Tissue-Specific Ablation of the LIF Receptor in the Murine Uterine Epithelium Results in Implantation Failure.

The cytokine leukemia inhibitory factor (LIF) is essential for rendering the uterus receptive for blastocyst implantation. In mice, LIF receptor expression (LIFR) is largely restricted to the uterine luminal epithelium (LE). LIF, secreted from the endometrial glands (GEs), binds to the LIFR, activating the Janus kinase-signal transducer and activation of transcription (STAT) 3 (Jak-Stat3) signaling pathway in the LE. JAK-STAT activation converts the LE to a receptive state so that juxtaposed blastocysts begin to implant. To specifically delete the LIFR in the LE, we derived a line of mice in which Cre recombinase was inserted into the endogenous lactoferrin gene (Ltf-Cre). Lactoferrin expression in the LE is induced by E2, and we demonstrate that Cre recombinase activity is restricted to the LE and GE. To determine the requirement of the LIFR in implantation, we derived an additional mouse line carrying a conditional (floxed) Lifrflx/flx gene. Crossing Ltf-Cre mice with Lifrflx/flx mice generated Lifrflx/Δ:LtfCre/+ females that were overtly normal but infertile. Many of these females, despite repeated matings, did not become pregnant. Unimplanted blastocysts were recovered from the Lifrflx/Δ:LtfCre/+ uteri and, when transferred to wild-type recipients, implanted normally, indicating that uterine receptivity rather than the embryo's competency is compromised. The loss of Lifr results in both the failure for STAT3 to translocate to the LE nuclei and a reduction in the expression of the LIF regulated gene Msx1 that regulates uterine receptivity. These results reveal that uterine expression of the LIFR is essential for embryo implantation and further define the components of the LIF signaling pathway necessary for effective implantation.

The Multifaceted Actions of Leukaemia Inhibitory Factor in Mediating Uterine Receptivity and Embryo Implantation.

Embryo implantation is mediated by the combined actions of the ovarian hormones E2 and P4 on the uterus. In addition, the pro-inflammatory cytokine, leukaemia inhibitory factor (LIF), plays a pivotal role in regulating uterine receptivity. LIF is expressed in the endometrial glands and has a robust action on the uterine luminal epithelium (LE). In mice, LIF is induced by nidatory E2 and functions to convert the LE from a non-receptive to an embryo-responsive state. LIF mediates its actions by activating the JAK-STAT pathway specifically in the LE. Activation of JAK-STAT pathway results in the induction of many additional pathways, including some 40 +  transcription factors, many of which initiate a cascade of changes affecting epithelial polarity, epithelial-mesenchymal interactions, angiogenesis, stromal cell decidualization, and inhibiting cell proliferation. This review discusses the role of LIF and the recent analysis of its action on the uterine LE in regulating endometrial receptivity and implantation.

The LIF-mediated molecular signature regulating murine embryo implantation.

The establishment of a receptive uterus is the prime requirement for embryo implantation. In mice, the E2-induced cytokine leukemia inhibitory factor (LIF) is essential in switching the uterine luminal epithelium (LE) from a nonreceptive to a receptive state. Here we define the LIF-mediated switch using array analysis and informatics to identify LIF-induced changes in gene expression and annotated signaling pathways specific to the LE. We compare gene expression profiles at 0, 1, 3, and 6 h, following LIF treatment. During the first hour, the JAK-STAT signaling pathway is activated and the expression of 54 genes declines, primarily affecting LE cytoskeletal and chromatin organization as well as a transient reduction in the progesterone, TGFbetaR1, and ACVR1 receptors. Simultaneously 256 genes increase expression, of which 42 are transcription factors, including Sox, Kfl, Hes, Hey, and Hox families. Within 3 h, the expression of 3987 genes belonging to more than 25 biological process pathways was altered. We confirmed the mRNA and protein distribution of key genes from 10 pathways, including the Igf-1, Vegf, Toll-like receptors, actin cytoskeleton, ephrin, integrins, TGFbeta, Wnt, and Notch pathways. These data identify novel LIF-activated pathways in the LE and define the molecular basis between the refractory and receptive uterine phases. More broadly, these findings highlight the staggering capacity of a single cytokine to induce a dynamic and complex network of changes in a simple epithelium essential to mammalian reproduction and provide a basis for identifying new routes to regulating female reproduction.

