Global, synchronous oscillations in cytosolic calcium and adherence in bradykinin-stimulated Madin-Darby canine kidney cells.

AIMS AND METHODS: Intercellular Ca2+ oscillations are a universal mode of signalling in both excitable and non-excitable cells. Here, we study the relationship between Ca2+ signalling and coherent changes in adhesion properties by measuring the transepithelial impedance across bradykinin-stimulated Madin-Darby canine kidney (MDCK) cell layers grown on a microelectrode. During hormone stimulation, the impedance is found to oscillate, reflecting that the cells undergo morphological/adhesive alterations with high spatio-temporal organization. The experiments are supplemented with parallel, digital imaging fluorescence microscopy of bradykinin-induced single-cell Ca2+ oscillations. RESULTS: In agreement with previous experiments, MDCK cells are found to elicit synchronous, multicellular Ca2+ oscillations in response to hormone stimulus. The periods of the Ca2+ oscillations and the electrical fluctuations are found to coincide. Further, blocking of gap junctions by 18alpha-glycyrrhetinic acid causes a loss of synchrony in Ca2+ signals and inhibition of impedance oscillations, emphasizing the importance of gap junctions in the signal transduction process. CONCLUSION: Based on these observations it is concluded that the co-ordinated adhesive changes in MDCK cells are a direct consequence of synchronized Ca2+ oscillations. Calcium signalling represents an efficient way of organizing physiological responses in a tissue. A possible functional implication of the structural changes might be to modulate transportation of various substances across the cell sheet.

Electrical impedance of cultured endothelium under fluid flow.

The morphological and functional status of organs, tissues, and cells can be assessed by evaluating their electrical impedance. Fluid shear stress regulates the morphology and function of endothelial cells in vitro. In this study, an electrical biosensor was used to investigate the dynamics of flow-induced alterations in endothelial cell morphology in vitro. Quantitative, real-time changes in the electrical impedance of endothelial monolayers were evaluated using a modified electric cell-substrate impedance sensing (ECIS) system. This ECIS/Flow system allows for a continuous evaluation of the cell monolayer impedance upon exposure to physiological fluid shear stress forces. Bovine aortic endothelial cells grown to confluence on thin film gold electrodes were exposed to fluid shear stress of 10 dynes/cm2 for a single uninterrupted 5 h time period or for two consecutive 30 min time periods separated by a 2 h no-flow interval. At the onset of flow, the monolayer electrical resistance sharply increased reaching 1.2 to 1.3 times the baseline in about 15 min followed by a sustained decrease in resistance to 1.1 and 0.85 times the baseline value after 30 min and 5 h of flow, respectively. The capacitance decreased at the onset of flow, started to recover after 15 min and after slightly overshooting the baseline values, decreased again with a prolonged exposure to flow. Measured changes in capacitance were in the order of 5% of the baseline values. The observed changes in endothelial impedance were reversible upon flow removal with a recovery rate that varied with the duration of the preceding flow exposure. These results demonstrate that the impedance of endothelial monolayers changes dynamically with flow indicating morphological and/or functional changes in the cell layer. This in vitro model system (ECIS/Flow) may be a very useful tool in the quantitative evaluation of flow-induced dynamic changes in cultured cells when used in conjunction with biological or biochemical assays able to determine the nature and mechanisms of the observed changes.

Correlated motion and oscillation of neighboring cells in vitro.

It has long been realized that fibroblastic and epithelial cells establish recognizable patterns in tissue culture. This behavior implies that neighboring cells interact with one another to produce organized populations. Interaction between cells that are separated by many intervening cells is also possible and is demonstrated here using a special configuration of a biosensor referred to as electric cell-substrate impedance sensing (ECIS). Normally the electrical impedance of a single electrode covered with a confluent cell layer is measured, and the morphological changes of the cells are reflected in the impedance. In this case the cells are cultured on two closely spaced electrodes whose impedances are measured independently as a function of time, and communication between the cell populations is revealed as a correlation between these two time series. We also report for the first time another striking manifestation of dynamic cell interaction, where confluent layers of Madin-Darby canine kidney cells (MDCK) on a single electrode are observed to oscillate in synchrony with a period of approximately 2.5 h.

