(202) In Vivo Function of Uterine Epithelial Progesterone Receptor in Regulating Uterine Fluid Movement During Early Pregnancy

Taylor Elijah Martin^1,2; Yuehuan Li²; Jonathan Matthew Hancock^1,2; Jackson Sundgren²; Xiaoqin Ye^1,2

1. Interdisciplinary Toxicology Program, The University of Georgia, Athens, GA, USA

2. Department of Physiology and Pharmacology, College of Veterinary Medicine, The University of Georgia, Athens, GA, USA

Abstract Text:

Uterine fluid absorption during early pregnancy facilitates uterine lumen closure, which enables intimate contact of the implanting embryo with the uterine luminal epithelium (LE) to initiate embryo implantation. Studies from ovariectomized rodent models revealed that ovarian hormones estrogen (E2) and progesterone (P4) induce uterine fluid secretion accumulation and absorption reduction, respectively, primarily through the uterine epithelium. P4 is primarily synthesized from the developing corpus luteum during early pregnancy and its increasing levels inversely correlate with uterine fluid volume during the preimplantation period. The uterine functions of P4 are primarily mediated through progesterone receptor (PR, encoded by Pgr).

Dysregulated uterine fluid volume has been associated with impaired early pregnancy both in animal models, as well as in women undergoing in vitro fertilization-embryo transfer. Previous studies have revealed correlations between pathologies such as hydrosalpinx, endometriosis, and polycystic ovary syndrome, and uterine fluid accumulation within the endometrium. Furthermore, such conditions have been correlated to P4 resistance, allowing a potential link between P4 resistance in these conditions to cases of uterine fluid accumulation.

Despite the known function of P4 in uterine fluid absorption, the mechanisms by which this occurs during early pregnancy remain largely unknown. Bulk uterine fluid absorption is expected to occur through ion channels, such as epithelial Na⁺ channel (ENaC), and aquaporins. In the uterine fluid, the dominant electrolyte is Na⁺. The transport of Na⁺ from the uterine fluid to the uterine epithelium by ENaC and potentially other sodium channels generates an osmotic gradient, which facilitates bulk fluid absorption through the uterine epithelium. Na⁺/K⁺ ATPases then facilitate the movement of sodium ions across the basal membrane of LE cells into the underlying stromal layer for gradient continuation. Calcium-activated chloride channel regulator 1 (CLCA1, encoded by Clca1) is a secreted protein that modulates activation of chloride channels involved in epithelial cell fluid secretion and mucin production.

Our lab developed a novel method using Alexa Hydrazide 488 (AH) to visualize uterine fluid absorption and demonstrated reduction of bulk uterine fluid absorption via LE from day post-coitum 0.5 (D0.5) @ 11 h to D3.5 @ 11 h. Our recent novel finding shows that bulk absorption is high on D3.5 @ 17 h, just before implantation initiation ~D4.0. In the epiPR^-/- (Pgr^f/-Wnt7a^Cre/+) mouse model with conditional deletion of PR in the uterine epithelium, there is reduced uterine fluid bulk absorption in the LE compared to that in the control Pgr^f/- LE on D3.5 @ 11 h and D3.5 @ 17 h, which is consistent with the enlarged epiPR^-/- uterine lumen. No obvious difference in uterine fluid bulk absorption between epiPR^-/- LE and Pgr^f/- LE has been observed on D0.5 @ 11 h. Preliminary immunohistochemistry data show decreased expression of AQP5 and ATP1A1 on D3.5 @ 17 h and increased expression of CLCA1 on D3.5 @ 17 h in epiPR^-/- LE compared to Pgr^f/-. These results suggest that uterine epithelial PR regulates the expression of channels in uterine fluid movement during early pregnancy. To obtain a comprehensive picture of the genes regulated by PR in the LE, we plan to perform mRNA-seq, which will provide novel insights into the intricate P4-PR signaling mechanisms regulating uterine fluid trafficking and other uterine functions during early pregnancy.