UPMC (University of Paris 06), CNRS, UMR7009, Biologie du Développement, Observatoire Océanologique, Villefranche-sur-Mer, France.
PLoS Biol. 2009 Nov;7(11):e1000248. doi: 10.1371/journal.pbio.1000248. Epub 2009 Nov 24.
Formation of the dorsal-ventral axis of the sea urchin embryo relies on cell interactions initiated by the TGFbeta Nodal. Intriguingly, although nodal expression is restricted to the ventral side of the embryo, Nodal function is required for specification of both the ventral and the dorsal territories and is able to restore both ventral and dorsal regions in nodal morpholino injected embryos. The molecular basis for the long-range organizing activity of Nodal is not understood. In this paper, we provide evidence that the long-range organizing activity of Nodal is assured by a relay molecule synthesized in the ventral ectoderm, then translocated to the opposite side of the embryo. We identified this relay molecule as BMP2/4 based on the following arguments. First, blocking BMP2/4 function eliminated the long-range organizing activity of an activated Nodal receptor in an axis rescue assay. Second, we demonstrate that BMP2/4 and the corresponding type I receptor Alk3/6 functions are both essential for specification of the dorsal region of the embryo. Third, using anti-phospho-Smad1/5/8 immunostaining, we show that, despite its ventral transcription, the BMP2/4 ligand triggers receptor mediated signaling exclusively on the dorsal side of the embryo, one of the most extreme cases of BMP translocation described so far. We further report that the pattern of pSmad1/5/8 is graded along the dorsal-ventral axis and that two BMP2/4 target genes are expressed in nested patterns centered on the region with highest levels of pSmad1/5/8, strongly suggesting that BMP2/4 is acting as a morphogen. We also describe the very unusual ventral co-expression of chordin and bmp2/4 downstream of Nodal and demonstrate that Chordin is largely responsible for the spatial restriction of BMP2/4 signaling to the dorsal side. Thus, unlike in most organisms, in the sea urchin, a single ventral signaling centre is responsible for induction of ventral and dorsal cell fates. Finally, we show that Chordin may not be required for long-range diffusion of BMP2/4, describe a striking dorsal-ventral asymmetry in the expression of Glypican 5, a heparin sulphated proteoglycan that regulates BMP mobility, and show that this asymmetry depends on BMP2/4 signaling. Our study provides new insights into the mechanisms by which positional information is established along the dorsal-ventral axis of the sea urchin embryo, and more generally on how a BMP morphogen gradient is established in a multicellular embryo. From an evolutionary point of view, it highlights that although the genes used for dorsal-ventral patterning are highly conserved in bilateria, there are considerable variations, even among deuterostomes, in the manner these genes are used to shape a BMP morphogen gradient.
海胆胚胎背腹轴的形成依赖于 TGFbeta Nodal 启动的细胞相互作用。有趣的是，尽管 nodal 表达仅限于胚胎的腹侧，但 Nodal 功能对于腹侧和背侧区域的特化都是必需的，并能够在 nodal 形态发生素注射胚胎中恢复腹侧和背侧区域。Nodal 的长程组织活性的分子基础尚不清楚。在本文中，我们提供的证据表明，Nodal 的长程组织活性是由腹侧外胚层中合成的一种中继分子保证的，然后该分子被转运到胚胎的另一侧。我们将这种中继分子鉴定为 BMP2/4，其依据如下。首先，阻断 BMP2/4 功能消除了轴拯救测定中激活的 Nodal 受体的长程组织活性。其次，我们证明 BMP2/4 和相应的 I 型受体 Alk3/6 功能对于胚胎背侧区域的特化都是必需的。第三，使用抗磷酸化 Smad1/5/8 免疫染色，我们表明，尽管 BMP2/4 配体在转录上是腹侧的，但它仅在胚胎的背侧触发受体介导的信号转导，这是迄今为止描述的 BMP 转位最极端的情况之一。我们进一步报告说，pSmad1/5/8 的模式沿背腹轴呈梯度分布，并且两个 BMP2/4 靶基因以嵌套模式表达，中心位于具有最高水平的 pSmad1/5/8 的区域，强烈表明 BMP2/4 作为一种形态发生素发挥作用。我们还描述了 Nodal 下游 chordin 和 bmp2/4 的非常异常的腹侧共表达，并证明 Chordin 在很大程度上负责将 BMP2/4 信号转导限制在背侧。因此，与大多数生物体不同，在海胆中，单个腹侧信号中心负责诱导腹侧和背侧细胞命运。最后，我们表明 Chordin 可能不需要 BMP2/4 的长程扩散，描述了 Glypican 5 的显著背腹不对称性，Glypican 5 是一种肝素硫酸蛋白聚糖，调节 BMP 迁移，并且表明这种不对称性依赖于 BMP2/4 信号转导。我们的研究为建立海胆胚胎背腹轴沿背腹轴建立位置信息的机制提供了新的见解，更普遍地为在多细胞胚胎中建立 BMP 形态发生素梯度的机制提供了新的见解。从进化的角度来看，它突出表明，尽管用于背腹模式形成的基因在两侧对称动物中高度保守，但即使在后口动物中，这些基因用于塑造 BMP 形态发生素梯度的方式也存在很大差异。
