Inhibiting Uterine PC6 Blocks Embryo Implantation: An Obligatory Role for a Proprotein Convertase in Fertility1

Podocalyxin molecular characteristics and endometrial expression: high conservation between humans and macaques but divergence in mice†

Podocalyxin (PODXL) is a newly identified key negative regulator of human endometrial receptivity, specifically down-regulated in the luminal epithelium at receptivity to permit embryo implantation. Here, we bioinformatically compared the molecular characteristics of PODXL among the human, rhesus macaque and mouse, determined by immunohistochemistry and in situ hybridization (mouse tissues) whether endometrial PODXL expression is conserved across the three species, and examined if PODXL inhibits mouse embryo attachment in vitro. The PODXL gene, mRNA and protein sequences showed greater similarities between humans and macaques than with mice. In all species, PODXL was expressed in endometrial luminal/glandular epithelia and endothelia. In macaques (n = 9), luminal PODXL was significantly down-regulated when receptivity is developed, consistent with the pattern found in women. At receptivity PODXL was also reduced in shallow glands, whereas endothelial expression was unchanged across the menstrual cycle. In mice, endometrial PODXL did not vary considerably across the estrous cycle (n = 16); however, around embryo attachment on d4.5 of pregnancy (n = 4), luminal PODXL was greatly reduced especially near the site of embryo attachment. Mouse embryos failed to attach or thrive when co-cultured on a monolayer of Ishikawa cells overexpressing PODXL. Thus, endometrial luminal PODXL expression is down-regulated for embryo implantation in all species examined, and PODXL inhibits mouse embryo implantation. Rhesus macaques share greater conservations with humans than mice in PODXL molecular characteristics and regulation, thus represent a better animal model for functional studies of endometrial PODXL for treatment of human fertility.

Alterations in Epithelial Cell Polarity During Endometrial Receptivity: A Systematic Review

BACKGROUND

Abnormal endometrial receptivity is one of the major causes of embryo implantation failure and infertility. The plasma membrane transformation (PMT) describes the collective morphological and molecular alterations occurring to the endometrial luminal epithelium across the mid-secretory phase of the menstrual cycle to facilitate implantation. Dysregulation of this process directly affects endometrial receptivity and implantation. Multiple parallels between these alterations to confer endometrial receptivity in women have been drawn to those seen during the epithelial-mesenchymal transition (EMT) in tumorigenesis. Understanding these similarities and differences will improve our knowledge of implantation biology, and may provide novel therapeutic targets to manage implantation failure.

METHODS

A systematic review was performed using the Medline (Ovid), Embase, and Web of Science databases without additional limits. The search terms used were "(plasma membrane* or cell membrane*) and transformation*" and "endometrium or endometrial." Research studies on the PMT or its regulation in women, discussing either the endometrial epithelium, decidualized stroma, or both, were eligible for inclusion.

RESULTS

A total of 198 articles were identified. Data were extracted from 15 studies that matched the inclusion criteria. Collectively, these included studies confirmed the alterations occurring to the endometrial luminal epithelium during the PMT are similar to those seen during the EMT. Such similarities included alterations to the actin cytoskeleton remodeling of adherens junctions, integrin expression and epithelial-stromal communication. These were also some differences between these processes, such as the regulation of tight junctions and mucins, which need to be further researched.

CONCLUSIONS

This review raised the prospect of shared and distinct mechanisms existing in PMT and EMT. Further investigation into similarities between the PMT in the endometrium and the EMT in tumorigenesis may provide new mechanistic insights into PMT and new targets for the management of implantation failure and infertility.

Osteopontin as a novel substrate for the proprotein convertase 5/6 (PCSK5) in bone

Seven proprotein convertases cleave the basic amino acid consensus sequence K/R-X_n-K/R↓ (where n=0, 2, 4 or 6 variable amino acids) to activate precursor proteins. Despite similarities in substrate specificity, basic amino acid-specific proprotein convertases have a distinct tissue distribution allowing for enzymatic actions on tissue-resident substrates. Proprotein convertase 5/6 (PC5/6) has two splice variants - soluble PC5/6A and membrane-bound PC5/6B - and is expressed during mouse development in many tissues including bone and tooth, but little is known about the substrates for PC5/6 therein. Osteopontin (OPN) is an abundant bone extracellular matrix protein with roles in mineralization, cell adhesion and cell migration, and it has putative consensus sequence sites for cleavage by PC5/6, which may modify its function in bone. Since PC5/6-knockout mouse embryos show developmental abnormalities, and reduced overall mineralization, we designed this study to determine whether OPN is a substrate of PC5/6. In silico analysis of OPN protein sequences identified four potential PC5/6 consensus cleavage sites in human OPN, and three sites - including a noncanonical sequence - in mouse OPN. Ex vivo co-transfections with human OPN revealed complete OPN cleavage reducing full-length OPN (~70kDa) to an N-terminal fragment migrating at ~50kDa and two C-terminal fragments at ~18kDa and ~16kDa. Direct cleavage of OPN by PC5/6A - the predominant isoform expressed in human osteoblast cells - was confirmed by cell-free enzyme-substrate assays and by mass spectrometry. The latter was also used to investigate potential cleavage sites. Co-transfections of PC5/6 and mouse OPN showed partial cleavage of OPN into a C-terminal OPN fragment migrating at ~30kDa and an N-terminal fragment migrating at ~29kDa. Micro-computed tomography of PC5/6-knockout embryos at E18.5 confirmed a reduction in mineralized bone, and in situ hybridization performed on cryo-sections of normal mouse bone using Pcsk5 and Opn anti-sense and control-sense cRNA probes indicated the co-localization of the expression of these genes in bone cells. This mRNA expression profile was supported by semi-quantitative RT-PCR using osteoblast primary cultures, and cultured MC3T3-E1 osteoblast and MLO-Y4 osteocyte cell lines. Immunoblotting for OPN from mouse bone extracts showed altered OPN processing in PC5/6-knockout mice compared to wildtype mice. OPN fragments migrated at ~25kDa and ~16kDa in wildtype bone and were not present in PC5/6-deficient bone. In conclusion, this study demonstrates that Pcsk5 is expressed in bone-forming cells, and that OPN is a novel substrate for PC5/6. Cleavage of OPN by PC5/6 may modify the function of OPN in bone and/or modulate other enzymatic cleavages of OPN, leading to alterations in the bone phenotype.

