Evidence of signaling and adhesion roles for β-catenin in the sponge Ephydatia muelleri.

beta-Catenin acts as a transcriptional coactivator in the Wnt/beta-catenin signaling pathway and a cytoplasmic effector in cadherin-based cell adhesion. These functions are ancient within animals, but the earliest steps in beta-catenin evolution remain unresolved due to limited data from key lineages-sponges, ctenophores, and placozoans. Previous studies in sponges have characterized beta-catenin expression dynamics and used GSK3B antagonists to ectopically activate the Wnt/beta-catenin pathway; both approaches rely upon untested assumptions about the conservation of beta-catenin function and regulation in sponges. Here, we test these assumptions using an antibody raised against beta-catenin from the sponge Ephydatia muelleri. We find that cadherin-complex genes coprecipitate with endogenous Em beta-catenin from cell lysates, but that Wnt pathway components do not. However, through immunostaining we detect both cell boundary and nuclear populations, and we find evidence that Em beta-catenin is a conserved substrate of GSK3B. Collectively, these data support conserved roles for Em beta-catenin in both cell adhesion and Wnt signaling. Additionally, we find evidence for an Em beta-catenin population associated with the distal ends of F-actin stress fibers in apparent cell-substrate adhesion structures that resemble focal adhesions. This finding suggests a fundamental difference in the adhesion properties of sponge tissues relative to other animals, in which the adhesion functions of beta-catenin are typically restricted to cell-cell adhesions.