Developmental Expression of Echinonectin, an Endogenous Lectin of the Sea Urchin Embryo

Echinonectin (EN) is a galactose-binding lectin present in eggs and embryos of the sea urchin Lytechinus variegatus. Recent studies have suggested that EN is a hyaline layer protein that may function as a substrate adhesion molecule (SAM) during development. We have used monoclonal and affinity-purified polyclonal antibodies that specifically recognize this protein to determine its spatial and temporal expression during embryogenesis. EN is stored in granules or vesicles in the unfertilized egg. After fertilization, these granules are rapidly redistributed to the apical cytoplasm of the zygote. Our results show that at subsequent stages of development the lectin is expressed by cells of all three germ layers, including cells of the developing gut, coelomic pouches, and ectoderm, and by both primary and secondary mesenchyme cells. In contrast to previous observations based solely upon light level immunofluorescent staining, immunoelectron microscopy demonstrates that EN is localized in intracellular, membrane-bounded vesicles. In epithelial cell types these vesicles have a highly polarized distribution and are found in the apical cortical cytoplasm. In mesenchyme cells the distribution of EN-containing vesicles is not obviously polarized. Steady-state levels of EN protein in the embryo remain almost constant from fertilization to the pluteus larva stage, Metabolic labeling studies show that synthesis of EN in L. variegatus begins immediately after fertilization and continues throughout embryogenesis. Monospecific antibodies raised against L. variegatus EN have also been used to determine whether this lectin is expressed in other echinoid species.