Human cytotrophoblast expression of the von Hippel-Lindau protein is downregulated during uterine invasion in situ and upregulated by hypoxia in vitro

Human Chorionic Villous Differentiation and Placental Development

In humans, the placenta provides the only fetomaternal connection and is essential for establishing a pregnancy as well as fetal well-being. Additionally, it allows maternal physiological adaptation and embryonic immunological acceptance, support, and nutrition. The placenta is derived from extra-embryonic tissues that develop rapidly and dynamically in the first weeks of pregnancy. It is primarily composed of trophoblasts that differentiate into villi, stromal cells, macrophages, and fetal endothelial cells (FEC). Placental differentiation may be closely related to perinatal diseases, including fetal growth retardation (FGR) and hypertensive disorders of pregnancy (HDP), and miscarriage. There are limited findings regarding human chorionic villous differentiation and placental development because conducting in vivo studies is extremely difficult. Placental tissue varies widely among species. Thus, experimental animal findings are difficult to apply to humans. Early villous differentiation is difficult to study due to the small tissue size; however, a detailed analysis can potentially elucidate perinatal disease causes or help develop novel therapies. Artificial induction of early villous differentiation using human embryonic stem (ES) cells/induced pluripotent stem (iPS) cells was attempted, producing normally differentiated villi that can be used for interventional/invasive research. Here, we summarized and correlated early villous differentiation findings and discussed clinical diseases.

JMJD6 Dysfunction Due to Iron Deficiency in Preeclampsia Disrupts Fibronectin Homeostasis Resulting in Diminished Trophoblast Migration

The mechanisms contributing to excessive fibronectin in preeclampsia, a pregnancy-related disorder, remain unknown. Herein, we investigated the role of JMJD6, an O₂- and Fe²⁺-dependent enzyme, in mediating placental fibronectin processing and function. MALDI-TOF identified fibronectin as a novel target of JMJD6-mediated lysyl hydroxylation, preceding fibronectin glycosylation, deposition, and degradation. In preeclamptic placentae, fibronectin accumulated primarily in lysosomes of the mesenchyme. Using primary placental mesenchymal cells (pMSCs), we found that fibronectin fibril formation and turnover were markedly impeded in preeclamptic pMSCs, partly due to impaired lysosomal degradation. JMJD6 knockdown in control pMSCs recapitulated the preeclamptic FN phenotype. Importantly, preeclamptic pMSCs had less total and labile Fe²⁺ and Hinokitiol treatment rescued fibronectin assembly and promoted lysosomal degradation. Time-lapse imaging demonstrated that defective ECM deposition by preeclamptic pMSCs impeded HTR-8/SVneo cell migration, which was rescued upon Hinokitiol exposure. Our findings reveal new Fe²⁺-dependent mechanisms controlling fibronectin homeostasis/function in the placenta that go awry in preeclampsia.

Human placental cytotrophoblast epigenome dynamics over gestation and alterations in placental disease

The human placenta and its specialized cytotrophoblasts rapidly develop, have a compressed lifespan, govern pregnancy outcomes, and program the offspring's health. Understanding the molecular underpinnings of these behaviors informs development and disease. Profiling the extraembryonic epigenome and transcriptome during the 2nd and 3rd trimesters revealed H3K9 trimethylation overlapping deeply DNA hypomethylated domains with reduced gene expression and compartment-specific patterns that illuminated their functions. Cytotrophoblast DNA methylation increased, and several key histone modifications decreased across the genome as pregnancy advanced. Cytotrophoblasts from severe preeclampsia had substantially increased H3K27 acetylation globally and at genes that are normally downregulated at term but upregulated in this syndrome. In addition, some cases had an immature pattern of H3K27ac peaks, and others showed evidence of accelerated aging, suggesting subtype-specific alterations in severe preeclampsia. Thus, the cytotrophoblast epigenome dramatically reprograms during pregnancy, placental disease is associated with failures in this process, and H3K27 hyperacetylation is a feature of severe preeclampsia.

CCN3 Signaling Is Differently Regulated in Placental Diseases Preeclampsia and Abnormally Invasive Placenta

OBJECTIVES

An adequate development of the placenta includes trophoblast differentiation with the processes of trophoblast migration, invasion, cellular senescence and apoptosis which are all crucial to establishing a successful pregnancy. Altered placental development and function lead to placental diseases such as preeclampsia (PE) which is mainly characterized by insufficient trophoblast invasion and abnormally invasive placenta (AIP) disorders (Placenta accreta, increta, or percreta) which are characterized by excessive trophoblast invasion. Both of them will cause maternal and fetal morbidity/mortality. However, the etiology of these diseases is still unclear. Our previous study has shown that the matricellular protein nephroblastoma overexpressed (NOV, CCN3) induces G0/G1 cell cycle arrest, drives trophoblast cells into senescence and activates FAK and Akt kinases resulting in reduced cell proliferation and enhanced migration capability of the human trophoblast cell line SGHPL-5. The present study focuses on whether CCN3 can alter cell cycle-regulated pathways associated with trophoblast senescence and invasion activity in pathological versus gestational age-matched control placentas.

METHODS

Cell cycle regulator proteins were investigated by immunoblotting and qPCR. For localization of CCN3, p16, p21, and Cyclin D1 proteins, co-immunohistochemistry was performed.

RESULTS

In early-onset PE placentas, CCN3 was expressed at a significantly lower level compared to gestational age-matched controls. The decrease of CCN3 level is associated with an increase in p53, Cyclin E1 and pRb protein expression, whereas the level of cleaved Notch-1, p21, Cyclin D1, pFAK, pAKT, and pmTOR protein decreased. In term AIP placentas, the expression of CCN3 was significantly increased compared to matched term controls. This increase was correlated to an increase in p53, p16, p21, Cyclin D1, cleaved Notch-1, pFAK, pAkt, and pmTOR whereas pRb was significantly decreased. However, in late PE and early AIP placentas, no significant differences in CCN3, p16, p21, Cyclin D1, p53, and cleaved Notch-1 expression were found when matched to appropriate controls.

CONCLUSIONS

CCN3 expression levels are correlated to markers of cell cycle arrest oppositely in PE and AIP by activating the FAK/AKT pathway in AIP or down-regulating in PE. This may be one mechanism to explain the different pathological features of placental diseases, PE and AIP.

Comparison of phosphorescent agents for noninvasive sensing of tumor oxygenation via Cherenkov-excited luminescence imaging

Cherenkov emission generated in tissue during radiotherapy can be harnessed for the imaging biochemistry of tissue microenvironments. Cherenkov-excited luminescence scanned imaging (CELSI) provides a way to optically and noninvasively map oxygen-related signals, which is known to correlate to outcomes in radiotherapy. Four candidate phosphorescent reagents PtG4, MM2, Ir(btb)2  (  acac  )  , and MitoID were studied for oxygen sensing, testing in a progressive series of (a) in solution, (b) in vitro, and (c) in subcutaneous tumors. In each test, the signal strength and response to oxygen were assessed by phosphorescence intensity and decay lifetime measurement. MM2 showed the most robust response to oxygen changes in solution, followed by PtG4, Ir(btb)2  (  acac  )  , and MitoID. However, in PANC-1 cells, their oxygen responses differed with Ir(btb)2  (  acac  )   exhibiting the largest phosphorescent intensity change in response to changes in oxygenation, followed by PtG4, MM2, and MitoID. In vivo, it was only possible to utilize Ir(btb)2  (  acac  )   and PtG4, with each being used at nanomole levels, to determine signal strength, lifetime, and pO2. Oxygen sensing with CELSI during radiotherapy is feasible and can estimate values from 1 mm regions of tissue when used in the configuration of this study. PtG4 was the most amenable to in vivo sensing on the timescale of external beam LINAC x-rays.

