Expression cloning of an ascidian syndecan suggests its role in embryonic cell adhesion and morphogenesis

Expression cloning of maternally expressed genes of the ascidian Ciona savignyi demonstrated that the overexpression of syndecan, a member of a multigene family of integral membrane heparan sulfate proteoglycans, resulted in a disturbance of cell adhesion and morphogenesis. The Ciona syndecan gene was expressed both maternally and zygotically. The maternal transcript was distributed evenly in fertilized eggs and early embryos up to the 32-cell stage without any special localization and then became barely detectable in the 64-cell and gastrula stages. The zygotic transcription became evident during neurulation, mainly in cells of epidermis, the central nervous system, and mesenchyme. Embryos with syndecan overexpression via RNA injection cleaved as did normal embryos, but showed loose blastomere adhesion after the 32-cell stage. Gastrulation occurred, but the closure of the blastopore was markedly delayed, resulting in larvae without normal morphology. About half of the syndecan-overexpressing embryos hatched, and differentiation of epidermis, endoderm, muscle, and notochord was evident. However, the formation of pigment cells of the sensory organs was markedly disturbed. These results indicate that an appropriate level of syndecan expression is required for normal cell adhesion and morphogenesis of the ascidian embryo.