OR09-05 GPBAR1- Understanding Its Function In The Secretory Endometrium
Authors: Saad-Naguib Michael , Timmins Patrick , George Lea , Samuels Cherie-Ann , Douglas Nataki , Babwah Andy Vi 
Published: October 2023

Abstract
Disclosure: M. Saad-Naguib: None. P. Timmins: None. L. George: None. C. Samuels: None. N. Douglas: None. A.V. Babwah: None.
Every successful pregnancy relies on implantation of an embryo into a receptive uterine endometrium. While implantation failure accounts for 50% of pregnancy failures, it remains the least understood aspect of pregnancy. In the mid-secretory (MS) phase of the menstrual cycle, under the influence of progesterone and cAMP, fibroblasts decidualize, a critical requirement for implantation. Decidualization involves the differentiation of stromal fibroblasts into secretory epithelioid cells that contribute to the formation of the decidua, the support of the pregnancy prior to placentation. A defective decidua can lead to implantation failure, but if implantation occurs, it could compromise placenta formation and function. Several studies have demonstrated the importance of G protein-coupled receptors (GPCRs) in mouse endometrial receptivity and decidualization. GPCRs are transmembrane signaling molecules that are ubiquitously expressed in the body and regulate almost every cellular and physiological process. They are major pharmaceutical targets, with 35% of all the drugs approved by the US FDA targeting GPCRs. To identify novel GPCR regulators of embryo implantation, we conducted bulk RNA sequencing of the human endometrium at the periovulatory (PO) and mid-secretory (MS) phase in natural menstrual cycles in healthy women of reproductive age and observed that genes encoding 11 GPCRs, not previously reported to regulate implantation, were differentially expressed between these two time points. Among these was the gene encoding the G protein-coupled bile acid receptor 1 (GPBAR1), and this was upregulated 4.6-fold in the MS vs PO phase. Single cell RNA sequence analysis showed that within the human endometrium, GPBAR1 mRNA expression is mostly and strongly localized to stromal fibroblasts. Upon activation by secondary bile acids, GPBAR1 couples to Gαs leading to an increase in intracellular cAMP levels. We hypothesize that in the MS endometrium, upon ligand binding, GPBAR1 produces cAMP and induces human endometrial stromal cell (HESC) decidualization. This hypothesis is being tested in vitro using primary HESCs. Interestingly, in the pregnant mouse, GPBAR1 mRNA is barely expressed at the implantation site on E7.5, and therefore unlikely plays a role in stromal cell decidualization in the mouse. Whether Gpbar1 is expressed in the pregnant mouse endometrium prior to implantation and plays a role in regulating endometrial receptivity is being investigated. Overall, our studies have uncovered a GPCR, that to our knowledge, has not been previously described in the human endometrium. Our data show that the expression of this GPCR is upregulated at the time of endometrial stromal cell decidualization, and therefore it might regulate decidualization and/or be required for decidual function in women. These possibilities are being examined.
Presentation: Friday, June 16, 2023