Leptin is differentially expressed and epigenetically regulated across monochorionic twin placenta with discordant fetal growth

Severely growth-discordant monochorionic (MC) twins offer a unique opportunity to study fetal and placental growth based on a similar genetic background and maternal host environment where the healthy twin serves as an ideal control. Differences in development of MC twins may therefore be due to differential epigenetic regulation of genes involved in placental development and function. Growth-discordant twins are known for abnormal angio-architecture in the placenta of the smaller twin. Since the reasons for this phenotype are mostly unknown this study was aimed to investigate the expression and regulation of genes known to be involved in angiogenesis. We studied 10 severely growth-discordant MC twin placentas (birthweight difference ≥20%) without twin-twin-transfusion syndrome and 5 growth-concordant MC twin placentas. Growth-discordant twin placentas were phenotyped by histology. Placental mRNA expression of 88 angiogenesis-related genes was measured by PCR array. ELISA assay and immunohistochemistry were used to confirm PCR results. EpiTYPTER for DNA methylation was used to determine if methylation ratios were responsible for differential gene expression. The PCR array analysis showed significant mRNA up-regulation in the placental share of the smaller twin for several genes. These included leptin (24.6-fold, P = 0.017), fms-like tyrosine kinase 1 (Flt1, 2.4-fold, P = 0.016) and Endoglin (Eng, 1.86-fold, P = 0.078). None of the other 84 angiogenesis-related genes showed significant differences. ELISA confirmed significantly increased leptin protein expression (49.22 versus 11.03 pg/ml, P = 0.049) in the smaller twin of the discordant growth cohort. Leptin expression in smaller twins' placentas was associated with elevated DNA methylation of the leptin promotor region suggesting the inhibition of binding of a transcriptional activator/inhibitor in that region. We attempted to overcome the limitation of sample size by careful patient selection. We minimized any bias in placental sampling by random sampling from two different sites and by avoiding sampling from areas with grossly visible abnormalities using a standardized sampling protocol. In conclusion, the smaller twin's placenta is characterized by differentially increased gene expressions for Flt1 and Eng mRNA that may be causally associated with the villous pathology driven by abnormal feto-placental angiogenesis. The substantial up-regulation of leptin mRNA may be epigenetically conferred and relevant to the post-natal risk of metabolic syndrome in intrauterine growth restriction offspring with placental pathology. Growth-discordant MC twins offer unique insights into the epigenetic basis of perinatal programming.

Emerging role of SUMOylation in placental pathology

INTRODUCTION

Small ubiquitin-like modifiers (SUMO) conjugate to target proteins in a dynamic, reversible manner to function as post-translational modifiers. SUMOylation of target proteins can impinge on their localization, in addition to their activity or stability. Differential expression of deSUMOylating enzymes (SENP 1 and 2) contributes to altered mammalian placental development and function in mice. Severe preeclampsia (sPE) is associated with abnormal placental development and chronic ischemic injury. Extra- and intracellular stimuli/stressors that include hypoxic-activated pathways are known modulators of SUMOylation. In this current study we hypothesized that placentas from sPE patients will display up regulation in the SUMO regulatory pathway.

METHODS

Utilizing qRT-PCR, immuno-blotting and Western techniques, we determined the expression levels of SUMO pathway genes in healthy and diseased placentas. We also exposed placental explants to hypoxia to study the effect on the SUMOylation pathway.

RESULTS

We observed steady-state expression of SUMO1-3, SUMO-conjugated enzyme-UBC9 and deSUMOylating enzymes - SENPs, throughout normal gestation. An elevated level of free SUMO1-3 and SUMO-protein conjugates was observed in sPE placentas. Furthermore, placental UBC9 levels were strikingly increased in the same sPE patients. Hypoxia-induced SUMOylation in first trimester placental explants.

DISCUSSION

Our data demonstrate an elevated steady-state of SUMOylation in sPE placentas compared with gestational aged-matched controls. The observed hyper-SUMOylation in sPE placentas correlates with elevated expression of UBC9 rather than with reduced expression of SENPs Hypoxia may contribute to alterations in placental SUMOylation pathway.

CONCLUSION

Increased placental SUMOylation may contribute to the pathogenesis of serious placental pathology that causes extreme preterm birth.

