Dependence of membrane potential on Ca2+ transport in cultured cytotrophoblasts of human immature placentas

Estrogenic endocrine disruptive components interfere with calcium handling and differentiation of human trophoblast cells

During development, calcium (Ca) is actively transported by placental trophoblasts to meet fetal nutritional and the skeletal mineralization needs. Maternal exposure to estrogenic pesticides, such as 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) and methoxychlor (MTC), has been shown to result in reproductive disorders and/or abnormal fetal development. In this study, we have examined the effects of exposure of trophoblastic cells to MTC and DTT, in comparison to 17beta-estradiol (E2) and diethylstilbestrol (DES), to test the hypothesis that cellular Ca handling is a target for these endocrine disruptive components. Treatment with DDT, MTC, DES, or E2 increased cellular Ca uptake, and the expression of trophoblast-specific human Ca binding protein (HCaBP) was down-regulated by both MTC and DDT. Treatment with MTC, DDT, and DES inhibited cell proliferation, induced apoptosis, and suppressed expression of several trophoblast differentiation marker genes. These effects were reversed by overexpression of metallothionein IIa, a gene highly responsive to cadmium and other metals. These results strongly suggest that trophoblast Ca handling functions are endocrinally modulated, and that their alteration by candidate endocrine disruptors, such as MTC and DDT, constitutes a possible pathway of the harmful effects of these components on fetal development.

Expression of calcium channels along the differentiation of cultured trophoblast cells from human term placenta

Placental transfer of maternal calcium (Ca2+) is carried out in vivo by the syncytiotrophoblast layer. Although this process is crucial for fetal development, it remains poorly understood. Cytotrophoblasts isolated from human term placenta undergo spontaneous syncytiotrophoblast-like morphological and biochemical differentiation in vitro and are thought to reflect in vivo syncytiotrophoblast. In the present study, we characterized the Ca2+ uptake potential and the expression of several Ca2+ channels by human trophoblasts during differentiation in vitro for up to 6 days. Secretion of hCG (specific differentiation marker) and uptake of Ca2+ by trophoblasts increased gradually as a function of days in culture. Both hCG secretion and Ca2+ uptake were maximal on Day 4 and declined on Days 5-6. Expression of the Ca2+ transporter proteins CaT1 and CaT2 was revealed by reverse transcription-polymerase chain reaction in cytotrophoblasts freshly isolated from human term placenta. In addition, messengers for two L-type Ca2+ channel isoforms (alpha(1C) and alpha(1D)) were also detected. Levels of CaT1, CaT2, and L-type Ca2+ channel mRNA increased gradually during culture, reaching a maximum between Days 2 and 3. In contrast to CaT1 and CaT2 expression that declined thereafter to levels observed on Day 1, L-type channel expression decreased by 50% but remained above the expression level of Day 1. Our results indicate that the pattern of CaT1 and CaT2 expression correlates with the Ca2+ uptake potential along the differentiation of cultured human trophoblasts isolated from term placenta. This correlation provides circumstantial evidence for a role of this family of channels in basal Ca2+ uptake by the syncytiotrophoblast.

Inwardly rectifying K(+) current and differentiation of human placental cytotrophoblast cells in culture

Ion transport is important for driving nutrient transport across the syncytiotrophoblast and yet is poorly understood. We have examined K(+)currents under basal conditions in cultured cytotrophoblast cells, at various stages of differentiation, using the whole cell patch clamp technique. Cytotrophoblast cells were isolated from human term placenta and maintained in culture for up to 3 days. Cells were studied at four stages of progressive morphological differentiation: (i) mononuclear cells, (ii) mononuclear cells in aggregates, (iii) small multinucleate cells and (iv) large multinucleate syncytiotrophoblast-like cells. In the conditions of whole cell recording the only K(+) selective current identified in all cell types was a strong inwardly rectifying current which was sensitive to Ba(2+) and Cs(+). This current was unaffected by intracellular ATP whereas intracellular GTPgammas caused either run down of the current or activated a linear current. The characteristics of the current described are consistent with those of the inwardly rectifying K(+) channel Kir2.1. The inwardly rectifying K(+) current was observed in three out of 19 (16 per cent ) mononuclear cells, seven out of 21 (33 per cent ) mononuclear aggregates, eight out of 21 (38 per cent ) small multinucleate cells and 16 out of 19 (84 per cent ) large multinucleate cells. This inwardly rectifying K(+) current is likely to have an important role in determining net K(+) diffusion across the syncytiotrophoblast cell membrane, perhaps increasing in importance as the cells terminally differentiate.

