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-hemolysin induces calcium oscillations in mammalian cells—the pore is on its own
,2
* Rudolf-Buchheim-Institute of Pharmacology, Justus-Liebig-University, Giessen, Germany; and
Institute of Medical Microbiology and Hygiene, Johannes-Gutenberg-University, Mainz, Germany
2Correspondence: Frankfurter Str. 107, Rudolf-Buchheim-Institute of Pharmacology, Justus-Liebig-University, 35392 Giessen, Germany. Email: Holger.Repp{at}pharma.med.uni-giessen.de; and Obere Zahlbacher Str. 67, Institute of Medical Microbiology and Hygiene, Johannes-Gutenberg-University, 55101 Mainz, Germany. Email: walev{at}mail.uni-mainz.de
ABSTRACT
Escherichia coli 
-hemolysin (HlyA), archetype of a bacterial pore-forming toxin, has been reported to deregulate physiological Ca2+ channels, thus inducing periodic low-frequency Ca2+ oscillations that trigger transcriptional processes in mammalian cells. The present study was undertaken to delineate the mechanisms underlying the Ca2+ oscillations. Patch-clamp experiments were combined with single cell measurements of intracellular Ca2+ and with flowcytometric analyses. Application of HlyA at subcytocidal concentrations provoked Ca2+ oscillations in human renal and endothelial cells. However, contrary to the previous report, the phenomenon could not be inhibited by the Ca2+ channel blocker nifedipine and Ca2+ oscillations showed no constant periodicity at all. Ca2+ oscillations were dependent on the pore-forming activity of HlyA: application of a nonhemolytic but bindable toxin had no effect. Washout experiments revealed that Ca2+ oscillations could not be maintained in the absence of toxin in the medium. Analogously, propidium iodide flux into cells occurred in the presence of HlyA, but cells rapidly became impermeable toward the dye after toxin washout, indicating resealing or removal of the membrane lesions. Finally, patch-clamp experiments revealed temporal congruence between pore formation and Ca2+ influx. We conclude that the nonperiodic Ca2+ oscillations induced by HlyA are not due to deregulation of physiological Ca2+ channels but derive from pulsed influxes of Ca2+ as a consequence of formation and rapid closure of HlyA pores in mammalian cell membranes.—Koschinski, A., Repp, H., Ünver, B., Dreyer, F., Brockmeier, D., Valeva, A., Bhakdi, S., and Walev, I. Why Escherichia coli -hemolysin induces calcium oscillations in mammalian cells—the pore is on its own.
Key Words: pore-forming toxins • RTX toxins • gram-negative bacteria • membrane lesions
ESCHERICHIA COLI -hemolysin (HlyA) IS prototypic of a large family of pore-forming toxins that are produced by many medically important gram-negative pathogens. Common to all are the presence of a Ca2+ binding nonapeptide repeat sequence and the requirement for posttranslational fatty acylation at one or two lysine residues for the acquisition of pore-forming activity (1
, 2)
. In the absence of Ca2+ or fatty acylation, binding capacity is retained but the toxins are unable to adopt the pore-forming configuration (3
, 4)
. The question of whether HlyA binds to specific cellular receptors is currently under investigation (5
, 6)
; the toxin binds to protein-free lipid bilayers to form pores of 
1 nm diameter (7)
, similar to the pore size described in animal cell membranes (8)
. Pores formed in planar lipid bilayers flicker between an open and closed state dependent on the membrane potential (7
, 9)
.
In addition to their direct cytotoxic capacity, pore-forming toxins can trigger a multitude of cellular responses that may in turn produce important long-range effects in the mammalian host organism. Many reactions are triggered by the uncontrolled flux of monovalent and divalent ions across the plasma membrane (10)
. Studies with S. aureus alpha-toxin and streptolysin O have disclosed that nucleated cells are able to repair a limited number of lesions (11
, 12)
and that this is accompanied by transcriptional responses such as the activation of NF-
B and subsequent cytokine production (13
, 14)
. Thus, similar to endotoxin, pore-forming toxins may provoke a large spectrum of long-term effects, and delineating the mechanisms underlying transcriptional activation represents a novel field of research with broad potential relevance.
