Enzymatic isolation of human trophoblast and culture on various substrates: comparison of first trimester with term trophoblast

Wide-ranging DNA methylation differences of primary trophoblast cell populations and derived cell lines: implications and opportunities for understanding trophoblast function

Difficulties associated with long-term culture of primary trophoblasts have proven to be a major hurdle in their functional characterization. In order to circumvent this issue, several model cell lines have been established over many years using a variety of different approaches. Due to their differing origins, gene expression profiles and behaviour in vitro, different model lines have been utilized to investigate specific aspects of trophoblast biology. However, generally speaking, the molecular mechanisms underlying functional differences remain unclear. In this study, we profiled genome-scale DNA methylation in primary first trimester trophoblast cells and seven commonly used trophoblast-derived cell lines in an attempt to identify functional pathways differentially regulated by epigenetic modification in these cells. We identified a general increase in DNA promoter methylation levels in four choriocarcinoma (CCA)-derived lines and transformed HTR-8/SVneo cells, including hypermethylation of several genes regularly seen in human cancers, while other differences in methylation were noted in genes linked to immune responsiveness, cell morphology, development and migration across the different cell populations. Interestingly, CCA-derived lines show an overall methylation profile more similar to unrelated solid cancers than to untransformed trophoblasts, highlighting the role of aberrant DNA methylation in CCA development and/or long-term culturing. Comparison of DNA methylation and gene expression in CCA lines and cytotrophoblasts revealed a significant contribution of DNA methylation to overall expression profile. These data highlight the variability in epigenetic state between primary trophoblasts and cell models in pathways underpinning a wide range of cell functions, providing valuable candidate pathways for future functional investigation in different cell populations. This study also confirms the need for caution in the interpretation of data generated from manipulation of such pathways in vitro.

Trophoblast invasion

The establishment of a pregnancy depends upon mechanisms of forming close contacts between embryonic/fetal and maternal tissues. The starting point is the attachment of the blastocyst to the uterine wall, and this critical step is followed by a series of different morphogenetic events leading to placentation. These processes depend on the invasive properties of extra-embryonic trophectoderm-derived cells that show their highest expression in species with haemochorial placentation, i.e. Insectivores, rodents, and primates including the human. In this review we will concentrate upon the human with occasional reference to other species.

Culture of syncytiotrophoblast for the study of human placental transfer. Part II: Production, culture and use of syncytiotrophoblast

The conditions necessary for producing syncytical syncytiotrophoblast are examined. Tissue disaggregation conditions, culture media composition, different extracellular matrices and the influence of placental gestational age are all assessed. The importance of evaluating the biochemical and functional differentiational state of the cells is also stressed. Evidence is summarized that syncytiotrophoblast in culture is morphologically and ultrastructurally very similar to syncytiotrophoblast in vivo, and what is so far known biochemically is largely consistent with what is known in vivo. Studies published to date on microvillous membrane uptake and release and relationships with intracellular metabolism using syncytiotrophoblast in conventional culture are outlined from the point of view of the advantages and potential of this model. The present state of development of the two-sided model is assessed, mentioning factors to be considered such as the supporting membrane to be used, accounting for passive diffusion and paracellular leak components of transport and dealing with quantitative effects in kinetic studies of the presence of the supporting membrane. It is concluded that satisfactory methods are now in place for preparing pure villous syncytial syncytiotrophoblast in culture from cytotrophoblast derived from term (but not early) placentae, suitable for studying microvillous membrane transport and relationships with intracellular metabolism. Cytotrophoblast from early gestational age placenta may require different conditions to form true syncytiotrophoblast. A two-sided model for studies of overall transfer, basal transport and basal control mechanisms is now available and possibly with some development should be a good model for such investigations.

Culture of syncytiotrophoblast for the study of human placental transfer. Part I: Isolation and purification of cytotrophoblast

Criteria for a successful model for the study of trans-syncytiotrophoblast transfer include isolating substantially pure trophoblast cells from placental villous tissue, and obtaining from them phenotypical villous syncytial syncytiotrophoblast during culture. For studies involving the basal membrane, including overall transfer, basal uptake and output, and controls acting at the basal membrane, a two-sided model is required with a separate compartment of culture medium in contact with the basal cell surface. All current methods of isolating cytotrophoblast, the precursor of syncytiotrophoblast, derive from the original tissue trypsinization method of Thiede (1960), which produces cultures of villous cytotrophoblast cells contaminated with other placental cell types. Lessons learned from successful and unsuccessful development of the model over 35 years are outlined, and recently established methods for purifying the isolated mixed cells discussed. These include sedimentation and centrifugation methods, immunological and receptor binding methods, and more selective release of trophoblast cells from tissue. Immuno flow cytometric cell sorting methods are potentially capable of isolating subpopulations of various phenotypical trophoblast types. We conclude that satisfactory methods are now available for isolating and purifying cytotrophoblast from early or late gestation human placenta.