Effects of cadmium on trophoblast calcium transport

Maternal exposure to cadmium (Cd) during pregnancy has been linked to low fetal birthweight, which may be attributed to placental damage and/or dysfunction in nutrient transport. Previous studies have suggested that Cd is accumulated in the placenta, and that placental transport of calcium (Ca) and zinc (Zn) is perturbed by Cd. To investigate the mechanism of Cd perturbation of Ca transport, we used JEG-3, a human choriocarcinoma cell line which exhibits trophoblastic properties, to analyse Cd effects in vitro. Treatment with Cd at low, physiologically relevant concentrations (e.g. 0.04 microM) did not result in obvious changes in cell morphology or integrity, whereas higher concentrations (> or = 0.16 microM) affected cell integrity. With lower concentrations of Cd treatment for 24 h, activities of cellular Ca uptake and transport, and Ca2+ binding were decreased, and intracellular [Ca2+] ([Ca2+]i) profile was also altered; however, membrane-associated Ca(2+)-activated ATPase activity remained relatively unchanged. Interestingly, cellular Ca uptake activity was unaffected by short-term (30 min) Cd pretreatment. The 24-h Cd treatment also resulted in elevated expression of the metal-binding protein, metallothionein, whereas the expression of a trophoblast-specific cytosolic Ca(2+)-binding protein (HCaBP) was drastically reduced. These results strongly suggest that Cd exposure significantly compromises the Ca handling ability of trophoblastic cells; this effect is probably not due to perturbations in Ca channel or membrane Ca pump activities, but rather a consequence of alterations in subcellular, cytosolic Ca2+ binding activities.

Guanidine transport in a human choriocarcinoma cell line (JAR)

PURPOSE

Many endogenous substances and xenobiotics are organic cations. Transplacental transport of organic cations is an important determinant of the delivery of these compounds to the fetus. The aim of this study was to determine the mechanisms of organic cation transport using the human choriocarcinoma cell line (JAR) as a model system with [14C]guanidine as a ligand.

METHODS

Uptake studies of [14C]guanidine were carried out in JAR cell monolayers on day 2 after plating.

RESULTS

[14C]guanidine uptake was temperature dependent, saturable (Km = 167 microM) and inhibited by many organic cations including amiloride, cimetidine, quinine, quinidine and nicotine. [14C]guanidine uptake exhibited a counterflux phenomenon indicative of a carrier-mediated process. The uptake of [14C]guanidine was sodium and pH-independent and could be driven by an inside-negative membrane potential difference.

CONCLUSIONS

This is the first demonstration of an electrogenic guanidine transporter in a human cell culture model. This transporter may play a role in the transplacental transport of many clinically used drugs and xenobiotics.

Effect of human chorionic gonadotrophin on chloride current in human syncytiotrophoblasts in culture

Human trophoblast differentiates in vivo and in vitro by the fusion of cytotrophoblastic cells to form syncytiotrophoblasts. A large amount of human chorionic gonadotrophin (hCG) is produced by the syncytiotrophoblasts, which express hCG luteinizing hormone (LH) receptors. Since recent investigations with electrophysiological techniques support the conclusion that hormonal effects can be mediated by modulations of the membrane ionic conductances of the cells, a perforated patch-clamp technique was used to investigate the possible presence of a chloride current evoked by hCG. The perifusion of hCG (500 mIU/ml) activated a time-independent current, which presents a linear current-voltage (I/V) relationship in symmetrical chloride concentrations. The reversal potential was -1.8 mV with 142 mM Cl- external solution and 134 mM cl- internal solution. This reversal potential shifted with changes in the transmembrane Cl- gradient. Moreover, this hCG-induced current was sensitive to 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (50 microM), to diphenylalamine-2-carboxylic acid (DPC) (0.5 mM) and to 9-AC (1 mM), three known chloride channel blockers. These results confirm the autocrine action of hCG in the physiology of the trophoblast.