Electric cell-substrate impedance sensing (ECIS) as a noninvasive means to monitor the kinetics of cell spreading to artificial surfaces.

This article describes the optimization of an experimental technique referred to as electric cell-substrate impedance sensing (ECIS) to monitor attachment and spreading of mammalian cells quantitatively and in real time. The method is based on measuring changes in AC impedance of small gold-film electrodes deposited on a culture dish and used as growth substrate. Based on experimental data and theoretical considerations we demonstrate that high-frequency capacitance measurements (f = 40 kHz) are most suited to follow the increasing surface coverage of the electrode due to cell spreading. The excellent time resolution of the method allowed an in-depth analysis of cell spreading kinetics under various experimental conditions. Using ECIS we studied the attachment and spreading of epithelial MDCK cells (strain II) on different protein coatings, and investigated the influence of divalent cations on spreading kinetics. We quantified the inhibitory effect of soluble peptides that mimic the recognition sequence of fibronectin and other extracellular matrix proteins (RGDS). We also applied the ECIS technique to monitor the detachment of confluent fibroblastic cell layers (WI38/VA-13) by means of these peptides.

Histamine alters endothelial barrier function at cell-cell and cell-matrix sites.

To determine how histamine regulates endothelial barrier function through an integrative cytoskeletal network, we mathematically modeled the resistance across an endothelial cell-covered electrode as a function of cell-cell, cell-matrix, and transcellular resistances. Based on this approach, histamine initiated a rapid decrease in transendothelial resistance predominantly through decreases in cell-cell resistance in confluent cultured human umbilical vein endothelial cells (HUVECs). Restoration of resistance was characterized by initially increasing cell-matrix resistance, with later increases in cell-cell resistance. Thus histamine disrupts barrier function by specifically disrupting cell-cell adhesion and restores barrier function in part through direct effects on cell-matrix adhesion. To validate the precision of our technique, histamine increased the resistance in subconfluent HUVECs in which there was no cell-cell contact. Exposure of confluent monolayers to an antibody against cadherin-5 caused a predominant decrease in cell-cell resistance, whereas the resistance was unaffected by the antibody to cadherin-5 in subconfluent cells. Furthermore, we observed an increase predominantly in cell-cell resistance in ECV304 cells that were transfected with a plasmid containing a glucocorticoid-inducible promoter controlling expression of E-cadherin. Transmission electron microscopy confirmed tens of nanometer displacements between adjacent cells at a time point in which histamine maximally decreased cell-cell resistance.

Cell-substrate contact: another factor may influence transepithelial electrical resistance of cell layers cultured on permeable filters.

Transepithelial resistance (TER) measurement has often been used to study the paracellular transport properties of epithelia grown on permeable filters, especially the barrier function of tight junctions. However, the TER value includes another source, the resistance caused by cell-substrate contact, that may give rise to a high TER value if cell-substrate separation is small. In this study we use electric cell-substrate impedance sensing (ECIS) to measure both paracellular resistance and the average cell-substrate distance of MDCK (II), HEp-2, and WI-38 VA13 cells. Comparing ECIS data with those from TER measurements of cell layers cultured on polycarbonate filters, we can obtain the approximate extra resistance resulting from cell-substrate contact for each cell type. The value of cell-substrate resistance was also estimated by two theoretical calculations that bracket the true values. Our results demonstrate that cell-substrate contact substantially influences the TER data measured using polycarbonate filters and that the extra resistance due to cell-substrate spaces depends on both cell type and filter property.

Cell adhesion force microscopy.

The adhesion forces of cervical carcinoma cells in tissue culture were measured by using the manipulation force microscope, a novel atomic force microscope. The forces were studied as a function of time and temperature for cells cultured on hydrophilic and hydrophobic polystyrene substrates with preadsorbed proteins. The cells attached faster and stronger at 37 degreesC than at 23 degreesC and better on hydrophilic than on hydrophobic substrates, even though proteins adsorb much better to the hydrophobic substrates. Because cell adhesion serves to control several stages in the cell cycle, we anticipate that the manipulation force microscope can help clarify some cell-adhesion related issues.

Assessment of rapid morphological changes associated with elevated cAMP levels in human orbital fibroblasts.

