Department of Cell and Developmental Biology, University of Pennsylvania School of Medicine, 1110 Biomedical Research Building 2/3, 421 Curie Boulevard, Philadelphia, PA 19104-6058, USA.
Dev Biol. 2011 Apr 15;352(2):367-81. doi: 10.1016/j.ydbio.2011.01.034. Epub 2011 Feb 3.
The Spemann organizer is an essential signaling center in Xenopus germ layer patterning and axis formation. Organizer formation occurs in dorsal blastomeres receiving both maternal Wnt and zygotic Nodal signals. In response to stabilized βcatenin, dorsal blastomeres express the closely related transcriptional activators, Siamois (Sia) and Twin (Twn), members of the paired homeobox family. Sia and Twn induce organizer formation and expression of organizer-specific genes, including Goosecoid (Gsc). In spite of the similarity of Sia and Twn sequence and expression pattern, it is unclear whether these factors function equivalently in promoter binding and subsequent transcriptional activation, or if Sia and Twn are required for all aspects of organizer function. Here we report that Sia and Twn activate Gsc transcription by directly binding to a conserved P3 site within the Wnt-responsive proximal element of the Gsc promoter. Sia and Twn form homodimers and heterodimers by direct homeodomain interaction and dimer forms are indistinguishable in both DNA-binding and activation functions. Sequential chromatin immunoprecipitation reveals that the endogenous Gsc promoter can be occupied by either Sia or Twn homodimers or Sia-Twn heterodimers. Knockdown of Sia and Twn together, but not individually, results in a failure of organizer gene expression and a disruption of axis formation, consistent with a redundant role for Sia and Twn in organizer formation. Furthermore, simultaneous knockdown of Sia and Twn blocks axis induction in response to ectopic Wnt signaling, demonstrating an essential role for Sia and Twn in mediating the transcriptional response to the maternal Wnt pathway. The results demonstrate the functional redundancy of Sia and Twn and their essential role in direct transcriptional responses necessary for Spemann organizer formation.
Spemann 组织者是爪蟾胚层模式形成和轴形成的重要信号中心。组织者的形成发生在接收母体 Wnt 和合子 Nodal 信号的背侧胚盘中。在稳定的β-catenin 作用下,背侧胚盘表达密切相关的转录激活因子 Siamois(Sia)和 Twin(Twn),它们是配对同源盒家族的成员。Sia 和 Twn 诱导组织者的形成和表达组织者特异性基因,包括 Goosecoid(Gsc)。尽管 Sia 和 Twn 的序列和表达模式相似,但不清楚这些因子在启动子结合和随后的转录激活中是否等效发挥作用,或者 Sia 和 Twn 是否是组织者功能的所有方面所必需的。在这里,我们报告 Sia 和 Twn 通过直接结合 Gsc 启动子 Wnt 反应性近端元件中的保守 P3 位点来激活 Gsc 转录。Sia 和 Twn 通过直接同源域相互作用形成同源二聚体和异源二聚体,并且在 DNA 结合和激活功能方面二聚体形式是不可区分的。连续的染色质免疫沉淀显示,内源性 Gsc 启动子可以被 Sia 或 Twn 同源二聚体或 Sia-Twn 异源二聚体占据。Sia 和 Twn 的同时敲低,而不是单独敲低,导致组织者基因表达失败和轴形成中断,这与 Sia 和 Twn 在组织者形成中的冗余作用一致。此外,Sia 和 Twn 的同时敲低阻断了对异位 Wnt 信号的轴诱导,表明 Sia 和 Twn 在介导对母体 Wnt 途径的转录反应中具有重要作用。这些结果表明了 Sia 和 Twn 的功能冗余性及其在 Spemann 组织者形成所必需的直接转录反应中的重要作用。