Indrani C. Bagchi

Endometrial Decidualization: Of Mice and Men

In murine and human pregnancies, embryos implant by attaching to the luminal epithelium and invading into the stroma of the endometrium. Under the influence of the steroid hormones estrogen and progesterone, the stromal cells surrounding the implanting embryo undergo a remarkable transformation event. This process, known as decidualization, is an essential prerequisite for implantation. It comprises morphogenetic, biochemical, and vascular changes driven by the estrogen and progesterone receptors. The development of mutant mouse models lacking these receptors has firmly established the necessity of steroid signaling for decidualization. Genomic profiling of mouse and human endometrium has uncovered a complex yet highly conserved network of steroid-regulated genes that supports decidualization. To advance our understanding of the mechanisms regulating implantation and better address the clinical challenges of infertility and endometrial diseases such as endometriosis, it is important to integrate the information gained from the mouse and human models.

The Antiproliferative Action of Progesterone in Uterine Epithelium Is Mediated by Hand2

A progesterone-regulated transcription factor regulates stromal-epithelial communication in early pregnancy. During pregnancy, progesterone inhibits the growth-promoting actions of estrogen in the uterus. However, the mechanism for this is not clear. The attenuation of estrogen-mediated proliferation of the uterine epithelium by progesterone is a prerequisite for successful implantation. Our study reveals that progesterone-induced expression of the basic helix-loop-helix transcription factor Hand2 in the uterine stroma suppresses the production of several fibroblast growth factors (FGFs) that act as paracrine mediators of mitogenic effects of estrogen on the epithelium. In mouse uteri lacking Hand2, continued induction of these FGFs in the stroma maintains epithelial proliferation and stimulates estrogen-induced pathways, resulting in impaired implantation. Thus, Hand2 is a critical regulator of the uterine stromal-epithelial communication that directs proper steroid regulation conducive for the establishment of pregnancy.

Bone Morphogenetic Protein 2 Functions via a Conserved Signaling Pathway Involving Wnt4 to Regulate Uterine Decidualization in the Mouse and the Human

A critical role of progesterone (P) during early pregnancy is to induce differentiation of the endometrial stromal cells into specialized decidual cells that support the development of the implanting embryo. The P-induced signaling pathways that participate in the formation and function of the decidual cells remain poorly understood. We report here that the expression of the bone morphogenetic protein 2 (BMP2), a morphogen belonging to the TGFβ superfamily, is induced downstream of P action in the mouse uterine stroma during decidualization. To determine the function of BMP2 during this differentiation process, we employed a primary culture system in which undifferentiated stromal cells isolated from pregnant mouse uterus undergo decidualization. When recombinant BMP2 was added to these stromal cultures, it markedly advanced the differentiation program. We also found that siRNA-mediated silencing of BMP2 expression in these cells efficiently blocked the differentiation process. Gene expression profiling experiments identified Wnt4 as a downstream target of BMP2 regulation in stromal cells undergoing decidualization. Attenuation of Wnt4 expression by siRNAs greatly reduced stromal differentiation in vitro, indicating that it is a key mediator of BMP2-induced decidualization. We also observed a remarkable induction in the expression of BMP2 in human endometrial stromal cells during decidualization in vitro in response to steroids and cAMP. Addition of BMP2 to these cultures led to a robust enhancement of Wnt4 expression and stimulated the differentiation process. Collectively, our studies uncovered a unique conserved pathway involving BMP2 and Wnt4 that mediates P-induced stromal decidualization in the mouse and the human.

WNT4 is a key regulator of normal postnatal uterine development and progesterone signaling during embryo implantation and decidualization in the mouse

WNT4, a member of the Wnt family of ligands, is critical for the development of the female reproductive tract. Analysis of Wnt4 expression in the adult uterus during pregnancy indicates that it may play a role in the regulation of endometrial stromal cell proliferation, survival, and differentiation, which is required to support the developing embryo. To investigate the role of Wnt4 in adult uterine physiology, conditional ablation of Wnt4 using the PRC” mouse model was accomplished. Ablation of Wnt4 rendered female mice subfertile due to a defect in embryo implantation and subsequent defects in endometrial stromal cell survival, differentiation, and responsiveness to progesterone signaling. In addition to altered stromal cell function, the uteri of PITe/+ Wnt4f/f (Wnt4d/d) mice displayed altered epithelial differentiation characterized by a reduction in the number of uterine glands and the emergence of a p63‐positive basal cell layer beneath the columnar luminal epithelial cells. The altered epithelial cell phenotype was further escalated by chronic estrogen treatment, which caused squamous cell metaplasia of the uterine epithelium in the Wnt4d/d mice. Thus, WNT4 is a critical regulator not only of proper postnatal uterine development, but also embryo implantation and decidualization.—Franco, H. L., Dai, D., Lee, K. Y., Rubel, C. S., Roop, D., Boerboom, D., Jeong, J.‐W., Lydon, J.‐P., Bagchi, I. C., Bagchi, M. K., DeMayo, F. J. WNT4 is a key regulator of normal postnatal uterine development and progesterone signaling during embryo implantation and decidualization in the mouse. FASEB J. 25, 1176–1187 (2011). www.fasebj.org

