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Cadherin Conservation and Localization in Sponge Tissue (Porifera)

Cell adhesion proteins are required to maintain the structural integrity of multicellular organisms. Cadherins are a highly conserved class of transmembrane proteins that form cell-cell adhesions known as adherens junctions. They are characterized by extracellular cadherin repeat domains that bind to other cadherin molecules and a cytoplasmic tail that is complexed with cytoskeletal proteins such as F-actin. Transcriptome sequencing in the sponge Ephydatia muelleri has shown that sponges express two putative classical cadherin homologs – members of the subfamily involved in homophilic adhesion at the adherens junction. Whether cadherin localization or function are conserved in sponge cell-cell junctions is unknown. Immunostaining and confocal microscopy were used to visualize cadherin localization in E. muelleri tissues using peptide antibodies raised against both extracellular and cytoplasmic epitopes of both expressed cadherins. Various fixation methods, as well as deglycosylation and antigen retrieval, were used to optimize antibody staining procedures. None of the antibodies labeled cell-cell junctions, but some showed differential staining patterns, labeling discrete structures (such as flagella) or cell types (migratory stem cells). Because there was no consistency between antibodies raised against different antigens from the same protein, we expect that these staining patterns are non-specific, but they may still be useful as cell-markers in future studies if their binding targets can be identified.