Department of Biological Sciences, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA.
Development. 2010 Apr;137(7):1149-57. doi: 10.1242/dev.048652. Epub 2010 Feb 24.
The gene regulatory network (GRN) that underlies the development of the embryonic skeleton in sea urchins is an important model for understanding the architecture and evolution of developmental GRNs. The initial deployment of the network is thought to be regulated by a derepression mechanism, which is mediated by the products of the pmar1 and hesC genes. Here, we show that the activation of the skeletogenic network occurs by a mechanism that is distinct from the transcriptional repression of hesC. By means of quantitative, fluorescent whole-mount in situ hybridization, we find that two pivotal early genes in the network, alx1 and delta, are activated in prospective skeletogenic cells prior to the downregulation of hesC expression. An analysis of the upstream regulation of alx1 shows that this gene is regulated by MAPK signaling and by the transcription factor Ets1; however, these inputs influence only the maintenance of alx1 expression and not its activation, which occurs by a distinct mechanism. By altering normal cleavage patterns, we show that the zygotic activation of alx1 and delta, but not that of pmar1, is dependent upon the unequal division of vegetal blastomeres. Based on these findings, we conclude that the widely accepted double-repression model is insufficient to account for the localized activation of the skeletogenic GRN. We postulate the existence of additional, unidentified repressors that are controlled by pmar1, and propose that the ability of pmar1 to derepress alx1 and delta is regulated by the unequal division of vegetal blastomeres.
原肠胚骨骼发育的基因调控网络（GRN）是理解发育性 GRN 结构和进化的重要模型。该网络的初始部署被认为受去阻遏机制调控，该机制由 pmar1 和 hesC 基因产物介导。在这里，我们证明了骨架形成网络的激活是通过一种与 hesC 转录抑制不同的机制发生的。通过定量荧光整体原位杂交，我们发现网络中的两个关键早期基因 alx1 和 delta 在 hesC 表达下调之前就在预期的骨骼生成细胞中被激活。对 alx1 的上游调控分析表明，该基因受 MAPK 信号和转录因子 Ets1 调控；然而，这些输入仅影响 alx1 表达的维持，而不是其激活，其激活是通过一种不同的机制发生的。通过改变正常的分裂模式，我们表明 alx1 和 delta 的合子激活，而不是 pmar1 的激活，依赖于植物半球的不均等分裂。基于这些发现，我们得出结论，广泛接受的双重抑制模型不足以解释骨架形成性 GRN 的局部激活。我们假设存在其他未被识别的受 pmar1 控制的抑制剂，并提出 pmar1 去抑制 alx1 和 delta 的能力受植物半球不均等分裂的调控。