Oscillations in the cytoplasmic concentration of free Ca2+ have recently been recognized to represent a central event in transcriptional regulation (15
, 16)
. Therefore, the finding that subcytocidal attack by HlyA is accompanied by periodic low-frequency Ca2+ oscillations, transcriptional activation, and interleukin-6 production in renal epithelial cells represented an important discovery that might be extrapolatable to other pore-forming agents (17)
. The question relating to the cause of the HlyA-induced periodic Ca2+ oscillations also appeared to be resolved in that publication: Ca2+ oscillations were reportedly suppressed in the presence of the Ca2+ channel blocker nifedipine. Thus, in addition to being a pore-forming toxin, HlyA appeared to be endowed with the capacity of influencing physiological Ca2+ channels in mammalian cells (17)
. The present investigation was undertaken to make a closer investigation of this putative bifunctionality of HlyA.
The capacity of the toxin to induce Ca2+ oscillations in mammalian cells was confirmed. However, Ca2+ oscillations showed no constant periodicity at all. Furthermore, a different conclusion was reached regarding the cause of Ca2+ oscillations. Evidence is presented that they are not due to toxin-dependent alterations of intrinsic Ca2+ channel activity but to pulses of Ca2+ flux through short-lived toxin pores, which are rapidly closed or removed from the plasma membrane of target cells.
MATERIALS AND METHODS
Cell culture
Human embryonic kidney (HEK293) cells were maintained in a humidified, 6% CO2-94% air atmosphere in a mixture of Dulbecco’s modified Eagle’s medium (DMEM) and Ham’s F-12 medium (1:1, v/v) containing 10% (v/v) fetal calf serum (PAN Systems, Aidenbach, Germany) and 2 mM L-glutamine without antibiotics. The human endothelial cell line EAhy926 (a kind gift of Cora-Jean S. Edgell, University of North Carolina, Chapel Hill, NC), a hybridoma derived from the fusion of human umbilical vein endothelial cells with the A549 cell line, was grown in DMEM/Nutrient Mix F-12 (1:1) with Glutamax I and pyrodoxin (Life Technologies, Paisley, Scotland) supplemented with 10% fetal calf serum (Life Technologies), 100 U/ml penicillin, and 100 µg/ml streptomycin (Life Technologies) at 37°C in humidified air containing 6% CO2. Human renal proximal tubule-derived cells (IHKE) were kindly provided by Dr. Michael Gekle (Institute of Physiology, University of Würzburg, Würzburg, Germany). The cells were originally from Dr. Steen Mollerup (Department of Toxicology, National Institute of Occupational Health, Oslo, Norway) who kindly permitted us to use them. Cells were grown in Ham’s F-12 modified DMEM medium supplemented with 1.18 g/l NaHCO3, 5.37 g/l HEPES, 5 mg/l human apo-transferrin, 5 mg/l bovine insulin, 36 µg/l hydrocortisone, 10 µg/l mouse epidermal growth factor, 5 µg/l Na-selenite, and 10% fetal calf serum. Cells were grown in a 37°C, 94% air-6% CO2 incubator.
Expression of HlyA, mutagenesis, and toxin purification
Hemolytically inactive mutant in which Lys at position 564 and 690 was replaced by Arg (K564R/K690R) (1)
was kindly provided by C. Hughes and V. Koronakis (Department of Pathology, Cambridge University, Cambridge, UK). Mutation of Ser-177 into Cys in cysteine-less wild-type (WT) HlyA was described previously (4)
. The same procedure was followed starting with the mutant K564R/K690R to form S177C/K564R/K690R. Mutant toxins containing Cys177 (S177C and S177C/K564R/K690R) were labeled with [3H]N-ethyl-maleimide. The binding capacity of these labeled toxins to erythrocytes was determined. The bindability of labeled mutant S177C and the hemolytically inactive mutant S177C/K564R/K690R was identical. Protein purification of toxins was carried out as described previously (1
,4)
.