Embryo-induced alterations in the molecular phenotype of primate endometrium.

Reproductive biomedicine has made significant advances in the area of assisted reproductive technologies in the last two and half decades. However, embryo implantation remains a major obstacle in securing high pregnancy rates. Various non-human primate models including rhesus, marmoset and baboon have been employed to elucidate in vivo mechanisms underlying the uterine events that initiate, sustain and complete implantation. This review collates the information available on the molecular profile of gestational endometrium in primates. Collectively, these studies reveal dynamic spatio-temporal changes in the expression of cytokines, growth factors, cell-adhesion molecules, cytoskeleton elements and other factors in the endometrium during the post-implantation phase of pregnancy. Considering that the endometrial events during the pre-implantation stages of pregnancy may dictate implantation success, we have developed a bonnet monkey (Macaca radiata) model where pregnancy can be detected at the pre-implantation stage. Using this model, we investigated some of the endometrial events that occur before the completion of implantation. Remarkable changes were observed in endometrial expression of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNFalpha), as well as expression of immunosuppressive factors such as transforming growth factor beta-2 (TGFbeta2), interleukin-6 (IL-6) and placental protein-14 (PP-14), even before the embryo starts invading the endometrium. This highlights the super-imposition of endometrial receptivity by embryonic stimuli, marked by differential expression and/or localization of the factors that regulate endometrial transformation for embryo survival, growth and development.

Expression of endometrial protein kinase a during early pregnancy in bonnet monkeys (Macaca radiata).

Embryo-induced signaling pathways are considered to be important for initiation and sustenance of pregnancy. However many of these pathways remain to be deciphered in primates. In the present study, differential display RT-PCR was used to identify genes or gene fragments that are differentially expressed in endometrium of bonnet monkeys (Macaca radiata) on Day 6 of pregnancy. Of several fragments found to be differentially expressed, a fragment of 567 base pair (named GG1) was characterized in detail. GG1 was highly represented in endometrium of pregnant animals compared with that of nonpregnant animals. Sequencing analysis revealed homology of this fragment to exons 7, 8, 9, and 10 and surprisingly to intron 6 of cAMP-dependent protein kinase A (PKA) regulatory type I alpha (tissue-specific extinguisher 1) (PRKAR1A). The increased expression of this fragment in gestational endometrium was confirmed by quantitative PCR studies. Two transcripts of 3.0 kilobase (kb) and 1.5 kb were detected in Northern blot probed with labeled GG1. Protein expressions of alpha regulatory (PRKAR1A) and alpha catalytic (PRKCA) subunits of PKA were also higher in gestational endometrium compared with that in nongestational endometrium. Further in vitro studies using human endometrial explants demonstrated regulation of PRKAR1A (or GG1) and prostaglandin-endoperoxide synthase 2 or cyclooxygenase 2 (PTGS2) by estradiol. This is the first study to date on the differential expression of PKA in primate endometrium during early pregnancy and its in vitro regulation by estradiol.

Endometrial modifications during early pregnancy in bonnet monkeys (Macaca radiata).