Endoglin (CD105) coordinates the process of endometrial receptivity for embryo implantation

Endoglin is a TGF-β receptor that is expressed in uterine endothelial and stromal cells in addition to trophoblast expression. However, the functional importance of endoglin in the embryo implantation process is not clear. We observed endoglin expression in the endometrium throughout the stages of its receptivity; however, its expression was enhanced during the receptive stage. Endoglin expression was predominant in epithelial cells of the lumen and glands, but showed a milder expression in stromal cells. Endoglin expression was initially observed in the primary decidual zone and later extended to the secondary decidua zone. Knockdown of endoglin via siRNA reduced the implantation sites along with the blastocyst numbers. Mouse blastocyst with endoglin-silenced endometrial epithelial cells (human and mouse origin) showed poor trophoblast outgrowth, which suggests an essential role for endoglin during endometrial receptivity. In conclusion, our findings reveal the association of endoglin with endometrial receptivity, which is important for embryo attachment.

The Microenvironment of Human Implantation: Determinant of Reproductive Success

Successful implantation requires synchronous development of embryo and endometrium. Endometrial receptivity results from progesterone-induced differentiation of endometrial cells, generally achieved during the mid-secretory phase of the cycle. Failure to properly develop receptivity results in failed or inadequate implantation and hence no ongoing pregnancy. The blastocyst undergoes final development, apposition, attachment and initiates invasion of the endometrial epithelium within the uterine cavity. Thus, the microenvironment provided by uterine fluid, particularly glandular secretions, is essential for implantation. Analysis of endometrial fluid has identified cytokines, chemokines, proteases, antiproteases and other factors that modulate blastocyst functions relevant to implantation. Exosomes/microvesicular bodies released from the endometrium (and likely also the embryo) are present in uterine fluid. These can transfer miRNA, proteins and lipids between cells, thus providing endometrial-embryo communication in the peri-implantation period. Understanding the uterine microenvironment, and its effects on endometrial-embryo interactions, will provide opportunities to modify current infertility treatments to improve success rates.

Development of a high-throughput assay for human proprotein convertase 5/6 for detecting uterine receptivity

Embryo implantation requires a healthy embryo and a receptive uterus. In women, the inner lining of the uterus, the endometrium, remains in a hostile state and becomes receptive for embryo implantation for only a short period during each menstrual cycle. Determining endometrial receptivity is vital in in vitro fertilization (IVF) treatment because the timing of embryo transfer needs to be synchronized with endometrial receptivity. We have previously demonstrated that proprotein convertase 5/6A (PC6) is highly expressed in the receptive endometrium and that PC6 is critical for receptivity establishment in women. Furthermore, endometrial PC6 is secreted into the uterine fluid, and levels correlate with receptivity status. Detection of PC6 in uterine fluids, therefore, would provide a nonsurgical assessment of endometrial receptivity. However, to date no assays are available for human PC6. In this study, we produced three PC6 monoclonal antibodies (mAbs) and developed a sandwich enzyme-linked immunosorbent assay (ELISA) for PC6 detection in human uterine fluids. The PC6 mAbs were confirmed to be highly specific to PC6, and the ELISA detected PC6 in human uterine fluids with a significantly higher level during the receptive phase. This newly established PC6 ELISA provides an important tool in the development of noninvasive strategies to detect endometrial receptivity in women.

