Dorsoventral specification of zebrafish CNS: Analysis with mutants

利用斑马鱼胚胎研究了脊椎动物中枢神经系统的背腹侧特征。底板位于脊椎动物神经管的腹侧中线。斑马鱼单眼针尖（oep）突变体底板发育严重受损。为了了解底板发育过程中的细胞行为和细胞间相互作用，我们研究了野生型和oep突变型细胞在遗传嵌合体中的分布和基因表达。结果表明，oep突变细胞在野生型环境中仅采用脊索命运。在接受野生型细胞的oep宿主中，野生型供体细胞经常贡献部分底板区域并表达底板标记物。对连续切片的仔细检查显示，当突变的神经细胞位于野生型细胞旁边时，抑制了邻近野生型细胞的底板分化。这种抑制性相互作用可能通过拮抗来自底板细胞的同源信号，在限制底板区域进入最腹中线方面发挥重要作用。端脑，产生大脑半球，从前脑的背前部发育而来。最近对小鼠的研究表明，Fgf8在前神经嵴中表达，是端脑区特异的局部信号分子。为了探讨Fgf信号在斑马鱼中枢神经系统发育中的作用，我们研究了Fgf受体下游介质之一的胞外信号调节激酶（extracellular signal regulated kinase）激活的空间模式，并发现它与fgf3在前神经区的表达非常相似。然后，我们将Ras的显性阴性形式或FGF受体抑制剂注射到斑马鱼胚胎中，并检测了前神经区域的缺陷。结果表明，Fgf信号通过Ras/MAPK级联介导，是腹侧端脑形成的必要条件。

The dorsoventral specification of vertebrate CNS has been examined using zebrafish embryos.1. The floor plate is located at the ventral midline of the neural tube in vertebrates. The floor-plate development is severely impaired in zebrafish one-eyed pinhead (oep) mutants. To understand the cell behavior and cell-cell interaction during floor plate development, we examined the distribution and gene expression of wild-type and oep mutant cells in genetic mosaics. The results indicate that oep mutant cells adopt only a notochord fate in a wild-type environment. In oep hosts that received wild-type cells, wild-type donor cells frequently contributed to part of the floor-plate region and expressed floor-plate markers. Careful examination of serial sections revealed that a mutant neural cell, when located next to the wild-type cells, inhibited floor-plate differentiation of the adjacent wild-type cells. This repressive interaction may play an important role in restricting the floor-plate region into the ventral-most midline by antagonizing homeogenetic signals from the floor-plate cells.2. Telencephalon, which gives rise to the cerebral hemisphere, develops from the dorso-anterior part of the forebrain. Recent studies with mice have shown that Fgf8 expressed in the anterior neural ridge functions as a local signaling molecule in the specification of the telencephalic region. To explore the role of Fgf signaling in zebrafish developing CNS, we have examined the spatial pattern of activation of the extracellular signal regulated kinase , one of the downstream mediator of FGF receptor, and show that it closely resembles the expression of fgf3 in the anterior neural region. We then injected the dominant-negative form of Ras or the inhibitor of the FGF receptor into zebrafish embryos and examined defects in anterior neural region. The results obtained suggest that Fgf signaling, mediated by Ras/MAPK cascade, is required for the establishment of the ventral telencephalon.