German Cancer Research Center (DKFZ), Division of Signaling and Functional Genomics, Heidelberg University, Department of Cell and Molecular Biology, Medical Faculty Mannheim, 69120 Heidelberg, Germany.
Cell Rep. 2013 Sep 26;4(6):1224-34. doi: 10.1016/j.celrep.2013.08.008. Epub 2013 Sep 12.
Wnt/β-catenin signaling plays an important role in embryonic development and adult tissue homeostasis. When Wnt ligands bind to the receptor complex, LRP5/6 coreceptors are activated by phosphorylation and concomitantly endocytosed. In vertebrates, Wnt ligands induce caveolin-dependent endocytosis of LRP6 to relay signal downstream, whereas antagonists such as Dickkopf promote clathrin-dependent endocytosis, leading to inhibition. However, little is known about how LRP6 is directed to different internalization mechanisms, and how caveolin-dependent endocytosis is mediated. In an RNAi screen, we identified the Rab GTPase RAB8B as being required for Wnt/β-catenin signaling. RAB8B depletion reduces LRP6 activity, β-catenin accumulation, and induction of Wnt target genes, whereas RAB8B overexpression promotes LRP6 activity and internalization and rescues inhibition of caveolar endocytosis. In Xenopus laevis and Danio rerio, RAB8B morphants show lower Wnt activity during embryonic development. Our results implicate RAB8B as an essential evolutionary conserved component of Wnt/β-catenin signaling through regulation of LRP6 activity and endocytosis.
Wnt/β-catenin 信号通路在胚胎发育和成人组织稳态中发挥着重要作用。当 Wnt 配体与受体复合物结合时，LRP5/6 核心受体通过磷酸化被激活，并随之被内吞。在脊椎动物中，Wnt 配体诱导 LRP6 依赖 caveolin 的内吞作用，以向下游传递信号，而拮抗剂如 Dickkopf 则促进网格蛋白依赖的内吞作用，从而抑制信号。然而，对于 LRP6 如何被导向不同的内化机制，以及 caveolin 依赖性内吞作用如何被介导，人们知之甚少。在 RNAi 筛选中，我们发现 Rab GTPase RAB8B 是 Wnt/β-catenin 信号所必需的。RAB8B 耗尽会降低 LRP6 的活性、β-catenin 的积累和 Wnt 靶基因的诱导，而 RAB8B 的过表达会促进 LRP6 的活性和内化，并挽救 caveolar 内吞作用的抑制。在非洲爪蟾和斑马鱼中，RAB8B 基因敲低的胚胎在发育过程中表现出较低的 Wnt 活性。我们的研究结果表明，RAB8B 通过调节 LRP6 的活性和内化，是 Wnt/β-catenin 信号通路的一个必不可少的进化保守成分。