Proprotein convertase 5/6 cleaves platelet-derived growth factor A in the human endometrium in preparation for embryo implantation

Establishment of endometrial receptivity is vital for successful embryo implantation. Proprotein convertase 5/6 (referred to as PC6) is up-regulated in the human endometrium specifically at the time of epithelial receptivity. PC6, a serine protease of the proprotein convertase family, plays an important role in converting precursor proteins into their active forms through specific proteolysis. The proform of platelet-derived growth factor A (pro-PDGFA) requires PC cleavage to convert to the active-PDGFA. We investigated the PC6-mediated activation of PDGFA in the human endometrium during the establishment of receptivity. Proteomic analysis identified that the pro-PDGFA was increased in the conditioned medium of HEC1A cells in which PC6 was stably knocked down by small interfering RNA (PC6-siRNA). Western blot analysis demonstrated an accumulation of the pro-PDGFA but a reduction in the active-PDGFA in PC6-siRNA cell lysates and medium compared with control. PC6 cleavage of pro-PDGFA was further confirmed in vitro by incubation of recombinant pro-PDGFA with PC6. Immunohistochemistry revealed cycle-stage-specific localization of the active-PDGFA in the human endometrium. During the non-receptive phase, the active-PDGFA was barely detectable. In contrast, it was localized specifically to the apical surface of the luminal and glandular epithelium in the receptive phase. Furthermore, the active-PDGFA was detected in uterine lavage with levels being significantly higher in the receptive than the non-receptive phase. We thus identified that the secreted PDGFA may serve as a biomarker for endometrial receptivity. This is also the first study demonstrating that the active-PDGFA localizes to the apical surface of the endometrium during receptivity.

Inhibition of embryo implantation in mice through vaginal administration of a proprotein convertase 6 inhibitor

Uterine proprotein convertase 6 (PC6) plays a critical role in embryo implantation in both mice and women. It was hypothesized that inhibiting uterine PC6 could prevent pregnancy. Vaginal administration of a PC6 inhibitor presents the ideal route for local drug delivery. A peptide-based PC6 inhibitor, C-30k-PEG Poly R that was previously shown to have properties of increased vaginal absorption and penetration was tested for its contraceptive potential in mice following vaginal administration. The study demonstrated that this approach could inhibit embryo implantation in some mice (24% completely and 47% partially inhibited).

Small molecule proprotein convertase inhibitors for inhibition of embryo implantation

Uterine proprotein convertase (PC) 6 plays a critical role in embryo implantation and is pivotal for pregnancy establishment. Inhibition of PC6 may provide a novel approach for the development of non-hormonal and female-controlled contraceptives. We investigated a class of five synthetic non-peptidic small molecule compounds that were previously reported as potent inhibitors of furin, another PC member. We examined (i) the potency of these compounds in inhibiting PC6 activity in vitro; (ii) their binding modes in the PC6 active site in silico; (iii) their efficacy in inhibiting PC6-dependent cellular processes essential for embryo implantation using human cell-based models. All five compounds showed potent inhibition of PC6 activity in vitro, and in silico docking demonstrated that these inhibitors could adopt a similar binding mode in the PC6 active site. However, when these compounds were tested for their inhibition of decidualization of primary human endometrial stromal cells, a PC6-dependent cellular process critical for embryo implantation, only one (compound 1o) showed potent inhibition. The lack of activity in the cell-based assay may reflect the inability of the compounds to penetrate the cell membrane. Because compound's lipophilicity is linked to cell penetration, a measurement of lipophilicity (logP) was calculated for each compound. Compound 1o is unique as it appears the most lipophilic among the five compounds. Compound 1o also inhibited another crucial PC6-dependent process, the attachment of human trophoblast spheroids to endometrial epithelial cells (a model for human embryo attachment). We thus identified compound 1o as a potent small molecule PC6 inhibitor with pharmaceutical potential to inhibit embryo implantation. Our findings also highlight that human cell-based functional models are vital to complement the biochemical and in silico analyses in the selection of promising drug candidates. Further investigations for compound 1o are warranted in animal models to test its utility as an implantation-inhibiting contraceptive drug.

Proprotein convertases subtilisin/kexin type 9, an enzyme turned escort protein: hepatic and extra hepatic functions

Proprotein Convertases Subtilisin/Kexin Type 9 (PCSK9) is a serine endoproteinase. Biosynthesized as a zymogen, it cleaves itself once, and then turns into an escort protein for transmembrane proteins, leading them into lysosomes for degradation. It is primarily produced and secreted by the liver. It attaches to the low-density lipoprotein receptor (LDLR) at the surface of hepatocytes and, after co-endocytosis, directs it into lysosomes where it is degraded. By downregulating LDLR, PCSK9 reduces hepatic clearance of LDL-cholesterol. Inborn or induced increase of this function causes hypercholesterolemia; its decrease causes hypocholesterolemia. This has been experimentally demonstrated ex vivo and in vivo, and corroborated by epidemiological studies associating PCSK9 genetic variations with plasma cholesterol levels. PCSK9 is now a proven target for inactivation in the treatment of hypercholesterolemia and associated atherosclerosis. However, it is still uncertain whether its severe or complete inactivation, combined with other predispositions, will be without undesirable side-effects. Some experimental data suggest that PCSK9 could contribute positively to the physiology of non-hepatic cells such as pancreatic islets β cells, adipocytes and macrophages, protecting them from excessive lipid uptake, in an endocrine, autocrine, or paracrine manner. Genetic variations that attenuate PCSK9 anti-LDLR activity are common in human populations. Their evolutionary significance still needs to be evaluated on the background of environmental pressures, such as infectious diseases, cold weather and famine, which have threatened survival and reproduction in the course of human prehistory and history.