Orbital fibroblasts exhibit a phenotype distinct from that of other types of fibroblasts. Addition of prostaglandin E2 (PGE2) to culture medium elicits a dramatic change in orbital fibroblast morphology. That response is mediated through the generation of cAMP. Orbital fibroblasts can generate high levels of PGE2 through induction by proinflammatory cytokines of prostaglandin endoperoxide H synthase-2 (PGHS-2). Here we compare the influence on fibroblast morphology of exogenous PGE2, forskolin, and 8-br-cAMP to that mediated through PGHS-2 induction by a lymphocyte-derived cytokine. Within a few hours, orbital fibroblasts treated with any of these test compounds appear under phase-contrast microscopy to exhibit a stellate morphology. When these changes were assessed quantitatively by electric cell-substrate impedance sensing (ECIS), it became evident that 8-br-cAMP, forskolin, and PGE2 initiated shape changes within 30 min of addition to the culture medium, while effects of the cytokine were first evident after approximately 3.5 h. Dermal fibroblasts failed to respond to any of these compounds with regard to changes in cellular morphology. Analysis of micromotion, manifested as small impedance fluctuations, revealed that orbital fibroblasts treated with 8-br-cAMP exhibit less motion than did untreated cells. These results suggest that orbital fibroblast shape can be altered by several compounds known to alter intracellular cAMP levels. They demonstrate the utility of ECIS in the assessment of very rapid and dynamic cellular events associated with changes in cell morphology.

Podokinesis in endothelial cell migration: role of nitric oxide.

Previously, we demonstrated the role of nitric oxide (NO) in transforming epithelial cells from a stationary to locomoting phenotype [E. Noiri, T. Peresleni, N. Srivastava, P. Weber, W.F. Bahou, N. Peunova, and M. S. Goligorsky. Am. J. Physiol. 270 (Cell Physiol. 39): C794-C802, 1996] and its permissive function in endothelin-1-stimulated endothelial cell migration (E. Noiri, Y. Hu, W. F. Bahou; C. Keese, I. Giaever, and M. S. Goligorsky, J. Biol: Chem. 272: 1747-1753, 1997). In the present study, the role of functional NO synthase in executing the vascular endothelial growth factor (VEGF)-guided program of endothelial cell migration and angiogenesis was studied in two independent experimental settings. First, VEGF, shown to stimulate NO release from simian virus 40-immortalized microvascular endothelial cells, induced endothelial cell transwell migration, whereas NG-nitro-L-arginine methyl ester (L-NAME) or antisense oligonucleotides to endothelial NO synthase suppressed this effect of VEGF. Second, in a series of experiments on endothelial cell wound healing, the rate of VEGF-stimulated cell migration was significantly blunted by the inhibition of NO synthesis. To gain insight into the possible mode of NO action, we next addressed the possibility that NO modulates cell matrix adhesion by performing impedance analysis of endothelial cell monolayers subjected to NO. The data showed the presence of spontaneous fluctuations of the resistance in ostensibly stationary endothelial cells. Spontaneous oscillations were induced by NO, which also inhibited cell matrix adhesion. This process we propose to term "podokinesis" to emphasize a scalar from of micromotion that, in the presence of guidance cues, e.g., VEGF, is transformed to a vectorial movement. In conclusion, execution of the program for directional endothelial cell migration requires two coexisting messages: NO-induced podokinesis (scalar motion) and guidance cues, e.g., VEGF, which imparts a vectorial component to the movement. Such a requirement for the dual signaling may explain a mismatch in the demand and supply with newly formed vessels in different pathological states accompanied by the inhibition of NO synthase.

Permissive role of nitric oxide in endothelin-induced migration of endothelial cells.

