Localization of TASK and TREK, Two-Pore Domain K+ Channels, in Human Cytotrophoblast Cells

TASK Channel Expression in Human Placenta and Cytotrophoblast Cells

OBJECTIVE

The multinucleate syncytiotrophoblast is the transporting epithelium of the human placental villus, formed throughout pregnancy by fusion and differentiation of underlying mononucleate cytotrophoblast cells. Similar to other epithelia, K+ channels will impact on syncytiotrophoblast transport properties during its development and differentiation. Therefore we investigated expression and activity of the two-pore domain K+ channels TASK1 and 2 in relation to gestation and differentiation, using villous tissue from first and third trimester and cultured cytotrophoblast cells at mononucleate and multinucleate stages of culture.

METHODS

Quantitative real-time polymerase chain reaction (PCR), immunofluorescence, and 86Rb+ (K) efflux were used to investigate TASK channel expression and function.

RESULTS

TASK2 mRNA expression was higher in first trimester than term (10 to 13 vs 38 to 40 weeks, P < .05). Other K+ alpha-subunit mRNAs, including TASK1, remained unaltered but the regulatory BKCa beta-subunit, like TASK2, was higher in first trimester than term (P < .001). Immunofluorescence showed that TASK2 had an intracellular localization within the trophoblast of first trimester villi but was less abundant and restricted to stem villi at term. TASK2 also showed intracellular localization in mononucleate cytotrophoblast cells in culture and expression was lost with multinucleation. By contrast, TASK1 was localised, independently of cell nucleation, to cytotrophoblast cell plasma membranes. 86Rb+ (K) efflux was measured from multinucleated cytotrophoblast cells. Both basal and pH 8.0-stimulated efflux was inhibited by the TASK1 antagonist anandamide (n = 5 for both conditions; P < .01 and P < .001, respectively).

CONCLUSION

TASK1 and 2 are expressed in placental trophoblast cells and TASK1 activity may have a role in regulating syncytiotrophoblast homeostasis and/or solute transport functions.

The effect of pH and ion channel modulators on human placental arteries

Chorionic plate arteries (CPA) are located at the maternofetal interface where they are able to respond to local metabolic changes. Unlike many other types of vasculature, the placenta lacks nervous control and requires autoregulation for controlling blood flow. The placental circulation, which is of low-resistance, may become hypoxic easily leading to fetal acidosis and fetal distress however the role of the ion channels in these circumstances is not well-understood. Active potassium channel conductances that are subject to local physicochemical modulation may serve as pathways through which such signals are transduced. The aim of this study was to investigate the modulation of CPA by pH and the channels implicated in these responses using wire myography. CPA were isolated from healthy placentae and pre-contracted with U46619 before testing the effects of extracellular pH using 1 M lactic acid over the pH range 7.4 - 6.4 in the presence of a variety of ion channel modulators. A change from pH 7.4 to 7.2 produced a 29±3% (n = 9) relaxation of CPA which increased to 61±4% at the lowest pH of 6.4. In vessels isolated from placentae of women with pre-eclampsia (n = 6), pH responses were attenuated. L-methionine increased the relaxation to 67±7% (n = 6; p<0.001) at pH 6.4. Similarly the TASK 1/3 blocker zinc chloride (1 mM) gave a maximum relaxation of 72±5% (n = 8; p<0.01) which compared with the relaxation produced by the TREK-1 opener riluzole (75±5%; n = 6). Several other modulators induced no significant changes in vascular responses. Our study confirmed expression of several ion channel subtypes in CPA with our results indicating that extracellular pH within the physiological range has an important role in controlling vasodilatation in the human term placenta.

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.

A role for two-pore potassium (K2P) channels in endometrial epithelial function

The human endometrial epithelium is pivotal to menstrual cycle progression, implantation and early pregnancy. Endometrial function is directly regulated by local factors that include pH, oxygen tension and ion concentrations to generate an environment conducive to fertilization. A superfamily of potassium channels characterized by two-pore domains (K2P) and encoded by KCNK genes is implicated in the control of the cell resting membrane potential through the generation of leak currents and modulation by various physicochemical stimuli. The aims of the study were to determine the expression and function of K2P channel subtypes in proliferative and secretory phase endometrium obtained from normo-ovulatory women and in an endometrial cancer cell line. Using immunochemical methods, real-time qRT-PCR proliferation assays and electrophysiology. Our results demonstrate mRNA for several K2P channel subtypes in human endometrium with molecular expression of TREK-1 shown to be higher in proliferative than secretory phase endometrium (P < 0.001). The K2P channel blockers methanandamide, lidocaine, zinc and curcumin had antiproliferative effects (P < 0.01) in an endometrial epithelial cancer cell line indicating a role for TASK and TREK-1 channels in proliferation. Tetraethylammonium- and 4-aminopyridine-insensitive outwards currents were inhibited at all voltages by reducing extracellular pH from 7.4 to 6.6. Higher expression of TREK-1 expression in proliferative phase endometrium may, in part, underlie linked to increased cell division. The effects of pH and a lack of effect of non-specific channel blockers of voltage-gated potassium channels imply a role for K2P channels in the regulation of human endometrial function.

