Hirate Y, Mieda M, Harada T, Yamasu K, Okamoto H
Laboratory for Developmental Gene Regulation, Brain Science Institute, RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
Mech Dev. 2001 Sep;107(1-2):83-96. doi: 10.1016/s0925-4773(01)00467-1.
Development of the tectum and the cerebellum is induced by a reciprocal inductive signaling between their respective primordia, the midbrain and the midbrain/hindbrain boundary (MHB). We set out to identify molecules that function in and downstream of this reciprocal signaling. Overexpression of LIM domain of the transcription factor Islet-3 (LIM(Isl-3)) leads to inhibition of this reciprocal signaling and to resultant defects in tectal and cerebellar development. We therefore searched for genes that may be either up- or down-regulated by overexpression of LIM(Isl-3) by comparing the gene expression profiles in the midbrain and the MHB of normal embryos and embryos in which Islet-3 function was repressed, using a combination of ordered differential display and whole-mount in situ hybridization. Among genes identified in this search, two cDNA fragments encoded Wnt1 and FGF8, which are already known to be essential for the reciprocal signaling between the midbrain and the MHB, confirming the effectiveness of our strategy. We identified four other partial cDNA clones that were specifically expressed around the MHB, ten cDNAs specifically expressed in the tectum, and three cDNAs expressed in neural crest cells including those derived from the midbrain level. The ephrin-A3 gene was specifically expressed in posterior tectum in a gradient that decreased anteriorly. Although ephrin-A2 and ephrin-A5 have been reported to be expressed in the corresponding region in mouse embryos, the superior/inferior colliculi, mouse ephrin-A3 is not expressed prominently in this region, suggesting that the role of ephrin-A3 in brain development may have been altered in the process of brain evolution.
顶盖和小脑的发育是由它们各自的原基——中脑和中脑/后脑边界（MHB）之间的相互诱导信号所引发的。我们着手鉴定在这种相互信号传导及其下游发挥作用的分子。转录因子胰岛-3（Isl-3）的LIM结构域过表达会导致这种相互信号传导受到抑制，并进而导致顶盖和小脑发育出现缺陷。因此，我们通过使用有序差异显示和全胚胎原位杂交相结合的方法，比较正常胚胎以及胰岛-3功能受到抑制的胚胎的中脑和MHB中的基因表达谱，来寻找可能被LIM（Isl-3）过表达上调或下调的基因。在此次搜索中鉴定出的基因中，两个cDNA片段编码Wnt1和FGF8，已知它们对于中脑和MHB之间的相互信号传导至关重要，这证实了我们策略的有效性。我们还鉴定出另外四个在MHB周围特异性表达的部分cDNA克隆、十个在顶盖中特异性表达以及三个在神经嵴细胞（包括源自中脑水平的神经嵴细胞）中表达的cDNA。ephrin-A3基因在后侧顶盖中呈梯度特异性表达，从前向后逐渐降低。尽管据报道ephrin-A2和ephrin-A5在小鼠胚胎的相应区域（上丘/下丘）表达，但小鼠ephrin-A3在该区域并未显著表达，这表明ephrin-A3在脑发育中的作用可能在脑进化过程中发生了改变。