Endothelin (ET) synthesis is enhanced at sites of ischemia or in injured vessels. The purpose of this study was to explore the possibility of autocrine stimulation of endothelial cell migration by members of the endothelin family. Experiments with microvascular endothelial cell transmigration in a Boyden chemotactic apparatus showed that endothelins 1 and 3, as well as a selective agonist of ETB receptor IRL-1620, equipotently stimulated migration. Endothelial cell migration was unaffected by the blockade of ETA receptor, but it was inhibited by ETB receptor antagonism. Based on our previous demonstration of signaling from the occupied ETB receptor to constitutive nitric oxide (NO) synthase (Tsukahara, H., Ende, H., Magazine, H. I., Bahou, W. F., and Goligorsky, M. S. (1994) J. Biol. Chem. 269, 21778-21785), we next examined the contribution of ET-stimulated NO production to endothelial cell migration. In three independent cellular systems, 1) migration and wound healing by microvascular endothelial cells, 2) wound healing by Chinese hamster ovary cells stably expressing ETB receptor with or without endothelial NO synthase, and 3) application of antisense oligodeoxynucleotides targeting endothelial NO synthase in human umbilical vein endothelial cells, an absolute requirement for the functional NO synthase in cell migration has been demonstrated. These findings establish the permissive role of NO synthesis in endothelin-stimulated migration of endothelial cells.

Histamine and thrombin modulate endothelial focal adhesion through centripetal and centrifugal forces.

We examined the contribution of actin-myosin contraction to the modulation of human umbilical vein endothelial cell focal adhesion caused by histamine and thrombin. Focal adhesion was measured as the electrical resistance across a cultured monolayer grown on a microelectrode. Actin-myosin contraction was measured as isometric tension of cultured monolayers grown on a collagen gel. Histamine immediately decreased electrical resistance but returned to basal levels within 3-5 min. Histamine did not increase isometric tension. Thrombin also immediately decreased electrical resistance, but, however, resistance did not return to basal levels for 40-60 min. Thrombin also increased isometric tension, ML-7, an inhibitor of myosin light chain kinase, prevented increases in myosin light chain phosphorylation and increases in tension development in cells exposed to thrombin. ML-7 did not prevent a decline in electrical resistance in cells exposed to thrombin. Instead, ML-7 restored the electrical resistance to basal levels in a shorter period of time (20 min) than cells exposed to thrombin alone. Also, histamine subsequently increased electrical resistance to above basal levels, and thrombin initiated an increase in resistance during the time of peak tension development. Hence, histamine and thrombin modulate endothelial cell focal adhesion through centripetal and centrifugal forces.

Impedance analysis of MDCK cells measured by electric cell-substrate impedance sensing.

Transepithelial impedance of Madin-Darby canine kidney cell layers is measured by a new instrumental method, referred to as electric cell-substrate impedance sensing. In this method, cells are cultured on small evaporated gold electrodes, and the impedance is measured in the frequency range 20-50,000 Hz by a small probing current. A model for impedance analysis of epithelial cells measured by this method is developed. The model considers three different pathways for the current flowing from the electrode through the cell layer: (1) in through the basal and out through the apical membrane, (2) in through the lateral and out through the apical membrane, and (3) between the cells through the paracellular space. By comparing model calculation with experimental impedance data, several morphological and cellular parameters can be determined: (1) the resistivity of the cell layer, (2) the average distance between the basal cell surface and substratum, and (3) the capacitance of apical, basal, and lateral cell membranes. This model is used to analyze impedance changes on removal of Ca2+ from confluent Mardin-Darby canine kidney cell layers. The method shows that reduction of Ca2+ concentration causes junction resistance between cells to drop and the distance between the basal cell surface and substratum to increase.

Prostaglandin E2 alters human orbital fibroblast shape through a mechanism involving the generation of cyclic adenosine monophosphate.