Hypoxia-induced TET1 facilitates trophoblast cell migration and invasion through HIF1α signaling pathway

Low oxygen is a typical extrinsic factor for the regulation of trophoblast biological function, including cell migration, invasion and proliferation. Ten-eleven translocation methylcytosine dioxygenase 1 (TET1), an enzyme converting 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC), is transcriptionally activated by hypoxia in cancer cells. Therefore, we focus on the role of TET1 on trophoblast function in a physiologically hypoxic environment (3% oxygen), which is related to early placentation. Here, we found that TET1 was highly expressed in first trimester villi compared with normal term placentas. In vitro, both TET1 mRNA and protein expression levels in JEG3 cells were increased following exposure to 3% oxygen, and the migration and invasion capacities of JEG3 cells were up-regulated. Furthermore, TET1 knockdown decreased the migration, invasion and proliferation of JEG3 cells exposed to 3% oxygen, and the expression of HIF1α and its downstream target genes was also decreased, which was related to hyper-methylation of the HIF1α promoter. Finally, increased HIF1α protein expression reversed the inhibitory effect of TET1 knockdown on the migration and invasion of JEG3 cells exposed to 3% oxygen. These data show that hypoxia-induced TET1 expression facilitates trophoblast cell migration and invasion through the HIF1α signaling pathway, which plays an important role during placentation.

Hypoxia Directs Human Extravillous Trophoblast Differentiation in a Hypoxia-Inducible Factor-Dependent Manner

Villous cytotrophoblasts are epithelial stem cells of the early human placenta, able to differentiate either into syncytiotrophoblasts in floating chorionic villi or extravillous trophoblasts (EVTs) at the anchoring villi. The signaling pathways regulating differentiation into these two lineages are incompletely understood. The bulk of placental growth and development in the first trimester occurs under low oxygen tension. One major mechanism by which oxygen regulates cellular function is through the hypoxia-inducible factor (HIF), a transcription factor complex stabilized under low oxygen tension to mediate cellular responses, including cell fate decisions. HIF is known to play a role in trophoblast differentiation in rodents; however, its role in human trophoblast differentiation is poorly understood. Using RNA profiling of sorted populations of primary first-trimester trophoblasts, we evaluated the first stage of EVT differentiation, the transition from epidermal growth factor receptor⁺ villous cytotrophoblasts into human leukocyte antigen-G⁺ proximal column EVT (pcEVT) and identified hypoxia as a major pcEVT-associated pathway. Using primary cytotrophoblasts, we determined that culture in low oxygen directs differentiation preferentially toward human leukocyte antigen-G⁺ pcEVT, and that an intact HIF complex is required for this process. Finally, using global RNA profiling, we identified integrin-linked kinase and associated cytoskeletal remodeling and adhesion to be among HIF-dependent pcEVT-associated signaling pathways. Taken together, we propose that oxygen regulates EVT differentiation through HIF-dependent modulation of various cell adhesion and morphology-related pathways.

Oxygen Sensitivity of Placental Trophoblast Connexins 43 and 46: A Role in Preeclampsia?

Several gap junction connexins have been shown to be essential for appropriate placental development and function. It is known that the expression and distribution of connexins change in response to environmental oxygen levels. The placenta develops under various oxygen levels, beginning at a low oxygen tension of approximately 2% and increasing to a tension of 8% after the onset of the uteroplacental circulation. Moreover, it has been shown that during preeclampsia (PE) placentas are subjected to chronic hypoxia. Therefore, we investigated oxygen sensitivity of placental connexins 43 and 46. Using the trophoblast cell line Jar, we demonstrated that the expression of connexin43 increased during acute hypoxia but decreased during chronic hypoxia. Chronic hypoxia resulted in the translocation of connexin43 from the membrane to the cytoplasm and in a reduction in its communication properties. In contrast, the expression of connexin46 was down-regulated during chronic hypoxia and was translocated from perinuclear areas to the cell membrane. Hypoxia-inducible factor (HIF) knockdown showed that the translocation of connexin43 but not that of connexin46 was HIF-2α dependent and was mediated by phosphoinositide 3-kinase. The up-regulation of connexin43 in combination with the down-regulation of connexin46 was confirmed in placental explants cultivated under low oxygen and in placentas with early-onset PE. Taken together, in Jar cells, placental connexins 43 and 46 are regulated during periods of low oxygen in opposite manners. The oxygen sensing of connexins in the trophoblast may play a role in physiological and pathophysiological oxygen conditions and thus may contribute to PE.

Acute hypoxia induces upregulation of microRNA-210 expression in glioblastoma spheroids

AIM

Tumor hypoxia and presence of tumor stem cells are related to therapeutic resistance and tumorigenicity in glioblastomas. The aim of the present study was therefore to identify microRNAs deregulated in acute hypoxia and to identify possible associated changes in stem cell markers.

MATERIALS & METHODS

Glioblastoma spheroid cultures were grown in either 2 or 21% oxygen. Subsequently, miRNA profiling was performed and expression of ten stem cell markers was examined.

RESULTS

MiRNA-210 was significantly upregulated in hypoxia in patient-derived spheroids. The stem cell markers displayed a complex regulatory pattern.

CONCLUSION

MiRNA-210 appears to be upregulated in hypoxia in immature glioblastoma cells. This miRNA may represent a therapeutic target although it is not clear from the results whether this miRNA may be related to specific cancer stem cell functions.

Uterine artery dysfunction in pregnant ACE2 knockout mice is associated with placental hypoxia and reduced umbilical blood flow velocity

Angiotensin-converting enzyme 2 (ACE2) knockout is associated with reduced fetal weight at late gestation; however, whether uteroplacental vascular and/or hemodynamic disturbances underlie this growth-restricted phenotype is unknown. Uterine artery reactivity and flow velocities, umbilical flow velocities, trophoblast invasion, and placental hypoxia were determined in ACE2 knockout (KO) and C57Bl/6 wild-type (WT) mice at day 14 of gestation. Although systolic blood pressure was higher in pregnant ACE2 KO vs. WT mice (102.3 ± 5.1 vs. 85.1 ± 1.9 mmHg, n = 5-6), the magnitude of difference was similar to that observed in nonpregnant ACE2 KO vs. WT mice. Maternal urinary protein excretion, serum creatinine, and kidney or heart weights were not different in ACE2 KO vs. WT. Fetal weight and pup-to-placental weight ratio were lower in ACE2 KO vs. WT mice. A higher sensitivity to Ang II [pD2 8.64 ± 0.04 vs. 8.5 ± 0.03 (-log EC50)] and greater maximal contraction to phenylephrine (169.0 ± 9.0 vs. 139.0 ± 7.0% KMAX), were associated with lower immunostaining for Ang II receptor 2 and fibrinoid content of the uterine artery in ACE2 KO mice. Uterine artery flow velocities and trophoblast invasion were similar between study groups. In contrast, umbilical artery peak systolic velocities (60.2 ± 4.5 vs. 75.1 ± 4.5 mm/s) and the resistance index measured using VEVO 2100 ultrasound were lower in the ACE2 KO vs. WT mice. Immunostaining for pimonidazole, a marker of hypoxia, and hypoxia-inducible factor-2α were higher in the trophospongium and placental labyrinth of the ACE2 KO vs. WT. In summary, placental hypoxia and uterine artery dysfunction develop before major growth of the fetus occurs and may explain the fetal growth restricted phenotype.

Energy status and HIF signalling in chorionic villi show no evidence of hypoxic stress during human early placental development

Early human placental and embryonic development occurs in a physiologically low oxygen environment supported by histiotrophic secretions from endometrial glands. In this study, we compare the placental metabolomic profile in the first, second and third trimesters to determine whether the energy demands are adequately met in the first trimester. We investigated whether hypoxia-inducible factors, HIF-1α and/or HIF-2α, might regulate transcription during the first trimester. First and second trimester tissue was collected using a chorionic villus sampling-like (CVS) technique. Part of each villus sample was frozen immediately and the remainder cultured under 2 or 21% O2 ± 1 mM H2O2, and ±the p38 MAPK pathway inhibitor, PD169316. Levels of HIF-1α were assessed by western blotting and VEGFA, PlGF and GLUT3 transcripts were quantified by RT-PCR. Term samples were collected from normal elective Caesarean deliveries. There were no significant differences in concentrations of ADP, NAD(+), lactate, and glucose, and in the ATP/ADP ratio, across gestational age. Neither HIF-1α nor HIF-2α could be detected in time-zero CVS samples. However, culture under any condition (2 or 21% O2 ± 1 mM H2O2) increased HIF-1α and HIF-2α. HIF-1α and HIF-2α were additionally detected in specimens retrieved after curettage. HIF-1α stabilization was accompanied by significant increases in VEGFA and GLUT3 and a decrease in PlGF mRNAs. These effects were suppressed by PD169316. In conclusion, our data suggest that first trimester placental tissues are not energetically compromised, and that HIF-1α is unlikely to play an appreciable role in regulating transcriptional activity under steady-state conditions in vivo. However, the pathway may be activated by stress conditions.

