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

Abstract

{beta}-catenin acts as a transcriptional co-activator 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 co-precipitate 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.