盖中 Ephrin-A 的水平足以抑制颞侧轴突分支并在体外产生分支特异性。我们的研究结果表明，沿 RGC 轴突的拓扑分支形成和分支是视网膜-顶盖映射中的关键事件。Ephrin-A 抑制 RGC 轴突在其正确 TZ 后方的分支，但单独不能解释拓扑分支，并且必须与其他分子活动合作以沿顶盖 A-P 轴产生适当的映射。
