Mechanizms of Regionalizaion of the tectum and cerebellum and retinotectal projection

峡部起组织者的作用，而Fgf8模仿组织活动。峡部组织者在中脑和后脑分别对顶盖和小脑的发育起作用。以往的研究表明，强的Fgf8信号诱导小脑分化。在本研究中，我们发现强烈的Fgf8信号激活了RAS-ERK信号级联反应。如果我们通过错误表达显性的RAS(dN-RAS)来关闭这一信号级联，y电穿孔，则后脑的命运改变为中脑，顶盖分化而不是小脑。然后，我们将重点放在Sprouty2上。Sprouty2与Fgf8重叠表达，但被认为是Ras-ERK信号级联的负调控因子。结果表明，Fgf8能迅速诱导Sprouty2的表达，即在电穿孔表达载体后2小时，抑制ERK的激活。Sprouty2的错误表达产生了与dN-RAS相似的结果，即后脑向中脑的命运改变。另一方面，Sprouty2显性阴性形式的错误表达导致了中-中脑边界的前移。结果表明，需要对RAS-ERK信号进行精细调控。

The isthmus works as an organizer, and Fgf8 mimics the organizing activity. The isthmic organizer plays a role for the development of the tectum and the cerebellum in the mesencephalon and in the metencephalon, respectively. It was previously shown that strong Fgf8 signal induces cerebellar differentiation. In the present study, we have shown that strong Fgf8 signal activates Ras-ERK signaling cascade. If we shut down this signaling cascade by misexpression of dominant negative form of Ras (DN-Ras), y electroporation the fate of the metencephalon changes to the mesencephalon, and the tectum differentiates in place of the cerebellum. Then, we focused on Sprouty2. Sprouty2 is expressed overlapping to Fgf8, but was indicated as a negative regulator of Ras-ERK signaling cascade. As indicated, Sprouty2 expression was induced rapidly by Fgf8, that is, 2 hours after electroporation of Fgf8 expression vector, and repressed ERK activation. Misexpression of Sprouty2 exerted similar results as that of DN-Ras, that is, fate change of the metencephalon to the mesencephalon. On the other hand, misexpression of dominant negative form of Sprouty2 resulted in anterior shift of the mes-metencephalic boundary. The results indicate the need of fine regulation of Ras-ERK signaling.