LIMK1 regulates human trophoblast invasion/differentiation and is down-regulated in preeclampsia

Successful human pregnancy requires extensive invasion of maternal uterine tissues by the placenta. Invasive extravillous trophoblasts derived from cytotrophoblast progenitors remodel maternal arterioles to promote blood flow to the placenta. In the pregnancy complication preeclampsia, extravillous trophoblasts invasion and vessel remodeling are frequently impaired, likely contributing to fetal underperfusion and maternal hypertension. We recently demonstrated in mouse trophoblast stem cells that hypoxia-inducible factor-2 (HIF-2)-dependent Lim domain kinase 1 (LIMK1) expression regulates invasive trophoblast differentiation by modulating the trophoblast cytoskeleton. Interestingly, in humans, LIMK1 activity promotes tumor cell invasion by modulating actin and microtubule integrity, as well as by modulating matrix metalloprotease processing. Here, we tested whether HIF-2α and LIMK1 expression patterns suggested similar roles in the human placenta. We found that LIMK1 immunoreactivity mirrored HIF-2α in the human placenta in utero and that LIMK1 activity regulated human cytotrophoblast cytoskeletal integrity, matrix metallopeptidase-9 secretion, invasion, and differentiation in vitro. Importantly, we also found that LIMK1 levels are frequently diminished in the preeclampsia setting in vivo. Our results therefore validate the use of mouse trophoblast stem cells as a discovery platform for human placentation disorders and suggest that LIMK1 activity helps promote human placental development in utero.

Oxygen regulates human cytotrophoblast migration by controlling chemokine and receptor expression

INTRODUCTION

Placental development involves the variation of oxygen supply due to vascular changes and cytotrophoblast invasion. Chemokines and their receptors play an important role during placental formation. Herein, the analysis of the chemokine/receptor pair CXCL12/CXCR4 and further chemokine receptors, such as CCR1, CCR7 and CXCR6 expression in human cytotrophoblasts was conducted.

METHODS

Human cytotrophoblasts were examined directly after isolation or after incubation with different oxygen tensions and a chemical HIF-stimulator for 12 h with realtime PCR, immunoblot, immunohistochemistry. Conditioned media of placental villi, decidua, and endothelial cells was used for ELISA analysis of CXL12. Cytotrophoblast migration assays were conducted applying conditioned media of endothelial cells, a CXCL12 gradient, and different oxygen level. Endometrial and decidual tissue was stained for CXCL12 expression.

RESULTS

An upregulation of CXCL12, CXCR4, CCR1, CCR7 and CXCR6 was observed after cytotrophoblast differentiation. Low oxygen supply upregulated CXCR4, CCR7 and CXCR6, but downregulated CXCL12 and CCR1. In contrast to the HIF associated upregulation of the aforementioned proteins, downregulation of CXCL12 and CCR1 seemed to be HIF independent. Cytotrophoblast migration was stimulated by low oxygen, the application of a CXCL12 gradient and endothelial cell conditioned media. CXCL12 was detected in endometrial vessels, glands and conditioned media of placental and decidual tissue, but not decidual vessels.

DISCUSSION/CONCLUSION

Taken together, oxygen supply and cytotrophoblast differentiation seem to be regulators of chemokine and receptor expression and function in human cytotrophoblasts. Therefore, this system seems to be involved in placental development, directed cytotrophoblast migration in the decidual compartment and a subsequent sufficient supply of the growing fetus.

Human trophoblast progenitors: where do they reside?

In humans, very little is known about the factors that regulate trophoblast (TB) specification, expansion of the initial TB population, and formation of the cytotrophoblast (CTB) populations that populate the chorionic villi. The absence of human trophoblast progenitor cell (hTPC) lines that can be propagated in vitro has been a limiting factor. Because attempts to derive TB stem cells from the trophectoderm of the human blastocyst have so far failed, investigators use alternative systems as cell culture models including TBs derived from human embryonic stem cells (hESCs), immortalized CTBs, and cell lines established from TB tumors. Additionally, the characteristics of mature TBs have been extensively studied using primary cultures of CTBs and explants of placental chorionic villi. However, none of these models can be used to study TB progenitor self-renewal and differentiation. Furthermore, the propagation of human TB progenitors from villous CTBs (vCTBs) has not been achieved. The downregulation of key markers of cell cycle progression in vCTBs by the end of the first trimester of pregnancy may indicate that these cells are not a source of human TB progenitors later in pregnancy. In contrast, mesenchymal cells of the villi and chorion continue to proliferate until the end of pregnancy. We recently reported isolation of continuously self-renewing hTPCs from chorionic mesenchyme and showed that they differentiated into the mature TB cell types of the villi, evidence that they can function as TB progenitors. This new cell culture model enables a molecular analysis of the seminal steps in human TB differentiation that have yet to be studied in humans. In turn, this information can be used to trace the origins of pregnancy complications that are associated with faulty TB growth and differentiation.

Comparative expression of hCG β-genes in human trophoblast from early and late first-trimester placentas

Human chorionic gonadotropin (hCG) displays a major role in pregnancy initiation and progression and is involved in trophoblast differentiation and fusion. However, the site and the type of dimeric hCG production during the first trimester of pregnancy is poorly known. At that time, trophoblastic plugs present in the uterine arteries disappear, allowing unrestricted flow of maternal blood to the intervillous space. The consequence is an important modification of the trophoblast environment, including a rise of oxygen levels from about 2.5% before 10 wk of amenorrhea (WA) to ∼8% after 12 WA. Two specific β-hCG proteins that differ from three amino acids have been described: type 1 (CGB7) and type 2 (CGB3, -5, and -8). Here, we demonstrated in situ and ex vivo on placental villi and in vitro in primary cultures of human cytotrophoblasts that type 1 and 2 β-hCG RNAs and proteins were expressed by trophoblasts and that these expressions were higher before blood enters in the intervillous space (8-9 vs. 12-14 WA). hCG was immunodetected in villous mononucleated cytotrophoblasts (VCT) and syncytiotrophoblast (ST) at 8-9 WA but only in ST at 12-14 WA. Furthermore, hCG secretion was fourfold higher in VCT cultures from 8-9 WA compared with 12-14 WA. Interestingly, VCT from 8-9 WA placentas were found to exhibit more fusion features. Taken together, we showed that type 1 and type 2 β-hCG are highly expressed by VCT in the early first trimester, contributing to the high levels of hCG found in maternal serum at this term.

Increased expression of the von Hippel-Lindau gene in the implantation site of human tubal pregnancy

The objective of this study was to investigate the expression of the von Hippel-Lindau (VHL) gene in tissues of human fallopian tube and tubal pregnancy. Twenty patients undergoing salpingectomy for tubal pregnancy were recruited into the study group. Tissues of tubal pregnancy were separated into both the implantation and non-implantation sites as the implantation group and the non-implantation group, respectively. Samples of ampullary fallopian tube during mid-secretory phase were collected from twenty patients with benign uterine disease as the control group. Immunohistochemistry, real-time reverse transcription polymerase chain reaction, and Western blotting analysis were performed to detect expressions of VHL mRNA and protein. The results showed that VHL immunostaining appeared in the cytoplasm of tubal epithelial cells. Expression of VHL mRNA in the implantation group was higher than that in the non-implantation group or the control group (P < 0.01). Intensity of VHL protein in the implantation group was increased compared with that in the non-implantation group (P < 0.05) or in the control group (P < 0.01). There was no difference on expressions of VHL mRNA and protein between the non-implantation group and the control group (P > 0.05). In conclusion, VHL mRNA and protein are present in human tubal tissues. The VHL gene expression is increased in the implantation site of tubal pregnancy, and locally elevated expression of the VHL gene might be associated with human tubal pregnancy.

