The Increase in Maternal Expression of <i>axin1</i> and <i>axin2</i> Contribute to the Zebrafish Mutant <i>Ichabod</i> Ventralized Phenotype

Abstract

β-Catenin is a central effector of the Wnt pathway and one of the players in Ca(+)-dependent cell-cell adhesion. While many wnts are present and expressed in vertebrates, only one β-catenin exists in the majority of the organisms. One intriguing exception is zebrafish that carries two genes for β-catenin. The maternal recessive mutation ichabod presents very low levels of β-catenin2 that in turn affects dorsal axis formation, suggesting that β-catenin1 is incapable to compensate for β-catenin2 loss and raising the question of whether these two β-catenins may have differential roles during early axis specification. Here we identify a specific antibody that can discriminate selectively for β-catenin1. By confocal co-immunofluorescent analysis and low concentration gain-of-function experiments, we show that β-catenin1 and 2 behave in similar modes in dorsal axis induction and cellular localization. Surprisingly, we also found that in the ich embryo the mRNAs of the components of β-catenin regulatory pathway, including β-catenin1, are more abundant than in the Wt embryo. Increased levels of β-catenin1 are found at the membrane level but not in the nuclei till high stage. Finally, we present evidence that β-catenin1 cannot revert the ich phenotype because it may be under the control of a GSK3β-independent mechanism that required Axin's RGS domain function.