Koos D S, Ho R K
Department of Molecular Biology, Princeton University, New Jersey 08544, USA.
Curr Biol. 1998 Nov 5;8(22):1199-206. doi: 10.1016/s0960-9822(07)00509-x.
In amphibians, the Nieuwkoop center--a primary inducing region--has a central role in the induction of dorsal mesodermal cells to form the Spemann organizer. In teleosts, such as the zebrafish, Danio rerio, the functional equivalent of the amphibian Spemann organizer is the dorsal shield. Historically, a small region of the teleost yolk syncytial layer (YSL), an extraembryonic tissue that underlies the entire blastoderm, has been implicated in dorsal shield specification. Difficulties in transplanting discrete regions of the YSL and the previous lack of localized expression patterns unique to the YSL have, however, hindered efforts to prove definitively that the YSL possesses Nieuwkoop-center-like activities.
Here, we describe the isolation and analysis of a new homeobox gene, called nieuwkoid, which is first expressed immediately following the mid-blastula transition on the dorsal side of the zebrafish pregastrula embryo. We found that, by the onset of gastrulation, nieuwkoid expression becomes localized to a restricted region of the YSL, directly underlying the future dorsal shield. Mis-expression of nieuwkoid in early zebrafish embryos was found to be sufficient for the induction of ectopic organizer regions and secondary axes. Mis-expression of nieuwkoid by cell transplantation or by direct injection into the YSL led to the non-autonomous induction of ectopic organizer gene expression.
The dynamic and restricted expression of the nieuwkoid gene, combined with its potent dorsalizing activity, suggests that nieuwkoid is an important component in the regionalization of the gastrula organizer, possibly characterizing and mediating an organizer-inducing/Nieuwkoop-center-like activity.
在两栖动物中，nieuwkoop中心（一个主要的诱导区域）在诱导背侧中胚层细胞形成斯佩曼组织者方面起着核心作用。在硬骨鱼中，如斑马鱼（Danio rerio），与两栖动物斯佩曼组织者功能相当的结构是背盾。从历史上看，硬骨鱼卵黄合胞体层（YSL）的一个小区域（YSL是位于整个胚盘下方的一种胚外组织）被认为与背盾的特化有关。然而，YSL离散区域移植的困难以及之前缺乏YSL特有的局部表达模式，阻碍了明确证明YSL具有类似nieuwkoop中心活性的研究努力。
在这里，我们描述了一个新的同源框基因nieuwkoid的分离和分析，该基因在斑马鱼原肠胚形成前胚胎的背侧，于囊胚中期转换后立即首次表达。我们发现，到原肠胚形成开始时，nieuwkoid的表达定位于YSL的一个受限区域，该区域直接位于未来背盾的下方。在斑马鱼早期胚胎中过表达nieuwkoid足以诱导异位组织者区域和次生轴的形成。通过细胞移植或直接注射到YSL中过表达nieuwkoid会导致异位组织者基因表达的非自主诱导。
nieuwkoid基因的动态和受限表达，加上其强大的背化活性，表明nieuwkoid是原肠胚组织者区域化的重要组成部分，可能表征并介导一种组织者诱导/类似nieuwkoop中心的活性。