Electrophysiological recording
Cells were plated in 35 mm dishes 48 h before the experiments. Cells (4x105 per dish) were washed with extracellular (bath) solution (in mM: 140 NaCl, 3 KCl, 2 MgCl2, 2 CaCl2, 10 glucose, and 10 HEPES, pH 7.35 adjusted with 4 mM NaOH), and whole-cell recordings were started within 5–10 min after the washing procedure, using a bath volume of 2 ml. Recording pipettes had a resistance of 5–10 M
when filled with pipette solution (in mM: 140 K+ glutamate, 10 NaCl, 2 MgCl2, and 10 HEPES, pH 7.3 adjusted with 4 mM KOH). A free intracellular Ca2+ concentration ([Ca2+]i) of 100 nm was obtained using 100 µM of the Ca2+ chelator BAPTA and a total Ca2+ concentration of 30 µM, assuming an apparent dissociation constant Kd of 0.24 µM (pH 7.3) for the Ca2+ BAPTA complex. Solutions were filtered through 0.2 µm pore size filters. Whole-cell current recording and data analysis were performed as described previously (18
,19)
. HlyA was applied with a pipette directly into the bath solution within 2–5 min after a stable recording configuration had been obtained. All measurements were performed at 20–22°C. Data are mean ± SEM unless stated otherwise.
Measurement of intracellular ATP
HEK293 cells in 96-well plates were treated with various concentrations of HlyA for 60 min, after which the supernatants were removed. Cellular ATP concentrations were determined after the cells were lysed with 0.2 ml of 0.5% (v/v) Triton X-100, using a commercial ATP-bioluminescence kit (Boehringer Mannheim). Intracellular ATP was expressed as the percentage of luminescence relative to that of negative controls (without toxin).
Ratiometric Ca2+ imaging
Cells were plated on 35 mm plastic petri dishes that had been modified for fluorescence measurements (18)
. Cells were loaded for 30 min at 37°C with the fluorescent Ca2+ indicator Fura 2-acetoxymethyl ester (AM) (Calbiochem-Novabiochem, Bad-Soden, Germany) dissolved in extracellular solution at a concentration of 5 µM. The solution contained 16 µM Pluronic F-127 (Molecular Probes, Eugene, OR) for a better dispersion of Fura 2-AM. Loaded cells were washed extensively with extracellular solution and further incubated for 10 min at 37°C. Measurements of [Ca2+]i were performed at room temperature with a Leica DM IRB inverse microscope and a VisiChrome imaging system (Visitron, Puchheim, Germany). Excitation was at 350 and 380 nm, and emission was determined at 510 nm. [Ca2+]i was determined ratiometrically from the captured data using the Metafluor imaging software.
Biometric analysis
A time period of 25.55 min (512 measurements, sampling interval 3 s) of each individual original Ca2+ tracing was subjected to a discrete Fourier transformation, followed by computation of the respective power spectrum. According to the sampling period (T=25.55 min), multifolds of the basic angular velocity (
0=2
/T) constitute the Fourier transformation, corresponding to wavelengths of 25.55, 12.775, 8.517, 6.388, etc., min. In addition, the signals of these data sets were first corrected by possible baseline drifts using a trend function that consisted in a second order polynomial function, i.e., a constant, a linear, and a quadratic term. The trend function was estimated by a Gaussian least square method. After subtraction of the trend, the signals were treated as described above. No further data manipulations or standardization was carried out.
Online supplemental material
Figure S1 is a time-lapse video of monitoring of [Ca2+]i in IHKE cells after exposure to HlyA (2.5 ng/ml). The original recording period was 90 min. Figure S2 shows Ca2+ tracings of 52 different IHKE cells after exposure to HlyA (2.5 ng/ml). Figure S3 shows a time-lapse video of [Ca2+]i in IHKE cells after exposure to HlyA (twice 2.5 ng/ml) in the presence of nifedipine. Ca2+ oscillations become visible after the change of the field of view. Figure S4 is a time-lapse video of the effect of HlyA (2.5 ng/ml) on [Ca2+]i in serum-starved (24 h) IHKE cells. Figure S5 shows Ca2+ tracings of 30 different serum-starved (24 h) IHKE cells exposed to HlyA (2.5 ng/ml).