Heparin promotes soluble VEGF receptor expression in human placental villi to impair endothelial VEGF signaling

BACKGROUND

Severe preeclampsia is characterized by hypertension, renal injury and placental dysfunction. Prothrombotic disorders are discovered in 10-20% of women with preeclampsia, providing the rationale for prescribing low-molecular-weight heparin (LMWH) in future pregnancies. Heparin has diverse molecular actions and appears to reduce the recurrence risk of preeclampsia in women without prothrombotic disorders. The placenta-derived anti-angiogenic splice-variant protein soluble vascular endothelial growth factor (VEGF) receptor-1 (sFLT1) is strongly implicated in the pathogenesis of the underlying endothelial dysfunction. As the placental syncytiotrophoblast is the principal source of sFLT1, we tested the hypothesis that heparin suppresses placental sFLT1 secretion.

METHODS AND RESULTS

First trimester placental villi exposed to LMWH (0.25-25 IU mL(-1)) in an in vitro explant model significantly increased the expression and release of sFLT1 by the syncytiotrophoblast into culture media, reducing phosphorylation of FLT1 and KDR receptors in cultured human umbilical vein endothelial cells. This response was significantly diminished in placental villi from healthy term pregnancies. Placental villi from severely preeclamptic pregnancies had a higher baseline sFLT1 release, compared with first trimester placental villi and did not respond to LMWH treatment. LMWH promoted villous cytotrophoblast proliferation (BrdU incorporation) and impaired syncytial fusion-differentiation, causing syncytiotrophoblast apoptosis (by caspase 3&7 activity and TUNEL staining) and necrosis (ADP/ATP ratio).

CONCLUSION

 LMWH promotes sFLT1 synthesis and release from first trimester placental villi in a manner similar to that of severely preeclamptic placental villi, which antagonizes VEGF signaling in endothelial cells. These effects in part are mediated by an interaction between heparin and the cytotrophoblasts that regenerates the overlying syncytiotrophoblast responsible for sFLT1 secretion into the maternal blood.

Villous trophoblast abnormalities in extremely preterm deliveries with elevated second trimester maternal serum hCG or inhibin-A

Elevated levels of the maternal prenatal screening markers hCG and inhibin-A, measured at 15-20 weeks gestation, increase the subsequent risk of severe pre-eclampsia and intra-uterine growth restriction (IUGR). Since both markers are produced by syncytiotrophoblast, we tested the hypothesis that these elevations were due to accelerated differentiation of the villous trophoblast compartment. We performed a retrospective study of 12 cases from our Placenta Clinic with total hCG and/or inhibin-A levels of ≥3.0 multiples of the median that subsequently delivered by 28 weeks gestation and compared their placental pathology findings with 24 gestational age-matched controls. Morphometric analysis demonstrated a 41% reduction in the volume ratio of Ki67 positive cytotrophoblast nuclei to total trophoblast in cases vs controls (Student's T-test; p = 0.028). Distal villous hypoplasia (DVH) was significantly more common in cases (10/12) than controls (4/24); Fisher's exact test, p = 0.002. Wave-like syncytial knot (WLSK) formation was significantly more common in cases (9/12) than controls (1/24); Fisher's exact test, p < 0.0001. WLSK formation was associated with DVH and resulted from accumulation of senescent/apoptotic syncytiotrophoblast nuclei along inherent lines of syncytial nuclear organization. Our data support the hypothesis that elevated second trimester maternal serum levels of total hCG and/or inhibin-A may result from premature accelerated differentiation of the villous cytotrophoblasts. The subsequent pathologic findings in the syncytiotrophoblast could render the pregnancy at risk of severe pre-eclampsia and IUGR.

Glial cell missing-1 transcription factor is required for the differentiation of the human trophoblast

Mammalian placentation is a highly regulated process and is dependent on the proper development of specific trophoblast cell lineages. The two major types of trophoblast, villous and extravillous, show mitotic arrest during differentiation. In mice, the transcription factor, glial cell missing-1 (Gcm1), blocks mitosis and is required for syncytiotrophoblast formation and morphogenesis of the labyrinth, the murine equivalent of the villous placenta. The human homolog GCM1 has an analogous expression pattern, but its function is presently unknown. We studied GCM1 function in the human-derived BeWo choriocarcinoma cell line and in first trimester human placental villous and extravillous explants. The GCM1 expression was either inhibited by siRNA and antisense oligonucleotides methods or upregulated by forskolin treatment. Inhibition of GCM1 resulted in an increased rate of proliferation, but prevented de novo syncytiotrophoblast formation in syncytially denuded floating villous explants. GCM1 inhibition prevented extravillous differentiation along the invasive pathway in extravillous explants on matrigel. By contrast, forskolin-induced expression of GCM1 reduced the rate of proliferation and increased the rate of syncytialization in the floating villous explant model. Our studies show that GCM1 has a distinct role in the maintenance, development and turnover of the human trophoblast. Alterations in GCM1 expression or regulation may explain several aspects of two divergent severe placental insufficiency syndromes, namely preeclampsia and intrauterine growth restriction, which cause extreme preterm birth.