Isolation and characterization of human placental trophoblast subpopulations from first-trimester chorionic villi

A method for the simultaneous preparation of highly enriched human placental trophoblast populations (villous and extravillous) from first-trimester placental villi (5 to 12 weeks) by using sequential trypsinization, percoll gradient centrifugation, and negative selection with anti-CD9 immunomagnetic separation is described. The purification method resulted in the isolation of four distinct trophoblast populations identified on the basis of morphology and phenotyping: (i) mononuclear villous cytotrophoblast cells which, through differentiation, become committed to syncytium formation; (ii) an extravillous trophoblast population which appeared as a "crazy pavement" and, with subsequent subculturing, differentiated morphologically to mononuclear cells; (iii) an extravillous trophoblast fraction which fused to form multinucleated trophoblast giant cells; and (iv) floating intermediate extravillous trophoblast cells which fused together to form cell clumps and which further differentiated to a mononuclear anchoring intermediate extravillous trophoblast. Short-term cultures of the freshly isolated cell fractions consisted of heterogeneous trophoblasts at different differentiation stages as determined by their varied biochemical and morphological properties. All the isolated trophoblast populations expressed the cytokeratin intermediate filament and the epithelium-specific cell-cell adhesion molecule E-cadherin. The isolated villous trophoblasts in culture expressed integrins alpha 6 and beta 4 and reduced levels of beta 1 subunits, whereas the proliferating extravillous trophoblast cultures expressed alpha 1, alpha 3, and alpha 5 and high levels of beta 1 integrin subunits, vitronectin receptor (alpha V beta 3/beta 5), and major histocompatibility complex class 1 molecules. Furthermore, the isolated trophoblast populations secreted metalloproteases (such as type IV collagenases [mainly 72- and 92-kDa enzymes, i.e., gelatinases A and B]) and urokinase plasminogen activator, as evaluated by substrate gel zymography. This method of isolation should facilitate in vitro studies of trophoblast proliferation, differentiation, invasion, virus interactions, cytokenesis, and immunology.

Non-Michaelis-Menten kinetics of zero-trans glucose uptake by trophoblast cells from human term placentae and by choriocarcinoma (JEG-3/JAR) cells

Maternal glucose is a major substrate for placental and fetal metabolism. The kinetics of its uptake into placental trophoblast cells has not been characterised yet and was therefore investigated in the present study. In addition to trophoblast cells isolated from human term placentae, JEG-3 and JAR choriocarcinoma cells were used. Measurements were carried out in 5 s intervals until 30 s with the non-metabolisable glucose analogue 3-O-[14C]methyl-D-glucose using confluent cells adhering to glass coverslips. L-[1-14C]glucose was used to correct for extracellular trapped tracer and diffusion. The uptake was rapid and saturable. It reached equilibrium after 30 s at 20 degrees C and could be inhibited by 0.4 mmol/l cytochalasin B up to 98%. The choriocarcinoma cells took up twice as much glucose as trophoblast cells. Fitting the experimental data to the Michaelis-Menten equation by non-linear regression failed to adequately describe the data, even when a contribution of diffusion to total uptake was considered. Introducing the Hill coefficient n into the Michaelis-Menten equation significantly improved the quality of the fits as was assessed by three statistical criteria. Using this equation modified for allosteric kinetics (v = k[To] [S]n)/(Km + [S]n)), parameters were calculated as Km = 12 mmol/l, Vmax = 17 fmol/l s-1 per cell, n = 1.1 for trophoblast cells; Km = 13 mmol/l, Vmax = 27 fmol/l s-1 per cell, n = 1.2 for JEG-3 cells and Km = 29 mmol/l, Vmax = fmol/l s-1 per cell, n = 1.4 for JAR cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Villous culture of first trimester human placenta--model to study extravillous trophoblast (EVT) differentiation