Transforming growth factor-beta 1 (TGF-B1) liberation from its latent complex during embryo implantation and its regulation by estradiol in mouse

Transforming growth factor-beta (TGF-B) plays an important role in embryo implantation; however, TGF-B requires liberation from its inactive latent forms (i.e., large latent TGF-B complex [LLC] and small latent TGF-B complex [SLC]) to its biologically active (i.e., monomer or dimer) forms in order to act on its receptors (TGF-BRs), which in turn activate SMAD2/3. Activation of TGF-B1 from its latent complexes in the uterus is not yet deciphered. We investigated uterine latent TGF-B1 complex and its biologically active form during implantation, decidualization, and delayed implantation. Our study, utilizing nonreducing SDS-PAGE followed by Western blotting and immunoblotting with TGF-B1, LTBP1, and latency-associated peptide, showed the presence of LLC and SLC in the uterine extracellular matrix and plasma membranous protein fraction during stages of the implantation period. A biologically active form of TGF-B1 (~17-kDa monomer) was highly elevated in the uterine plasma membranous compartment at the peri-implantation stage (implantation and nonimplantation sites). Administration of hydroxychloroquine (an inhibitor of pro-TGF-B processing) at the preimplantation stage was able to block the liberation of biologically active TGF-B1 from its latent complex at the postimplantation stage; as a consequence, the number of implantation sites was reduced at Day 5 (1000 h), as was the number of fetuses at Day 13. The inhibition of TGF-B1 showed reduced levels of phosphorylated SMAD3. Further, the delayed-implantation mouse model showed progesterone and estradiol coordination to release the active TGF-B1 form from its latent complex in the receptive endometrium. This study demonstrates the importance of liberation of biologically active TGF-B1 during the implantation period and its regulation by estradiol.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.

PEGylation of a proprotein convertase peptide inhibitor for vaginal route of drug delivery: in vitro bioactivity, stability and in vivo pharmacokinetics

Uterine proprotein convertase (PC) 6 is critical for embryo implantation in mice and women. It is also one of the PC family members that play a vital role in HIV infectivity. We hypothesized that inhibiting PC6 in the female reproductive tract (vagina, cervix and uterus), may protect women from both pregnancy and HIV infection. One key requirement to prove this concept in an animal model is a vaginally deliverable PC6 inhibitor. Nona-D-arginine (Poly R) is a potent peptide PC inhibitor and is able to inhibit HIV in cell culture. We modified Poly R by PEGylation with different strategies and determined their biochemical properties in vitro and in vivo. PEGylation at the C-terminus, regardless of the PEG size (30 kDa or 1239 Da) did not compromise the inhibitory potency of Poly R. In contrast, PEGylation at both termini (1239 Da) dramatically reduced its inhibitory activity. Poly R and C-PEGylated Poly Rs also showed equal potency in inhibiting a PC6-dependent cellular process critical for embryo implantation. Poly R and the equipotent C-PEGylated Poly Rs were further tested for their serum stability in vitro and pharmacokinetics in vivo following vaginal administration in mice. All Poly Rs were equally stable in mouse serum in vitro for 24h; C-PEGylated Poly Rs showed enhanced vaginal absorption and penetration across the vaginal mucosa/epithelium. This is the first report that C-terminal PEGylation significantly enhances the therapeutic properties of Poly R for vaginal drug delivery. Our findings also provide important insights into future design of Poly R derivatives.

Innate immunity, decidual cells, and preeclampsia

Preeclampsia (PE) manifested by hypertension and proteinuria complicates 3% to 8% of pregnancies and is a leading cause of fetal-maternal morbidity and mortality worldwide. It may lead to intrauterine growth restriction, preterm delivery, and long-term sequelae in women and fetuses, and consequently cause socioeconomic burden to the affected families and society as a whole. Balanced immune responses are required for the maintenance of successful pregnancy. Although not a focus of most studies, decidual cells, the major resident cell type at the fetal-maternal interface, have been shown to modulate the local immune balance by interacting with other cell types, such as bone marrow derived-immune cells, endothelial cells, and invading extravillous trophoblasts. Accumulating evidence suggests that an imbalanced innate immunity, facilitated by decidual cells, plays an important role in the pathogenesis of PE. Thus, this review will discuss the role of innate immunity and the potential contribution of decidual cells in the pathogenesis of PE.