RESULTS
Formation of HlyA pores in HEK293 cells
HEK293 cells were initially used to characterize the pore-forming effects of HlyA in the whole-cell configuration of the patch-clamp technique. HEK293 cells possess only low endogenous ion channel activity at certain membrane potentials (20)
and are well suited for electrophysiological experiments to measure the effect of pore-forming toxins. This cell type already has been successfully used in our group to study pore formation by listeriolysin of Listeria monocytogenes (18)
. Pore formation was observed within seconds after application of HlyA and followed a steep concentration-response curve (Fig. 1
A). At a concentration of 5 ng/ml HlyA, 230 s elapsed from the opening of the first pore to the opening of the tenth pore. A 10-fold higher concentration reduced this time span to 40 s. With the patch-clamp-technique, a determination of single pores is possible, since each pore opening becomes visible by a stepwise change in the membrane current trace. As a negative membrane holding potential of –50 mV was used, the membrane current trace steps down during a pore opening and up during a pore closing. Thus, Fig. 1A
also demonstrates that the HlyA pores did not remain continuously open but oscillated between open and closed states (see also Figs. 4B
and 5B and C
). Despite ongoing pore formation, the cells remained viable when toxin concentrations of 
10 ng/ml were employed, and no decreases in cellular ATP levels were noted (Fig. 1B
).
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HlyA induces nonperiodic Ca2+ oscillations in renal cells
IHKE cells were loaded with Fura 2-AM and used to study the effect of HlyA on [Ca2+]i. IHKE cells were chosen as human equivalent to primary rat proximal tubule (RPT) cells that were used in a previous study to demonstrate the effect of HlyA on [Ca2+]i (17)
. IHKE cells are easily accessible for electrophysiological and Ca2+ measurements. Within 30 s after application of 5 ng/ml HlyA to the cells, [Ca2+]i increased and exhibited oscillations during further monitoring (Fig. 2
A-E). It was previously reported that HlyA provoked periodic Ca2+ oscillations in RPT cells, with a constant periodicity of Ca2+ peaks of 12 min (17)
. However, we found large variations of oscillation kinetics and peak-to-peak spans in human tubule cells. Figure 2A-D
shows Ca2+ tracings of four individual cells after application of HlyA, already giving an impression of the broad variability of the Ca2+ oscillations. False-color images of [Ca2+]i in individual cells are demonstrated in Fig. 2E
. A time-lapse video of Ca2+ monitoring of IHKE cells is available online (Fig. S1), as well as Ca2+ tracings of 50 different IHKE cells (Fig. S2), demonstrating impressively that the Ca2+ oscillations showed no constant periodicities of the peaks and large variations of the peak-to-peak spans. We then applied the identical mathematical method that was previously used to identify HlyA-induced periodic Ca2+ oscillations of low frequency (17)
, i.e., a power spectrum analysis that is based on a discrete Fourier transformation. Despite the marked heterogeneity of the original Ca2+ tracings, the analysis of the data sets yielded a periodicity of Ca2+ peaks with a dominant wavelength of 12.8 min, which was in remarkable agreement with the previously reported value of 12.0 min in RPT cells (17)
. As a control, tracings monitored in the absence of HlyA were also analyzed. Surprisingly, a periodicity of peaks with a wavelength of 12 min was again computed, although no Ca2+ elevations existed in the control tracings. Furthermore, analysis of a signal consisting only of white noise superimposed to a discretely drifting baseline yielded the same peak value. Thus, it became clear that this mathematical model was not adequate for a proper interpretation of the measured Ca2+ data. Indeed, due to the properties of discrete Fourier transformation, the power spectrum will provide information for discrete frequencies only, depending on the sampling period and sampling frequency. In the experimental constellation we used, which is comparable to the constellation described in the previous report (17)
, the power spectrum is computed for wavelengths of 25.5, 12.8, 8.5, etc., min. The discrete Fourier transformation power spectrum analysis is not suitable to trace oscillations for these low frequencies, since variations in baseline do produce prominent peaks at the same discrete frequencies, which can lead to a fundamental misinterpretation of measured data.