During implantation and subsequent placentation the human extravillous trophoblast (EVT) cells invade the endometrium and maternal vasculature within the uterus. The origin of the EVT and signals triggering its differentiation, migration and invasion are poorly understood. First and second trimester human chorionic villi explants were used as a source of EVT and a variety of substrates which resemble extracellular matrix (ECM) in vivo have been tested to induce EVT differentiation and migration. The obtained results demonstrate that villous explants from both 5-7 and 8-10 weeks of gestation give rise to EVT cells in vitro if maintained on the surface of Matrigel or decidual extract supplemented collagen gel. Fetal calf serum (FCS) supplemented media was essential for EVT differentiation and villous trophoblast viability. Immunostaining of both EVT cells and cells from the cytotrophoblastic column with monoclonal antibody Ki67 (cell proliferation marker) indicate that EVT cells differentiate in vitro by proliferation from the tip of anchoring villi. These mononucleated, round-shaped, migrating cells are HLA-A,B,C class I antigen (W6/32) antibody and low molecular weight cytokeratin positive, and do not immunostain with PAI-1 (plasminogen activator inhibitor) and HPL antibodies. Differentiation of EVT was restricted to first trimester villous tissue; explants from second trimester placentae did not give rise to EVT. Tissue viability as monitored by glucose utilization, lactate, progesterone and hCG production rates correlated with EVT differentiation. The production rates for hCG demonstrated significant variation among individual placentae and was maintained constant for 10 days consistently only in explants cultured on decidual extract supplemented collagen matrix. The described villous tissue culture system may be, therefore, a unique in vitro model to study proliferation and differentiation of EVT from cytotrophoblastic columns, the regulation of EVT proliferation and differentiation, the role of ECM in the induction of the migration and the interaction of extravillous and villous trophoblast at the level of the cytotrophoblastic column.

Uptake of 67Cu by isolated human trophoblast cells

The isolated human trophoblast was used as a system to analyze the effects of different physiological ligands on cellular uptake of copper. The results show that the uptake of copper by these cells follows a similar pattern for the ligands tested (histidine, albumin and ceruloplasmin) as that for copper chloride. The process follows a typical hyperbolic curve at 37 degrees C. The initial phase of uptake follows a linear pattern during 30 min at 37 degrees C and at least 60 min at 4 degrees C from which the uptake rate is calculated. However, a significant decrease in the uptake rate is observed for albumin. The effect of histidine on stimulating copper transport is observed in the presence of serum, a phenomenon which is considered to be due to the release of copper that is bound to albumin. These results support the role of ceruloplasmin as a copper transport protein which releases copper at the cell surface, and a subsequent transport of the released copper in a manner similar to that of copper chloride or copper-histidine complexes.

Regulation of progesterone secretion in human syncytiotrophoblast in culture by human chorionic gonadotropin

In the present investigation we sought to define the specific sites in the pathway of placental progesterone biosynthesis that underlie the action of human chorionic gonadotropin (hCG). When the cells were challenged with dibutryl cAMP (dbcAMP), forskolin or isobutylmethylxanthine, they produced significantly higher amounts of progesterone which in the presence of the hCG antibody was reduced to the level of the control set of cells. Trophoblast cells cultured in serum free medium with 25-hydroxycholesterol (25-OHC) produced increased amounts of progesterone. In the presence of hCG antibody at a concentration which neutralized the secreted hCG, the steroid production was completely blocked, even when the 25-OHC was added to the medium. Also, direct quantitation of the cytochrome P450 SCC enzyme in the absence of hCG indicated a significant decrease. The exogenous addition of low density lipoproteins (LDL) increased the progesterone secretion by the trophoblast cells in culture. Neutralization of hCG by the antibodies, however, drastically reduced the LDL induced progesterone secretion, which was restored by the addition of dbcAMP to the medium. Based on these findings, we suggest a stimulatory effect of hCG on normal trophoblast cells at the level of LDL utilization and cytochrome P450 SCC enzyme. Since dbcAMP could mimic these actions of hCG, the data suggest a possible autocrine/paracrine role of hCG on the trophoblast cells. An additive effect of hCG and cAMP on progesterone secretion observed in our studies, indicate that apart from hCG, adenylate cyclase activity may also be regulated by other factors.

