Transcriptional Activation by Wnt Signaling

描述(由申请人提供)： Wnt家族分泌的糖蛋白通过进化保守的信号级联促进β-连环蛋白的核积累，β-连环蛋白与DNA结合蛋白TCF家族的成员相互作用，激活靶基因转录。TCFs与特定的序列结合，但共识是如此松散，以至于在整个基因组中都可以找到潜在的TCFs结合位点。尽管如此，我们发现TCF与WNT目标中的特定位置结合，对应于WNT响应元件(WRE)。WRE的系统突变揭示了额外基序的存在，这些基序与TCF结合位点作用，介导Wnt信号的激活。这些基序(称为辅助位点)在其他几个WE中也被发现，全基因组范围内对保守的TCF/辅助位点簇的搜索已经确定了其他可能的WE。将测试这些元素中的Helper站点的功能意义。将探索辅助位点如何与TCF结合位点相互作用的分子机制，并将表征与其结合的反式作用因子(S)。β-连环蛋白与TCF的结合可将其从转录抑制因子转化为激活剂。我们发现Wnt信号促进了整个靶基因的组蛋白乙酰化，这与转录的激活有关。这种广泛的修饰似乎是为了对抗使WNT靶标沉默的因子的作用。将详细探讨这些过程之间的机制和关系。我们的数据表明，Wnt靶标的转录转换涉及染色质结构的变化，远远超出了之前所认识的。项目简介：Wnt信号通路在细胞发育过程中的命运决定中起着重要作用，是维持成人组织中干细胞种群所必需的。该途径的错误调控在许多人类癌症中起着因果作用。我们的研究旨在了解TCF位点以外的基序在WRE功能中的作用，这将导致在许多重要的生物背景下使用更好的生物信息学方法来识别Wnt靶标。我们关于染色质修饰在调节TCF转录开关中的作用的工作将成为研究哺乳动物系统中这些过程的范例。

DESCRIPTION (provided by applicant): Secreted glycoproteins of the Wnt family act through an evolutionarily conserved signaling cascade which promotes the nuclear accumulation of beta-catenin, which interacts with members of the TCF family of DNA-binding proteins to activate target gene transcription. TCFs binds to specific sequences, but the consensus is so loose that potential TCF binding sites can be found throughout the genome. Despite this, we find that TCF is bound to specific locations in Wnt targets, corresponding to Wnt response elements (WREs). Systematic mutagenesis of a WRE revealed the presence of additional motifs that act with TCF binding sites to mediate activation by Wnt signaling. These motifs (called Helper sites) are found in several other WREs, and genome-wide searches for conserved TCF/Helper site clusters have identified other putative WREs. The functional significance of the Helper sites in these elements will be tested. The molecular mechanism of how Helper sites interact with TCF binding sites will be explored, and the trans-acting factor(s) that bind to it will be characterized. Binding of beta-catenin to TCF converts it from a transcriptional repressor to an activator. We have found that Wnt signaling promotes histone acetylation throughout target loci, which is correlated with activation of transcription. This widespread modification appears to be required to antagonize the action of factors that silence Wnt targets. The mechanisms and relationship between these processes will be explored in detail. Our data indicates that the transcriptional switch at Wnt targets involves changes in chromatin structure far beyond what was previously recognized. PROJECT NARRATIVE: The Wnt signaling pathway plays important roles in cell fate decisions during development, and is required for the maintenance of stem cell populations in adult tissues. Misregulation of the pathway plays a causal role in many human cancers. Our studies to understand the role of motifs besides TCF sites that contribute to WRE function will lead to better bioinformatic methods to identify Wnt targets in many important biological contexts. Our work on the role of chromatin modifications in regulating the TCF transcriptional switch will serve as a paradigm to study these processes in mammalian systems.