|
We next determined the statistical distribution of the time spans between directly neighboring Ca2+ peaks in Fig. 2A-D
and 48 other Ca2+ tracings. Figure 3
shows that the most frequent peak-to-peak span was 70 s and that spans in the range of 1–3 min occurred more frequently compared to longer time spans, whereas peak-to-peak spans above 10 min were very rare events. Importantly, these peak-to-peak spans represent the time span between a Ca2 +peak to its direct neighbor. They should not be confused with peak periodicity or frequency of consecutive peaks. When the toxin concentration was increased, the peak-to-peak span distribution shifted to shorter time spans, as illustrated in Fig. 3
, inset, by a typical Ca2+ tracing where two different HlyA concentrations were applied consecutively. The finding that the peak-to-peak span distribution is dependent on HlyA concentration clearly refutes the idea that the HlyA molecule might have the inherent feature to induce Ca2+ oscillations of a discrete frequency.
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To address the question of a possible cell-type limitation of our observations, we further investigated the Ca2+ response to HlyA in HEK293 and EAhy926 cells. The EAhy926 line is representative of human vascular endothelial cells and was used to have a complement to IHKE cells, which have an epithelial nature. In HEK293 and EAhy926 cells, Ca2+ oscillations were also observed but with no evidence of a constant periodicity (data not shown).
The results presented yet were obtained with nonsynchronized cells. Thus, one might speculate that the observed nonperiodic Ca2+ oscillations could be cell-cycle dependent. To address this question, IHKE cells were subjected to 12–24 h serum starvation, and then Ca2+ measurements were performed. Despite synchronization of the cell cycle, HlyA (2.5 ng/ml)-induced Ca2+ oscillations were still nonperiodic, and no synchronization of Ca2+ elevations was observable. A time-lapse video of Ca2+ monitoring of serum-starved IHKE cells is available online (Fig. S4), as well as 30 individual Ca2+ tracings of serum-starved IHKE cells (Fig. S5). These data clearly show that also in synchronized cells, the Ca2+ response to HlyA is nonperiodic and not synchronous.
Nifedipine does not inhibit Ca2+ oscillations
When voltage-gated L-type Ca2+ channels in IHKE cells were blocked by nifedipine (100 µM), Ca2+ oscillations were still observable after application of HlyA (Fig. 2F
, Fig. S3). This finding contrasted with the observations in RPT cells where nifedipine (100 µM) abolished the Ca2+ response to HlyA (17)
. A striking phenomenon that we observed during Ca2+ measurements in the presence of nifedipine can account for this difference. Those cells that were in the microscopic field of view for several minutes, i.e., irradiated by ultraviolet (UV) light, did indeed not show Ca2+ oscillations after application of HlyA (Fig. 2F
, Fig. S3). Once adjusted, the field of view is usually not changed during the further Ca2+-monitoring period. However, when, after application of HlyA, the field of view was changed to cells that had not been in the optical path before, i.e., had not "seen" UV light, Ca2+ oscillations were observed (Fig. 2F
, Fig. S3). This phenomenon was reproduced in three independent experiments in IHKE cells and also in HEK293 and EAhy926 cells. Thus, the previously observed elimination of HlyA-induced Ca2+ oscillations by nifedipine was essentially an experimental artifact.
Additionally, HlyA still led to Ca2+ oscillations in HEK293 cells when a combination of Ca2+ channel blockers was present, including nifedipine (100 µM); Cd2+ (2 mM), which blocks all types of voltage-gated Ca2+ channels (21)
; and SK&F 96365 (25 µM), a blocker of receptor-operated Ca2+ channels (22)
. This shows that the effect of HlyA on [Ca2+]i is not dependent on the activity of endogenous membrane Ca2+ channels.
