, 2002), in the skin ( Figure 1E). Interestingly, there was also increased expression of the Wnt-responsive transcription factor Lef1 in a nonautonomous manner in the underlying mesenchymal tissue, including the meninges ( Figure 1F). This made us wonder whether there was Wnt signaling-dependent expression of a Wnt ligand in the skin that was then signaling to the underlying tissues. Wnt6 is normally expressed high laterally and low dorsally in the skin of the head ( Figure 1C) and is absent from the midline. However, in the Msx2-Cre;Ctnnb1lox(ex3) mice, Wnt6 expression covered the entire dorsal surface

of the head. We also found that Wnt6 was elevated about 1.5-fold compared to control by using quantitative UMI-77 concentration real time PCR (qRT-PCR) on mRNA isolated from whole head ( Figure S2C). Expression of Wnt10b, another skin-specific Wnt, was not altered ( Figure 1D), indicating that persistent activation of the Wnt signaling pathway in the skin leads to specific upregulation of Wnt6 expression. Beyond the skin and calvarial defects, we found that in the Msx2-Cre;Ctnnb1lox(ex3) mice, the main cortical commissural pathway, the corpus callosum, failed to form ( Figure 1G). However, in the same mutants, other commissural pathways, including the anterior commissure and hippocampal commissures, were still formed (although the hippocampal commissure was slightly

smaller in size than normal) ( Figure 1G). At E17.5, a day before the mutant embryos die, it was BMS-354825 cost apparent that the callosal axons stopped at the cortical midline, rather than crossing formed Probst bundles (asterisk in Figure 1G), which are aberrant axonal tracts made up of callosal axons that fail to cross the midline ( Paul et al., 2007). These callosal defects were also observed in horizontal sections

of mutant animals ( Figure S1A) and showed full penetrance from 14 mutant embryos that we analyzed. Because the failure of corpus callosum formation is a dramatic midline structural defect, we wondered how excess Wnt signaling in the dorsal skin might cause this phenotype. There have also been numerous studies showing strain differences in the appearance of all corpus callosum defects in mice with some strains (e.g., 129 and Balb/c); however, our colonies of Msx2-Cre and Ctnnb1lox(ex3) mice have been extensively crossed into the CD-1 background, not noted for defects in the corpus callosum. One possible cause of agenesis of the corpus callosum could be defects in the development of the cortical projection neurons. This phenotype has often been observed in mutant animals for the transcription factors governing maturation of the cortical callosal neurons that comprise layer II/III (Alcamo et al., 2008, Armentano et al., 2006, Britanova et al., 2008, Molyneaux et al., 2007, Paul et al., 2007, Piper et al., 2009 and Shu et al., 2003).