Expression of Alpha-2-Beta-1 and Claudin-4 Protein at the Uterine-Placental Interface Suggests Roles in the Transport of Proteins and Nutrients from the Mother to Fetus in Pigs.

Abstract

In pigs, endometrial functions are regulated primarily by progesterone (P4) and placental factors including estrogen. The high P4 levels throughout pregnancy maintain secretion of histotroph from uterine epithelia, necessary for placentation and conceptus development. The apical domain of endometrial luminal epithelium (LE) mediates the complex between the uterus and trophoblast/chorion throughout pregnancy in the epitheliochorial placenta of pigs. LE cells also interact with adjacent LE cells and the underlying basement membrane through their basolateral domains. Multiple adhesion molecules and junctional complexes attach adjacent membranes and regulate the paracellular transport of molecules into the lumen. We previously reported that long-term P4 treatment altered expression of α2β1, a collagen and laminin integrin receptor normally found at the basal surface of epithelial cells, and claudin-4, a tight junction component that defines charge selectivity of tight junctions normally found at the apical junctional complex of epithelia, such that these proteins were highly expressed at the lateral borders between adjacent LE cells. The present study was conducted to determine whether similar changes occur during normal pregnancy. Gilts were hysterectomized on Day 9, 12, or 15 of the estrous cycle, or Day 10, 12, 15, 20, 25, 30, 35, 40, 50, 60, or 85-90 of pregnancy. α2β1 and claudin-4 proteins were localized in frozen sections of the uterine-placental interface by immunofluorescence. Endometrial expression of α2β1 was similar for the estrous cycle and pregnancy. Low levels of α2β1 protein were detected at the basal surface of Day 10 LE, but expression expanded to the lateral surfaces and was induced in GE only between Day 15 and 20. However, LE expression remained high through Day 90 except at the tips of the uterine folds extending into placental tissue. This may represent sites of nutrient and protein transport between maternal and fetal tissues, since expression of the glucose transporter, SLC2A2, was detected in the same LE cells. α2β1 was detected at moderate levels in trophoblast by Day 25, increased in chorion by Day 35, and was maintained for the duration of gestation. Similar to α2β1, no differences in claudin-4 expression were detected between the estrous cycle and pregnancy. Claudin-4 was present in the tight junctions of both LE and GE at Day 10, then expression expanded to the entire basolateral surfaces of both epithelia by Day 15. Claudin-4 levels remained high in LE and GE through Day 20 of gestation, but became limited to the tight junctions of LE and were not detectable in GE by Day 25. This expression pattern was maintained through Day 85 of pregnancy. Claudin-4 was also present in tight junctions of the trophoblast and chorion throughout pregnancy, except in chorionic epithelia that form areolae to absorb secretions from uterine glands. In areolae, claudin-4 was distributed along the entire basolateral surfaces of chorionic cells. Collectively, these results indicate that α2β1 and claudin-4 are prominently expressed in the endometrial and placental epithelia of the entire uterine-placental interface of pigs. Temporal and spatial changes in the distribution of these proteins along the basolateral surfaces of discrete epithelial cell populations suggest roles in the transport of proteins and nutrients from the mother to fetus. (poster)