Orbital fibroblasts from patients with Graves' ophthalmopathy, when treated with prostaglandin E2 (PGE2), become stellate and develop prominent cellular processes. In this paper, we describe results of studies designed to characterize the action of PGE2 on orbital fibroblast shape changes in vitro. Orbital and dermal fibroblasts were incubated with PGE2, one of several prostanoid analogues, 8-br-cAMP or forskolin and were then visualized by phase-contrast microscopy. Other studies involved seeding cells in special chambers equipped with electrodes for cell sensing using electric cell-substrate impedance sensing (ECIS) to detect changes in shape. PGE2 (10(-7) mol/L) elicited a rapid and dramatic alteration in the shape of orbital fibroblasts but not those derived from the skin. Cells became stellate and developed prominent cytoplasmic processes that extended out from the central area containing the cell nucleus. The effects were stereoselective in that a number of structurally related compounds, including Sulprostone, PGI2, PGF2 alpha, thromboxane A2, thromboxane B2, and 11 deoxy,16,16 dimethyl PGE2 failed to elicit a similar shape change. Butaprost (10(-5) mol/L), a specific EP2 agonist, elicited a similar shape-change as that observed with PGE2. 16,16-dimethyl PGE2, a nonselective agonist, could mimic the action of PGE2. The effect of PGE2 was apparent at 10(-8) mol/L, maximal at a concentration of 10(-7) mol/L and took 4-8 hr to evolve completely. Cycloheximide (10 micrograms/mL) and actinomycin D (1 micrograms/mL) failed to block the shape change. The morphologic change could be reproduced by addition of 8-br-cAMP (3 mmol/L) and by forskolin (5 mumol/L). Moreover, PGE2 and Butaprost treatment elicited in orbital cultures a massive increase in endogenous cAMP production while analogues not affecting cell shape failed to influence cyclic nucleotide generation. Three strains of orbital fibroblasts from patients with Graves' ophthalmopathy and three from normal orbits were tested and all responded to PGE2 (10(-7) mol/L). Four strains of dermal fibroblasts failed to respond to PGE2. The changes in orbital fibroblast morphology were accompanied by a marked decrease in monolayer impedance as assessed by electric cell-substrate impedance sensing. The earliest effects were apparent within 30 min using this sensitive technique. The widely recognized roles of PGE2 and related compounds in the mediation of the inflammatory response make our current findings in orbital fibroblasts of potential importance to the pathogenesis of Graves' ophthalmopathy.

pH changes in pulsed CO2 incubators cause periodic changes in cell morphology.

It is common in animal cell culture to utilize media that are buffered with bicarbonate ions and elevated CO2 concentrations in the incubator gas. Many modern incubators maintain these concentrations by monitoring the level of CO2 and injecting the gas when the value drops below a certain set point. This feedback system results in small oscillations in the incubator CO2 concentration of about 0.5%, resulting in oscillating pH values in the medium bathing the cells. These oscillations are very small and cannot be detected using a standard pH combination electrode, and until now no one has noticed any effect on the cells. In this study, however, we demonstrate that these periodic changes can result in detectable changes in cell morphology and therefore do affect cell behavior. Using electric cell-substrate impedance sensing (ECIS) to detect subtle alterations in cell morphology, we have measured cellular changes that when analyzed with the Fast Fourier Transform have a periodicity that matches those of the CO2 addition. This effect becomes particularly prominent when the culture medium has been acidified by cell metabolism reducing the liquid's buffer capacity. We have further analyzed the cyclical morphological changes employing a model of cell-electrode interactions in the ECIS measurement. We conclude that: (1) the periodic changes in cell morphology are most apparent if the medium is partially spent and if the surface to volume ratio of the culture vessel is high and (2) for confluent WI-38/VA13 fibroblasts and bovine pulmonary endothelial cells (B3B3), the main periodic morphological changes manifest themselves primarily as alterations in paracellular resistance.

Effect of the pSV2-neo plasmid on NIH 3T3 cell motion detected electrically.

Advantage was taken of DNA transfection techniques to investigate the effect of the pSV2-neo plasmid and its derivatives on recipient NIH 3T3 cell motion. Cell spreading and motion were followed by a newly developed electrical method to monitor cell morphology, referred to as electric cell-substrate impedance sensing. Using this method, we found that the eukaryotic-prokaryotic shuttle vector pSV2-neo had a strong effect on the recipient NIH 3T3 cell spreading and cell motion. However, two new neo plasmids, pSK-neo and pSP-neo, which were constructed by modifying the pSV2-neo plasmid, did not have a significant effect on the recipient cell activities. The results suggest that there may be some sequences in pSV2-neo which affect recipient cell behavior.

Prostaglandin E2 elicits a morphological change in cultured orbital fibroblasts from patients with Graves ophthalmopathy.