Establishment of human trophoblast progenitor cell lines from the chorion

Placental trophoblasts are key determinants of in utero development. Mouse trophoblast (TB) stem cells, which were first derived over a decade ago, are a powerful cell culture model for studying their self-renewal or differentiation. Our attempts to isolate an equivalent population from the trophectoderm of human blastocysts generated colonies that quickly differentiated in vitro. This finding suggested that the human placenta has another progenitor niche. Here, we show that the chorion is one such site. Initially, we immunolocalized pluripotency factors and TB fate determinants in the early gestation placenta, amnion, and chorion. Immunoreactive cells were numerous in the chorion. We isolated these cells and plated them in medium containing fibroblast growth factor which is required for human embryonic stem cell self-renewal, and an inhibitor of activin/nodal signaling. Colonies of polarized cells with a limited lifespan emerged. Trypsin dissociation yielded continuously self-replicating monolayers. Colonies and monolayers formed the two major human TB lineages-multinucleate syncytiotrophoblasts and invasive cytotrophoblasts (CTBs). Transcriptional profiling experiments revealed the factors associated with the self-renewal or differentiation of human chorionic TB progenitor cells (TBPCs). They included imprinted genes, NR2F1/2, HMGA2, and adhesion molecules that were required for TBPC differentiation. Together, the results of these experiments suggested that the chorion is one source of epithelial CTB progenitors. These findings explain why CTBs of fully formed chorionic villi have a modest mitotic index and identify the chorionic mesoderm as a niche for TBPCs that support placental growth.

GROα regulates human embryonic stem cell self-renewal or adoption of a neuronal fate

Previously we reported that feeders formed from human placental fibroblasts (hPFs) support derivation and long-term self-renewal of human embryonic stem cells (hESCs) under serum-free conditions. Here, we show, using antibody array and ELISA platforms, that hPFs secrete ∼6-fold higher amounts of the CXC-type chemokine, GROα, than IMR 90, a human lung fibroblast line, which does not support hESC growth. Furthermore, immunocytochemistry and immunoblot approaches revealed that hESCs express CXCR, a GROα receptor. We used this information to develop defined culture medium for feeder-free propagation of hESCs in an undifferentiated state. Cells passaged as small aggregates and maintained in the GROα-containing medium had a normal karyotype, expressed pluripotency markers, and exhibited apical-basal polarity, i.e., had the defining features of pluripotent hESCs. They also differentiated into the three primary (embryonic) germ layers and formed teratomas in immunocompromised mice. hESCs cultured as single cells in the GROα-containing medium also had a normal karyotype, but they downregulated markers of pluripotency, lost apical-basal polarity, and expressed markers that are indicative of the early stages of neuronal differentiation-βIII tubulin, vimentin, radial glial protein, and nestin. These data support our hypothesis that establishing and maintaining cell polarity is essential for the long-term propagation of hESCs in an undifferentiated state and that disruption of cell-cell contacts can trigger adoption of a neuronal fate.

Antibody treatment promotes compensation for human cytomegalovirus-induced pathogenesis and a hypoxia-like condition in placentas with congenital infection

Human cytomegalovirus (HCMV) is the major viral cause of birth defects worldwide. Affected infants can have temporary symptoms that resolve soon after birth, such as growth restriction, and permanent disabilities, including neurological impairment. Passive immunization of pregnant women with primary HCMV infection is a promising treatment to prevent congenital disease. To understand the effects of sustained viral replication on the placenta and passive transfer of protective antibodies, we performed immunohistological analysis of placental specimens from women with untreated congenital infection, HCMV-specific hyperimmune globulin treatment, and uninfected controls. In untreated infection, viral replication proteins were found in trophoblasts and endothelial cells of chorionic villi and uterine arteries. Associated damage included extensive fibrinoid deposits, fibrosis, avascular villi, and edema, which could impair placental functions. Vascular endothelial growth factor and its receptor fms-like tyrosine kinase 1 (Flt1) were up-regulated, and amniotic fluid contained elevated levels of soluble Flt1 (sFlt1), an antiangiogenic protein, relative to placental growth factor. With hyperimmune globulin treatment, placentas appeared uninfected, vascular endothelial growth factor and Flt1 expression was reduced, and sFlt1 levels in amniotic fluid were lower. An increase in the number of chorionic villi and blood vessels over that in controls suggested compensatory development for a hypoxia-like condition. Taken together the results indicate that antibody treatment can suppress HCMV replication and prevent placental dysfunction, thus improving fetal outcome.

Benzopyrene and experimental stressors cause compensatory differentiation in placental trophoblast stem cells

Stress causes decreased cell accumulation in early periimplantation embryos and the placental trophoblast stem cells derived from them. Benzopyrene and many other stressors activate stress enzymes that lead to suppressed stem cell accumulation through diminished proliferation and increased apoptosis. Trophoblast stem cells proliferate and a subpopulation of early postimplantation trophoblast cells differentiate to produce the first placental hormones that arise in the implanting conceptus. These hormones mediate antiluteolytic effects that enable the continuation of a successful implantation. The normal determination and differentiation of placental trophoblast stem cells is dependent upon a series of transcription factors. But, these transcription factors can also be modulated by stress through the activity of stress enzymes. This review enumerates and analyzes recent reports on the effects of benzopyrene on placental function in terms of the emerging paradigm that placental differentiation from stem cells can be regulated when insufficient production of stem cells is caused by stress. In addition, we review the other effects caused by benzopyrene throughout placental development.

Beyond oxygen: complex regulation and activity of hypoxia inducible factors in pregnancy

In the first trimester the extravillous cytotrophoblast cells occlude the uterine spiral arterioles creating a low oxygen environment early in pregnancy, which is essential for pregnancy success. Paradoxically, shallow trophoblast invasion and defective vascular remodelling of the uterine spiral arteries in the first trimester may result in impaired placental perfusion and chronic placental ischemia and hypoxia later in gestation leading to adverse pregnancy outcomes. The hypoxia inducible factors (HIFs) are key mediators of the response to low oxygen. We aimed to elucidate mechanisms of regulation of HIFs and the role these may play in the control of placental differentiation, growth and function in both normal and pathological pregnancies. The Pubmed database was consulted for identification of the most relevant published articles. Search terms used were oxygen, placenta, trophoblast, pregnancy, HIF and hypoxia. The HIFs are able to function throughout all aspects of normal and abnormal placental differentiation, growth and function; during the first trimester (physiologically low oxygen), during mid-late gestation (where there is adequate supply of blood and oxygen to the placenta) and in pathological pregnancies complicated by placental hypoxia/ischemia. During normal pregnancy HIFs may respond to complex alterations in oxygen, hormones, cytokines and growth factors to regulate placental invasion, differentiation, transport and vascularization. In the ever-changing environment created during pregnancy, the HIFs appear to act as key mediators of placental development and function and thereby are likely to be important contributors to both normal and adverse pregnancy outcomes.

Human embryonic stem cells as a model for embryotoxicity screening

Reproductive toxicity encompasses harmful effects of various agents on all aspects and stages of the reproductive cycle, including infertility and the induction of adverse effects in the embryo/fetus. In developing a model for reproductive toxicity screening, it is important to define the stage of the human reproductive cycle that this specific model is going to recreate in vitro and to identify molecular targets that are critical for this stage of development. In this review, we focus our discussion on modeling pre-implantation embryotoxicity. The rationale for this is that despite advances on both clinical and biological levels, many unresolved infertility cases may be due to our lack of knowledge regarding environmental influences on this short, but critical stage of development. Data from in vitro fertilization practice suggest that the early-dividing embryo is very sensitive to numerous factors present in its microenvironment. In vivo, as the embryo travels down the oviduct, physical or chemical insults can directly damage the embryo and/or prevent implantation, and cause infertility. Multiple lines of evidence point to the differences between mouse and human pre-implantation development and between mouse and human embryonic stem cells (hESCs). In light of these data we present the case that hESCs and their derivatives are better suited as in vitro models for human pre-implantation development than their mouse counterparts. We then describe some of the most promising hESC-based systems that are used today to model certain aspects of development in the human pre-implantation embryo and that have the potential to be used for embryo toxicity screening tests in the near future. Described systems model two major events during differentiation of the human pre-implantation embryo: differentiation of the trophectoderm and segregation of the inner cell mass into epiblast and hypoblast. The first event is replicated in vitro by triggering either direct or indirect (through embryoid body stage) differentiation into trophectoderm. The second event can be modeled using the recently described system of high-throughput generation of embryoid bodies that recapitulate segregation of inner cell mass. We conclude by discussing the potential of these existing models in toxicology studies and the possibilities for their improvement in the future.

