Ovine osteopontin: I. Cloning and expression of messenger ribonucleic acidin the uterus during the periimplantation period

Trophoblast-derived interferon tau (IFN tau) acts on the endometrium to inc rease secretion of several proteins during the pregnancy recognition period in ruminants. One of these is a 70-kDa acidic protein that has not been id entified. Our hypothesis was that the 70-kDa acidic protein is osteopontin (OPN). OPN is an acidic glycoprotein that fragments upon freezing and thawi ng or treatment with proteases including thrombin. OPN contains a Gly-Arg-G ly-Asp-Ser (GRGDS) sequence that binds to cell surface integrins to promote cell-cell attachment and cell spreading. Using antisera to recombinant hum an OPN, both 70-kDa and 45-kDa proteins were identified in uterine flushing s from pregnant ewes by Western blotting. A clone containing the entire ovi ne OPN cDNA coding sequence was isolated by screening a Day 15 pregnant ovi ne endometrial cDNA library with a partial ovine OPN cDNA. In pregnant ewes , steady-state levels of OPN endometrial mRNA increased (P < 0.01) after Da y 17. In both cyclic and pregnant ewes, in situ hybridization analysis show ed that OPN mRNA was localized on unidentified immune cells within the stra tum compactum of the endometrium. In pregnant ewes, OPN mRNA was also expre ssed by the glandular epithelium. Results suggest that progesterone and/or IFN tau induce expression and secretion of OPN by uterine glands during the periimplantation period and that OPN may induce adhesion between luminal e pithelium and trophectoderm to facilitate superficial implantation.