Gtpbp2 is a positive regulator of Wnt signaling and maintains low levels of the Wnt negative regulator Axin.
Gtpbp2 is a positive regulator of Wnt signaling and maintains low levels of the Wnt negative regulator Axin.
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DOI：
10.1186/s12964-016-0138-x
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发表时间：
2016-08-02
期刊：
影响因子：
--
通讯作者：
Thomsen GH
中科院分区：
文献类型：
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作者：
Gillis WQ;Kirmizitas A;Iwasaki Y;Ki DH;Wyrick JM;Thomsen GH
由稳定的β-连环蛋白转导的典型Wnt信号在动物中在维持干细胞多能性、调节细胞分化和指导正常胚胎发育方面发挥相似的作用。Wnt/β-catenin信号转导失调导致疾病和出生缺陷，多种调节过程控制该途径以确保其正常功能并与其他信号转导系统整合。我们以前确定了GTP结合蛋白2（Gtpbp 2）作为一种新的BMP信号转导调节因子，然而进一步的研究表明，Gtpbp 2也可以影响Wnt信号转导，这是一个新的发现。在非洲爪蟾胚胎中Gtpbp 2的敲低导致严重的轴向缺陷，并减少Spemann-Mangold组织者基因的表达。Gtpbp 2敲低阻断对异位Wnt 8配体的反应，例如外胚层组织外植体中的组织者基因诱导和整个胚胎中的次级轴的诱导。然而，异位Nodal 2的组织者基因诱导不受Gtpbp 2敲低的影响。通过激活经典途径中连续点的Wnt信号转导进行的上位性测试表明，Gtpbp 2需要在Dishevelled和Gsk 3 β的下游，但在β-连环蛋白的上游，这与先前报道的Axin 1过表达在非洲爪蟾胚胎中的作用相似。聚焦于非洲爪蟾胚胎中的Axin，我们发现Gtpbp 2的敲低提高了内源性或外源性Axin蛋白水平。此外，Gtpbp 2融合蛋白与Dishevelled共定位，并与Axin和Gsk 3b共免疫沉淀。我们的结论是，Gtpbp 2是非洲爪蟾胚胎中典型的Wnt/β-catenin信号所必需的。我们的数据表明，Gtpbp 2抑制Axin蛋白（β-连环蛋白破坏复合物的限速组分）的积累，使得Axin蛋白水平与Gtpbp 2水平呈负相关。我们的Gtpbp 2-Wnt上位性结果与先前报道的Axin过表达的影响、Gtpbp 2与Axin的物理相互作用以及Axin蛋白水平升高与Gtpbp 2敲低后Wnt响应性丧失之间的相关性的相似性支持该模型。多种致癌Wnt通路突变需要低Axin水平，因此Gtpbp 2抑制剂的开发可能提供一种新的治疗策略，以提高Axin水平并抑制癌症和其他Wnt相关疾病中的异常β-连环蛋白信号传导。本文的在线版本（doi：10.1186/s12964-016-0138-x）包含补充材料，可供授权用户使用。
Canonical Wnt signals, transduced by stabilized β-catenin, play similar roles across animals in maintaining stem cell pluripotency, regulating cell differentiation, and instructing normal embryonic development. Dysregulated Wnt/β-catenin signaling causes diseases and birth defects, and a variety of regulatory processes control this pathway to ensure its proper function and integration with other signaling systems. We previously identified GTP-binding protein 2 (Gtpbp2) as a novel regulator of BMP signaling, however further exploration revealed that Gtpbp2 can also affect Wnt signaling, which is a novel finding reported here. Knockdown of Gtpbp2 in Xenopus embryos causes severe axial defects and reduces expression of Spemann-Mangold organizer genes. Gtpbp2 knockdown blocks responses to ectopic Wnt8 ligand, such as organizer gene induction in ectodermal tissue explants and induction of secondary axes in whole embryos. However, organizer gene induction by ectopic Nodal2 is unaffected by Gtpbp2 knockdown. Epistasis tests, conducted by activating Wnt signal transduction at sequential points in the canonical pathway, demonstrate that Gtpbp2 is required downstream of Dishevelled and Gsk3β but upstream of β-catenin, which is similar to the previously reported effects of Axin1 overexpression in Xenopus embryos. Focusing on Axin in Xenopus embryos, we find that knockdown of Gtpbp2 elevates endogenous or exogenous Axin protein levels. Furthermore, Gtpbp2 fusion proteins co-localize with Dishevelled and co-immunoprecipitate with Axin and Gsk3b. We conclude that Gtpbp2 is required for canonical Wnt/β-catenin signaling in Xenopus embryos. Our data suggest a model in which Gtpbp2 suppresses the accumulation of Axin protein, a rate-limiting component of the β-catenin destruction complex, such that Axin protein levels negatively correlate with Gtpbp2 levels. This model is supported by the similarity of our Gtpbp2-Wnt epistasis results and previously reported effects of Axin overexpression, the physical interactions of Gtpbp2 with Axin, and the correlation between elevated Axin protein levels and lost Wnt responsiveness upon Gtpbp2 knockdown. A wide variety of cancer-causing Wnt pathway mutations require low Axin levels, so development of Gtpbp2 inhibitors may provide a new therapeutic strategy to elevate Axin and suppress aberrant β-catenin signaling in cancer and other Wnt-related diseases. The online version of this article (doi:10.1186/s12964-016-0138-x) contains supplementary material, which is available to authorized users.