A role for ions in barrier recovery after acute perturbation

The epidermal cutaneous permeability barrier can be disrupted by treatment with topical solvents. Recent studies have shown that barrier recovery, measured by the recovery of transepidermal water loss towards normal, is inhibited by high extracellular Ca++ and K+, and accelerated by low extracellular concentrations of these ions. To examine the effects of Ca++ or K+ fluxes on barrier recovery, we tested the effects on transepidermal water loss recovery of agents that modify these fluxes. K+ channel agonists or blockers modified the inhibitory effects on barrier recovery induced by raised extracellular Ca++ and K+. In addition, Na+/K+ adenosine 5' triphosphatase inhibitors reversed the inhibitory effects of high extracellular Ca++ and K+. Our results suggest that barrier recovery requires both Ca++ and K+ fluxes and are consistent with the hypothesis that both verapamil or dihydropyridine-sensitive Ca++-permeable channels and Ca++-sensitive K+ channels participate in epidermal permeability barrier homeostasis.

Whole-cell outward currents in freshly dissociated rat placental cells

1. We applied whole cell voltage clamp techniques to freshly dissociated rat placental cells (20-21 days gestation). Tetraethylammonium (TEA)-sensitive outward currents were recorded in about 50% of cells. 2. The outward current had a reversal potential of -50 mV which is more positive than the potassium equilibrium potential (-82 mV). 3. The bath solution without NaCl decreased the outward current amplitude, while the elimination of only Na ions from the bath solution did not modify the outward current. 4. The results suggest a possible contribution of chloride ions to the outward currents in rat placental cells.

Chloride channels of high conductance in the microvillous membrane of term human placenta

The patch clamp technique has been used to study ion channels in the undisturbed microvillous membrane of the human placental syncytiotrophoblast. In villi from 55 placentae delivered by caesarean section, high resistance seals were achieved in approximately 30 percent of attempts. Of these, a large conductance chloride channel was identified in seven inside-out and two 'cell' attached patches. The channel had the following properties: (a) a slope conductance of 313 +/- 9 pS, (b) the presence of sub-conductance states, (c) voltage dependency, being open predominantly between +/- 20 mV and inactivating at more extreme potentials and (d) inhibition by DIDS (4-acetamido-4'-diisothiocyanostilbene 2,2-disulphonic acid). These are characteristic features of 'maxi' chloride channels which have been identified in a variety of cell types (Gogelein, 1988). The role of the chloride channel in ion transport by or homeostasis of the syncytiotrophoblast has yet to be determined.

Parathyroid-like regulation of parathyroid-hormone-related protein release and cytoplasmic calcium in cytotrophoblast cells of human placenta

Immunohistochemical staining of human placenta revealed intense reactivity for amino terminal and midregional parathyroid-hormone-related protein (PTHrp) in the cytotrophoblast cells and weaker staining in the syncytiotrophoblasts. The cytotrophoblasts also displayed conspicuous surface staining with the monoclonal antibodies E11 and G11, which recognize a Ca2+ receptor mechanism regulating hormone release of parathyroid cells. Cytotrophoblasts enriched on Percoll gradients or by linking surface-bound E11 to magnetic beads revealed biphasic elevation of cytoplasmic Ca2+ ([Ca2+]i) upon a stepwise rise of external Ca2+ from 0.5 to 3.0 mM, with a half-maximal effect at 1.75 mM. Individual cytotrophoblasts identified by their E11 reactivity disclosed a temporary increase of [Ca2+]i upon elevation of external Mg2+, while Mn2+ triggered both a [Ca2+]i transient and an influx of itself. These effects were efficiently blocked by the G11 antibody. Depolarization with K+ or addition of the voltage-dependent Ca2+ channel blocker verapamil had only marginal effects on [Ca2+]i. Raised extracellular calcium inhibited release of PTHrp from the cells, and this inhibition was blocked by the G11 antibody. The virtually parathyroid-identical Ca2+ regulation of [Ca2+]i may mediate feedback control of PTHrp release from the cytotrophoblasts and thereby participate in the regulation of placental Ca2+ transport.

