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

Expression of an electrically silent voltage-gated potassium channel in the human placenta

Human placental expression of K(V)9.3, a voltage-gated K channel linked to tissue oxygenation responses, has been suggested at the messenger RNA level but tissue localisation has not been described. We aimed to: (1) produce an antibody to human K(V)9.3 and (2) assess channel expression and distribution in human placental tissue. We determined human placental protein expression and localisation using an antibody to K(V)9.3. Antibody specificity was confirmed by Western blotting. Staining was observed in syncytiotrophoblast microvillous membrane, endothelial cells (in intermediate, stem villi and chorionic plate blood vessels) and vascular smooth muscle cells (large diameter vessels only) by immunohistochemistry. Expression was unchanged in tissue from women with small-for-gestational age babies. It was concluded that K(V)9.3 is localised to human placental vascular tissues and syncytiotrophoblast.

Differential expression of potassium channels in placentas from normal and pathological pregnancies: targeting of the K(ir) 2.1 channel to lipid rafts

Potassium channels play important physiological roles in human syncytiotrophoblasts (hSTBs) from placenta, an epithelium responsible for maternal-fetal exchange. Basal and apical plasma membranes differ in their lipid and protein composition, and the latter contains cholesterol-enriched microdomains. In placental tissue, the specific localization of potassium channels is unknown. Previously, we described two isolated subdomains from the apical membrane (MVM and LMVM) and their respective microdomains (lipid rafts). Here, we report on the distribution of K(ir)2.1, K(v)2.1, TASK-1, and TREK-1 in hSTB membranes and the lipid rafts that segregate them. Immunoblotting experiments showed that these channels are present mainly in the apical membrane from healthy hSTBs. Apical expression versus basal membrane was 84 and 16% for K(ir)2.1 and K(v)2.1, 60 and 30% for TREK-1, and 74 and 26% for TASK-1. Interestingly, K(v)2.1 showed differences between apical membrane subdomains: 26 ± 8% was located in the LMVM and 59 ± 9% in MVM. In pathological placentas, the expression distribution changed in the basal membrane: preeclampsia shifted to 50% and intrauterine growth restriction to 42% for TASK-1 and both pathologies increased to 25% for K(ir)2.1 and K(v)2.1, K(ir)2.1 appeared to be associated with rafts that were sensitive to cholesterol depletion in healthy, but not in pathological, placentas. K(v)2.1 and TREK-1 emerged in the nonraft fractions. The precise membrane localization of ion channels in hSTB membranes is necessary to understand the physiological events.

Placental chloride channels: a review

The human placental syncytiotrophoblast (hSTB) is a polarized epithelial structure, that forms the main barrier to materno-fetal exchange. The chloride (Cl(-)) channels in other epithelial tissues contribute to several functions, such as maintenance of the membrane potential, volume regulation, absorption and secretion. Additionally, the contributions of Cl(-) channels to these functions are demonstrated by certain diseases and knock-out animal models. There are multiple lines of evidence for the presence of Cl(-) channels in the hSTB, which could contribute to different placental functions. However, both the mechanism by which these channels are involved in the physiology of the placenta, and their molecular identities are still unclear. Furthermore, a correlation between altered Cl(-) channels functions and pathological pregnancies is beginning to emerge. This review summarizes recent developments on conductive placental chloride transport, and discusses its potential implications for placental physiology.