K+ efflux through two-pore domain K+ channels is required for mouse embryonic development

Numerous studies have suggested that K+ channels regulate a wide range of physiological processes in mammalian cells. However, little is known about the specific function of K+ channels in germ cells. In this study, mouse zygotes were cultured in a medium containing K+ channel blockers to identify the functional role of K+ channels in mouse embryonic development. Voltage-dependent K+ channel blockers, such as tetraethylammonium and BaCl2, had no effect on embryonic development to the blastocyst stage, whereas K2P channel blockers, such as quinine, selective serotonin reuptake inhibitors (fluoxetine, paroxetine, and citalopram), gadolinium trichloride, anandamide, ruthenium red, and zinc chloride, significantly decreased blastocyst formation (P<0.05). RT-PCR data showed that members of the K2P channel family, specifically KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9, were expressed in mouse oocytes and embryos. In addition, their mRNA expression levels, except Kcnk3, were up-regulated by above ninefold in morula-stage embryos compared with 2-cell stage embryos (2-cells). Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed in the membrane of oocytes, 2-cells, and blastocysts. Each siRNA injection targeted at Kcnk2, Kcnk10, Kcnk4, Kcnk3, and Kcnk9 significantly decreased blastocyst formation by ∼38% compared with scrambled siRNA injection (P<0.05). The blockade of K2P channels acidified the intracellular pH and depolarized the membrane potential. These results suggest that K2P channels could improve mouse embryonic development through the modulation of gating by activators.

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.

Novel expression and regulation of voltage-dependent potassium channels in placentas from women with preeclampsia

Preeclampsia is associated with structural/functional alterations in placental and maternal vasculature. Voltage-dependant potassium channels encoded by KCNQ1-5 genes have been detected in several types of blood vessels where they promote vascular relaxation. Voltage-dependant potassium channel function can be modulated by KCNE1-5-encoded accessory proteins. The aim of this study was to determine whether KCNQ and KCNE genes are differentially expressed in placentas from women with preeclampsia compared with normotensive controls and to examine any differences in those who delivered preterm (<37 weeks) or term. Placental biopsies (from midway between the cord and periphery) were obtained, with consent, from white European control (n=24; term) and preeclamptic (n=22; of whom 8 delivered before 37 weeks' gestation) women. KCNQ/KCNE and GAPDH mRNA expressions were determined by quantitative RT-PCR. Protein expression/localization was assessed using immunohistochemistry. KCNQ3 and KCNE5 mRNA expressions were significantly upregulated in preeclampsia (median [interquartile range]: 1.942 [0.905 to 3.379]) versus controls (0.159 [0.088 to 0.288]; P=0.001) and exhibited a strong positive correlation with each other (P<0.001), suggesting a novel heterodimer. Enhanced protein expression of KCNQ3 and KCNE5 in preeclampsia was confirmed with localization mainly restricted to the syncytiotrophoblast. KCNQ4 and KCNE1 isoforms were suppressed in placentas from term preeclamptic women versus controls (P≤0.05). KCNQ1 mRNA expression was increased and KCNQ5 decreased in the preterm preeclamptic group versus controls (P<0.05). In summary, voltage-dependant potassium channels are expressed and markedly modulated in placentas from preeclamptic women. Differential expression of isoforms may lead to altered cell proliferation. The correlation between KCNQ3 and KCNE5 expression is indicative of a novel channel complex and warrants further investigation.

