Chloride transport by human placental microvillous membrane vesicles

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.

Gestational profile of Na+/H+ exchanger and Cl−/HCO3− anion exchanger mRNA expression in placenta using real-time QPCR

The onset of maternal blood flow (10-12 weeks gestation) results in increased oxygenation of the placenta. We investigated whether the expressions of Na+/H+ exchanger (NHE) and Cl-/HCO3- anion exchanger (AE), thought to have an important role in maintaining intracellular pH of the syncytiotrophoblast and fetal pH homeostasis, are altered at the same time as this increase in blood flow. Real-time quantitative PCR was used to examine steady state levels of NHE (NHE1, 2, 3) and AE (AE1, 2) mRNA expression in early (6-9 weeks) and late (10-13 weeks) first trimester and full-term (38-40 weeks) placentas. beta-Actin, IF2B and GAPDH mRNA was also measured. None of the genes showed a significant difference in expression between the early and late first trimester groups. However, NHE2 (p < 0.001) and GAPDH (p < 0.05) mRNA expression significantly increased 18- and 3.7-fold between early first trimester and term. In conclusion, this study provides additional evidence that GAPDH is an unsuitable housekeeping gene for normalization of transcript levels in placenta. The expression of NHE and AE in the villous placenta is not altered concomitant with the onset of maternal blood flow. However, NHE2 transcripts appear to be gestationally regulated, which may contribute to changes in NHE activity.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Microvillar ion channels: cytoskeletal modulation of ion fluxes

The recently presented theory of microvillar Ca(2+)signaling [Lange, K. (1999) J. Cell. Physiol.180, 19-35], combined with Manning's theory of "condensed counterions" in linear polyelectrolytes [Manning, G. S. (1969). J. Chem. Phys.51, 924-931] and the finding of cable-like ion conductance in actin filaments [Lin, E. C. & Cantiello, H. F. (1993). Biophys. J.65, 1371-1378], allows a systematic interpretation of the role of the actin cytoskeleton in ion channel regulation. Ion conduction through actin filament bundles of microvilli exhibits unique nonlinear transmission properties some of which closely resemble that of electronic semiconductors: (1) bundles of microfilaments display significant resistance to cation conduction and (2) this resistance is decreased by supply of additional energy either as thermal, mechanical or electromagnetic field energy. Other transmission properties, however, are unique for ionic conduction in polyelectrolytes. (1) Current pulses injected into the filaments were transformed into oscillating currents or even into several discrete charge pulses closely resembling that of single-channel recordings. Discontinuous transmission is due to the existence of counterion clouds along the fixed anionic charge centers of the polymer, each acting as an "ionic capacitor". (2) The conductivity of linear polyelectrolytes strongly decreases with the charge number of the counterions; thus, Ca(2+)and Mg(2+)are effective modulator of charge transfer through linear polyelectrolytes. Field-dependent formation of divalent cation plugs on either side of the microvillar conduction line may generate the characteristic gating behavior of cation channels. (3) Mechanical movement of actin filament bundles, e.g. bending of hair cell microvilli, generates charge translocations along the filament structure (mechano-electrical coupling). (4) Energy of external fields, by inducing molecular dipoles within the polyelectrolyte matrix, can be transformed into mechanical movement of the system (electro-mechanical coupling). Because ionic transmission through linear polyelectrolytes is very slow compared with electronic conduction, only low-frequency electromagnetic fields can interact with the condensed counterion systems of linear polyelectrolytes. The delineated characteristics of microvillar ion conduction are strongly supported by the phenomenon of electro-mechanical coupling (reverse transduction) in microvilli of the audioreceptor (hair) cells and the recently reported dynamics of Ca(2+)signaling in microvilli of audio- and photoreceptor cells. Due to the cell-specific expression of different types and combinations of ion channels and transporters in the microvillar tip membrane of differentiated cells, the functional properties of this cell surface organelle are highly variable serving a multitude of different cellular functions including receptor-mediated effects such as Ca(2+)signaling, regulation of glucose and amino acid transport, as well as modulation of membrane potential. Even mechanical channel activation involved in cell volume regulation can be deduced from the systematic properties of the microvillar channel concept. In addition, the specific ion conduction properties of microfilaments combined with their proposed role in Ca(2+)signaling make microvilli the most likely cellular site for the interaction with external electric and magnetic fields.

