Simplet/Fam53b is required for Wnt signal transduction by regulating β-catenin nuclear localization

• DOI: 10.1242/dev.108415
• Publication Date: 2014-09-02T11:56:45Z
• Authors: Caghan Kizil; Beate Küchler; Jia-Jiun Yan; Günes Özhan; Enrico Moro; Francesco Argenton; Michael Brand; Gilbert Weidinger; Christopher L. Antos
• Keywords: 0301 basic medicine; Blotting, Western; physiology [Wnt Signaling Pathway]; Mice, Transgenic; genetics [RNA, Small Interfering]; Mice; 03 medical and health sciences; metabolism [Wnt Proteins]; ddc:570; physiology [Gene Expression Regulation, Developmental]; Animals; Humans; Immunoprecipitation; metabolism [Cell Nucleus]; RNA, Small Interfering; Luciferases; Wnt Signaling Pathway; In Situ Hybridization; beta Catenin; Cell Nucleus; genetics [Wnt Proteins]; Gene Expression Regulation, Developmental; metabolism [beta Catenin]; Immunohistochemistry; Fam53b protein, mouse; Wnt Proteins; RNA Interference

Canonical β-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of β-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating β-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating β-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two β-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of β-catenin. Conversely, overexpression of smp increases β-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the β-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for β-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating β-catenin nuclear localization: the protein colocalizes with β-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces β-catenin nuclear localization and transcriptional activity; human SMP mediates β-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human β-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of β-catenin-dependent Wnt signal transduction.