Expression and localization of two-pore domain K(+) channels in bovine germ cells

Two-pore domain K(+) (K(2P)) channels that help set the resting membrane potential of excitable and nonexcitable cells are expressed in many kinds of cells and tissues. However, the expression of K(2P) channels has not yet been reported in bovine germ cells. In this study, we demonstrate for the first time that K(2P) channels are expressed in the reproductive organs and germ cells of Korean cattle. RT-PCR data showed that members of the K(2P) channel family, specifically KCNK3, KCNK9, KCNK2, KCNK10, and KCNK4, were expressed in the ovary, testis, oocytes, embryo, and sperm. Out of these channels, KCNK2 and KCNK4 mRNAs were abundantly expressed in the mature oocytes, eight-cell stage embryos, and blastocysts compared with immature oocytes. KCNK4 and KCNK3 were significantly increased in eight-cell stage embryos. Immunocytochemical data showed that KCNK2, KCNK10, KCNK4, KCNK3, and KCNK9 channel proteins were expressed at the membrane of oocytes and blastocysts. KCNK10 and KCNK4 were strongly expressed and distributed in oocyte membranes. These channel proteins were also localized to the acrosome sperm cap. In particular, KCNK3 and KCNK4 were strongly localized to the post-acrosomal region of the sperm head and the equatorial band within the sperm head respectively. These results suggest that K(2P) channels might contribute to the background K(+) conductance of germ cells and regulate various physiological processes, such as maturation, fertilization, and development.

K+channel inhibition modulates the biochemical and morphological differentiation of human placental cytotrophoblast cells in vitro

Maintaining placental syncytiotrophoblast, a specialized multinucleated transport epithelium, is essential for normal human pregnancy. Syncytiotrophoblast continuously renews through differentiation and fusion of cytotrophoblast cells, under paracrine control by syncytiotrophoblast production of human chorionic gonadotropin (hCG). We hypothesized that K+channels participate in trophoblast syncytialization and hCG secretion in vitro. Two models of normal-term placenta were used: 1) isolated cytotrophoblast cells and 2) villous tissue in explant culture. Cells and explants were treated with K+channel modulators from 18 h, and day 3, onward, respectively. Culture medium was analyzed for hCG, to assess secretion, as well as for lactate dehydrogenase (LDH), to indicate cell/tissue integrity. hCG was also measured in cytotrophoblast cell lysates, indicating cellular production. Syncytialization of cytotrophoblast cells was assessed by immunofluorescent staining of desmosomes and nuclei. Over 18–66 h, mononucleate cells fused to form multinucleated syncytia, accompanied by a 28-fold rise in hCG secretion. 1 mM Ba2+stimulated cytotrophoblast cell hCG secretion at 66 h compared with control, whereas 5 mM tetraethylammonium (TEA) inhibited hCG secretion by >90%. 0.1–1 mM 4-aminopyridine (4-AP) reduced cytotrophoblast cell hCG secretion and elevated cellular hCG; without altering cellular integrity or syncytialization. In villous explants, hCG secretion was not altered by 1 mM Ba2+but inhibited by 5 mM 4-AP and 5/10 mM TEA, without affecting LDH release. Anandamide, pinacidil, and cromakalim were without effect in either model. In conclusion, 4-AP- and TEA-sensitive K+channels (e.g., voltage-gated and Ca2+-activated) regulate trophoblast hCG secretion in culture. If these K+channels participate in hCG secretion in situ, they may regulate trophoblast turnover in health and disease.

Expression and function of potassium channels in the human placental vasculature

In the placental vasculature, where oxygenation may be an important regulator of vascular reactivity, there is a paucity of data on the expression of potassium (K) channels, which are important mediators of vascular smooth muscle tone. We therefore addressed the expression and function of several K channel subtypes in human placentas. The expression of voltage-gated (Kv)2.1, KV9.3, large-conductance Ca2+-activated K channel (BKCa), inward-rectified K+ channel (KIR)6.1, and two-pore domain inwardly rectifying potassium channel-related acid-sensitive K channels (TASK)1 in chorionic plate arteries, veins, and placental homogenate was assessed by RT-PCR and Western blot analysis. Functional activity of K channels was assessed pharmacologically in small chorionic plate arteries and veins by wire myography using 4-aminopyridine, iberiotoxin, pinacidil, and anandamide. Experiments were performed at 20, 7, and 2% oxygen to assess the effect of oxygenation on the efficacy of K channel modulators. KV2.1, KV9.3, BKCa, KIR6.1, and TASK1 channels were all demonstrated to be expressed at the message level. KV2.1, BKCa, KIR6.1, and TASK1 were all demonstrated at the protein level. Pharmacological manipulation of voltage-gated and ATP-sensitive channels produced the most marked modifications in vascular tone, in both arteries and veins. We conclude that K channels play an important role in controlling placental vascular function.