External electric field effects on photosynthetic membrane vesicles. Kinetic characterization of two electrophotoluminescence phases in hypotonically swollen chloroplasts

Strong externally applied electrical field pulses are known to stimulate delayed luminescence from preilluminated blebs (hypotonically swollen vesicles originating from thylakoid membranes of broken chloroplasts) by up to 3 orders of magnitude. This phenomenon is known as electrophotoluminescence. Previous analysis showed the kinetics of the electrophotoluminescence to be biphasic, displaying a rapid (R) phase which decays towards a slower one (S) (Ellenson, J.L. and Sauer, K. (1976) Photochem. Photobiol. 23, 113–123). We demonstrate that these two components represent different processes. At low pH, a good kinetic separation is obtained between the two phases, which become distinct, with the S phase manifesting also an initial rise period. Under these conditions, it is possible to estimate separately the approximate rise times of the two phases. It is shown that the R and S components have a different dependence on the pH and on the time between the actinic flash and onset of the field. The field dependence is also different, with the S phase requiring a lower threshold field than R. From these observations, it is concluded that the R and S luminescence components are formed by different precursors. The difference in behaviour of the two phases during formation of the bleb indicates that the precursors of the R and S phases belong to different parts of the bleb. We suggest that R precursors are located in the wall of the swollen thylakoid and S precursors in the membrane formations which are attached to this wall.     

Electric field-induced lateral mobility of photosystem I in the photosynthetic membrane: A study by electrophotoluminescence

Electrophoretic movement of photosystem I (PS I) along the photosynthetic membrane of hypotonically swollen thylakoid vesicles was studied by analyzing the electric field-stimulated delayed luminescence (electrophotoluminescence) emitted from PS I. The electrophoretic mobility was inferred from the differences in electrophotoluminescence (EPL) of the photosynthetic vesicles in presence and absence of trains of low amplitude (<80 V/cm) prepulses of 1 ms duration at 4 ms spacing. The average apparent electric mobility, determined from the time course of EPL increase on one hemisphere or its decrease on the other one, as function of prepulse length and intensity was of the order of 3 · 10^-5 cm²V^-1s^-1. The assymetric distribution of the PS I reached a steady state when the diffusional, electrostatic, and elastic forces balanced the electrophoretic driving force. A lateral diffusion coefficient of ~5 · 10^-9 cm²s^-1 was found for the PS I complex from the diffusional relaxation after cessation of the electric field pulse train. Experimental conditions such as concentration, temperature, and viscosity of the aqueous solution were not critical for the effect. Between 23 and 150 electron charges per moving particle were estimated from the measured electrophoretic mobility.     

Electroporation of the photosynthetic membrane: structural changes in protein and lipid-protein domains.

A biological membrane undergoes a reversible permeability increase through structural changes in the lipid domain when exposed to high external electric fields. The present study shows the occurrence of electric field-induced changes in the conductance of the proton channel of the H(+)-ATPase as well as electric field-induced structural changes in the lipid-protein domain of photosystem (PS) II in the photosynthetic membrane. The study was carried out by analyzing the electric field-stimulated delayed luminescence (EPL), which originates from charge recombination in the protein complexes of PS I and II of photosynthetic vesicles. We established that a small fraction of the total electric field-induced conductance change was abolished by N,N'-dicyclohexylcarbodiimide (DCCD), an inhibitor of the H(+)-ATPase. This reversible electric field-induced conductance change has characteristics of a small channel and possesses a lifetime < or = 1 ms. To detect electric field-induced changes in the lipid-protein domains of PS II, we examined the effects of phospholipase A2 (PLA2) on EPL. Higher values of EPL were observed from vesicles that were exposed in the presence of PLA2 to an electroporating electric field than to a nonelectroporating electric field. The effect of the electroporating field was a long-lived one, lasting for a period > or = 2 min. This effect was attributed to long-lived electric field-induced structural changes in the lipid-protein domains of PS II.     