Calcium channels activated by endothelin-1 in human trophoblast

Ca2+ transfer across the syncytiotrophoblast (ST) of the human placenta is essential for normal fetal development. However, the nature of Ca2+ conductance in the ST and the mechanisms by which it is regulated are poorly understood. With the major signal transduction pathway of endothelin-1 (ET1) acting via phospholipase C (PLC) and Ca2+, we used ET1 to analyse the nature of Ca2+ channels on cultured trophoblastic cells by means of cytofluorimetric analysis using the ratiometric Ca2+ indicator Indo-1. Results indicate that ET1 (10(-7) M) stimulates a biphasic (transient and sustained) increase in [Ca2+]i in trophoblastic cells. This response is mediated by the endothelin receptor B (ETB) coupled to PLC, since treatment with BQ788 (10(-6) M) or U73122 (2 microM) totally abolished the response. Persistence of the rapid transient rise in [Ca2+]i in Ca2+-free extracellular medium confirms the release of Ca2+ from intracellular stores in response to ET1 stimulation. Furthermore, abolition of the sustained increase in [Ca2+]i in Ca2+-free extracellular medium argues in favour of the entry of Ca2+ during the plateau phase. Abolition of this plateau phase by Ni2+ (1 mM) in the presence of extracellular Ca2+ confirmed the existence of an ET1-induced Ca2+ entry. No evidence for the presence of voltage-operated channels was demonstrated during ET1 action since nifedipine (10(-6) M) did not reduce the Ca2+ response and depolarization with a hyper-potassium solution had no effect. Pharmacological studies using the imidazole derivatives SK&F96365 (30 microM) and LOE 908 (10 microM) partially inhibited the ET1-evoked Ca2+ response, thus providing evidence for the presence of both store-operated Ca2+ channels and non-selective cationic channels in the human ST.

A low conductance, non-selective cation channel from human placenta

Non-selective cation channels have been identified in the plasma membranes of many different cells. Previous research using fluorescent techniques has demonstrated the presence of cation conductances in membranes from human trophoblast. The purpose of this work was to explore, by electrophysiological methods, a non-selective cation channel in apical membranes from human placenta. Human placental apical membranes were purified by differential centrifugation and reconstituted in giant liposomes. These giant liposomes were then used for electrophysiological studies and were probed for the presence of cation channels by the patch-clamp method. The channel identified had a linear current-potential relationship with a conductance of around 16 pS in symmetrical Na(+) solution. Under asymmetrical conditions the reversal potential was close to the reversal potential for Na(+). The channel was equally permeable to sodium and potassium and the permeability sequence was NH+4>Cs(+) approximately Rb(+)>Na(+) approximately K(+)>Li(+). The channel also showed permeability to calcium and barium. The channel was insensitive to calcium but was blocked by millimolar concentration of Mg(2+). We have demonstrated the presence of a low conductance, non-selective cation channel in placental apical membranes. These channels share some properties with non-selective cation channels previously described in other different cells. The precise role of these channels in placental physiology has yet to be determined.

Properties of two multisubstate Cl- channels from human syncytiotrophoblast reconstituted on planar lipid bilayers

We describe the first successful reconstitution of placental ionic channels on planar lipid bilayers. An apical plasma membrane-enriched vesicle fraction from human syncytiotrophoblast at term was prepared by following isotonic agitation, differential centrifugation, and Mg2+-induced selective precipitation of nonapical membranes, and its purity was assessed by biochemical and morphological marker analysis. We have already reported that, unlike previous patch-clamp studies, nonselective cation channels were incorporated in most cases, a result consistent with the higher permeability for cations as compared with Cl- and with the low apical membrane potential difference at term revealed by fluorescent probe partition studies, and microelectrode techniques. In this paper, we report that Cl--selective channels were incorporated in 4% of successful reconstitutions (14 out of 353) and that their analysis revealed two types of activity. One of them was consistent with a voltage-dependent, 100-pS channel while the other was consistent with the lateral association of 47-pS conductive units, giving rise to multibarrelled, DIDS-sensitive channels of variable conductance (300 to 650 pS). The latter displayed a very complex behavior which included cooperative gating of conductive units, long-lived substates, voltage-dependent entry into an apparent inactivated state, and flickering activity. The role of the reported Cl- channels in transplacental ion transport and/or syncytium homeostasis remains to be determined.

