Ovine osteopontin: II. Osteopontin and alpha(v)beta(3) integrin expressionin the uterus and conceptus during the periimplantation period

Osteopontin (OPN) is an acidic 70-kDa glycoprotein that is cleaved by prote ases to yield 45-kDa and 24-kDa fragments. The 70-kDa and 45-kDa proteins c ontain a Gly-Arg-Cly-Asp-Ser (GRGDS) sequence that binds to cell surface in tegrins (primarily alpha(v)beta(3) heterodimer) to promote cell-cell attach ment and cell spreading. A 70-kDa acidic protein was previously detected by two-dimensional (2D) PAGE in Day 17 pregnant endometrial cytosolic extract s using Stainsall and identified as immunoreactive OPN using Western blotti ng. Three forms of immunoreactive OPN proteins (70, 45, and 24 kDa) were de tected by 1D PAGE and Western blot analysis of endometrial extracts. OPN pr otein in endometrial extracts did not differ between cyclic and pregnant ew es. However, the amount of 45-kDa OPN increased in uterine flushings from p regnant ewes between Days 11 and 17. Immunoreactive OPN was localized to lu minal and glandular epithelia of both cyclic and pregnant ewes, and to trop hectoderm of Day 19 conceptuses. The alpha(v) and beta(3) integrins were de tected on Day 19 endometrium and conceptuses by immunofluorescence. It was reported that OPN mRNA increases in the uterine glands of pregnant ewes and secretion of OPN protein into the uterine lumen increases during early pre gnancy. The present results demonstrate accumulation of OPN protein on endo metrial LE and conceptus trophectoderm. Therefore, it is hypothesized that progesterone and/or interferon-tau induce expression, secretion and/or prot eolytic cleavage of OPN by uterine epithelium. Secreted OPN is then availab le as ligand for alpha(v)beta(3) integrin heterodimer on trophectoderm and uterus to 1) stimulate changes in morphology of conceptus trophectoderm and 2) induce adhesion between luminal epithelium and trophectoderm essential for implantation and placentation.