Corbit Kevin C, Shyer Amy E, Dowdle William E, Gaulden Julie, Singla Veena, Chen Miao-Hsueh, Chuang Pao-Tien, Reiter Jeremy F
Department of Biochemistry and Biophysics, Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California 94158-2324, USA.
Nat Cell Biol. 2008 Jan;10(1):70-6. doi: 10.1038/ncb1670. Epub 2007 Dec 16.
Primary cilia are microtubule-based organelles involved in signal transduction and project from the surface of most vertebrate cells. Proteins that can localize to the cilium, for example, Inversin and Bardet-Biedl syndrome (BBS) proteins, are implicated in both beta-catenin-dependent and -independent Wnt signalling. Given that Inversin and BBS proteins are found both at the cilium and elsewhere in the cell, the role of the cilium itself in Wnt signalling is not clear. Using three separate mutations that disrupt ciliogenesis (affecting Kif3a, Ift88 and Ofd1), we show in this study that the primary cilium restricts the activity of the canonical Wnt pathway in mouse embryos, primary fibroblasts, and embryonic stem cells. Interestingly, unciliated cells activate transcription only in response to Wnt stimulation, but do so much more robustly than ciliated cells. Loss of Kif3a, but not other ciliogenic genes, causes constitutive phosphorylation of Dishevelled (Dvl). Blocking the activity of casein kinase I (CKI) reverses this constitutive Dvl phosphorylation and abrogates pathway hyper-responsiveness. These results suggest that Kif3a restrains canonical Wnt signalling both by restricting the CKI-dependent phosphorylation of Dvl and through a separate ciliary mechanism. More generally, these findings reveal that, in contrast to its role in promoting Hedgehog (Hh) signalling, the cilium restrains canonical Wnt signalling.
初级纤毛是基于微管的细胞器，参与信号转导，从大多数脊椎动物细胞表面伸出。能够定位于纤毛的蛋白质，例如Inversin和巴德-比德尔综合征（BBS）蛋白，与β-连环蛋白依赖性和非依赖性Wnt信号传导有关。鉴于Inversin和BBS蛋白在纤毛和细胞其他部位均有发现，纤毛本身在Wnt信号传导中的作用尚不清楚。在本研究中，我们利用三种破坏纤毛发生的独立突变（影响Kif3a、Ift88和Ofd1）表明，初级纤毛在小鼠胚胎、原代成纤维细胞和胚胎干细胞中限制经典Wnt通路的活性。有趣的是，无纤毛细胞仅在Wnt刺激下激活转录，但比有纤毛细胞激活得更强烈。Kif3a的缺失而非其他纤毛发生基因的缺失会导致Dishevelled（Dvl）的组成型磷酸化。阻断酪蛋白激酶I（CKI）的活性可逆转这种组成型Dvl磷酸化，并消除通路的高反应性。这些结果表明，Kif3a通过限制CKI依赖性的Dvl磷酸化以及通过一种独立的纤毛机制来抑制经典Wnt信号传导。更普遍地说，这些发现揭示，与纤毛在促进刺猬信号（Hh）传导中的作用相反，纤毛会抑制经典Wnt信号传导。