Cytomegalovirus infection in the human placenta: maternal immunity and developmentally regulated receptors on trophoblasts converge

During human pregnancy, CMV infects the uterine-placental interface with varied outcomes from fetal intrauterine growth restriction to permanent birth defects, depending on the level of maternal immunity and gestational age. Virus spreads from infected uterine blood vessels, amplifies by replicating in decidual cells, and disseminates to the placenta in immune complexes. Cytotrophoblasts--epithelial cells of the placenta--differentiate along two distinct pathways. In the first, cells fuse into syncytiotrophoblasts covering the surface of chorionic villi that transport substances from the maternal to fetal bloodstream. In the second, cells invade the uterine interstitium and blood vessels, remodel the vasculature and form anchoring villi. CMV initiates replication in cytotrophoblast progenitor cells of floating villi, whereas syncytiotrophoblasts are spared. This extraordinary pattern of focal infection in underlying cells hinges on virion receptors being upregulated as villous cytotrophoblasts begin to differentiate. Expression of developmentally regulated receptors could explain viral replication in spatially distinct maternal and fetal compartments. Reduced invasiveness of infected cells could impair remodeling of the uterine vasculature, restrict maternal blood flow and access of the fetus to nutrients causing intrauterine growth restriction.

Abnormal heart development and lung remodeling in mice lacking the hypoxia-inducible factor-related basic helix-loop-helix PAS protein NEPAS

Hypoxia-inducible factors (HIFs) are crucial for oxygen homeostasis during both embryonic development and postnatal life. Here we show that a novel HIF family basic helix-loop-helix (bHLH) PAS (Per-Arnt-Sim) protein, which is expressed predominantly during embryonic and neonatal stages and thereby designated NEPAS (neonatal and embryonic PAS), acts as a negative regulator of HIF-mediated gene expression. NEPAS mRNA is derived from the HIF-3alpha gene by alternative splicing, replacing the first exon of HIF-3alpha with that of inhibitory PAS. NEPAS can dimerize with Arnt and exhibits only low levels of transcriptional activity, similar to that of HIF-3alpha. NEPAS suppressed reporter gene expression driven by HIF-1alpha and HIF-2alpha. By generating mice with a targeted disruption of the NEPAS/HIF-3alpha locus, we found that homozygous mutant mice (NEPAS/HIF-3alpha(-)(/)(-)) were viable but displayed enlargement of the right ventricle and impaired lung remodeling. The expression of endothelin 1 and platelet-derived growth factor beta was increased in the lung endothelial cells of NEPAS/HIF-3alpha-null mice. These results demonstrate a novel regulatory mechanism in which the activities of HIF-1alpha and HIF-2alpha are negatively regulated by NEPAS in endothelial cells, which is pertinent to lung and heart development during the embryonic and neonatal stages.

Examination of distinct fetal and maternal molecular pathways suggests a mechanism for the development of preeclampsia

In pregnancy, the maternal spiral arteries must widen to nourish the growing fetus. It is this critical step in placental development that is commonly defective in the pathology of preeclampsia. Other features often observed in the placental pathology of preeclampsia include fewer invasive trophoblasts, shallow trophoblast invasion and placental thrombosis and atherotic-like changes. In this review, we propose that there are two distinct pathways, maternal and fetal, which converge on narrow spiral arteries. The unmodified (along the fetal pathway) or blocked (along the maternal pathway) spiral artery, or a combination of the two, may in turn lead to placental insufficiency and induce the maternal cascade of events leading to preeclampsia. We suggest a paradigm for the molecular developmental events that cause preeclampsia through narrow spiral arteries and focus on early events that may cause failed remodeling or blockage of the arteries, which then lead to placental insufficiency and ultimately the hypoxic placenta associated with preeclampsia. We propose that examination of the molecular mechanisms of maternal and fetal pathways that lead to the development of preeclampsia may aid researchers to focus on new potential factors in this molecular basis and ultimately in treatment of this disease.

Placental angiogenesis markers sFlt-1 and PlGF: response to cigarette smoke

OBJECTIVE

Excess soluble vascular endothelial growth factor receptor, fms-like tyrosine kinase-1 (sFlt-1), and reduced placental growth factor (PlGF) mediate the genesis of preeclampsia. Cigarette smoking reduces the risk of preeclampsia. We hypothesized that placental secretion of sFlt-1 and PlGF was affected by exposure to cigarette smoke extract.

STUDY DESIGN

Term placental villous explants were cultured with cigarette smoke extract. Media were analyzed for sFlt-1 and PlGF. Apoptosis was measured by TUNEL staining. Results are reported as sFlt-1 or PlGF picogram/milliliter/milligram wet weight of explant.

RESULTS

Exposure to cigarette smoke extract reduced secretion of sFlt-1 in a dose-dependent manner. There was no difference in apoptosis. In contrast with sFlt-1, PlGF did not decline when incubated with cigarette smoke extract.

CONCLUSION

Exposure of placental villous explants to cigarette smoke extract results in a proangiogenic state with reduced sFlt-1 and relative abundance of PlGF. This is the reverse of changes that are seen in preeclampsia and may explain the reduction of preeclampsia in smokers.

Human placental hypoxia-inducible factor-1alpha expression correlates with clinical outcomes in chronic hypoxia in vivo

Placental hypoxia is causally implicated in fetal growth restriction and preeclampsia, with both occurring more frequently at high altitude (>2700 m; HA). The nuclear transcription factor hypoxia-inducible factor (HIF) may facilitate placental oxygen transport at HA by increasing erythropoiesis and placental angiogenesis. We therefore investigated HIF expression and its regulatory mechanisms in placentas from normal pregnancies at high (3100 m), moderate (1600 m), and sea level (75 m) altitudes. Moderate-altitude and sea level placentas did not differ, but HIF-1alpha and the von Hippel-Lindau tumor suppressor protein were overexpressed in HA placentas. The ability of von Hippel-Lindau tumor suppressor protein to form the E3 ubiquitin protein ligase complex, required for HIF-1alpha degradation, was unaltered in HA placentas. mRNA for factor-inhibiting HIF, a negative modulator of HIF-1alpha transactivation, was increased, but protein levels were diminished. Elevated HIF-1alpha likely contributed to the significant increase we report in HIF-1alpha downstream target proteins, transforming growth factor beta3 in the placenta, and vascular endothelial growth factor and erythropoietin in the maternal circulation. These circulating markers and lowered birth to placental weight ratios correlated with increased HIF-1alpha, thereby linking molecular and systemic physiological data. The HA response to chronic hypoxia resembles preeclampsia in several aspects, illustrating the utility of the HA model in understanding placental pathologies.

The hypoxia-inducible factor is stabilized in circulating hematopoietic stem cells under normoxic conditions

The hypoxia-inducible factor (HIF) transcriptional system enables cell adaptation to limited O(2) availability, transducing this signal into patho-physiological responses such as angiogenesis, erythropoiesis, vasomotor control, and altered energy metabolism, as well as cell survival decisions. However, other factors beyond hypoxia are known to activate this pleiotropic transcription factor. The aim of this study was to characterize HIF in human hematopoietic stem cells (HSCs) and evidence is provided that granulocyte colony stimulating factor-mobilized CD34+- and CD133+-HSCs express a stabilized cytoplasmic form of HIF-1alpha under normoxic conditions. It is shown that HIF-1alpha stabilization correlates with down-regulation of the tumour suppressor von Hippel-Lindau protein (pVHL) and is positively controlled by NADPH-oxidase-dependent production of reactive oxygen species, indicating a specific O(2)-independent post-transcriptional control of HIF in mobilized HSCs. This novel finding is discussed in the context of the proposed role of HIF as a mediator of progenitor cell recruitment to injured ischemic tissues and/or in the control of the maintenance of the undifferentiated state.