Proprotein convertases in post-menopausal endometrial cancer: distinctive regulation and non-invasive diagnosis

Proprotein convertases (PCs) play critical roles in cleaving precursor proteins (growth factors, hormones, receptors and adhesion molecules) for activation. PCs are implicated in a number of cellular functions, including oncogenesis. Endometrial cancer is the most common gynecological cancer in the developed world, but the involvement of PCs is unclear. To characterize the role of PCs in endometrial cancer, we assessed expression of seven PCs (PC1/3, PC2, PACE4, PC4, furin, PC5/6 and PC7) by RT-PCR in six well characterized endometrial cancer cell lines. Expression was variable in all lines, with furin being most consistently expressed in all cell lines tested. We next determined the cellular localization and expression levels of four ubiquitously expressed PCs (furin, PACE4, PC5/6 and PC7) in post-menopausal endometrial biopsies from control (n=7) and endometrial cancer patients (n=30) by immunohistochemistry. Furin increased in tumors, whereas PC5/6, PACE4 and PC7 expression was reduced with increasing cancer grades. Uterine lavage is a non-invasive source material for evaluating the endometrium. We thus assessed whether total PC activity was altered in uterine lavage of endometrial cancer patients (n=36) compared to controls (n=10). PC activity was detected in all uterine lavage samples, and significantly elevated in all grades of endometrial cancer. This study demonstrates a complex association between individual PCs and endometrial cancer. Importantly, we show that monitoring the total PC activity in uterine lavage may provide a rapid and non-invasive method for the diagnosis of endometrial cancer in postmenopausal women.

Angiopoietin-like gene expression in the mouse uterus during implantation and in response to steroids

The purpose of this work was to determine if and where Angiopoietin-like genes are expressed in the mouse uterus during the implantation period of pregnancy and to determine if uterine expression of such genes is controlled by estrogen or progesterone. We found that all six known murine angiopoietin-like genes were expressed in the mouse uterus during implantation. The expression of four genes was controlled by either estrogen or progesterone. Only the levels of angiopoietin-like 4 (Angptl4) mRNA dramatically increased in implantation segments of the uterus during decidualization and was conceptus-independent. Due to this increased expression and the fact that angiopoietin-like 4 protein plays a role in lipid metabolism and angiogenesis in other tissues, only the expression of Angptl4 was further examined in the uterus and developing placenta. Angptl4 mRNA was localized to subpopulations of the endometrial stromal fibroblast and endothelial cell populations during decidualization. It was also localized to the ectoplacental cone, trophoblast giant cells and parietal endoderm of the conceptus at this time. By mid-pregnancy, Angptl4 mRNA was localized mainly to the mesometrial lymphoid aggregate region plus mesometrial endothelial cells of the uterus, as well as in various cell types of the conceptus. Additional work showed that Angptl4 expression increases in mouse endometrial stromal cells as they undergo decidualization in vitro. As in other cell types, the expression of Angptl4 in endometrial stromal cells was increased in response to an agonist of the peroxisome proliferator activated receptors. Taken together, the results of this work support the hypothesis that locally expressed Angptl4 might play a role in local uterine/placental lipid metabolism and vascular changes during implantation and thus provide a basis for future research.

Proprotein convertase 5/6 is critical for embryo implantation in women: regulating receptivity by cleaving EBP50, modulating ezrin binding, and membrane-cytoskeletal interactions

Establishment of endometrial receptivity is vital for successful embryo implantation; its failure causes infertility. Epithelial receptivity acquisition involves dramatic structural changes in the plasma membrane and cytoskeleton. Proprotein convertase 5/6 (PC6), a serine protease of the proprotein convertase (PC) family, is up-regulated in the human endometrium specifically at the time of epithelial receptivity and stromal cell decidualization. PC6 is the only PC member tightly regulated in this manner. The current study addressed the importance and mechanisms of PC6 action in regulating receptivity in women. PC6 was dysregulated in the endometrial epithelium during the window of implantation in infertile women of three demographically different cohorts. Its critical role in receptivity was evidenced by a significant reduction in mouse blastocyst attachment of endometrial epithelial cells after PC6 knockdown by small interfering RNA. Using a proteomic approach, we discovered that PC6 cleaved the key scaffolding protein, ezrin-radixin-moesin binding phosphoprotein 50 (EBP50), thereby profoundly affecting its interaction with binding protein ezrin (a key protein bridging actin filaments and plasma membrane), EBP50/ezrin cellular localization, and cytoskeleton-membrane connections. We further validated this novel PC6 regulation of receptivity in human endometrium in vivo in fertile vs. infertile patients. These results strongly indicate that PC6 plays a key role in regulating fundamental cellular remodeling processes, such as plasma membrane transformation and membrane-cytoskeletal interface reorganization. PC6 cleavage of a crucial scaffolding protein EBP50, thereby profoundly regulating membrane-cytoskeletal reorganization, greatly extends the current knowledge of PC biology and provides substantial new mechanistic insight into the fields of reproduction, basic cellular biology, and PC biochemistry.

Inhibition of proprotein convertase 5/6 activity: potential for nonhormonal women-centered contraception

BACKGROUND

Proprotein convertase 5/6 (PC6) is critical for endometrial epithelial receptivity and stromal cell decidualization for embryo implantation in women. We hypothesized that inhibiting PC6 could block implantation for contraception. The aim of this study was to prove this concept using human cell models and rabbits.

STUDY DESIGN

A potential PC6 inhibitor, C1239-PEG-Poly R, was biochemically confirmed to be a potent PC6 inhibitor. The potential contraceptive action of the inhibitor was then tested in decidualization of primary human endometrial stromal cells in a human trophoblast spheroid attachment model and in vivo in rabbits.

