Wnt/β-catenin activity in hindbrain specification. (A) The loss of Wnt/β-catenin activity by either Dkk1 (middle panel) or Wnt3a-MO (right panel) eliminates the expression of the hindbrain inducing gene, Meis3, in Xenopus embryos (Elkouby et al., 2010). (B) This loss is accompanied by the elimination of HoxD1 and Gbx2 hindbrain-promoting homeobox transcription factors, and posterior expansion of the forebrain Otx2 marker in Dkk1-overexpressing Xenopus embryos (right column) (Elkouby et al., 2010). (C) Consequential to these early effects (A, B), posterior neural cell fates are missing from neurula-stage Xenopus embryos knocked down for Wnt3a (right column), which no longer express hindbrain Krox20 and HoxB3 markers and cannot differentiate to primary neuron (N-Tub) or NC (FoxD3). In these embryos, the anterior Anf1 gene is caudally expanded (Elkouby et al., 2010). (D) Chick hindbrain-level neural plate explants do not express Otx2, but express Gbx2 and Krox20 (upper row). When such explants are treated with the Frz8-CRD protein, a secreted inhibitory form of the Frz8 Wnt receptor, they do not express hindbrain Gbx2 and Krox20 markers, and expression of the forebrain Otx2 marker is induced (bottom row). These explants have undergone a hindbrain to forebrain transformation, thus demonstrating the need for Wnt activity in hindbrain development (Nordstrom et al., 2002).