The present study was undertaken to investigate endometrial modifications that occur before embryo invasion in bonnet monkeys (Macaca radiata). These changes were analysed in luminal epithelium, glandular epithelium and stroma of endometrial functionalis on Day 6 post ovulation from pregnant and non-pregnant animals (n = 4 each) by transmission electron microscopy. Distinct features (i.e. loss of columnar shape by epithelial cells, changes in mitochondrial size and diffused apicolateral gap junctions) were observed in the luminal and glandular epithelium in pregnant animals. Stromal compaction was also observed in pregnant animals. Further, immunogold localisation studies demonstrated significantly higher expression (P < 0.05) of oestrogen receptor alpha, an oestrogen-regulated gene, in the glandular epithelium and stroma of the endometrium in pregnant animals compared with non-pregnant animals. Expression of two other genes known to be regulated by oestradiol, namely beta-actin and cyclo-oxygenase-1, were also significantly higher (P < 0.05) in the endometria of pregnant animals. These studies demonstrate marked changes in the endometrium before embryo invasion in bonnet monkeys. These studies also indicate altered oestrogenic activity in the uterine milieu before embryo invasion.

Enhanced expressions of endometrial tumour necrosis factor alpha and its receptors during early pregnancy in bonnet monkeys.

Tumour necrosis factor alpha (TNF-alpha), a pro-inflammatory cytokine may play an active role in stimulating inflammatory reactions during pregnancy. However, the expression of endometrial TNF-alpha has not been investigated especially during early pregnancy, a phenomenon invariably accompanied by inflammatory reaction. In the present study, the endometrial expressions of TNF-alpha and its receptors (TNFR1 and TNFR2) during early pregnancy, when the embryo lies free in the zona hatched state in the uterine lumen, were analyzed by immunohistochemistry. The endometrial expressions of TNF-alpha, TNFR1 and TNFR2 were found to be significantly up-regulated (p < 0.05) in the glandular epithelium on day 6 post-ovulation in pregnant animals. The alteration in the expression of these molecules may contribute to the induction of local inflammatory reactions during implantation.

Endometrial expression of immunomodulatory cytokines and their regulators during early pregnancy in bonnet monkeys (Macaca radiata).

BACKGROUND: It is well established that endometrium undergoes extensive histological changes during implantation and subsequent stages of pregnancy in rodents as well as primates. Our previous investigation using a non-human primate model has demonstrated that morphological alterations are initiated even before the embryo invades the endometrium. The present study was undertaken to determine whether the embryo-induced morphological changes are accompanied by any alteration in the protein levels of the immunomodulatory cytokines and their regulators in the preimplantation stage endometrium. METHODS: The endometrial expression of immunosuppressive factors such as transforming growth factor beta2 (TGFbeta2), glycodelin (PP14), leukaemia inhibitory factor (LIF) and interleukin-6 (IL-6) were analysed on day 6 post-ovulation in pregnant and non-pregnant bonnet monkeys (Macaca radiata) using immunohistochemical methods. RESULTS: The endometrial expression of TGFbeta2, TGFbeta2 receptor, PP14 and IL-6 were significantly up-regulated (p < 0.05) in pregnant animals as compared to non-pregnant animals, whereas the expression of LIF and its receptor remained unaltered in pregnant animals. CONCLUSIONS: Expression levels of some immunomodulatory cytokines in endometrium are significantly increased even before the embryo invades the endometrium. The altered cytokine expression profile in endometrium probably contributes towards generating a conducive environment for the embryo survival, growth and development in the uterus.

Progesterone receptors: various forms and functions in reproductive tissues.

The unequivocal role of progesterone in a variety of events like ovulation, mammary gland development, establishment and maintenance of pregnancy etc are well established. Also the data are accumulating on its role in male reproductive events. In vertebrates and humans, the biological activity of progesterone is mediated by two progesterone receptor proteins PR-A and PR-B, that arise from the same gene and are the members of nuclear receptor superfamily of transcriptional factors. Several studies have demonstrated that the blockage of progesterone receptor using antiprogestins impairs folliculogenesis, ovulation, implantation and pregnancy. Progesterone receptor (PR), have also been detected in human spermatozoa. However, unlike the conventional PR, sperm PR was localized on the membrane and showed distinct characteristics in terms of its size. There are data to demonstrate the inhibition of progesterone driven functions such as hyperactive motility, acrosome reaction on neutralization of sperm membrane PR with specific antibodies against PR. Further significant decrease in the % of PR positive spermatozoa was observed in infertile cases as compared to the fertile men. This indicated that PR can serve as the marker to define the fertilizing potential of the spermatozoa. Recently we have also shown that the PR is expressed in human testis. This reinforced that this PR protein is an inherent testicular protein and not a secretion of accessory reproductive organs. This review compiles the major observations on the forms of the progesterone receptor in various reproductive tissues.

