The Role of Endometrial Selectins and Their Ligands on Bovine Conceptus Attachment to the Uterine Epithelium During Peri-Implantation Period1

A successful pregnancy depends on the blastocyst's implantation to the maternal endometrium; however, the initial interaction between blastocyst and uterine epithelium has not been well characterized. The objectives of this study were to determine if selectins and their ligands were expressed in the bovine conceptus and/or uterus during the periattachment period and to study whether selectins were associated with conceptus attachment to the uterine epithelium. Through the RNA-sequence analysis of bovine conceptuses on Days 17, 20, and 22 (Day 0 = day of estrus), only the SELL ligand, podocalyxin (PODXL), and P-selectin (SELP) ligand, SELPLG, were found. Quantitative PCR analysis confirmed the presence of PODXL and SELPLG in these conceptuses and revealed that SELL, mRNA and protein, detected in the uterine epithelium but not in conceptuses increased during the periattachment period. In the cultured endometrial epithelial cells (EECs), SELL transcript was up-regulated when uterine flushings from Day 20 pregnant animals were placed onto these cells. SELL was also up-regulated when cultured EECs were treated with progesterone, EGF, or bFGF, but not with IFNT. In the coculture system with EECs and bovine trophoblast CT-1 cells, SELL expression in EECs was effectively reduced by its small interfering RNA; however, IFNT, a marker for CT-1 cell attachment to EECs, was not reduced, nor was a transcription factor of IFNT, CDX2. These observations suggest that the conceptus could attach to the uterine epithelium through the use of endometrial SELL and embryonic selectin ligands, possibly initiating the conceptus attachment process in the bovine species.