Antibodies to a renal Na+/glucose cotransport system localize to the apical plasma membrane domain of polar mouse embryo blastomeres.

Mouse preimplantation embryos were examined for the cell surface expression of epitopes that cross-react with antibodies to a 75-kDa subunit of a purified porcine renal brush border Na+/glucose cotransport system. A Na+ cotransport system is hypothesized to reside in the apical plasma membrane domain of mouse polar blastomeres and to be associated with the induction of their apical-basal polarity. Western blot analysis showed that unfertilized oocytes as well as preimplantation embryos contain a cross-reacting antigen with an apparent molecular weight of about 75,000. Embryos and their isolated blastomeres were double-labeled and assayed by indirect immunofluorescence (IIF) for the expression of epitopes (visualized by labeling with rabbit antiserum or mouse monoclonal IgG to cotransporter followed by the appropriate rhodamine-conjugated second antibodies) and for the development of cell surface polarity (visualized by the apical restriction of fluoresceinated succinylated concanavalin A binding; FS Con A). IIF did not detect these epitopes until after the second cleavage when 4-cell embryos expressed low-to-moderate levels. Although epitopes were expressed on all surfaces of 4-cell blastomeres, some blastomeres expressed more epitopes on their apical surfaces than on their basolateral ones. All precompaction 8-cell embryos expressed epitopes, with expression being greater apically on some blastomeres. The level of expression appeared to reach a maximum on morulae and to decline on cavitating embryos. Assays performed on isolated blastomeres from postcompaction embryos showed that by the 16-cell stage epitope expression appeared to become restricted to FS Con A-labeled apical plasma membrane domains and was no longer evident on basolateral domains. This apparent apical restriction of epitope expression was confirmed by electron microscopic examination of immunogold-labeled isolated polar 16-cell blastomeres. These results demonstrate that preimplantation mouse embryos contain an antigen(s) that is immunologically and structurally similar to a 75-kDa renal Na+/glucose cotransporter. The onset of cell surface expression of this antigen precedes development of the stable polar phenotype.