Fibroblasts derived from distinct anatomical regions appear to differ in regard to their behavior in culture. These differences may reflect functions of these cells in vivo that are tissue specific. Moreover, intrinsic differences in fibroblasts may underlie the site-specific connective tissue manifestations associated with systemic disease. We have demonstrated previously that orbital fibroblasts exhibit different cytokine response domains and protein synthetic programs when compared to those emanating from the skin. In the present communication, we demonstrate that prostaglandin E2 (PGE2) elicits in cultured human orbital fibroblasts from patients with Graves ophthalmopathy a rapid and dramatic change in cell morphology in vitro as assessed by phase-contrast and scanning electron microscopy. The central areas of the cells become elevated with respect to the plane of the substratum and are stellate, with long processes that touch neighboring cells. These changes occur within 6 hr of prostanoid addition to culture medium at an apparent concentration threshold of approximately 10 nM. Shape changes are accompanied by marked alterations in monolayer impedance as assessed by electric cell-substrate impedance sensing as described previously. Both morphologic and impedance changes elicited by PGE2 revert over 24 hr toward those found in untreated cells despite the continued presence of the prostanoid in the culture medium. In contrast, dermal fibroblasts fail to respond to PGE2. These observations define a previously unrecognized phenotypic attribute of orbital fibroblasts. Intrinsic differences in these cells may account for the anatomic site-selective vulnerability of the orbit in Graves ophthalmopathy. The culture system described here may be useful for studying the morphogenic actions of prostanoids.

A morphological biosensor for mammalian cells.

An electrical biosensor is described that can continuously track morphological changes of adherent cells providing quantitative data from both sparse and confluent cultures. The method is capable of detecting vertical motion of cells of the order of 1 nm, much below the resolution of an optical microscope.

Monitoring electropermeabilization in the plasma membrane of adherent mammalian cells.

When an electrical potential of order one volt is induced across a cell membrane for a fraction of a second, temporary breakdown of ordinary membrane functions may occur. One result of such a breakdown is that molecules normally excluded by the membrane can now enter the cells. This phenomenon, generally referred to as electropermeabilization, is known as electroporation when actual pores form in the membrane. This paper presents a unique approach to the measurement of pore formation and closure in anchored mammalian cells. The cells are cultured on small gold electrodes, and by constantly monitoring the impedance of the electrode with a low-amplitude AC signal, small changes in cell morphology, cell motion, and membrane resistance can be detected. Because the active electrode is small, the application of a few volts across the cell-covered electrode causes pore formation in the cell membrane. In addition, the heat transfer is very efficient, and the cells can be porated in their regular growth medium. By this method, the formation and resealing of pores due to applied electric fields can be followed in real time for anchorage-dependent cells.

Electrical resistance method for measuring volume changes in monolayer cultures applied to primary astrocyte cultures.

An electrical resistance method was developed to measure volume changes in substratum-attached monolayer cultures. Astrocytes in primary monolayer cultures prepared from neonatal rat cerebral cortex were placed in a confined channel containing a balanced salt solution, and the electrical resistance of the channel was measured using an applied alternating current. If the volume of the cells increases, then the volume of the solution within the channel available for current flow decreases by the same amount, resulting in an increase in the measured resistance through the channel. If the volume of the cells decreases, a decrease in resistance would be recorded. This method allows continuous measurements of volume changes in real time. When primary astrocyte monolayers were exposed to hyposmotic solutions (93-193 mosmol/kgH2O), they showed a rapid initial swelling and, in the continued presence of hyposmotic media, a characteristic regulatory volume decrease (RVD) in which there was a return to normal cell volume within approximately 20 min. Astrocytes exposed to hyperosmotic media (343-493 mosmol/kgH2O) gave a decrease in electrical resistance, indicating shrinkage. Putative endogenous effectors of astrocytic swelling, such as high extracellular K+ and glutamate, resulted in a much slower onset of swelling and no sign of RVD. This system can reliably measure the average change in cell monolayer volume to 1-2% and thus provides a sensitive means of continuous measurements of changes in cell volume in monolayer cultures.

Monitoring motion of confluent cells in tissue culture.

Cell movements in confluent fibroblastic cell layers have been followed with a new instrumental method. Using the electric cell-substrate impedance sensor (ECIS), cell motions on the scale of nanometers are manifested as fluctuations in the impedance of small gold film electrodes that serve as cell substrata. These fluctuations have been recorded and numerically processed for cells at 27 and 37 degrees C, for cells deprived of all external nutrients over extended periods of time, and for cells exposed to different levels of cytochalasin D. Results suggest that the movements in confluent layers detected via ECIS can be correlated with cell metabolic activity.