Morphological events in the primate endometrium in the presence of a preimplantation embryo, detected by the serum preimplantation factor bioassay.

BACKGROUND: Hormonal modulation of the endometrium towards receptivity is well established; however, the role of embryonic stimuli in modulation of the endometrium prior to implantation, especially in primates, is unknown. The aim of the present study was to evaluate the endometrial histology when the embryo was present in its vicinity prior to implantation. METHODS: Preimplantation factor (PIF) bioassay was used as a tool to detect the presence of an embryo in the uterine lumen of mated bonnet monkeys (Macaca radiata) (n=9). The control group comprised seven non-mated animals. The specificity of the PIF bioassay for the presence of an embryo was tested by studies in pregnant humans and monkeys. The effects of embryonic stimuli on the endometrial morphology were analysed by routine haematoxylin-eosin staining. The expressions of CD34, an endothelial cell marker, alpha-smooth muscle actin (alpha-SMA), a marker for blood vessel maturation, and prolactin, a marker of endometrial decidualization, were studied by immunohistochemistry. RESULTS: That PIF is embryo specific was established by its presence in sera of pregnant humans, monkeys and also in embryo culture media. Six mated bonnet monkeys were found to be PIF positive. Morphologically, the endometria from these PIF-positive animals showed the presence of the pre-epithelial plaque reaction, increased angiogenesis and stromal compaction. The significantly increased number of CD34- and alpha-SMA-positive blood vessels (P<0.05) in the endometria of PIF-positive animals indicated increased angiogenesis in response to embryonic stimuli. The endometrial expression of immunoreactive prolactin was also significantly increased (P<0.05) in the PIF-positive animals, indicating decidualization. CONCLUSIONS: Using PIF as a marker to detect early pregnancy in bonnet monkeys, we have shown that the embryo induces a pre-epithelial plaque type of reaction, increased angiogenesis and decidual reaction in the endometrium prior to implantation.

Role of progesterone in structural and biochemical remodeling of endometrium.

The endometrial response to the varying levels of ovarian steroids is exhibited as alterations in its form and function. These changes in endometrial morphology and physiology, especially those observed during the implantation window are prerequisites to support embryo attachment and invasion. However the state of endometrial receptivity to embryo results from an operative network of several molecular events triggered by estrogen, progesterone and probably some other factors, yet to be discovered. It is well established that estrogen and progesterone are the critical endocrine determinants of endometrial functions. However the precise delineation of hormone driven events and their interaction is yet to be ascertained. Several attempts have been made to understand these cascades, however most of these studies have been conducted in vitro using one or the other component of endometrial tissues. We have attempted to investigate in vivo morphological and biochemical/molecular changes in endometrium in response to neutralization of progesterone synthesis/ function in two primate animal models. In one of the models, ovariectomized rhesus monkeys, artificial menstrual cycles were simulated and subsequent effects on the _expression of various genes were investigated in presence and absence of sufficient progesterone levels. The results coincided with those observed in the endometrium of the other model, bonnet monkeys presenting normal hypothalamus-ovarian-pituitary function but displaying retarded endometrial growth due to blocked progesterone receptor. A significant decline was observed in the expression of transforming growth factor beta, transforming growth factor beta receptor, leukaemia inhibitory factor, whereas no remarkable changes were observed in the expression of estrogen receptor and progesterone receptors in response to neutralization of progesterone synthesis/function in these two animal models. Taking support from the inferences drawn from previously published in vitro studies and our data from in vivo studies conducted in these two models, we propose a hypothesis supporting a potential link between the expressions of transforming growth factor beta, leukaemia inhibitory factor, cyclooxygenases and integrins.