Expression of breast cancer resistance protein (BCRP/ABCG2) in human placenta throughout gestation and at term before and after labor

Breast cancer resistance protein, BCRP, is a multidrug resistance protein that is highly expressed in the human placenta. In cancer tissues, this protein actively extrudes a wide variety of chemically and structurally unrelated chemotherapeutic drugs and other compounds. Studies in mice have shown that in the absence of BCRP activity in the placenta, there is a 2-fold increase in the uptake in BCRP substrates into fetus. This suggests that in the placenta, BCRP extrudes compounds that would otherwise cross the syncytiotrophoblast cells into fetal circulation. The purpose of this study was to examine the expression and localization of BCRP in the human placenta throughout gestation. Tissues from 6–13, 16–19, 24–29, 32–35, and 38–41 weeks of gestation were used. Real time RT-PCR analysis demonstrated that the mRNA levels of BCRP in the placenta do not change significantly as gestation progressed. However, Western blot analysis revealed that the protein levels increased towards the end of gestation. We demonstrated that BCRP is localized to the syncytiotrophoblast of the placenta and in some fetal blood vessels within the placenta. Tissues from the early stages of pregnancy (6–13 weeks) showed fewer BCRP positive blood vessels than term tissues (38–41 weeks).

Bi-potential Behaviour of Cytotrophoblasts in First Trimester Chorionic Villi

Murine trophoblast stem (TS) cells express fibroblast growth factor receptor 2 (FGFR2) and are maintained in their proliferative state by fibroblast growth factor 4 (FGF4). We show in this report that in the first trimester human placenta FGFR2 expression is similarly found in a subset of villous cytotrophoblast and in proximal anchoring columns. Western analysis demonstrated declining FGFR2 protein expression as gestation advanced, suggesting a similar role for FGF in early human trophoblast proliferation. Mouse TS cell differentiation is known to occur along two distinct transcriptionally-regulated pathways; extravillous trophoblast (EVT) cells invade the uterine wall to promote maternal blood flow whilst syncytiotrophoblast lines chorionic villi in the labyrinth. Similar differentiation steps occur in the human placenta though the fate of human trophoblast stem cells is presently unknown. To investigate the mechanisms underlying human cytotrophoblast differentiation we have developed a novel cultured floating first trimester villous explant model in which denuded first trimester villi spontaneously regenerate syncytiotrophoblast following 48 h of culture. Addition of FGF4 and heparin inhibited syncytiotrophoblast regeneration in favor of forming clumps of cytotrophoblast. Proximal cells in these clumps were FGFR2 immuno-reactive and proliferative, intermediate parts expressed alpha5beta1-integrin, while the distal portion expressed HLA-G and the invasive integrin alpha1beta1 indicating differentiation to the EVT phenotype. In contrast, non-denuded villi exposed to FGF4 exhibited similar proliferation of the cytotrophoblast; however, these cells did not express any of the invasive EVT markers. We conclude that FGFR2-positive chorionic cytotrophoblasts exhibit bi-potential behaviour, being capable of forming either syncytiotrophoblast or EVT. We suggest bipotential trophoblast progenitor cells persist during first trimester human placental development.

Trophoblast differentiation: progenitor cells, fusion and migration -- a workshop report

Challenge lies ahead in unravelling the role played by trophoblast and its repertoire of expressed genes in normal human placental development, growth and pathology. Specific technical advances will clearly be required for characterisation of function. In particular, improvements in our repertoire of in vitro models are needed before many of the key questions can be answered. Recent advances in the study of human trophoblast differentiation are discussed.