Regulation of cell volume via microvillar ion channels

A novel mechanism of cellular volume regulation is presented, which ensues from the recently introduced concept of transport and ion channel regulation via microvillar structures (Lange K, 1999, J Cell Physiol 180:19-35). According to this notion, the activity of ion channels and transporter proteins located on microvilli of differentiated cells is regulated by changes in the structural organization of the bundle of actin filaments in the microvillar shaft region. Cells with microvillar surfaces represent two-compartment systems consisting of the cytoplasm on the one side and the sum of the microvillar tip (or, entrance) compartments on the other side. The two compartments are separated by the microvillar actin filament bundle acting as diffusion barrier ions and other solutes. The specific organization of ion and water channels on the surface of microvillar cell types enables this two-compartment system to respond to hypo- and hyperosmotic conditions by activation of ionic fluxes along electrochemical gradients. Hypotonic exposure results in swelling of the cytoplasmic compartment accompanied by a corresponding reduction in the length of the microvillar diffusion barrier, allowing osmolyte efflux and regulatory volume decrease (RVD). Hypertonic conditions, which cause shortening of the diffusion barrier via swelling of the entrance compartment, allow osmolyte influx for regulatory volume increase (RVI). Swelling of either the cytoplasmic or the entrance compartment, by using membrane portions of the microvillar shafts for surface enlargement, activates ion fluxes between the cytoplasm and the entrance compartment by shortening of microvilli. The pool of available membrane lipids used for cell swelling, which is proportional to length and number of microvilli per cell, represents the sensor system that directly translates surface enlargements into activation of ion channels. Thus, the use of additional membrane components for osmotic swelling or other types of surface-expanding shape changes (such as the volume-invariant cell spreading or stretching) directly regulates influx and efflux activities of microvillar ion channels. The proposed mechanism of ion flux regulation also applies to the physiological main functions of epithelial cells and the auxiliary action of swelling-induced ATP release. Furthermore, the microvillar entrance compartment, as a finely dispersed ion-accessible peripheral space, represents a cellular sensor for environmental ionic/osmotic conditions able to detect concentration gradients with high lateral resolution. Volume regulation via microvillar surfaces is only one special aspect of the general property of mechanosensitivity of microvillar ionic pathways.

Identification and biochemical localization of a Na-K-Cl cotransporter in the human placental cell line BeWo

Several transport systems mediating the placental transport of Na, K and Cl have been described, but whether the trophoblast membrane also expresses a Na-K-Cl cotransporter that mediates the coupled movement of all three ions remains unclear. Here we show that BeWo cells, a human trophoblastic cell line, exhibit bumetanide-sensitive (86)Rb (a K surrogate) uptake. Entry via this route accounts for approximately 17% of the (86)Rb influx with the remainder being mediated largely via the Na,K-ATPase. The activity of the bumetanide-sensitive transporter was rapidly elevated (>40%) upon subjecting cells to an acute hyperosmotic challenge signifying a potential role in cell volume regulation. Antibodies to the Na-K-Cl cotransporter identified a single band of approximately 200 kDa on Western blots of fractionated BeWo membranes. This immunoreactivity colocalized with that of the Na,K-ATPase (a basal membrane marker), but was absent from membranes enriched with placental alkaline phosphatase (an apical membrane marker). These findings show for the first time, that a Na-K-Cl cotransporter is expressed in a human placental cell line which may be involved in regulating trophoblast cell volume.

Regulation of human placental chloride channel by arachidonic acid and other cis unsaturated fatty acids

OBJECTIVE

Arachidonic acid has been implicated in the modulation of various transport processes, including conductive chloride transport in brush border membranes in the human placenta. The purpose of this work was to explore the effects of some cis unsaturated fatty acids on the electrophysiologic properties of the maxi chloride channels present in apical membranes from human placenta.

STUDY DESIGN

Apical membrane chloride channels from human term placentas were reconstituted in giant liposomes. These cell-sized liposomes, generated by a cycle of dehydration and rehydration, are suitable for electrophysiologic studies by the patch-clamp method.