RESULTS

The PC6 inhibitor C1239-PEG-Poly R inhibited in a dose-dependent manner both decidualization and spheroid attachment. Vaginal delivery of 200 μL of the inhibitor at a final concentration of 5 mM to rabbits over a 3-day period starting 6 days after mating resulted in a 60% decrease in implantation and, hence, pregnancy.

CONCLUSIONS

This study presents proof of concept that PC6 inhibition has the potential to block embryo implantation, providing nonhormonal contraception for women.

Embryo implantation is closely associated with dynamic expression of proprotein convertase 5/6 in the rabbit uterus

BACKGROUND

Proprotein convertase 5/6 (PC5/6) is critical for embryo implantation in women, regulating both uterine epithelial receptivity and stromal cell decidualization. PC5/6 is likewise essential for implantation in mice, but involved only in decidualization. An alternative animal model is required to address the function of PC5/6 in the uterine epithelium. This study aimed to establish whether PC5/6 is associated with embryo implantation in rabbits.

METHODS

Virgin New-Zealand white rabbits aged 3-4 moths were mated with males of the same strain, or pseudo-pregnancy induced. After mating, uterine tissues were collected over a 10 day (d) period (n = 3 per time point) for RNA, protein and histological analyses to determine the temporal and spatial uterine expression pattern of PC5/6 during the initial stages of pregnancy or induced pseudo-pregnancy.

RESULTS

PC5/6 mRNA was up-regulated just prior to embryo attachment on d6, and the elevated expression was maintained throughout implantation on d6.5-10. Western analysis revealed a preferential up-regulation of PC5/6 in the implantation sites. Immunohistochemical analysis identified that both the amount and cellular localization of PC5/6 changed with increasing pregnancy stages. Before embryo attachment, PC5/6 was low and localised in the luminal and glandular epithelium. It increased on d6.5 in the basal glands and mucosal folds, and then strongly intensified on d7-10 in the multinucleated luminal symplasma and decidual cells at the site of embryo implantation. In contrast, the pseudo-pregnant uterus displayed relatively low and static PC6 mRNA expression throughout the 10 days, with no obvious changes in either PC5/6 level or cellular localization.

CONCLUSIONS

These findings demonstrate that embryo implantation in the rabbit is closely associated with dynamic expression of uterine PC5/6, and that the rabbit may be an appropriate model to investigate the function of PC5/6 in the uterine epithelium during embryo attachment.

PC6 levels in uterine lavage are closely associated with uterine receptivity and significantly lower in a subgroup of women with unexplained infertility

BACKGROUND Embryo implantation requires a healthy embryo and a receptive uterus. Uterine incompetence contributes significantly to implantation failure and infertility. To date, there are no reliable biochemical methods that can determine whether the uterus is receptive. Proprotein convertase 5/6 (PC6) is tightly regulated in the uterus and critical for receptivity and implantation; its secretory nature predicts PC6 to be secreted into the uterine cavity. The present study examines whether PC6 is detectable in uterine lavage and whether there is any correlation between secreted PC6 levels and uterine receptivity. METHODS Western blotting determined the presence of PC6 protein in uterine lavage. A sensitive and high-throughput activity assay was established and validated. This assay was applied to 103 lavages collected from different phases of the menstrual cycle from women with proven fertility or unexplained infertility. RESULTS Uterine lavage contained PC6 protein with levels paralleling enzymatic activity. PC6 levels were significantly higher in the receptive than in the non-receptive phase in fertile women, and the putative receptive phase levels in a subgroup of women with unexplained infertility were significantly lower than in the fertile counterparts. CONCLUSIONS PC6 levels in uterine lavage are significantly elevated in the luteal phase of fertile women and markedly reduced in a subgroup of women with unexplained infertility. Uterine fluid is a valuable source of material to evaluate uterine function. Detection of PC6 in uterine fluid may lead to the development of a rapid and relatively non-surgical assessment of uterine receptivity.

A proteomic protocol to identify physiological substrates of pro-protein convertases

Proprotein convertases (PCs) convert pro-proteins into their bioactive forms through limited proteolytic cleavage, thereby regulating the temporal and spatial activation of a large number of functionally important proteins. This "converting" process is involved in a wide range of essential physiological and pathological processes, making PCs valuable therapeutic targets. One of the challenges in the field of PC research has been to identify the physiological substrates of a particular PC in a specific tissue or cellular process. Proteomics provides an unprecedented opportunity to identify novel PC substrates in a physiological context. Here we provide a detailed practical procedure utilizing two-dimensional fluorescent differential gel electrophoresis (2D-DiGE) and tandem mass spectrometry techniques, in combination with other standard molecular and biochemical methods, to identify and subsequently validate novel PC6 substrates in a critical uterine event called decidualization. This method is applicable to the study of any PC members and their relevant cellular processes.

