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First published online 18 February 2009
doi: 10.1242/dev.032912

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The Wnt antagonists Frzb-1 and Crescent locally regulate basement membrane dissolution in the developing primary mouth

¹ Whitehead institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02102, USA.
² Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA.

^* Author for correspondence (e-mail: sive{at}wi.mit.edu)

Accepted 27 January 2009

The primary mouth forms from ectoderm and endoderm at the extreme anterior of the embryo, a conserved mesoderm-free region. In Xenopus, a very early step in primary mouth formation is loss of the basement membrane between the ectoderm and endoderm. In an unbiased microarray screen, we defined genes encoding the sFRPs Frzb-1 and Crescent as transiently and locally expressed in the primary mouth anlage. Using antisense oligonucleotides and `face transplants', we show that frzb-1 and crescent expression is specifically required in the primary mouth region at the time this organ begins to form. Several assays indicate that Frzb-1 and Crescent modulate primary mouth formation by suppressing Wnt signaling, which is likely to be mediated by β-catenin. First, a similar phenotype (no primary mouth) is seen after loss of Frzb-1/Crescent function to that seen after temporally and spatially restricted overexpression of Wnt-8. Second, overexpression of either Frzb-1 or Dkk-1 results in an enlarged primary mouth anlage. Third, overexpression of Dkk-1 can restore a primary mouth to embryos in which Frzb-1/Crescent expression has been inhibited. We show that Frzb-1/Crescent function locally promotes basement membrane dissolution in the primary mouth primordium. Consistently, Frzb-1 overexpression decreases RNA levels of the essential basement membrane genes fibronectin and laminin, whereas Wnt-8 overexpression increases the levels of these RNAs. These data are the first to connect Wnt signaling and basement membrane integrity during primary mouth development, and suggest a general paradigm for the regulation of basement membrane remodeling.

Key words: Primary mouth, Xenopus, Wnt, sFRP, Frzb-1, Crescent, Basement membrane, Laminin, Fibronectin

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