A mutation in the Gsk3-binding domain of zebrafish Masterblind/Axin1 leadsto a fate transformation of telencephalon and eyes to diencephalon

Zebrafish embryos homozygous for the masterblind (mb1) mutation exhibit a s triking phenotype in which the eyes and telencephalon are reduced or absent and diencephalic fates expand to the front of the brain. Here we show that mb1(-/-) embryos carry an amino-acid change at a conserved site in the Wnt pathway scaffolding protein, Axin1. The amino-acid substitution present in the mbl allele abolishes the binding of Axin to Gsk3 and affects Tcf-depen dent transcription. Therefore, Gsk3 activity may be decreased in mbl(-/-) e mbryos and in support of this possibility, overexpression of either wild-ty pe Axin1 or Gsk3 beta can restore eye and telencephalic fates to mb1(-/-) e mbryos. Our data reveal a crucial role for Axin1-dependent inhibition of th e Wnt pathway in the early regional subdivision of the anterior neural plat e into telencephalic, diencephalic, and eye-forming territories.