Membrane potential difference and intracellular cation concentrations in human placental trophoblast cells in culture

1. The electrochemical gradients for Na+ and K+ were assessed in a cell culture model of trophoblast differentiation. 2. Membrane potential difference (Em), intracellular water and Na+ and K+ contents were measured in choriocarcinoma cells (JAr cell line; 96% of which are undifferentiated trophoblast cells) and in mononucleate and multinucleate (differentiated) cytotrophoblast cells isolated from the human placenta at term. 3. There was a significant fall in Em from -57 mV in JAr cells, to -48 and -40 mV in mono-and multinucleate cytotrophoblast cells, respectively. Treatment with ouabain (1 mM for 15 min) depolarized the JAr cell membrane by 15 mV but did not affect cytotrophoblast cell membrane potential. 4. Intracellular K+ concentration was similar in JAr, mono- and multinucleate cytotrophoblast cells but Na+ concentration was higher in mononucleate cytotrophoblast cells compared with JAr cells. 5. Ouabain treatment (3 mM for 15 min) caused a small increase (4.5%) in cell water in mononucleate cytotrophoblast cells but lowered K+ (approximately 30%) and increased Na+ concentration (approximately 125%) in all the trophoblast cells studied. 6. The K+ equilibrium potential (EK) was more negative than Em in all cells and the difference between EK and Em was smaller in JAr cells (-25 mV) than in mono- and multinucleate cytotrophoblast cells (-33 and -43 mV, respectively). 7. The Na+ equilibrium potential (ENa) was positive in the trophoblast cells and the difference between ENa and Em was 122, 100 and 100 mV in JAr, mono- and multinucleate cytotrophoblast cells, respectively. 8. These results suggest that the electrochemical gradient for K+ is affected by the stage of trophoblast cell differentiation. In contrast, the electrochemical gradient for Na+ is similar in mono- and multinucleate cytotrophoblast cells.

A chloride channel from human placenta reconstituted into giant liposomes

OBJECTIVE

Ion channels play important roles in epithelial transport, but they are difficult to access for conventional electrophysiologic studies in intact placenta. The purpose of this work was to explore the suitability of purified trophoblast plasma membrane as a source of ion channels for reconstitution in artificial lipid membranes.

STUDY DESIGN

Human placental brush border membranes were purified by differential and gradient centrifugation and fused with small liposomes. Giant liposomes were then generated by a cycle of dehydration and rehydration. These giant liposomes are suitable for electrophysiologic studies and were probed for the presence of active ion channels by the patch-clamp method.

RESULTS

The results reported here indicate the presence of a high conductance chloride channel showing some similarities with "maxi" chloride channels described in secreting and absorbing epithelia. The channel had a slight outward rectification with conductances of 232 and 300 pS at negative and positive potentials, respectively.

CONCLUSIONS

For the first time successful reconstitution of a human placental ion channel is achieved in a system suited for electrophysiologic studies. The chloride channel described might play a role in transplacental transport.

The effect of osmolarity on human placental mitochondria function

Human placental explants survive large changes in osmolarity, but the mechanism for this property is unknown. The goal of this work was to examine the effect of osmolarity on human placental mitochondria. Mitochondria from human term placenta were isolated by differential centrifugation, and incubated in the presence of different concentrations of sucrose or KCl, to modify the osmolarity of the media. Rat liver mitochondria were used as control. The parameters studied were: respiration rate, adenine nucleotide hydrolysis, calcium transport, membrane potential, and mitochondrial morphology. Stimulation of the mitochondrial respiration rate and an increase in Ca2+ transport was observed in the presence of K+. With sucrose, Ca2+ transport showed a complex kinetic behavior, whereas the respiratory control was slightly diminished. Although the ATPase activity was enhanced in the absence of a respiratory substrate, no change in ATP hydrolysis due to osmolarity was observed. ADP hydrolysis was inhibited by a high K+ concentration, but not by sucrose. The membrane potential was not modified by osmolarity, even in the absence of sucrose or K+ in the medium. Mitochondria isolated with KCl showed aggregation, whereas dispersed mitochondria were observed with sucrose. This study showed that sucrose-induced changes in osmolarity, does not modify metabolic and transport properties of human placental mitochondria, whereas KCl-induced osmolarity changes does affect these functions.