Mechanizms of Regionalizaion of the tectum and cerebellum and retinotectal projection

地峡就像一个组织者，Fgf8模仿了这种组织活动。峡组织者分别在中脑顶盖和后脑小脑的发育中起作用。先前的研究表明，强烈的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Ras-ERK信号级联。如果我们通过Ras显性阴性形式（DN-Ras）的错误表达来关闭这一信号级联，通过电穿孔，中脑的命运改变为中脑，顶盖代替小脑分化。然后，我们专注于Sprouty2。Sprouty2的表达与Fgf8重叠，但被认为是Ras-ERK信号级联的负调控因子。结果表明，Fgf8可快速诱导Sprouty2表达，即在Fgf8表达载体电穿孔后2小时即可表达，并抑制ERK的激活。Sprouty2的错表达与DN-Ras的结果相似，即中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脑后端边界前移。结果提示需要对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.