EXPRESSION OF SYNDECAN-1 CHANGES DURING THE DIFFERENTIATION OF VISCERAL AND PARIETAL ENDODERM FROM MURINE F9 TERATOCAR CINOMA CELLS

F9 teratocarcinoma stem cells treated with retinoic acid differentiate in suspension into embryoid bodies with an outer layer of visceral en doderm surrounding a core of largely undifferentiated cells. The visce ral endoderm-containing embryoid bodies, when plated onto an extracell ular matrix coating, give rise to parietal endoderm outgrowth. These i n vitro cell cultures mimic both geometrically and biochemically the d ifferentiation of visceral and parietal endoderm in the early mouse em bryo and, thus, were used as a model system for the study of molecular and cellular mechanisms underlying the differentiation of the extraem bryonic endoderm lineages. We have investigated the expression of synd ecan-1, an integral membrane proteoglycan that binds to multiple compo nents of the extracellular matrix and basic FGF, during visceral endod erm differentiation and parietal endoderm outgrowth. Syndecan-1 immuno staining is detected on ail cell surfaces in the undifferentiated embr yoid bodies and in the differentiating embryoid bodies prior to the fo rmation of the visceral endoderm. Following the differentiation of vis ceral endoderm, syndecan-1 localizes predominantly to the basal surfac e of this epithelial layer, while syndecan-1 staining in the core of d ifferentiated embryoid bodies is faint. Quantitation of cell associate d syndecan-1 indicates that syndecan-1 is down-regulated during embryo id body differentiation. However, northern analysis shows that the amo unts of steady-state syndecan-1 mRNA are the same in undifferentiated versus differentiated embryoid bodies, suggesting post-transcriptional regulation of syndecan-1 expression in the differentiating embryoid b ody. Analysis of syndecan-1 distribution in the outgrowth culture by i mmunofluorescence demonstrates that syndecan-1 is absent from the cell surface of parietal endoderm. However, a substantial amount of syndec an-1 is detected inside parietal endoderm cells. While all three cell types release syndecan-1 ectodomain into the culture medium, the parie tal endoderm outgrowth releases more syndecan-1 ectodomain than the di fferentiated embryoid body. These data suggest that the post-transcrip tional control and post-translational shedding of syndecan-1 from the cell surface are developmentally regulated during the differentiation of visceral to parietal endoderm and the migration of parietal endoder m.