Scholpp Steffen, Lohs Claudia, Brand Michael
Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany.
Development. 2003 Oct;130(20):4881-93. doi: 10.1242/dev.00683. Epub 2003 Aug 13.
Specification of the forebrain, midbrain and hindbrain primordia occurs during gastrulation in response to signals that pattern the gastrula embryo. Following establishment of the primordia, each brain part is thought to develop largely independently from the others under the influence of local organizing centers like the midbrain-hindbrain boundary (MHB, or isthmic) organizer. Mechanisms that maintain the integrity of brain subdivisions at later stages are not yet known. To examine such mechanisms in the anterior neural tube, we have studied the establishment and maintenance of the diencephalic-mesencephalic boundary (DMB). We show that maintenance of the DMB requires both the presence of a specified midbrain and a functional MHB organizer. Expression of pax6.1, a key regulator of forebrain development, is posteriorly suppressed by the Engrailed proteins, Eng2 and Eng3. Mis-expression of eng3 in the forebrain primordium causes downregulation of pax6.1, and forebrain cells correspondingly change their fate and acquire midbrain identity. Conversely, in embryos lacking both eng2 and eng3, the DMB shifts caudally into the midbrain territory. However, a patch of midbrain tissue remains between the forebrain and the hindbrain primordia in such embryos. This suggests that an additional factor maintains midbrain cell fate. We find that Fgf8 is a candidate for this signal, as it is both necessary and sufficient to repress pax6.1 and hence to shift the DMB anteriorly independently of the expression status of eng2/eng3. By examining small cell clones that are unable to receive an Fgf signal, we show that cells in the presumptive midbrain neural plate require an Fgf signal to keep them from following a forebrain fate. Combined loss of both Eng2/Eng3 and Fgf8 leads to complete loss of midbrain identity, resulting in fusion of the forebrain and the hindbrain primordia. Thus, Eng2/Eng3 and Fgf8 are necessary to maintain midbrain identity in the neural plate and thereby position the DMB. This provides an example of a mechanism needed to maintain the subdivision of the anterior neural plate into forebrain and midbrain.
前脑、中脑和后脑原基的特化发生在原肠胚形成过程中，以响应使原肠胚胚胎形成模式的信号。原基建立后，每个脑区被认为在诸如中脑 - 后脑边界（MHB，或峡部）组织者等局部组织中心的影响下，在很大程度上独立于其他脑区发育。后期维持脑区划分完整性的机制尚不清楚。为了研究前神经管中的此类机制，我们研究了间脑 - 中脑边界（DMB）的建立和维持。我们发现，DMB的维持既需要特定中脑的存在，也需要功能性的MHB组织者。Pax6.1是前脑发育的关键调节因子，其表达受到Engrailed蛋白Eng2和Eng3的后部抑制。在原脑原基中异常表达eng3会导致pax6.1下调，相应地，前脑细胞会改变其命运并获得中脑特征。相反，在同时缺乏eng2和eng3的胚胎中，DMB向尾侧移入中脑区域。然而，在这些胚胎中，前脑和后脑原基之间仍保留着一片中脑组织。这表明存在另一种维持中脑细胞命运的因子。我们发现Fgf8是该信号的一个候选因子，因为它对于抑制pax6.1从而独立于eng2/eng3的表达状态使DMB向前移位既必要又充分。通过检查无法接收Fgf信号的小细胞克隆，我们发现假定的中脑神经板中的细胞需要Fgf信号来防止它们遵循前脑命运。Eng2/Eng3和Fgf8的共同缺失导致中脑特征完全丧失，从而导致前脑和后脑原基融合。因此，Eng2/Eng3和Fgf8对于维持神经板中的中脑特征并由此定位DMB是必需的。这提供了一个维持前神经板划分为前脑和中脑所需机制的例子。