Expression of the multidrug resistance P-glycoprotein, (ABCB1 glycoprotein) in the human placenta decreases with advancing gestation

The multidrug resistance p-glycoprotein (P-gp), encoded by the ABCB1 gene, is a plasma membrane protein that actively extrudes a wide variety of substances from cells. Preliminary studies in mice have shown that the ABCB1/P-gp can protect the fetus from a number of toxic substances. ABCB1/P-gp is expressed in the human placenta and is potentially capable of protecting the fetus from a large number of drugs and toxins, including herbicides and pesticides. The protein can also extrude various steroids including certain glucocorticoids and may therefore play an important role in regulating fetal access of glucocorticoids. The aim of the present study was to examine the expression profile and cellular localization of ABCB1/P-gp in human placenta throughout gestation. We hypothesized that there would be gestational age-related changes in the expression of the protein. ABCB1/P-gp mRNA was measured by Real-Time PCR using specific probes in tissues obtained from 6 weeks gestation to term. ABCB1/P-gp mRNA levels in placental tissue obtained at 6-10 weeks (n=5) and 24-35 weeks (n=5) were significantly higher than in tissues obtained at term (38-41 weeks gestation) by elective C-section (n=6) or following labor (n=6). The profile of ABCB1/P-gp protein levels, quantified using Western analysis, demonstrated a similar decrease with advancing gestation. At all gestational ages ABCB1/P-gp was localized by immunohistochemistry to the syncytiotrophoblast. In term tissues, it appeared to be localized to some areas of the villi and not others. Together, these data indicate that with advancing gestation there is a decrease in the level of ABCB1/P-gp in the human placenta indicating that the fetus may be more susceptible to toxic insults in the latter part of gestation. Further, the reduction in ABCB1/P-gp expression may contribute to the increased transfer of maternal cortisol to the fetus that is known to occur in late gestation.

Complex Patterns of GCM1 mRNA and Protein in Villous and Extravillous Trophoblast Cells of the Human Placenta

The Gcm1 gene encodes a transcription factor that is essential for both syncytiotrophoblast differentiation and formation of chorionic villi in mice. Its early expression is very unusual in that it defines a subset of trophoblast cells in the chorion, a layer that otherwise contains trophoblast stem cells. While Gcm1 mRNA expression initiates independently within the chorion, the subsequent maintenance of mRNA expression as well as the onset of protein accumulation is dependent on contact with allantoic mesoderm. Previous studies have shown that human GCM1 mRNA and protein are detectable in the placenta, but their patterns have not been compared nor precisely localized. We, therefore, conducted the present study to determine if the human mRNA and protein are subject to the same complexities of regulation as the mouse. In situ hybridization studies showed that the GCM1 mRNA was expressed in villous cytotrophoblast cells, but only a subset and never within cells immediately at the base of columns. Interestingly, the mRNA was detected throughout the cytotrophoblast columns. GCM1 protein expression studies demonstrated that the transcription factor was present mainly within the nuclei of a subset of cytotrophoblast cells, consistent with its role as a transcription factor. Feint cytoplasmic staining of the transcription factor was found in the syncytiotrophoblast but not in aggregated syncytial nuclei. Nuclear immuno-reactivity for the GCM1 protein was detected in occasional nuclei in the distal part of the column. Therefore, GCM1 expression is regulated both at the transcriptional and translational level. Overall, these studies show that the general features of GCM1 mRNA and protein expression in the human placenta are conserved with the mouse. They also highlight the fact that villous cytotrophoblast cells are extremely heterogeneous with respect to GCM1 expression, a factor that should be considered when using isolated cytotrophoblast cells for culture studies.

Genes, development and evolution of the placenta

Through studies of transgenic and mutant mice, it is possible to describe molecular pathways that control the development of all major trophoblast cell subtypes and structures of the placenta. For example, the proliferation of trophoblast stem cells is dependent on FGF signalling and downstream transcription factors Cdx2, Eomes and Err2. Several bHLH transcription factors regulate the progression from trophoblast stem cells to spongiotrophoblast and to trophoblast giant cells (Id1/2, Mash2, Hand1, Stra13). Intercellular actions critical for maintaining stable precursor cell populations are dependent on the gap junction protein Cx31 and the growth factor Nodal. Differentiation towards syncytiotrophoblast as well as the initiation of chorioallantoic (villous) morphogenesis is regulated by the Gcm1 transcription factor, and subsequent labyrinth development is dependent on Wnt, HGF and FGF signalling. These insights suggest that most of the genes that evolved to regulate placental development are either identical to ones used in other organ systems (e.g., FGF and epithelial branching morphogenesis), were co-opted to take on new functions (e.g., AP-2gamma, Dlx3, Hand1), or arose via gene duplication to take on a specialized placental function (e.g., Gcm1, Mash2). Many of the human orthologues of these critical genes show restricted expression patterns that are consistent with a conserved function. Such information is aiding the comparison of the human and mouse placenta. In addition, the prospect of a conserved function clearly suggests potential mechanisms for explaining complications of human placental development.