Pore formation and Ca2+ oscillations do not occur in cells treated with nonhemolytic HlyA
IHKE and HEK293 cells were treated with the HlyA mutant S177C/K564R/K690R, which is nonhemolytic but fully bindable. In no case were pore formation and Ca2+ oscillations ever noted at concentrations of up to 500 ng/ml. An example of the course of [Ca2+]i in IHKE cells with consecutive exposure to nonhemolytic HlyA mutant and to WT HlyA is shown in Fig. 2G
. These results demonstrated that the ability of HlyA to provoke Ca2+ oscillations was not related simply to binding to the plasma membrane but required formation of pores.
Pore formation and Ca2+ oscillations cease rapidly on removal of HlyA
We then addressed the question of whether the effect of HlyA on [Ca2+]i persists after removal of the toxin from the extracellular medium. Figure 4
A shows a superposition of the Ca2+ responses of 31 IHKE cells after application of HlyA (10 ng/ml). Shortly after the start of a bath perfusion to wash out HlyA from the extracellular medium, [Ca2+]i decreased and reached the initial concentration after 3 min. A second application of HlyA (5 ng/ml) again led to Ca2+ oscillations, which disappeared within some minutes after restart of the bath perfusion. The disappearance of the Ca2+ oscillations after washout of HlyA followed the same time course as observed for the disappearance of pore formation. Figure 4B
shows a typical whole-cell registration of pore formation in a single IHKE cell after application of HlyA. Open pores were no longer observed 4 min after the start of the bath perfusion. Reapplication of HlyA again led to pore formation.
The most straightforward explanation for the above findings was that after formation, the HlyA pores rapidly disappeared from the plasma membrane, either through closure or by removal from the afflicted membrane area. To corroborate this, influx of propidium iodide was investigated in parallel. As shown in Fig. 4C-E
, cells became permeable to the dye during incubation with HlyA, but membranes returned to the impermeable state shortly after toxin removal. This confirmed that pores formed by HlyA were short lived.
Evidence that Ca2+ oscillations result from Ca2+ influx through HlyA pores followed by Ca2+ redistribution
We then simultaneously monitored pore formation by HlyA (10 ng/ml) and [Ca2+]i in IHKE cells. We found an increase in [Ca2+]i directly after pore opening and a good temporal correlation between formation and closure of HlyA pores and Ca2+ elevations (Fig. 5
A, B). We hypothesized that the decrease after an elevation might reflect the restoration of Ca2+ homeostasis by physiological Ca2+ redistribution after HlyA pore closure. To test this possibility, IHKE cells were incubated with thapsigargin (1 µM), an inhibitor of endoplasmic reticular Ca2+ ATPases. We found that the effect of thapsigargin depended on the concentration of HlyA. In the presence of thapsigargin and 2.5 ng/ml HlyA, most IHKE cells showed typical Ca2+ oscillations. In the presence of thapsigargin and 5 ng/ml HlyA, some IHKE cells showed typical Ca2+ oscillations even during several hours of monitoring. The major part of the cells exhibited Ca2+ oscillations only for a limited period of 30 min, and thereafter a continuous increase in [Ca2+]i was observed. Some cells showed no Ca2+ oscillations but only a continuous elevation in [Ca2+]i. When thapsigargin-treated IHKE cells were exposed to 10 ng/ml HlyA, most cells responded with a continuous increase in [Ca2+]i. In IHKE cells that were not treated with thapsigargin, HlyA concentrations of at least 20 ng/ml were necessary to induce a continuous elevation in [Ca2+]i.
DISCUSSION
The results of this study led us to a new explanation for Ca2+ oscillations that occur in mammalian cells treated with HlyA. Three human cell-types were used for this investigation, namely renal embryonic, renal tubule epithelial, and vascular endothelial cells. The Ca2+ oscillations are not thought to be due to any direct or indirect influence of HlyA on intrinsic Ca2+ channels. Instead, they are proposed to result from pulsed influxes of Ca2+ through short-lived toxin pores, which we believe are rapidly sealed or removed from the membrane. Given the fundamental relevance of cytoplasmic Ca2+ oscillations for many cell functions, this conceptual remodeling will be of importance for future investigations in the field. Arguments in support of the proposal are manifold.
