Distinct DNA binding sites contribute to the TCF transcriptional switch in C. elegans and Drosophila.
Distinct DNA binding sites contribute to the TCF transcriptional switch in C. elegans and Drosophila.
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DOI：
10.1371/journal.pgen.1004133
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发表时间：
2014-02
期刊：
影响因子：
4.5
通讯作者：
Cadigan KM
中科院分区：
文献类型：
--
作者：
Bhambhani C;Ravindranath AJ;Mentink RA;Chang MV;Betist MC;Yang YX;Koushika SP;Korswagen HC;Cadigan KM
通过信号传导途径调节基因表达通常通过转录开关发生，其中负责信号依赖性基因激活的转录因子在没有信号传导的情况下抑制相同的靶标。T细胞因子（TCF）是Wnt/β-连环蛋白途径中的转录因子，其控制多细胞动物中的许多细胞命运指定事件。TCF转录开关由许多辅助调节因子介导，这些辅助调节因子有助于抑制或激活Wnt靶基因。通常认为TCF的DNA识别对于靶基因定位是重要的，但在实际开关中不起作用。TCF/穿山甲（苍蝇TCF）和一些脊椎动物TCF亚型通过两个不同的结构域结合DNA，一个高迁移率族（HMG）结构域和一个C-钳，分别识别称为HMG和辅助位点的DNA基序。在这里，我们证明了POP-1（C。elegans TCF）也通过HMG和辅助位点相互作用激活靶基因。辅助位点增强了合成增强子在几种组织中检测Wnt/β-catenin信号的能力，并揭示了POP-1在调节C. elegans排便周期搜索HMG-Helper位点簇允许鉴定在头部肌肉和肠道中活跃的新POP-1靶基因。虽然辅助位点和C-夹对于蠕虫和苍蝇Wnt靶标的激活是必不可少的，但它们在Wnt信号传导不存在的情况下对于靶标的TCF依赖性抑制是必需的。这些数据表明，TCF-DNA结合的根本变化有助于Wnt刺激后发生的转录开关。细胞的DNA必须被正确地“读取”，以便正确的基因被表达。转录因子是主要的“DNA阅读器”，这些蛋白质与特定的DNA序列结合。以线虫为模型系统，我们研究了一种特殊的转录因子POP-1的DNA结合规则，POP-1介导Wnt信号传导，这是一种重要的细胞间通讯途径。除了其已知的DNA结合位点，我们发现POP-1识别额外的序列，称为辅助位点，这是激活Wnt靶标所必需的。我们利用这些知识发现Wnt信号在线虫肠道的起搏细胞中是活跃的，这些细胞控制排便，这是一种与脊椎动物心跳相似的节律行为。POP-1在调节Wnt靶点、在无信号传导的情况下抑制靶基因以及在信号刺激时激活它们方面具有双重作用。令人惊讶的是，我们发现辅助位点只需要激活而不是抑制，这也是果蝇的情况。因此，这项工作揭示了POP-1 DNA结合中意想不到的复杂性，这可能与其在干细胞生物学和癌症中发挥重要作用的人类对应物有关。
Regulation of gene expression by signaling pathways often occurs through a transcriptional switch, where the transcription factor responsible for signal-dependent gene activation represses the same targets in the absence of signaling. T-cell factors (TCFs) are transcription factors in the Wnt/ß-catenin pathway, which control numerous cell fate specification events in metazoans. The TCF transcriptional switch is mediated by many co-regulators that contribute to repression or activation of Wnt target genes. It is typically assumed that DNA recognition by TCFs is important for target gene location, but plays no role in the actual switch. TCF/Pangolin (the fly TCF) and some vertebrate TCF isoforms bind DNA through two distinct domains, a High Mobility Group (HMG) domain and a C-clamp, which recognize DNA motifs known as HMG and Helper sites, respectively. Here, we demonstrate that POP-1 (the C. elegans TCF) also activates target genes through HMG and Helper site interactions. Helper sites enhanced the ability of a synthetic enhancer to detect Wnt/ß-catenin signaling in several tissues and revealed an unsuspected role for POP-1 in regulating the C. elegans defecation cycle. Searching for HMG-Helper site clusters allowed the identification of a new POP-1 target gene active in the head muscles and gut. While Helper sites and the C-clamp are essential for activation of worm and fly Wnt targets, they are dispensable for TCF-dependent repression of targets in the absence of Wnt signaling. These data suggest that a fundamental change in TCF-DNA binding contributes to the transcriptional switch that occurs upon Wnt stimulation. The DNA of cells must be correctly “read” so that the proper genes are expressed. Transcription factors are the primary “DNA readers”, and these proteins bind to specific DNA sequences. Using nematodes as a model system, we investigated the rules of DNA binding for a particular transcription factor, called POP-1, which mediates Wnt signaling, an important cell-cell communication pathway. In addition to its known DNA binding site, we found that POP-1 recognizes additional sequences, termed Helper sites, which are essential for activation of Wnt targets. We used this knowledge to discover that Wnt signaling is active in pacemaker cells in the nematode intestine, which control defecation, a rhythmic behavior with parallels to the vertebrate heartbeat. POP-1 has a dual role in regulating Wnt targets, repressing target genes in the absence of signaling and activating them upon signal stimulation. Surprisingly, we found that Helper sites are only required for activation and not repression, and that this is also the case in the fruit fly Drosophila. This work thus reveals an unexpected complexity in POP-1 DNA binding, which is likely to be relevant for its human counterparts, which play important roles in stem cell biology and cancer.
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