Gap junction communication between uterine stromal cells plays a critical role in pregnancy-associated neovascularization and embryo survival

In the uterus, the formation of new maternal blood vessels in the stromal compartment at the time of embryonic implantation is critical for the establishment and maintenance of pregnancy. Although uterine angiogenesis is known to be influenced by the steroid hormones estrogen (E) and progesterone (P), the underlying molecular pathways remain poorly understood. Here, we report that the expression of connexin 43 (Cx43), a major gap junction protein, is markedly enhanced in response to E in uterine stromal cells surrounding the implanted embryo during the early phases of pregnancy. Conditional deletion of the Cx43 gene in these stromal cells and the consequent disruption of their gap junctions led to a striking impairment in the development of new blood vessels within the stromal compartment, resulting in the arrest of embryo growth and early pregnancy loss. Further analysis of this phenotypical defect revealed that loss of Cx43 expression resulted in aberrant differentiation of uterine stromal cells and impaired production of several key angiogenic factors, including the vascular endothelial growth factor (Vegf). Ablation of CX43 expression in human endometrial stromal cells in vitro led to similar findings. Collectively, these results uncovered a unique link between steroid hormone-regulated cell-cell communication within the pregnant uterus and the development of an elaborate vascular network that supports embryonic growth. Our study presents the first evidence that Cx43-type gap junctions play a critical and conserved role in modulating stromal differentiation, and regulate the consequent production of crucial paracrine signals that control uterine neovascularization during implantation.

Peroxisome Proliferator-Activated Receptor γ Is a Target of Progesterone Regulation in the Preovulatory Follicles and Controls Ovulation in Mice

ABSTRACT The progesterone receptor (PR) plays a critical role during ovulation. Mice lacking the PR gene are anovulatory due to a failure in the rupture of the preovulatory follicles. The pathways that operate downstream of PR to control ovulation are poorly understood. Using gene expression profiling, we identified peroxisome proliferator-activated receptor γ (PPARγ) as a target of regulation by PR in the granulosa cells of the preovulatory follicles during the ovulatory process. To investigate the function of PPARγ during ovulation, we created a conditional knockout mouse in which this gene was deleted via Cre-Lox-mediated excision in granulosa cells. When these mutant mice were subjected to gonadotropin-induced superovulation, the preovulatory follicles failed to rupture and the number of eggs released from the mutant ovaries declined drastically. Gene expression analysis identified endothelin-2, interleukin-6, and cyclic GMP-dependent protein kinase II as novel targets of regulation by PPARγ in the ovary. Our studies also suggested that cycloxygenase 2-derived metabolites of long-chain fatty acids function as endogenous activating ligands of PPARγ in the preovulatory follicles. Collectively, these studies revealed that PPARγ is a key mediator of the biological actions of PR in the granulosa cells and activation of its downstream pathways critically controls ovulation.

De novo synthesis of estrogen in pregnant uterus is critical for stromal decidualization and angiogenesis

Implantation is initiated when the embryo attaches to the uterine luminal epithelium during early pregnancy. Following this event, uterine stromal cells undergo steroid hormone-dependent transformation into morphologically and functionally distinct decidual cells in a unique process known as decidualization. An angiogenic network is also formed in the uterine stromal bed, critically supporting the early development of the embryo. The steroid-induced mechanisms that promote stromal differentiation and endothelial proliferation during decidualization are not fully understood. Although the role of ovarian progesterone as a key regulator of decidualization is well established, the requirement of ovarian estrogen (E) during this process remains unresolved. Here we show that the expression of P450 aromatase, a key enzyme that converts androgens to E, is markedly induced in mouse uterine stromal cells undergoing decidualization. The aromatase then acts in conjunction with other steroid biosynthetic enzymes present in the decidual tissue to support de novo synthesis of E. This locally produced E is able to support the advancement of the stromal differentiation program even in the absence ovarian E in an ovariectomized, progesterone-supplemented pregnant mouse model. Administration of letrozole, a specific aromatase inhibitor, to these mice blocked the stromal differentiation process. Gene expression profiling further revealed that the intrauterine E induces the expression of several stromal factors that promote neovascularization in the decidual tissue. Collectively, these studies identified the decidual uterus as a novel site of E biosynthesis and uncovered E-regulated maternal signaling pathways that critically control uterine differentiation and angiogenesis during early pregnancy.

Signaling by hypoxia-inducible factors is critical for ovulation in mice.

