Dual repression modes triggered by constitutively stabilized beta-catenin in the dorsal neural tube underlie cell fate alterations of the roof plate, neural crest, and dorsal interneurons

The roof plate that covers the dorsal neural tube is a key organizer or signaling center that generates morphogenetic signaling proteins to regulate neural crest cell production and dorsal CNS patterning. However, knowledge remains limited regarding the molecular mechanism regulating roof plate cell fate and function. Loss of function of Wnt/ß-catenin signaling in the roof plate or dorsal neural folds can prevent neural tube closure, neural crest cell production, and dorsal CNS patterning. In this study, we carried out conditional activation of ß-catenin in the dorsal neural folds of mice and found dramatic cell fate alterations, which include anteriorly expressed Cdx2 of the caudal-type homeobox gene in the dorsal neural tube, swapped expression patterns of the roof plate markers Bmp6 and Lmx1a with their respective homologues Bmp4 and Lmx1b, ectopically expressed Msx1 of the neural crest inducer in the migratory stream and dorsal root ganglia, and completely abolished expression of the neural crest cell differentiation marker Sox10 and the dorsal spinal interneuron precursor marker Atoh1. Our in vitro studies using a neural crest cell line demonstrate that the full-length ß-catenin, not the signaling defective ß-catenin significantly upregulates Msx1 and represses Sox10 and Atoh1 expression, which can be rescued by Msx1 knockdown, supporting an indirectly repressive role of the stabilized ß-catenin acting through Msx1 on Sox10 and Atoh1 expression. Intriguingly, Wnt1 and Wnt3a are drastically abolished in the roof plate of the ß-catenin stabilized embryos. However, Msx1 knockdown does not upregulate or rescue Wnt1 and Wnt3a expression when they are repressed by ß-catenin in vitro. Further in vitro studies reveal a direct repression of Wnt1 and Wnt3a by ß-catenin transcriptional complex, implicating a novel negative feedback loop of Wnt/ß-catenin signaling pathway at the ligand level. Together, these results suggest that constitutively stabilized ß-catenin triggers dual repression modes to prevent specification and differentiation of multiple cell lineages derived from the dorsal neural tube, but it may not prevent the presumptively undifferentiated Msx1-expressing neural crest cell precursors migrating out the dorsal neural tube to form the dorsal root ganglia, and that a finely tuned Wnt/ß-catenin signaling level is crucial for proper cell fate determination and functional modulation of the dorsal neural tube and its derivatives.