Ion conductances in the microvillous and basal membrane vesicles isolated from human placental syncytiotrophoblast

Experiments were performed to characterize the ionic conductances in microvillous and basal membranes from human placenta. Microvillous and basal membranes were prepared from term placental tissue by homogenization, magnesium precipitation, differential and sucrose density gradient centrifugation. The relative permeabilities of sodium, potassium and chloride were measured using the bi-ionic potential technique which employs a fluorescent probe [diS-C3-(5)] which partitions into membranes in a potential-dependent manner. The permeabilities of sodium and chloride relative to potassium were determined by measuring their effects on a known membrane potential produced by a potassium gradient. In microvillous membranes PNa/PK = 0.25 and PClPK = 0.19 while in basal membranes, PNa/PK = 1.31 and PCl/PK = 0.03. Measurements of chloride permeability relative to sodium confirmed these results. The cation conductances were inhibited by quaternary ammonium compounds. Addition of tetramethylammonium altered the relative permeabilities in a pattern suggesting a block of potassium conductance while tetraethylammonium appeared to block both sodium and potassium conductances.

Inwardly rectifying currents in human term placental cells

Using the whole cell voltage clamp technique, inwardly rectifying currents were observed in most cells isolated from human term placentas. Reversal potentials, estimated by extrapolation, as well as those estimated by tail currents, were found to be between -80mV and -90mV. These potentials are close to the potassium equilibrium potential (theoretical value: -82 mV). In addition, the inwardly rectifying currents were blocked by 1 mM CsCl. The elevation of [K]e increased the current amplitudes. Furthermore, the holding currents were inwardly shifted when the holding potential was kept at -70mV. The extrapolated reversal potentials changed linearly on a semilogarithmic graph with a slope of 57 mV/decade of [K]e. This value is close to the theoretical calculation (58 mV/decade). Consequently, it is suggested from these studies, that inwardly rectifying currents observed in human term placental cells are carried mainly by potassium ions.

Membrane potential and input resistance in syncytiotrophoblast of human term placenta in vitro

Direct measurements of electrical potential and input resistance were performed on the syncytial membrane facing maternal blood. The potentials varied according to the external potassium ion levels and, to a lesser extent, with sodium ion concentrations. Ouabain provoked a rapid and consistent depolarization which indicates an electrogenic component. Amiloride slightly hyperpolarized the membrane, confirming the predominant role of potassium ions in the control of the diffusional component of the membrane potential. The electrical potential revealed in this study might play a part in the maternofetal transport of ions.

Ca2+-binding protein of the human placenta. Characterization, immunohistochemical localization and functional involvement in Ca2+ transport

The Ca2+-binding protein (HCaBP) of the human placenta was studied with respect to its biochemical properties, tissue and cellular distribution, and possible involvement in placental Ca2+ transport. Optimal Ca2+ binding by the HCaBP occurs at pH 7-8 and in 100 mM-Na+ and 3 mM-Ca2+. The HCaBP possesses at least 10 Ca2+-binding sites with a Kd of 5 X 10(-6) M ([Ca2+]). Highly specific rabbit-derived anti-HCaBP antibodies were used for HCaBP immunoquantification and immunohistochemistry, which revealed that the HCaBP is localized in the chorionic villi and is primarily associated with the trophoblastic cells of the placenta. In addition, an 'in vitro' cell-free assay system for Ca2+ uptake was constructed with microsomal membranes isolated from term placental tissues. Ca2+ uptake by the placental microsomal fraction exhibited characteristics indicative of active Ca2+ transport such as temperature-dependence, saturability and energetic requirement. In this system, preincubation of microsomal membranes with anti-HCaBP antibodies inhibited Ca2+ uptake, suggesting that the HCaBP is functionally involved in placental membrane Ca2+ uptake.