The steroid hormone progesterone, acting via its nuclear receptor, is a major regulator of the process of ovulation. Female mice lacking progesterone receptor (PGR) exhibit an anovulatory phenotype due to failure in follicular rupture. To identify the PGR-regulated pathways that control ovulation, we analyzed global changes in gene expression in the ovaries of wild-type and Pgr-null mice subjected to gonadotropin-induced superovulation. Our analysis uncovered several genes whose expression was reduced in the Pgr-null ovaries compared with the wild-type ovaries immediately preceding ovulation. Interestingly, these genes included three hypoxia-inducible factors (HIFs): HIF-1 alpha, HIF-2 alpha, and HIF-1 beta. These transcription factors form alphabeta-heterodimers, which regulate the transcription of specific cellular genes, thereby mediating adaptive response of the tissue to low-oxygen levels. We observed that the expression of mRNAs and proteins corresponding to HIF-1 alpha, HIF-2 alpha, and HIF-1 beta was induced in a PGR-dependent manner, specifically in the granulosa cells of the preovulatory follicles. Inhibition of the HIF transcriptional activity by echinomycin, a small-molecule inhibitor that suppresses the binding of HIF alphabeta-heterodimers to target genes, blocked ovulation by preventing the rupture of the preovulatory follicles. Echinomycin specifically inhibited the expression of genes that are known regulators of ovulation, such as a disintegrin and metalloproteinase with thrombospondin-like motifs-1 and endothelin-2. Furthermore, echinomycin reduced the expression of vascular endothelial growth factor A, a key factor controlling vascularization/angiogenesis during ovulation. Collectively, these findings unveiled a novel ovarian role for the HIF transcription factors during the ovulatory period in mice.

Stromal progesterone receptors mediate induction of Indian hedgehog (IHH) in uterine epithelium and its downstream targets in uterine stroma

Uterine receptivity to embryo implantation depends on appropriate progesterone (P4) and estrogen stimulation. P4 rapidly stimulates production of the morphogen Indian hedgehog (IHH) in murine uterine epithelium as well as downstream molecules in the hedgehog pathway such as Patched homolog 1 (PTCH1) and nuclear receptor subfamily 2, group F, member 2 (NR2F2) in uterine stroma. Studies using IHH-null mice indicate that IHH is obligatory for the normal P4 response in the uterus. To determine whether IHH induction in uterine epithelium is mediated through P4 receptor (PR) in epithelium (E) and/or stroma (S), we produced tissue recombinants using uteri from neonatal PR knockout (ko) mice and wild-type (wt) mice containing PR in S and/or E or lacking PR altogether using a tissue recombinant methodology and assessed their response to P4. In tissue recombinants containing wt-S (wt-S + wt-E and wt-S + ko-E), P4 induced Ihh mRNA expression at 6 h that was 6-fold greater than in oil-treated controls (P < 0.05; n = 6) in both types of tissue recombinants despite the absence of epithelial PR in wt-S + ko-E grafts. Conversely, Ihh mRNA expression was unaffected by P4 in ko-S + ko-E and ko-S + wt-E grafts despite epithelial PR expression in the latter. Nr2f2 and Ptch1 mRNA expression was similar in that it was stimulated by P4 only in recombinants containing stromal PR. These results indicate that stromal PR is both necessary and sufficient for P4 stimulation of epithelial IHH as well as downstream events such as PTCH1 and NR2F2 increases in stroma.

Msx Homeobox Genes Critically Regulate Embryo Implantation by Controlling Paracrine Signaling between Uterine Stroma and Epithelium

The mammalian Msx homeobox genes, Msx1 and Msx2, encode transcription factors that control organogenesis and tissue interactions during embryonic development. We observed overlapping expression of these factors in uterine epithelial and stromal compartments of pregnant mice prior to embryo implantation. Conditional ablation of both Msx1 and Msx2 in the uterus resulted in female infertility due to a failure in implantation. In these mutant mice (Msx1/2 d/d), the uterine epithelium exhibited persistent proliferative activity and failed to attach to the embryos. Gene expression profiling of uterine epithelium and stroma of Msx1/2 d/d mice revealed an elevated expression of several members of the Wnt gene family in the preimplantation uterus. Increased canonical Wnt signaling in the stromal cells activated β-catenin, stimulating the production of a subset of fibroblast growth factors (FGFs) in these cells. The secreted FGFs acted in a paracrine manner via the FGF receptors in the epithelium to promote epithelial proliferation, thereby preventing differentiation of this tissue and creating a non-receptive uterus refractory to implantation. Collectively, these findings delineate a unique signaling network, involving Msx1/2, Wnts, and FGFs, which operate in the uterus at the time of implantation to control the mesenchymal-epithelial dialogue critical for successful establishment of pregnancy.