Cadmium effect on zinc metabolism in human trophoblast cells: involvement of cadmium-induced metallothionein

Isolated trophoblast cells from human placenta at term were used to investigate the effect of cadmium exposure on the placental transport of zinc. Cells were exposed to cadmium acetate (0-2 microM) for 18 h. These concentrations were found to be non-cytotoxic as determined by protein recovery and the leakage of lactate dehydrogenase. Primary trophoblast cultures showed a cadmium-dependent increase in metallothionein content. After incubation with 65Zn for different times 65Zn contents were compared between control and cadmium-exposed cells. Zinc uptake by the cells after cadmium pre-exposure was increased in a cadmium dose-dependent manner. The additional amount of zinc that was taken up by the cadmium pre-exposed cells with respect to the control cells remained constant during a 30-min incubation of the same cells in a metal-free medium. The cytosol of cadmium-exposed cells showed a higher content of zinc than that of control cells. Most of the zinc in the cytosol is in the metallothionein fraction. We have thus demonstrated that cadmium-induced metallothionein binds zinc in the trophoblast, making this essential element less available to the fetal circulation.

Binding, uptake and efflux of 65Zn by isolated human trophoblast cells

The isolated trophoblast from human placenta at term has been used as model for placental transport. Binding to the cells showed to be of low specificity and very limited (less than 10%) in presence of serum. Uptake of zinc was 90% reduced at 4 degrees C and depended on the presence of sulfhydryl groups (as suggested with the inhibition produced by N-ethylmaleimide). There is considerable efflux (50%) of zinc from preloaded cells and the process was enhanced by presence of histidine in the medium. The model proved to be useful in studying the mode of zinc transport. The results indicate that the uptake of zinc by the trophoblast is probably the limiting process in the placental transport of this metal.

Purification of human cytotrophoblast from term amniochorion by flow cytometry

Term cytotrophoblast do not express polymorphic MHC Class I antigens, unlike other fetal and maternal cells in the amniochorion/decidua. This allows cytotrophoblast to be isolated and purified from this tissue, utilizing 4E, a monoclonal antibody specific for HLA-B, which labels only non-trophoblast. We have developed a method using enzymic dispersion and Percoll gradient centrifugation, followed by flow cytometry, that yields, on average, a total of 5 X 10(6) term extravillous cytotrophoblast, 97 per cent pure. The availability of highly purified extravillous cytotrophoblast, for the first time, permits precise investigation of trophoblast function.

Two-sided culture of human placental trophoblast. Morphology, immunocytochemistry and permeability properties

We describe the culture of human term placental trophoblast cells on cell-free amniotic membrane, with medium on both sides. Over the course of 2 days, the isolated cells, initially simple, mononucleated and probably cytotrophoblast, form a confluent layer of multinucleated syncytial cells with morphological and immunocytochemical properties of syncytiotrophoblast. This layer becomes polarized with respect to morphology, alkaline phosphatase distribution and hCG secretion. Contamination with amnion cells, and with other cell types that are present in placental tissue, was less than 1 per cent. A preliminary investigation of the permeability properties of the preparation showed that the trophoblast cell layer, rather than the amniotic membrane, was rate-limiting to transtrophoblast transfer, but that possible effects of the supporting membrane should be considered. The transtrophoblast transfer of D-glucose and the non-metabolisable analogue, 3-O-methyl-D-glucose (3OMG), had saturable and non-saturable/leak components in both directions, indicating that carrier-mediated processes were involved. The non-metabolisable amino acid 2-aminoisobutyrate (AIB) was both accumulated within the trophoblast cells, and transferred by saturable and non-saturable processes from the microvillous side, but no saturable accumulation or transfer was observed from the basal side, at the concentrations tried. The results suggest that this model may prove suitable for studies of transtrophoblast transfer.

Interferon production by cultured human trophoblast induced with double stranded polyribonucleotide

Human trophoblast cultures were established from term placentae. Upon exposure to polyriboinosinic-polyribocytidylic acid (poly(I:C] the cultures produced interferon. Results of neutralization experiments and indirect immunofluorescence studies indicated that the trophoblast produces beta-interferon. The fusion of trophoblast cells into syncytia was accompanied by a restriction in interferon release after stimulation with poly(I:C). It was also demonstrated that the malignant choriocarcinoma cell line JAR produced less interferon than the non-transformed cytoorsyncytiotrophoblast.