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First published online 30 November 2005
doi: 10.1242/dev.02178

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Tsukushi controls ectodermal patterning and neural crest specification in Xenopus by direct regulation of BMP4 and X-delta-1 activity

¹ Division of Developmental Neurobiology, Graduate School of Medical Sciences, Kumamoto University, Kumamoto 860-8556, Japan.
² The 21st Century COE program "Cell Fate Regulation Research and Education Unit", Kumamoto University, Kumamoto 860-0811, Japan.
³ Department of Anatomy, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK
⁴ PRESTO, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
⁵ Department of Oncology, The Hutchison/MRC Research Centre, University of Cambridge, Hills Road, Cambridge CB2 2XZ, UK.

^* Author for correspondence (e-mail: ohta9203{at}gpo.kumamoto-u.ac.jp)

Accepted 25 October 2005

In Xenopus, ectodermal patterning depends on a mediolateral gradient of BMP signaling, higher in the epidermis and lower in the neuroectoderm. Neural crest cells are specified at the border between the neural plate and the epidermis, at intermediate levels of BMP signaling. We recently described a novel secreted protein, Tsukushi (TSK), which works as a BMP antagonist during chick gastrulation. Here, we report on the Xenopus TSK gene (X-TSK), and show that it is involved in neural crest specification. X-TSK expression accumulates after gastrulation at the anterior-lateral edges of the neural plate, including the presumptive neural crest region. In gain-of-function experiments, X-TSK can strongly enhance neural crest specification by the dorsolateral mesoderm or X-Wnt8 in ectodermal explants, while the electroporation of X-TSK mRNA in the lateral ectoderm of embryos after gastrulation can induce the expression of neural crest markers in vivo. By contrast, depletion of X-TSK in explants or embryos impairs neural crest specification. Similarly to its chick homolog, X-TSK works as a BMP antagonist by direct binding to BMP4. However, X-TSK can also indirectly regulate BMP4 mRNA expression at the neural plate border via modulation of the Delta-Notch signaling pathway. We show that X-TSK directly binds to the extracellular region of X-delta-1, and modulates Delta-dependent Notch activity. We propose that X-TSK plays a key role in neural crest formation by directly regulating BMP and Delta activities at the boundary between the neural and the non-neural ectoderm.

Key words: Ectoderm, Neural crest, Neural plate, Epidermis, X-TSK, BMP, Notch, Xenopus

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