Stage-specific alterations in the apical membrane glycoproteins of endometrial epithelial cells related to implantation in rabbits.

The rabbit endometrial epithelium undergoes differentiation prior to the time of blastocyst implantation, including loss of surface negativity and a change in glycocalyx morphology. Nonpregnant (estrous) and pseudopregnant rabbits were used to study specific alterations in proteins and saccharide composition of the luminal epithelial membrane and its glycocalyx related to the acquisition of receptivity to implantation. Pregnant animals were used to study further modification of the luminal surface by implanting blastocysts. The apical surface of luminal epithelial cells was solubilized by a 15-min intraluminal incubation of 1% Triton X-100 containing protease inhibitors. Proteins in extract solutions were separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Three new polypeptides (24 kDa, 42 kDa and 58 kDa) were identified in uteri from receptive rabbits. Binding of succinyl Wheat Germ Agglutinin (sWGA) and Ricinus communis Agglutinin (RCA-I) lectins to the 24 kDa and 42 kDa components on Western blots of extracts separated by SDS-PAGE identified them as glycoproteins. Additionally, other polypeptides (26 kDa, 80-86 kDa and 145 kDa) showed changes in affinity for WGA, RCA-I or concanavalin A (Con A), depending on the hormonal state. Correlating with these findings was an increased binding of these lectins to intact nonciliated cells in uteri of receptive rabbits compared to estrous animals; ciliated cells bound Dolichos biflorus Agglutinin (DBA) specifically, regardless of the hormonal condition. Treatment of uteri from estrous animals, or Western blots of proteins from these animals, with neuraminidase prior to lectin exposure suggested the presence of glycoproteins having a sialic acid-D-galactose terminus in nonreceptive rabbits. Reduced binding of lectin to intact cells at implantation sites and to blots of proteins isolated from these sites, compared to nonimplantation sites, was noted. These results provide evidence for stage-specific alterations in protein and saccharide composition of the apical surface of endometrial epithelium prior to implantation, and indicate that implanting blastocysts further modify the luminal surface.