Identification of the gene coding for the Endo B murine cytokeratin and its methylated, stable inactive state in mouse nonepithelial cells.

The Endo B type-I keratin intermediate filament protein is first expressed at the 4- to 8-cell stage of mouse development. In the adult, its expression is restricted to a variety of simple epithelial cell types. To investigate the mechanisms responsible for the restricted expression of Endo B, the gene coding for Endo B has been identified from among the five different Endo B genes found in the mouse genome by Southern hybridization analysis and cloning all or part of four of the genes. Nuclear run-on experiments demonstrate that Endo B expression is regulated at the level of transcription. The 5' end of the active gene, designated Endo beta 1, was found to be highly methylated and in a relatively nuclease-resistant chromatin conformation in fibroblasts and myoblasts that do not express Endo B, but undermethylated and relatively sensitive to nuclease digestion in endodermal cells or F9 embryonal carcinoma cells. The inactive state of the Endo B beta 1 gene in fibroblast appears to be very stable, because somatic cell hybrids formed by the fusion of HeLa cells, which express the homologous human protein, keratin 18, and mouse fibroblasts, continue to express keratin 18 but do not activate Endo B expression. Similarly, the fusion of mouse endodermal cells and fibroblasts results in hybrids that do not extinguish Endo B expression. These results suggest that Endo B transcription is limited by two different mechanisms. In somatic cells such as fibroblasts or myoblasts, expression may be restricted by methylation and a stable, nonpermissive transcriptional state. However, in embryonal carcinoma cells, the Endo B beta 1 gene is undermethylated and in a relatively nuclease-sensitive conformation, but it is restricted by an additional, negative regulatory mechanism.

Intermediate filament protein synthesis in preimplantation murine embryos.

The synthesis of two extraembryonic endodermal cytoskeletal proteins (Endo B, Mr = 50,000; Endo A, Mr = 55,000) was detected by immunoprecipitation at the 4- to 8-cell stage of preimplantation mouse development. The first detectable synthesis of both proteins occurs at about the same time as the earliest allocation of cells to the trophectodermal lineage. Both Endo A and B were identified in the two-dimensional gel pattern of blastocyst cytoskeletal proteins prepared by nonionic detergent and high-salt extraction. Endo A and B were identified as the y and x blastocyst cytoskeletal proteins, respectively, previously described by other investigators. Antibodies to Endo B are shown to react with intermediate filaments at the electron microscopic level, confirming that Endo B is an authentic intermediate filament protein. Previously, the TROMA 1 monoclonal antibody prepared by other investigators was shown to react specifically with Endo A and to decorate trophoblast cytoskeletons but did not react with the inner cell mass of blastocysts. Endo B antibodies are now also shown to decorate trophoblast cytoskeletons.

New globoseries glycosphingolipids in human teratocarcinoma reactive with the monoclonal antibody directed to a developmentally regulated antigen, stage-specific embryonic antigen 3.

Glycolipids in a cultured human teratocarcinoma cell line (2102Ep) were investigated. The major glycolipids in these cells are globoseries glycolipids having the following structures: (formula; see text) Synthesis of these structures by serial addition of galactose, fucose, and N-acetylneuraminic acid to globoside (Gb4) in this teratocarcinoma is obvious, although further elongation of Gb4 in human cells and tissues has not been previously found with the exception of the presence of a small quantity of Forssman glycolipid in some tissues in the human population (Fs+ group) and in some human cancers. The latter four glycolipids (b-e), with the common internal structure R leads to 3GalNAc beta 1 leads to 3Gal alpha 1 leads to 4R', were all reactive to a monoclonal antibody directed to the 4- to 8-cell stage of murine embryos, known as the stage-specific embryonic antigen 3 (SSEA-3 (Shevinsky, L. H., Knowles, B. B., Damjanov, I., and Solter, D. (1982) Cell 30, 697-705]; structure (c) showed the strongest reactivity. These findings, together with the demonstration of the glycolipid nature of SSEA-1 antigen (Kannagi, R., Nudelman, E., Levery, S. B., and Hakomori, S. (1982) J. Biol. Chem. 257, 14865-14874), indicate that cell surface glycolipids play significant roles as differentiation antigens during the course of embryogenesis.

Monoclonal antibody to murine embryos defines a stage-specific embryonic antigen expressed on mouse embryos and human teratocarcinoma cells.

A murine stage-specific embryonic antigen (SSEA3) is defined by reactivity with a monoclonal antibody prepared by immunization of a rat with 4- to 8-cell-stage mouse embryos. This antigenic determinant, present on oocytes, becomes restricted first to the inner cell mass at the blastocyst stage, and later to the primitive endoderm. Murine teratocarcinoma stem cells do not react with this antibody, whereas human teratocarcinoma stem cells are SSEA3-positive. This antigenic determinant is not expressed on a variety of other human and murine cell lines, but is found on the surface of human erythrocytes. It is a carbohydrate and is present on both cell-surface glycolipids and glycopeptides. These results demonstrate the feasibility of identifying stage-specific antigenic determinants with monoclonal antibody prepared against embryos. The need for thorough screening on a variety of cell types to establish developmentally important cross-reactivities is also emphasized.

Cell-surface antigens of a clonal human embryonal carcinoma cell line: morphological and antigenic differentiation in culture.

A cloned human embryonal carcinoma (EC) cell line has been derived from a testicular teratocarcinoma, and reproducibly forms EC tumors when injected into athymic (nu/nu) mice. These human EC cells are characterized by a newly described stage-specific embryonic antigen, SSEA-3. Unlike murine EC cells, they express major histocompatibility antigens (HLA-A, B, C and beta 2-microglobulin) but do not express the embryonic antigen SSEA-I. We also report that these cells appear to be capable of differentiation and that this can be induced by initiating cultures at low cell density. Differentiation is marked by the appearance of morphologically distinct cells and by the induction of SSEA-I, whereas the expression of other antigens, including SSEA-3, is initially diminished. This well-characterized system of human EC cells provides a model for the future investigation of other human teratocarcinoma cell lines and for the analysis of cellular differentiation during early human development.

A murine stage-specific embryonic antigen (SSEA-2) is expressed on some murine SV40-transformed cells.

A stage-specific embryonic antigen-2 (SSEA-2) found on murine preimplantation embryos is maximally expressed on 4- to 8-cell stage embryos and is present in decreasing amounts of morulae and blastocysts. This antigenic determinant is also expressed on murine teratocarcinoma cells, sperm, and some, but not all, SV40-transformed mouse cell lines. Analysis of solubilized immunoprecipitates by SDS-gel electrophoresis indicates that this cell surface molecule is not the 54,000 m.w. protein shared by teratocarcinoma and SV40-transformed cell lines.