Fan Xiang, Hagos Engda G, Xu Bo, Sias Christina, Kawakami Koichi, Burdine Rebecca D, Dougan Scott T
Department of Cellular Biology, The University of Georgia, Athens, GA 30602, USA.
Dev Biol. 2007 Oct 15;310(2):363-78. doi: 10.1016/j.ydbio.2007.08.008. Epub 2007 Aug 10.
In many vertebrates, extra-embryonic tissues are important signaling centers that induce and pattern the germ layers. In teleosts, the mechanism by which the extra-embryonic yolk syncytial layer (YSL) patterns the embryo is not understood. Although the Nodal-related protein Squint is expressed in the YSL, its role in this tissue is not known. We generated a series of stable transgenic lines with GFP under the control of squint genomic sequences. In all species, nodal-related genes induce their own expression through a positive feedback loop. We show that two tissue specific enhancers in the zebrafish squint gene mediate the response to Nodal signals. Expression in the blastomeres depends upon a conserved Nodal response element (NRE) in the squint first intron, while expression in the extra-embryonic enveloping layer (EVL) is mediated by an element upstream of the transcription start site. Targeted depletion experiments demonstrate that the zebrafish Nodal-related proteins Squint and Cyclops are required in the YSL for endoderm and head mesoderm formation. Thus, Nodal signals mediate interactions between embryonic and extra-embryonic tissues in zebrafish that maintain nodal-related gene expression in the margin. Our results demonstrate a high degree of functional conservation between the extra-embryonic tissues of mouse and zebrafish.
在许多脊椎动物中，胚外组织是诱导胚层并使其形成特定模式的重要信号中心。在硬骨鱼中，胚外卵黄合胞体层（YSL）使胚胎形成特定模式的机制尚不清楚。尽管与Nodal相关的蛋白Squint在YSL中表达，但其在该组织中的作用尚不清楚。我们利用squint基因组序列控制下的绿色荧光蛋白（GFP）构建了一系列稳定的转基因品系。在所有物种中，与Nodal相关的基因通过正反馈环诱导自身表达。我们发现斑马鱼squint基因中的两个组织特异性增强子介导了对Nodal信号的反应。卵裂球中的表达取决于squint基因第一内含子中一个保守的Nodal反应元件（NRE），而在胚外包被层（EVL）中的表达则由转录起始位点上游的一个元件介导。靶向缺失实验表明，斑马鱼中与Nodal相关的蛋白Squint和Cyclops在YSL中是内胚层和头部中胚层形成所必需的。因此，Nodal信号介导了斑马鱼胚胎组织和胚外组织之间的相互作用，从而维持边缘区域与Nodal相关基因的表达。我们的结果表明，小鼠和斑马鱼的胚外组织之间存在高度的功能保守性。