Developmental regulation of human cytomegalovirus receptors in cytotrophoblasts correlates with distinct replication sites in the placenta

Cytomegalovirus (CMV), the major viral cause of congenital disease, infects the uterus and developing placenta and spreads to the fetus throughout gestation. Virus replicates in invasive cytotrophoblasts in the decidua, and maternal immunoglobulin G (IgG)-CMV virion complexes, which are transcytosed by the neonatal Fc receptor across syncytiotrophoblasts, infect underlying cytotrophoblasts in chorionic villi. Immunity is central to protection of the placenta-fetal unit: infection can occur when IgG has a low neutralizing titer. Here we used immunohistochemical and function-blocking methods to correlate infection in the placenta with expression of potential CMV receptors in situ and in vitro. In placental villi, syncytiotrophoblasts express the virion receptor epidermal growth factor receptor (EGFR) but lack integrin coreceptors, and virion uptake occurs without replication. Focal infection can occur when transcytosed virions reach EGFR-expressing cytotrophoblasts that selectively initiate expression of alphaV integrin. In cell columns, proximal cytotrophoblasts lack receptors and distal cells express integrins alpha1beta1 and alphaVbeta3, enabling virion attachment. In the decidua, invasive cytotrophoblasts expressing coreceptors upregulate EGFR, thereby dramatically increasing susceptibility to infection. Our findings indicate that virion interactions with cytotrophoblasts expressing receptors in the placenta (i) change as the cells differentiate and (ii) correlate with spatially distinct sites of CMV replication in maternal and fetal compartments.

Hypoxia and placental remodelling

In the first trimester of pregnancy fetal trophoblast cells invade the maternal uterine spiral arteries leading to loss of the vascular cells from the vessel wall and remodelling of the extracellular matrix. This is crucial to ensure that sufficient blood can reach the developing fetus. Impaired arterial remodelling is a feature of the major pregnancy pathologies pre-eclampsia and fetal growth restriction. Despite its importance, little is known about the regulation of this process. We have shown, using in vitro culture models and ex vivo explant models, that trophoblast cells play an active role in remodelling spiral arteries, and have implicated apoptotic events in this process. Further we have shown that trophoblast-derived factors such as Fas-ligand, tumor necrosis factor-related apoptosis inducing ligand (TRAIL) are important regulators of this process. The oxygen tension within the uteroplacental environment will vary with gestational age and will depend on the extent of trophoblast invasion and artery remodelling. Fluctuations in oxygen tension may be an important determinant of cellular events both during invasion towards uterine vessels and during the remodelling process. The components of this process known to be regulated by oxygen are reviewed, including lessons that can be learned from pregnancies at high altitude. In addition, data on the effect of varying oxygen tension on trophoblast production of pro-apoptotic factors and susceptibility of vascular smooth muscle cells to induction of apoptosis are described.

Dynamic HIF1A Regulation During Human Placental Development1

The human placenta is a unique organ in terms of oxygenation as it undergoes a transition from a low to a more oxygenated environment. This physiological switch in oxygen tension is a prerequisite for proper placental development and involves the hypoxia inducible factor (HIF). HIF is stable and initiates gene transcription under hypoxia, whereas in normoxia, interaction with the von Hippel-Lindau tumor suppressor protein (VHL) leads to rapid degradation of the HIF1A subunit. The degradation requires formation of a multiprotein complex (VHLCBC) and hydroxylation of HIF1A proline residues via members of the egg-laying-defective nine (EGLN) family. Herein, we have investigated the regulatory mechanisms of HIF1A expression during human placental development. Expression of HIF1A and VHL was high at 7-9 wk of gestation, when oxygen tension is low, and decreased when placental oxygen tension increases (10-12 wk of gestation). During early placentation, HIF1A localized in cytotrophoblasts, while VHL was present in syncytiotrophoblasts. At 10-12 wk, VHL appeared in cytotrophoblast cells, which coincided with the disappearance of HIF1A. At the same time the association of VHL and Cullin 2 as well as ubiquitination of HIF1A was maximal. EGLN1, EGLN2, and EGLN3 were also temporally expressed in an oxygen-dependent fashion, with greatest mRNA expression at 10-12 wk of gestation. Inhibition of EGLN activity increased HIF1A stability in villous explants and stimulated transforming growth factor beta 3 (TGFB3) expression consistent with promoter analyses showing that HIF1A transactivates TGFB3. These data demonstrate that during placental development, HIF1A is regulated by temporal and spatial changes in expression and association of molecules forming the multi-protein VHLCBC complex as well as prolyl hydroxylase activities.

Hypoxia up-regulated angiogenin and down-regulated vascular cell adhesion molecule-1 expression and secretion in human placental trophoblasts

OBJECTIVE

Many processes that are involved in cellular invasion, including blastocyst implantation, placental development, and rapidly growing tumors, occur in reduced oxygen environments. It has been surmised that oxygen tension could regulate the cytotrophoblast ability to differentiate and, as a consequence, to express proteins that are critical for placentation. The objective of the current investigation was therefore to test the hypothesis that placental tissues and trophoblast cells in culture, under low oxygen tension, release angiogenic factors that could affect vascular behavior and invasive potential, thus providing a link between abnormal placentation and maternal vascular abnormality.

METHODS

Functionally active term placental explant culture and trophoblast cultures were used to demonstrate the secretion profiles of angiogenin and vascular cell adhesion molecule-1 (VCAM-1), and the real-time quantitative reverse transcriptase polymerase chain reaction (RT-PCR) technique was employed to demonstrate the mRNA expression under both normoxic and hypoxic conditions.

RESULTS

A significant increase in the secretion (P <.01) and mRNA expression (P <.01) of angiogenin and a significant decrease in the secretion (P <.04) and mRNA expression (P <.03) of VCAM-1 from both term placental explants and trophoblast cultures subjected to hypoxia in vitro were observed.

CONCLUSION

Because the primary defect in uteroplacental insufficiency is placental maldevelopment probably associated with hypoxia in situ, this study provides molecular evidence to indicate that the differential expression and secretion of angiogenic factors may play an important role in these pathologic conditions.

Oxygen, epigenetics and stem cell fate

Coordinated interactions between the embryo/fetus and its environment are critical for proper development. In addition to acting as metabolic substrates for cellular homeostasis, basic physiological factors, such as oxygen tension, have a profound influence on developmental outcomes. Since the mammalian embryo resides in a physiologically hypoxic environment during gestation, understanding its responses to oxygen deprivation on a cellular level is critical. In this review, we analyze interactions between the hypoxia-inducible factor family of transcriptional regulators and epigenetic mechanisms governing chromatin structure. The ability of hypoxia-inducible factors to interact physically with histone deacetylase (HDAC) enzymes and modulate nuclear HDAC activities places them in the pivotal position of integrating physiological and epigenetic effectors. Multiple embryonic and extra-embryonic stem cell populations in mice and humans rely on this interaction – an important determinant of stem cell fate. Dissection of the pathways involved will provide novel insights into the metabolic as well as molecular determinants of the stem cell niches that allow self-renewal of progenitors in an undifferentiated state.