First, we were unable to confirm the suppressive effect of the Ca2+ channel blocker nifedipine on HlyA-induced Ca2+ oscillations in any of the three cell-types studied. It is known that nifedipine is very sensitive to UV and daylight (up to 450 nm). It is readily converted into degradation products, which are no longer active against Ca2+ channels but instead have cellular as well as toxic effects (23
24
25)
. The degradation products might lead to cell damage, with the consequence of missing Ca2+ oscillations in UV-irradiated cells. In conclusion, the previously observed elimination of HlyA-induced Ca2+ oscillations by nifedipine was essentially not due to Ca2+ channel blockade, and the sole support for the postulated crucial involvement of intrinsic L-type Ca2+ channels in HlyA-provoked Ca2+ oscillations (17)
ceased to exist. Furthermore, we found that the effect of HlyA on [Ca2+]i is not dependent on the activity of any voltage-gated or receptor-operated membrane Ca2+ channels.
Secondly, washout experiments provided an important further clue. Removal of HlyA from the medium resulted in rapid disappearance of the pores. At the same time, the ability of propidium iodide to diffuse into the cells was lost and, strikingly, Ca2+ oscillations also ceased. The conclusion was rather inescapable that, after formation in mammalian cell membranes, HlyA pores are short lived and disappear within seconds, either by rapid closure or by removal from the membrane.
Final support for the pore theory of Ca2+ oscillations came from simultaneous patch-clamp measurements of pore formation and Ca2+ measurements. These experiments directly showed that Ca2+ elevations occurred as a consequence of opening of the pores formed by HlyA. Pore openings occur at random, and the open probability depends on HlyA concentration. Since a rise in [Ca2+]i is an effect that is directly conditioned by a pore opening, it is also expected to happen stochastically, and the probability should be dependent on toxin concentration. In contrast to a previous report (17)
, we found indeed no evidence for the existence of a constant periodicity of the Ca2+ peaks, and we observed that the time spans between consecutive Ca2+ peaks depended on HlyA concentration. This concentration dependence definitely rules out the idea that HlyA molecules may switch on in the target cell a periodic "Ca2+ oscillation run" with one defined frequency.
The concept thus took shape that Ca2+ fluxed from the extracellular compartment through the stochastically opened HlyA pores to the cytosol and that this was followed by pore closure and a rapid decline of the elevated cytoplasmic Ca2+ concentration, indicating restoration of cellular Ca2+ homeostasis. This concept is supported by the observed dependence of the Ca2+ response on HlyA concentration in IHKE cells treated with thapsigargin. It appears that cells respond to Ca2+ influx through HlyA pores with uptake into intracellular Ca2+ stores combined with discharge by plasma membrane Ca2+ pumps. If a part of these systems is cut off, e.g., by thapsigargin, the amount of Ca2+ influx determines whether Ca2+ homeostasis can still be maintained over time or not. As the amount of Ca2+ influx depends on the number of HlyA pores, it can be well explained that the effect of thapsigargin is dependent on HlyA concentration.
In summary, a novel concept has emerged to account for the intriguing finding that a pore-forming toxin can provoke Ca2+ oscillations in living cells. The experiments described herein provide evidence that an RTX toxin forms very short-lived pores in cell membranes. In fact, similar rapid pore closure has been noted in artificial lipid bilayers (7
8
9)
. Thus, the very short pore life may be an intrinsic property that is governed by the target bilayer itself. Nonperiodic ("chaotic") Ca2+ oscillations resulting from the interplay of formation and disappearance of the pores along with cellular Ca2+ redistribution will obviously vary depending on toxin concentration and susceptibility and may provide the starting point of myriad reactions, but on a very individual basis, in cells attacked by pore-forming toxins such as HlyA.
ACKNOWLEDGMENTS
We thank C. Zibuschka and S. Weis for expert technical assistance. This work was supported by grants from the Deutsche Forschungsgemeinschaft (RE 1046/1–2 and SFB 490, project C1/D3).
FOOTNOTES
1 These authors contributed equally to this work. 
Received for publication August 18, 2005. Accepted for publication January 3, 2006.
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tp1–10). Insets show courses of pore formation in single cells after exposure to (1)