RESULTS

Low micromolar concentrations of arachidonic acid reversibly inhibit maxi chloride channels in excised patches. Other cis unsaturated fatty acids, such as oleic and linoleic acids, show similar blockade. The inhibition was dose dependent. The maxi chloride channel can also be inhibited by 4,4 -diisothiocyanatostilbene-2,2 -disulfonic acid, a known chloride channel inhibitor.

CONCLUSIONS

Our results identify the apical membrane maxi chloride channel as a possible electrophysi ologic counterpart of 4,4 -diisothiocyanatostilbene-2, 2 -disulfonic acid and cis unsaturated fatty acid-inhibited conductance previously described in brush border membranes of the human placenta. From a functional point of view the control of these channels by arachidonic acid may be of great importance in placental physiologic characteristics. Regulation of chloride channels could be important in the control of electrolyte and fluid transfer across the placenta. In addition, if these channels contribute to setting the membrane potential their regulation could have consequences for nutrient transport and delivery to the fetus. The electrophysiologic identification of these channels and their regulation might help to unravel their possible role in transplacental transport in normal and pathologic placental tissue.

Chloride transport across syncytiotrophoblast microvillous membrane of first trimester human placenta

There are significant changes in the activity of some placental transporters between first trimester and term. However, chloride transport has previously been studied only in the term placenta. Therefore. in this study, we investigated chloride transport mechanisms in syncytiotrophoblast microvillous membrane (MVM) vesicles from first trimester human placentas and compared them with those in vesicles from term placentas. 36Cl- uptake into MVM vesicles was linear up to 45 s and had reached equilibrium by 1 h for both first trimester and term vesicles. In first trimester MVM at 0 mV, 0.1 mM diisothiocyano-2,2'-disulfonic stilbene (DIDS) blocked 25+/-3% (n=8) of 36Cl- uptake at 30 s (initial rate), which was similar to the 30+/-7% (n=6) inhibition by DIDS in term MVM. In the presence of a 25 mV inside-positive electrical potential difference, induced by imposition of a K+ gradient after preincubation with 200 microM valinomycin, 0.5 mM diphenylamine-2-carboxylate (DPC) significantly blocked 30+/-4% of 36Cl- uptake at 30 s by first trimester MVM (p < 0.01); 18+/-5% (n=8) of total uptake was inhibited by DPC but not by DIDS. There was a similar 15+/-3% (n=6) component of 36Cl- uptake by term MVM, which was inhibited by DPC but not by DIDS. Using Western blotting, it was shown that the anion exchanger-1 protein was expressed in first trimester MVM in quantitatively similar amounts to that in term MVM. This study suggests that there is both an anion exchanger and a DPC-sensitive conductance in MVM of first trimester placenta with activity similar to that of term human placenta.

Mechanisms of chloride transport across the syncytiotrophoblast basal membrane in the human placenta

Chloride transport mechanisms in isolated plasma membrane vesicles were studied to characterize pathways for transcellular transport of chloride. Microvillous membrane (MVM) and basal membranes (BM) vesicles were isolated from term placentae. Western blot analysis of the anion exchanger isoform 1 (AE1) demonstrated that the density of AE1 was 12-fold higher on the MVM compared to the BM. At 30 sec, the Cl- uptake in the absence of a potential difference (p.d.) was 457.3 +/- 69.7 and 111.0 +/- 29.1 pmol/mg protein in MVM and BM, respectively (mean +/- SEM, n=6). Chloride transport pathways were characterized using diisothiocyano-2'2-disulphonic stilbene. (DIDS, 0.1 mM) and diphenylamine-2-carboxylate (DPC, 0.5 mM) in the absence or presence of inside positive membrane potentials. Anion exchange (DIDS-sensitive uptake at zero mV) was found in the MVM only. Both MVM and BM showed increased chloride uptake in the presence of inside positive potentials, suggesting the presence of chloride conductance pathways. The chloride uptake with a 25-mV inside positive p.d. could be inhibited by both DIDS and DPC in MVM and BM. However greater potentials (50 mV) showed no significant inhibition by DIDS or DPC in BM. In conclusion, the anion exchanger is unlikely to contribute significantly to chloride fluxes across BM. The data also suggest the presence of Cl- conductance pathways in both the MVM and BM which are sensitive to both DIDS and DPC.

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.