An evolutionary model for identifying genetic adaptation to high altitude

Coordinated maternal/fetal responses to pregnancy are required to ensure continuous O2 delivery to the developing organism. Mammals employ distinctive reproductive strategies that afford their young an improved chance of survival through the completion or the reproductive period. Thus, mortality prior to the end of the reproductive period is concentrated in the earliest phases of the lifecycle. At high altitude, fetal growth restriction reduces birth weight and likely compromises survival during the early postnatal period. Population variation in the frequency of the altitude-associated increase in intrauterine growth restriction (IUGR) demonstrates that multigenerational Tibetan and Andean high-altitude populations are protected compared with shorter duration, European or Han (Chinese) residents. This experiment of nature permits testing the hypothesis that genetic factors (a) influence susceptibility to altitude-associated IUGR, (b) act on maternal vascular adjustments to pregnancy determining uteroplacental blood flow, and (c) involve genes which regulate and/or are regulated by hypoxia-inducible factors (HIFs). Serial, studies during pregnancy as well as postpartum in Andean and European residents of high (3600 m) and low (300 m) altitude will permit evaluation of whether uteroplacental O2 delivery is lower in the European than Andean women and, if so, the physiological factors responsible. Comparisons of HIF-targeted vasoactive substances and SNPs in or near HIF-regulatory or targeted genes will permit determination of whether these regions are distinctive in the Andean population. Studies coupling genetic and genomic approaches with more traditional physiological measures may be productively employed for determining the genetic mechanisms influencing physiological adaptation to high altitude.

The regulation of trophoblast differentiation by oxygen in the first trimester of pregnancy

In the first trimester of human pregnancy villous cytotrophoblasts are able to differentiate to form either the overlying syncytiotrophoblast layer or, in anchoring villi, extravillous trophoblasts which grow out from the villi and invade into the maternal decidua, acting to both physically attach the placenta to the decidua, and modify the maternal spiral arteries to sustain pregnancy. During the first 10-12 weeks of gestation, extravillous trophoblast plugs block the spiral arteries and prevent maternal blood flow entering the intervillous space, thereby creating an environment of physiological hypoxia in which placental and fetal development occur. As extravillous trophoblasts migrate away from the villus they differentiate from a proliferative to an invasive phenotype. The hypoxic environment of the first trimester is believed to play an important role in the regulation of trophoblast differentiation. However, there is currently a large body of conflicting experimental evidence concerning this topic. This review examines the experimental evidence to date on the role of oxygen in trophoblast differentiation.

Human placental explants in culture: approaches and assessments

Placental explant cultures in vitro are useful for studying tissue functions including cellular uptake, production and release of secretory components, cell interactions, proliferation, growth and differentiation, gene delivery, pharmacology, toxicology, and disease processes. A variety of culture conditions are required to mimic in utero environments at different times of gestation including differing oxygen partial pressures, extracellular matrices and culture medium. Optimization of explant methods is examined for first and third trimester human placental tissue and the biological processes under investigation.

O2 enhancement of human trophoblast differentiation and hCYP19 (aromatase) gene expression are mediated by proteasomal degradation of USF1 and USF2

When cultured in 20% O(2), human cytotrophoblasts fuse to form the syncytiotrophoblast with marked induction of hCYP19 (aromatase) gene expression. When cultured in 2% O(2), cytotrophoblast fusion and induced hCYP19 expression are prevented. These effects of hypoxia are mediated by increased expression of mammalian achaete/scute homologue-2 (Mash-2), which increases levels of upstream stimulatory factors 1 and 2 (USF1/2) and their binding as heterodimers to E-boxes surrounding the hCYP19 promoter. In studies to define mechanisms for O(2) regulation of syncytiotrophoblast differentiation, we found that hypoxia and overexpression of Mash-2 markedly increased cyclin B1 levels in cultured trophoblasts and the proportion of cells at the G(2)/M transition. Unlike USF proteins, USF1/2 mRNA levels are unaffected by O(2) tension. To determine whether increased O(2) might enhance proteasomal degradation of USF1/2, human trophoblasts were cultured in 2% or 20% O(2) with or without proteasome inhibitors. In cells cultured in 20% O(2), proteasome inhibitors increased USF1/2 protein levels and blocked spontaneous induction of hCYP19 expression, cell fusion, and differentiation. Like hypoxia, inhibitory effects of proteasome inhibitors on hCYP19 expression were mediated by increased binding of USF1/2 to the E-boxes. In human trophoblast cells cultured in 20% O(2), increased polyubiquitylation of USF1/2 proteins was observed. Thus, early in gestation when the placenta is relatively hypoxic, increased USF1/2 may block trophoblast differentiation and hCYP19 gene expression. In the second trimester, increased O(2) tension promotes proteasomal degradation of USF1/2, resulting in syncytiotrophoblast differentiation and induction of hCYP19 expression.

Hypoxia-inducible factor-dependent histone deacetylase activity determines stem cell fate in the placenta

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor composed of HIFα and the arylhydrocarbon receptor nuclear translocator(ARNT/HIF1β). Previously, we have reported that ARNT function is required for murine placental development. Here, we used cultured trophoblast stem (TS)cells to investigate the molecular basis of this requirement. In vitro, wild-type TS cell differentiation is largely restricted to spongiotrophoblasts and giant cells. Interestingly, Arnt-null TS cells differentiated into chorionic trophoblasts and syncytiotrophoblasts, as demonstrated by their expression of Tfeb, glial cells missing 1 (Gcm1) and the HIV receptor CXCR4. During this process, a region of the differentiating Arnt-null TS cells underwent granzyme B-mediated apoptosis,suggesting a role for this pathway in murine syncytiotrophoblast turnover. Surprisingly, HIF1α and HIF2α were induced during TS cell differentiation in 20% O2; additionally, pVHL levels were modulated during the same time period. These results suggest that oxygen-independent HIF functions are crucial to this differentiation process. As histone deacetylase(HDAC) activity has been linked to HIF-dependent gene expression, we investigated whether ARNT deficiency affects this epigenetic regulator. Interestingly, Arnt-null TS cells had reduced HDAC activity,increased global histone acetylation, and altered class II HDAC subcellular localization. In wild-type TS cells, inhibition of HDAC activity recapitulated the Arnt-null phenotype, suggesting that crosstalk between the HIFs and the HDACs is required for normal trophoblast differentiation. Thus, the HIFs play important roles in modulating the developmental plasticity of stem cells by integrating physiological, transcriptional and epigenetic inputs.

Expression of von Hippel Lindau (pVHL) protein in placentae from normal pregnant women and women with preeclampsia

The hypoxia inducible transcription factors, HIF-1alpha and -2alpha proteins, are overexpressed in placentae from women with preeclampsia (Biol Reprod 2001;64:499-506; Biol Reprod 2001;64:1019-1020). Normally, these proteins are regulated in an oxygen-dependent manner being rapidly degraded by the ubiquitin-mediated proteasomal pathway. Recent studies have shown that the tumor suppressor protein, von Hippel Lindau (VHL), targets HIF for ubiquitinylation under nonhypoxic conditions. The objectives of the present work were: (1) to investigate VHL protein expression in normal pregnant (NP), preeclamptic (PE), and preterm (without PE) placentae, (2) to test whether VHL protein is hypoxia inducible in term and first trimester placental villous explants, and (3) to analyze the ontogeny of VHL protein expression in the human placenta. To begin evaluating the potential contribution of VHL to HIF overexpression in preeclamptic placentae, we analyzed the levels of the VHL protein in both normal and preeclamptic placentae (n=7 each). We hypothesized a deficiency of VHL protein in preeclamptic placentae. Eight biopsy sites were tested in each placenta and protein extracts were made. Western analysis was performed using VHL specific antibodies. Human renal adenocarcinoma (ACHN) cell extracts and extracts from COS-7 cells transfected with a VHL expression vector were used as positive controls. In a total of 112 biopsy sites that were analyzed (56 each for normal and preeclamptic placentae), the composite densitometry ratios (PE/NP) for the long (28 kDa) and short (19 kDa) forms of VHL were 1.09+/-0.2 and 1.16+/-0.11, respectively (both p=NS vs 1.0). A ratio of 1.0 indicates equal expression by preeclamptic and normal placentae. The same placentae exhibited composite densitometry (PE/NP) ratios of 1.97+/-0.23 and 1.68+/-0.20 for HIF-1alpha and -2alpha proteins, respectively (both p<0.05 vs 1.0). In a separate analysis, the protein expression of the short form of VHL was also comparable among NP, PE and preterm (n=6) placentae. VHL immunoreactivity was localized to cells within the basal plate and the syncytiotrophoblast. Despite induction of HIF proteins by hypoxia in first and term villous explants, there was no significant upregulation of VHL proteins. Finally, the expression of both the short and long forms of VHL protein decreased with gestational age (both p<0.05 by ANOVA), and in villous tissue from first trimester placentae VHL immunoreactivity was predominantly localized to the cytotrophoblast. These results suggest that (1) deficiency of VHL protein does not account for HIF-alpha overexpression in preeclamptic placentae, (2) VHL protein is not regulated by hypoxia in either first trimester or term placental villous explants, and (3) VHL protein expression in the placenta decreases as a function of gestational age.

