Transient, localized accumulation of alpha-spectrin during sea urchin morphogenesis

The mRNA and protein of alpha-spectrin in the sea urchin embryo is shown here to be transiently overexpressed in cells initiating certain morphogenetic changes. This expression was detected by use of a 4.5-kb cDNA clone that encodes alpha-spectrin isolated from a Lytechinus variegatus cDNA library. DNA sequence analysis demonstrated 80% similarity of this cDNA to human nonerythroid alpha-spectrin. Sea urchin alpha-spectrin RNA accumulated to a uniform basal level in all cells of the embryo with a marked, transient increased accumulation in the primary mesenchyme cells of the vegetal plate just prior to gastrulation. Cells of the endoderm also express increased levels of spectrin during gastrulation and these levels remained high in the myoepithelial cells of the sphincter constrictions that separates the gut into foregut, midgut, and hindgut domains. Antibodies made to recombinant sea urchin spectrin synthesized in Escherichia coli were used to localize the protein in situ. These experiments showed that all cells of the embryo accumulated alpha-spectrin protein confined to the cortical regions of cell-cell contact. In epithelial cells, this localization showed a distinct honeycomb pattern. Primary mesenchyme cells and the myoepithelial cells of the gut sphincters showed significantly enhanced spectrin signal corresponding to the pronounced RNA localization pattern seen by in situ RNA hybridization. These data demonstrated that alpha-spectrin from the sea urchin is a highly conserved member of the spectrin superfamily of proteins and that its elevated expression in specific cell types anticipates overt morphogenesis.