Posttranslational activation of bone morphogenetic protein 2 is mediated by proprotein convertase 6 during decidualization for pregnancy establishment

Bone morphogenetic proteins (BMPs) require major posttranslational modifications to become biologically active. One such key modification is endoproteolytic cleavage of the initially synthesized nonactive precursor protein to release the mature ligand. Here we show in a physiological context of uterine stromal decidualization that BMP2 cleavage is mediated by proprotein convertase 5/6 (PC6). Decidualization is a uterine remodeling event critical for embryo implantation. Deletion or knockdown of either BMP2 or PC6 inhibits decidualization causing implantation failure and female infertility. In this study we provide biochemical and physiological evidence that PC6 proteolytically activates BMP2. We used freshly isolated primary human endometrial stromal cells and demonstrated that PC6 was the sole member of the PC family significantly up-regulated during decidualization. The precursor form of BMP2 was reduced, whereas its active form was increased during decidualization. Inhibition of PC6 activity inhibited decidualization, and this was accompanied by a total blockade of BMP2 activation. Addition of recombinant active BMP2 partially rescued the decidualization arrest caused by PC6 inhibition. PC6 processed BMP2 at the KREKR(282) downward arrow cleavage site, and mutating this site prevented the cleavage. This study thus demonstrates for the first time that the proteolytic activation and thus bioavailability of BMP2 is controlled by PC6.

Family planning: today and in the future

This review covers the state of contraceptive development noting new entries in the clinic (mainly steroidal and different delivery methods) and novel leads for nonsteroidal female- and male-methods in the pipeline. The time taken to market and the absence of partnerships with industry are stressed as major factors for the slow progress in their development.

Society for Reproductive Biology Founders' Lecture 2009. Preparing fertile soil: the importance of endometrial receptivity

The human endometrium is receptive for implantation of a blastocyst for only 4–5 days in each menstrual cycle. Failure of implantation is a major reason for infertility in women and the inability to achieve endometrial receptivity is responsible for much of the failure of reproductive technologies. Endometrial receptivity requires changes in the uterine luminal and glandular cells, particularly in terms of their secretory capacity and altered expression of adhesion molecules. In parallel with these changes, decidualisation (differentiation) of the endometrial stroma is initiated in women during the receptive phase, regardless of the presence of a blastocyst. Increased leucocyte numbers are also important. The microenvironments provided by the endometrium during the receptive phase and that support implantation are highly complex and constantly changing as implantation progresses. The present review provides a comprehensive overview of the cellular and molecular events of human implantation. It also summarises work from our laboratories emphasising the functional importance of proprotein convertase 6, along with key cytokines (interleukin-11, leukaemia inhibitory factor, activin A) and chemokines (including CX3CL1 and CCL14), during implantation. Of particular importance is how these mediators contribute to receptivity and how they are disturbed in infertile women. Factors that are critical for uterine receptivity may also be manipulated to provide new contraceptive strategies for women.

Dynamic changes in messenger RNA profiles of bovine endometrium during the oestrous cycle

During the oestrous cycle, the bovine endometrium exhibits characteristic morphological and functional changes, which are mainly induced by progesterone (P(4)), oestrogens and oxytocin. We studied the response of the endometrium to this changing hormonal environment at the transcriptome level using a custom-made cDNA microarray. Endometrium samples were recovered from Simmental heifers on days 0 (oestrus), 3.5 (metoestrus), 12 (dioestrus) and 18. The latter group was divided into animals with high (late dioestrus) and low P(4) levels (preoestrus). Significance analysis of microarrays revealed 269 genes exhibiting significant changes in their transcript levels during the oestrous cycle in distinct temporal patterns. Two major types of expression profiles were observed, which showed the highest mRNA levels during the oestrus phase or the highest levels during the luteal phase respectively. A minor group of genes exhibited the highest mRNA levels on day 3.5. Gene ontology (GO) analyses revealed GO categories related to extracellular matrix remodelling, transport, and cell growth and morphogenesis enriched at oestrus, whereas immune response and particular metabolic pathways were overrepresented at dioestrus. Generation of gene interaction networks uncovered the genes possibly involved in endometrial remodelling (e.g. collagen genes, TNC, SPARC, MMP2, MEP1B, TIMP1, TIMP2, HTRA1), regulation of angiogenesis (e.g. ANGPTL2, TEK, NPY, AGT, EPAS1, KLF5 ), regulation of invasive growth (e.g. PCSK5, tight junction proteins, GRP, LGALS1, ANXA2, NOV, PLAT, MET, TDGF1, CST6, ITGB4), cell adhesion (e.g. MUC16, LGALS3BP) and embryo feeding (e.g. SLC1A1, SLC11A2, SLC16A1, SEPP1, ENPP1). Localisation of mRNA expression in the endometrium was analysed for CLDN4, CLDN10, TJP1, PCSK5, MAGED1, and LGALS1.

As the world grows: contraception in the 21st century

Contraceptives that are readily available and acceptable are required in many poorer countries to reduce population growth and in all countries to prevent maternal morbidity and mortality arising from unintended pregnancies. Most available methods use hormonal steroids or are variations of barrier methods. Reports from several fora over the last 12 years have emphasized the number of unwanted pregnancies and resultant abortions, which indicate an unmet need for safe, acceptable, and inexpensive contraceptive methods. This unmet need can be assuaged, in part, by development of new nonhormonal contraceptive methods. This Review addresses the contribution that the "omic" revolution can make to the identification of novel contraceptive targets, as well as the progress that has been made for different target molecules under development.

