Mouse preimplantation blastocysts adhere to cells expressing the transmembrane form of heparin-binding EGF-like growth factor

Abstract

Previous studies have shown that heparin-binding epidermal growth factor (EGF)-like growth factor (HB-EGF) mRNA is synthesized in the mouse uterine luminal epithelium, temporally, just prior to implantation, and spatially, only at the site of blastocyst apposition (Das, S. K., Wang, X. N., Paria, B. C., Damm, D., Abraham, J. A., Klagsbrun, M., Andrews, G. K. and Dey, S. K. (1994) Development 120, 1071–1083). HB-EGF is synthesized as a transmembrane protein (HB-EGFTM) that can be processed to release the soluble growth factor. An antibody that cross-reacts only with the transmembrane form detected HB-EGFTM in uterine luminal epithelium in a spatial manner similar to that of HB-EGF mRNA. HB-EGFTM is a juxtacrine growth factor that mediates cell-cell contact. To ascertain if HB-EGFTM could be an adhesion factor for blastocysts, a mouse cell line synthesizing human HB-EGFTM was co-cultured with mouse blastocysts. Cells synthesizing HB-EGFTM adhered to day-4 mouse blasto-cysts more extensively than parental cells or cells synthesizing a constituitively secreted form of HB-EGF. Adhesion of cells synthesizing HB-EGFTM to blastocysts was inhibited by excess recombinant HB-EGF but less so by TGF-α. Adhesion was also inhibited by the synthetic peptide P21 corresponding to the HB-EGF heparin binding domain, and by incubating the blastocysts with heparinase. In addition, adhesion to delayed implanting dormant blastocysts, which lack EGF receptor (EGFR), was diminished relative to normal blastocysts. These results suggested that adhesion between blastocysts and cells synthesizing HB-EGFTM was mediated via interactions with both blastocyst EGFR and heparan sulfate proteoglycan (HSPG). It was concluded that HB-EGFTM, which is synthesized exclusively in the luminal epithelium at the site of blastocyst apposition, and which is a juxtacrine adhesion factor for blastocysts, could be one of the mediators of blastocyst adhesion to the uterus in the process of implantation.