ABSTRACT
Adenomatous polyposis coli (APC) regulates the activity of ß-catenin, an integral component of Wnt signaling. However, the selective role of the APC-ß-catenin pathway in cerebral cortical development is unknown. Here we genetically dissected the relative contributions of APC-regulated ß-catenin signaling in cortical progenitor development, a necessary early step in cerebral cortical formation. Radial progenitor-specific inactivation of the APC-ß-catenin pathway indicates that the maintenance of appropriate ß-catenin-mediated Wnt tone is necessary for the orderly differentiation of cortical progenitors and the resultant formation of the cerebral cortex. APC deletion deregulates ß-catenin, leads to high Wnt tone, and disrupts Notch1 signaling and primary cilium maintenance necessary for radial progenitor functions. ß-Catenin deregulation directly disrupts cilium maintenance and signaling via Tulp3, essential for intraflagellar transport of ciliary signaling receptors. Surprisingly, deletion of ß-catenin or inhibition of ß-catenin activity in APC-null progenitors rescues the APC-null phenotype. These results reveal that APC-regulated ß-catenin activity in cortical progenitors sets the appropriate Wnt tone necessary for normal cerebral cortical development.