Abstract: Tissue non-specific alkaline phosphatase (TNSALP) regulates postnatal phosphate homeostasis, but its role in utero-placental phosphate availability remains poorly understood. Gilts were bred and hysterectomized on Day 60 or Day 90 of gestation (n = 6/day). Phosphate was less abundant in allantoic and amniotic fluids on Day 90 compared to Day 60. TNSALP protein was immunolocalized, and enzymatic activity was quantified and localized in endometrial and chorioallantois tissues. Day had no effect on TNSALP activity in the chorioallantois. In contrast, endometrial TNSALP activity was lower on Day 90 compared to Day 60. Phosphate abundance in allantoic fluid correlated positively with endometrial TNSALP activity on Day 60 but not Day 90. TNSALP protein was abundantly expressed in the endometrium and chorioallantois on both days investigated, with localization to the endometrial, chorionic, and areolar epithelia, as well as stromal cells and endothelium. TNSALP activity was detected in the endothelium of the blood vessels in both the endometrium and chorioallantois, and on the basal surface of the endometrial glands on Day 60 but not Day 90. The endometrial stratum compactum stroma had strong TNSALP activity on Day 60. Weak TNSALP activity was present in the areolar epithelium, with a modest increase in activity on Day 90 compared to Day 60. TNSALP activity was present in the columnar chorionic epithelial cells, with an apparent decrease in activity in the chorioallantois on Day 90 compared to Day 60. These data reveal spatiotemporal changes in TNSALP localization and activity, suggesting its involvement in regulating phosphate availability at the utero-placental interface in swine.
Lay summary: Phosphate is an essential nutrient for fetal growth, but how it is managed during pregnancy is not fully understood. This study explored the role of an enzyme called tissue non-specific alkaline phosphatase (TNSALP) in regulating phosphate availability in the uterus and placenta in pigs in mid- and late pregnancy. Phosphate levels decreased in the fluids surrounding the fetus in late pregnancy. TNSALP was present in the uterus and placenta, and the amount of the enzyme varied depending on the tissue and stage of pregnancy and correlated with changes in phosphate levels. These findings suggest that TNSALP plays a key role in managing phosphate transport from the mother to the fetus in pregnancy to support fetal development.
Keywords: alkaline phosphatase; phosphate; placenta; porcine; pregnancy; uterus.