Follistatin is critical for mouse uterine receptivity and decidualization

Abstract

Embryo implantation remains a significant challenge for assisted reproductive technology, with implantation failure occurring in ∼50% of in vitro fertilization attempts. Understanding the molecular mechanisms underlying uterine receptivity will enable the development of new interventions and biomarkers. TGFβ family signaling in the uterus is critical for establishing and maintaining pregnancy. Follistatin (FST) regulates TGFβ family signaling by selectively binding TGFβ family ligands and sequestering them. In humans, FST is upregulated in the decidua during early pregnancy, and women with recurrent miscarriage have lower endometrial expression of FST during the luteal phase. Because global knockout of Fst is perinatal lethal in mice, we generated a conditional knockout (cKO) of Fst in the uterus using progesterone receptor-cre to study the roles of uterine Fst during pregnancy. Uterine Fst-cKO mice demonstrate severe fertility defects and deliver only 2% of the number of pups delivered by control females. In Fst-cKO mice, the uterine luminal epithelium does not respond properly to estrogen and progesterone signals and remains unreceptive to embryo attachment by continuing to proliferate and failing to differentiate. The uterine stroma of Fst-cKO mice also responds poorly to artificial decidualization, with lower levels of proliferation and differentiation. In the absence of uterine FST, activin B expression and signaling are up-regulated, and bone morphogenetic protein (BMP) signals are impaired. Our findings support a model in which repression of activin signaling by FST enables uterine receptivity by preserving critical BMP signaling.female infertility | implantation failure | TGFβ signaling | activin antagonism M ouse models are powerful tools for improving our understanding of uterine receptivity because mice can be easily manipulated with well-established genetic tools and experimental approaches (1). Studies using mice have shown that TGFβ family growth factors play critical roles in establishing uterine receptivity. The TGFβ family operates through a shared mechanism. The signaling cascade is initiated by binding of homodimeric or heterodimeric ligands to a cell-surface receptor complex composed of a type 2 receptor kinase and its partner type 1 receptor kinase. Once the ligand binds, the type 2 receptor phosphorylates and activates the type 1 receptor. The activated type 1 receptor kinase phosphorylates receptor SMADs, enabling them to form a complex with the co-SMAD, SMAD4. The resulting heterotrimeric SMAD complex concentrates in the nucleus where it regulates gene expression in conjunction with of a multitude of cofactors. Uterine conditional knockout (cKO) of TGFβ superfamily growth factors bone morphogenetic protein 2 (BMP2), type 1 receptors ALK2 and ALK3, and type 2 receptor bone morphogenetic protein receptor 2 (BMPR2) in mice have shown that local uterine TGFβ family signaling plays key roles in regulating embryo attachment and invasion as well as endometrial stromal cell decidualization (2-5)....

Uterine double-conditional inactivation ofSmad2andSmad3in mice causes endometrial dysregulation, infertility, and uterine cancer

“…Notably, members of the transforming growth factor β (TGF β) family are involved in many cellular processes and serve as principal regulators of numerous biological functions, including female reproduction. Previous studies have shown the TGF β family to have key roles in ovarian folliculogenesis and ovulation (13, 14), decidualization (15, 16), implantation (17, 18), placentation (17, 19), uterine receptivity (15), and uterine development (20, 21), with disruption in the TGF β family causing reproductive diseases and cancer (22–25).…”

Characterization of the role for cadherin 6 in the regulation of human endometrial receptivity

“…In the mid-secretory phase human endometrium, in contrast to the localization of CDH6 in the apical and lateral surface of endometrial luminal epithelium, E-cadherin protein expression is reduced and shows minimal levels [41]. Such an expression reduction has been proven essential for embryo implantation and invasion in mice [17]. It is likely that selective changes in expression of E-cadherin and CDH6, along with other notable cadherin family members that have been reported in the human endometrium [12,13,20] contribute to a functional transition that is required for successful embryo implantation.…”

“…An in vitro functional study in a non-receptive endometrial epithelial cell line AN3-CA demonstrated that forced overexpression of E-cadherin in these cells significantly increased their receptivity to trophoblast-like spheroids formed by the BeWo choriocarcinoma cells [16]. By contrast, downregulation of E-cadherin in the lateral surface of the uterine epithelium in both human (in vitro) and mouse (in vivo) models may serve as a key mechanism to facilitate the loss of apical-basal polarity in the epithelial layer [17,18]. Such change is likely to avoid mutual repulsion between the embryo and the polarized endometrial luminal epithelial surface to facilitate embryo attachment and invasion [19].…”

Expression of follistatin is associated with egg formation in the oviduct of laying hens

“…Female mice lacking follistatin display abnormal oviduct and uterine morphology, a reduced number of uterine glands (Holdsworth‐Carson et al., 2015). In another study, the conditional ablation of FST in the mouse uterus found uterine luminal epithelium to be unresponsive to the estradiol and progesterone and was unable to differentiate (Fullerton et al., 2017). Eggshell biomineralization takes place in the uterine lumen, which requires active synthesis and transport of several molecules across the uterine epithelium (Sah et al., 2018).…”