Endometrial biopsy-induced gene modulation: first evidence for the expression of bladder-transmembranal uroplakin Ib in human endometrium

OBJECTIVE

To explore the possibility that endometrial injury modulates the expression of specific genes that may increase uterine receptivity.

DESIGN

Controlled clinical study.

SETTING

Clinical IVF unit and academic research center.

PATIENT(S)

IVF patients with 28- to 30-day menstrual cycles.

INTERVENTION(S)

Endometrial biopsies from two groups of patients were collected on days 20-21 of their spontaneous menstrual cycle. The experimental, but not the control, group underwent biopsies on days 11-13 and 21-24 of their preceding cycle.

MAIN OUTCOME MEASURE(S)

Global endometrial gene expression and specific analysis of uroplakin Ib (UPIb) mRNA level throughout the menstrual cycle.

RESULT(S)

Local injury modulated the expression of a wide variety of genes. One of the prominently up-regulated genes was the bladder transmembranal protein, UPIb, whose expression by the endometrium is shown here for the first time. Endometrial UPIb mRNA increases after biopsy in the same cycle wct 2with an additional elevation in the following cycle. Immunohistochemical analysis localized the UPIb protein to the glandular-epithelial cells. Genes encoding other membrane proteins such as adipose differentiation-related protein and mucin 1, transmembrane, were also up-regulated.

CONCLUSION(S)

The biopsy-induced increase in the expression of UPIb and other genes encoding membrane proteins supports the possible importance of the membrane structure and stability during implantation. The specific role of UPIb in uterine receptivity should be elucidated.

Pro-protein convertases (PCs) other than PC6 are not tightly regulated for implantation in the human endometrium

Pro-protein convertases (PCs) are a family of serine proteases (furin, PC1/3, PC2, PACE4, PC4, PC5/6, PC7/8) responsible for post-translational processing and activation of inactive precursors of many regulatory proteins. Endometrial PC6 is critical for implantation in mice and for decidualization of human endometrial stromal cells (ESCs). This study investigated the endometrial expression of other PCs during the menstrual cycle and early pregnancy to elucidate potential redundancies. Furin, PC4, PACE4, and PC7 along with PC6 transcripts were detected in total endometrial RNA, whereas PC1 and PC2 transcription levels were negligible. Quantitative RT-PCR demonstrated highest levels of furin mRNA during menstruation and lowest levels during the proliferative phase. Furin protein was immunolocalized in endometrial luminal and glandular epithelia, stromal fibroblasts, endothelia, and leukocytes. PACE4 and PC7 proteins were also immunodetected in endometrial stroma and glands. Total furin, PC7, and PACE4 proteins were constitutive in both stromal and glandular compartments throughout the cycle and during first trimester pregnancy. Furthermore, Furin and PC7 transcription was unaltered during decidualization of ESCsin vitroin contrast to PC6 which is significantly up-regulated during decidualization. Thus, whereas PC6 is tightly regulated during endometrial preparation for implantation, furin, PACE4, and PC7 are constitutively expressed in human endometrium, but must be considered if PC6 is to be targeted for manipulation of fertility.

Deletion of the gene encoding proprotein convertase 5/6 causes early embryonic lethality in the mouse

PC5 belongs to the proprotein convertase family and activates precursor proteins by cleavage at basic sites during their transit through the secretory pathway and/or at the cell surface. These precursors include prohormones, proreceptors, growth factors, adhesion molecules, and viral glycoproteins. The Pcsk5 gene encodes two alternatively spliced isoforms, the soluble PC5A and transmembrane PC5B. We have carefully analyzed the expression of PC5 in the mouse during development and in adulthood by in situ hybridization, as well as in mouse tissues and various cell lines by quantitative reverse transcription-PCR. The data show that adrenal cortex and intestine are the richest sources of PC5A and PC5B, respectively. To better define the specific physiological roles of PC5, we have generated a mouse Pcsk5(Delta4)-deficient allele missing exon 4 that encodes the catalytic Asp173. While Delta4/+ heterozygotes were healthy and fertile, genotyping of progeny obtained from Delta4/+ interbreeding indicated that Delta4/Delta4 embryos died between embryonic days 4.5 and 7.5. These data demonstrate that Pcsk5 is an essential gene.

Decidual differentiation of stromal cells promotes Proprotein Convertase 5/6 expression and lefty processing

Lefty/Ebaf polypeptides, novel members of the TGF-beta superfamily, are involved in endometrial differentiation and embryo implantation. Recently, we showed that, during undisturbed estrous cycle, lefty is present in mouse uterine horn primarily in a precursor form. Here, we show that decidual differentiation of endometrial stroma leads to increased lefty (approximately 3.1- to 3.6-fold in vivo and 5- to 8-fold in vitro) and processing of its precursor primarily to its long form. This event occurs on d 5 of pregnancy, and is paralleled by proprotein convertase (PC)5/6 up-regulation (approximately 6-fold increase for PC5A and 3-fold increase for PC5B) in decidualized uterine horn, independent of embryo implantation. Among the known convertases, only PC5/6A processes lefty to its long form. Taken together, the findings show that decidualized differentiation of stroma, which is a prerequisite for embryo implantation, leads to processing of lefty by PC5/6A.