The homeobox gene bozozok promotes anterior neuroectoderm formation in zebrafish through negative regulation of BMP2/4 and Wnt pathways

The neuroectoderm of the vertebrate gastrula was proposed by Nieuwkoop to b e regionalized into forebrain, midbrain, hindbrain and spinal cord by a two -step process. In the activation step, the Spemann gastrula organizer induc es neuroectoderm with anterior character, followed by posteriorization by a transforming signal, Recently, simultaneous inhibition of BMP and Wnt sign aling was shown to induce head formation in frog embryos. However, how the inhibition of BMP and Wnt signaling pathways specify a properly patterned h ead, and how they are regulated in vivo, is not understood. Here we demonst rate that the loss of anterior neural fates observed in zebrafish bozozok ( boz) mutants occurs during gastrulation due to a reduction and subsequent p osteriorization of neuroectoderm. The neural induction defect was correlate d with decreased chordino expression and consequent increases in bmp2b/4 ex pression, and was suppressed by overexpression of BMP antagonists, Whereas expression of anterior neural markers was restored by ectopic BMP inhibitio n in early boz gastrulae, it was not maintained during later gastrulation, The posteriorization of neuroectoderm in boz was correlated with ectopic do rsal wnt8 expression, Overexpression of a Wnt antagonist rescued formation of the organizer and anterior neural fates in boz mutants. We propose that boz specifies formation of anterior neuroectoderm by regulating BMP and Wnt pathways in a fashion consistent with Nieuwkoop's two-step neural patterni ng model, boz promotes neural induction by positively regulating organizer- derived chordino and limiting the antineuralizing activity of BMP2b/4 morph ogens. In addition, by negative regulation of Wnt signaling, boz promotes o rganizer formation and limits posteriorization of neuroectoderm in the late gastrula.