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Evidence of Signaling and Adhesion Roles for &bgr;‐Catenin in the Sponge Ephydatia muelleri

Abstract &bgr;‐Catenin acts as a transcriptional coactivator in the Wnt/&bgr;‐catenin signaling pathway and a cytoplasmic effector in cadherin‐based cell adhesion. These functions are ancient within animals, but the earliest steps in &bgr;‐catenin evolution remain unresolved due to limited data from key lineages—sponges, ctenophores, and placozoans. Previous studies in sponges have characterized &bgr;‐catenin expression dynamics and used GSK3B antagonists to ectopically activate the Wnt/&bgr;‐catenin pathway; both approaches rely upon untested assumptions about the conservation of &bgr;‐catenin function and regulation in sponges. Here, we test these assumptions using an antibody raised against &bgr;‐catenin from the sponge Ephydatia muelleri. We find that cadherin‐complex genes coprecipitate with endogenous Em &bgr;‐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 &bgr;‐catenin is a conserved substrate of GSK3B. Collectively, these data support conserved roles for Em &bgr;‐catenin in both cell adhesion and Wnt signaling. Additionally, we find evidence for an Em &bgr;‐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 &bgr;‐catenin are typically restricted to cell‐cell adhesions.