Changes in the Pattern of Adherens Junction-Associated β-Catenin Accompany Morphogenesis in the Sea Urchin Embryo

Abstract

β-Catenin was originally identified biochemically as a protein that binds E-cadherin in cultured cells and that interaction was later shown to be essential for cadherin function. Independently,armadillo,the β-catenin homolog inDrosophila melanogaster,was identified as a segment polarity gene necessary for the transduction ofwingless (Wnt)signals during embryonic and larval development. Recently, several investigations have also shown that β-catenin plays a critical role in axial patterning of earlyXenopus,zebrafish, and mouse embryos. In these systems, the localization of β-catenin to the plasma membrane, cytosol, and nucleus is predictive of its role in cell adhesion and signaling. In order to examine the potential role of β-catenin in regulating cell adhesion during embryogenesis, we cloned β-catenin in the sea urchinLytechinus variegatusand characterized its subcellular distribution in cells undergoing morphogenetic movements. Indicative of a role in the establishment and maintenance of cell adhesion, β-catenin is associated with lateral cell–cell contacts and accumulates at adherens junctions from cleavage stages onward. At gastrulation, changes in junctional β-catenin localization accompany several morphogenetic events. The epithelial–mesenchymal conversion that characterizes the ingression of both primary and secondary mesenchyme cells coincides with a rapid and dramatic loss of adherens junction-associated β-catenin. In addition, epithelial cells in the archenteron display a significant decrease in adherens junction-associated β-catenin levels as they undergo convergent–extension movements. These data are consistent with a role for β-catenin in regulating cell adhesion and adherens junction function during gastrulation in the sea urchin embryo.