The adverse effects of maternal smoking on the human placenta: A review

Studies of placental pathologies associated with maternal cigarette smoking have led to many interesting observations. For example, maternal smoking impairs human placental development by changing the balance between cytotrophoblast (CTB) proliferation and differentiation. It is likely that chronic exposure to tobacco constituents in early pregnancy can affect placental development directly or indirectly by reducing blood flow, which creates a pathologically hypoxic environment. To understand this process at a molecular level, tissue samples from non-smoking and smoking mothers were studied to determine whether active and/or passive cigarette smoke exposure affects CTB expression of molecules that govern cellular responses to oxygen tension: the von Hippel-Lindau tumor suppressor protein (pVHL), hypoxia-inducible transcription factors (HIFs) and vascular endothelial growth factor-A (VEGF). The results show that maternal smoking dysregulates CTB expression of all three types of molecules. In addition, cell columns and proliferating cells were reduced while there was a corresponding increase in cell islands. All three phenomena were most obvious in the placentas of heavy smokers. Interestingly, a subset of the aforementioned effects can be detected in samples obtained from women who were passively exposed to cigarette smoke during pregnancy. These observations suggest that tobacco constituents exert direct effects on CTB proliferation and differentiation and help to explain the mechanisms by which smoking negatively effects human pregnancy outcome.

The human cytotrophoblastic cell, a mononuclear chameleon

The human placenta represents an abundant; easily accessible and unlimited study material (at birth a human placenta provides about 500 g of trophoblast). Cytotrophoblastic cells (CTB) are one constituent of the human placenta and represent epithelial cells with fascinating properties: They are able to fuse to form syncytia, can behave like immotile polarized epithelial cells, can phenocopy stromal fibroblasts or endothelial cells or undergo a mesenchymal-like transformation that converts them into non proliferative and highly invasive cells. Like a chameleon, CTB are thus able to adapt to their immediate environment by phenocopying their neighbor cells. This review describes the different routes that CTB follow during their differentiation pathways, the regulation of these at the molecular level, it gives also an overview of the pathologies associated with faulty pathways and describes the usual phenotypic markers used to identify the different CTB subsets. This review is intended to stimulate investigators not acquainted with the field of placental biology to use CTB as a model to study important biological functions in vitro, such as cell fusion, cell invasion and cell transformation.

Trophoblast differentiation during embryo implantation and formation of the maternal-fetal interface

Trophoblasts, the specialized cells of the placenta, play a major role in implantation and formation of the maternal-fetal interface. Through an unusual differentiation process examined in this review, these fetal cells acquire properties of leukocytes and endothelial cells that enable many of their specialized functions. In recent years a great deal has been learned about the regulatory mechanisms, from transcriptional networks to oxygen tension, which control trophoblast differentiation. The challenge is to turn this information into clinically useful tests for monitoring placental function and, hence, pregnancy outcome.

Maternal adaptation to high-altitude pregnancy: an experiment of nature--a review

A long and productive history of studies at high altitude has demonstrated that chronic hypoxia plays a key role in the aetiology of intrauterine growth restriction (IUGR) and pre-eclampsia. Susceptibility to altitude-associated IUGR varies among high-altitude populations in relation to their duration of altitude exposure, with multigenerational residents demonstrating one-third the birth weight fall present in shorter-resident groups. Higher uteroplacental blood flow during pregnancy in multigenerational high-altitude residents suggests that such population differences are due, at least in part, to differences in maternal vascular responses to pregnancy. We hypothesize that natural selection acting on hypoxia-inducible factor (HIF)-targeted or -regulatory genes has enabled maternal vascular adaptation to pregnancy in long-resident high-altitude groups. Preliminary evidence in support of this hypothesis demonstrates that the potent HIF-targeted vasoconstrictor, endothelin-1 (ET-1), is differentially regulated by pregnancy and chronic hypoxia in Andean vs European residents of high altitude. Andeans show the normal, pregnancy-associated fall in ET-1 levels previously reported at low altitude, whereas Europeans have higher ET-1 levels and little pregnancy-associated change, like pre-eclamptic women. Single nucleotide polymorphisms (SNPs) in the ET-1 gene also differ in Andeans compared with low-altitude populations. We conclude that high altitude serves as an experiment of nature for elucidating genetic factors underlying susceptibility to complications of pregnancy and fetal life. Such studies may be important for identifying persons at risk for these complications at any altitude.

Gap junctions are required for trophoblast proliferation in early human placental development

Little is known about the role of gap junctional intercellular communication (GJIC) in human trophoblast differentiation, particularly during the formation of extravillous trophoblast (EVT) cell columns and their subsequent differentiation into invasive cells. We have identified transcripts for five connexin gap junction proteins in the early human placenta (Cx32, Cx37, Cx40, Cx43 and Cx45). Of these, Cx40 and Cx45 proteins immunolocalize to EVT in anchoring cell columns. Cx40 expression is prominent in the anchoring column throughout the first trimester of pregnancy (6-14 weeks gestation). We used first trimester placental villous explant cultures to determine the functional significance of the inhibition of GJIC in EVT cell proliferation and differentiation using two known GJIC inhibitors, carbenoxolone (CBX) and heptanol. The morphology of EVT outgrowths changed dramatically upon GJIC-blockade, from compact and organized outgrowths into a scattered group of rounded individual trophoblast cells, reminiscent of an early invasive phenotype. Furthermore, the inhibition of GJIC in placental explants by CBX or heptanol induced a switch away from the proliferative and towards an invasive EVT phenotype, as evident from (a) the loss of the proliferation marker Ki67 and (b) an increase in the invasive marker alpha1 integrin. We also utilized antisense oligonucleotides to inhibit Cx40 protein expression in placental explants. Cx40 antisense treatment also resulted in the abolishment of outgrowth EVT cell proliferation (as determined by Ki67 immunostaining). Together, these results suggest that gap junctions composed particularly of Cx40 channels are required for the proliferation of EVT cells in anchoring cell columns, and that a loss of GJIC contributes to differentiation to the invasive EVT phenotype.

Greater vascularity, lowered HIF-1/DNA binding, and elevated GSH as markers of adaptation to in vivo chronic hypoxia

Vascularity is increased in placentas from high- compared with low-altitude pregnancies. An angiogenic response to hypoxia may protect an organ from further hypoxic insult by increasing blood flow and oxygen delivery to the tissue. We hypothesized that increased placental vascularity is sufficient to adapt to high altitude. Therefore, indexes of hypoxic stress would not be present in placentas from successful high-altitude pregnancies. Full-thickness placental biopsies were 1) collected and frozen in liquid nitrogen within 5 min of placental delivery and 2) fixed in formalin for stereologic analyses at high (3,100 m, n = 10) and low (1,600 m, n = 10) altitude. Hypoxia-inducible transcription factor (HIF-1) activity was analyzed by ELISA. Western blot analyses were used to evaluate HIF-1alpha, HIF-1beta, HIF-2alpha, von Hippel-Lindau protein, VEGF, Flt-1, enolase, and GAPDH. Magnetic resonance spectroscopy was used to evaluate endogenous metabolism. The ratio of placental capillary surface density to villous surface density was 70% greater at high compared with low altitude. HIF-1 activity and HIF-1-associated proteins were unchanged in placentas from high- vs. low-altitude pregnancies. Placental expression of HIF-1-mediated proteins VEGF, Flt-1, enolase, and GAPDH were unchanged at high vs. low altitude. Succinate, GSH, phosphomonoesters, and ADP were elevated in placenta from high compared with low altitude. Placentas from uncomplicated high-altitude pregnancies have greater vascularity and no indication of significant hypoxic stress at term compared with placentas from low altitude.