Electroporation of the photosynthetic membrane: A study by intrinsic and external optical probes

The study examines the relationship between electric field-induced conductivity and permeability changes in a biological membrane (electroporation) and the amplitude-duration parameters of the externally applied electric field. These reversible changes were characterized in giant photosynthetic membrane vesicles by means of the calibrated response of an intrinsic voltage-sensitive optical probe (electrophotoluminescence) and by the uptake studies of dextran-FITC fluorescent probes of different molecular weights. We quantitatively monitored electric field-induced conductivity changes by translating the electrophotoluminescence changes into conductivity changes. This was carried out by measuring the attenuation of the electrophotoluminescent signal after the addition of known amounts of gramicidin. The results demonstrate that electroporation involves the reversible formation of discrete holes in the membrane having radii <5.8 nm. The total area of the electric field-induced holes was 0.075% of the total surface of the vesicle. The formation of the electropores was affected differently by the electric field strength than by its duration. Increase in electric field strength caused increase in the total area of the vesicle that undergoes electroporation. Increase in the duration of the electric field increases the area of single electropores. Each of the two electric parameters can be rate limiting for the dynamics of electropore formation. These results are in accordance with the model of electroporation based on electric field-induced expansion of transient aqueous holes.     

A novel method for measuring membrane conductance changes by a voltage-sensitive optical probe.

This study presents a method whose principles enable using a voltage-sensitive optical probe, to quantitatively measure conductivity changes elicited in membrane vesicles and cells. The procedure is based on the fact that the amplitude of the transmembrane potential difference, established across a membrane by an external electric field, is decreased when membrane conductivity is increased upon incorporation of ionophores into the membrane. The method was applied to osmotically swollen thylakoid membranes whose membrane conductivity was changed by the addition of gramicidin or ionomycin. The electric field induced stimulated luminescence from photosystem I (electrophotoluminescence-EPL) was used as a voltage-sensitive optical probe. We calculated the induced conductance changes by using a calibrated EPL vs external electric field response curve and measuring the ionophore-mediated attenuation of the EPL signal. The calculated ionophore-unmodified conductance of the thylakoid membrane yields a value of 171 +/- 56 nS/cm. The value of the membrane conductance, modified by 10 nM gramicidin was found to be 190 +/- 56 nS/cm. The modified membrane conductance and the membrane conductance changes induced by 1 microM ionomycin in the presence of CaCl2 were found to be 186 +/- 3 nS/cm and 15 +/- 3 nS/cm, respectively.     

Electrophotoluminescence and the electrical properties of the photosynthetic membrane. II. Electric field-induced electrical breakdown of the photosynthetic membrane and its recovery

Preilluminated suspensions of swollen thylakoid vesicles (‘blebs’) were exposed to uni- and bipolar pairs of identical electric field pulses of variable duration, intensity and spacing. The resulting field-stimulated luminescence (electrophotoluminescence) was used as an intrinsic, voltage-sensitive optical probe to monitor electrical phenomena at the membrane level. The application of a pair of voltage pulses of opposite polarity made it possible to produce electric changes in the membrane by the first pulse and to analyse these effects by a second pulse of opposite polarity. It was found that the relative amplitudes of the two electrophoto-luminescence signals depended on the intensity of the applied electric field and on the time interval (t*) between the two pulses. When t* varied from 0.4 to 12 ms, the second stimulated luminescence signal was at first much smaller than the first one and then increased exponentially until the two signals were equal for t* ≥ 3 ms. We analysed these differences between the two field-stimulated luminescence signals as a measure of the electrical breakdown of the membrane, induced during the first pulse. In this way a distinction between irreversible and reversible breakdown could be made with an estimation of the recovery kinetics of the reversible breakdown, which was found to be complete within 3 ms. Irreversible breakdown of the membrane was found to increase with lengthening the exposure time from 0.1 to 1.3 ms especially when applying high electric field of at least 2000 V/cm.     

External electric field effects on prompt and delayed fluorescence in chloroplasts

An electric field pulse was applied to a suspension of osmotically swollen spinach chloroplasts after illumination with a saturating flash in the presence of DCMU. In addition to the stimulation of delayed fluorescence by the electric field, discovered by Arnold and Azzi (Arnold, W.A. and Azzi, R. (1971) Photochem. Photobiol. 14, 233–240) a sudden drop in fluorescence yield was observed. The kinetics of this fluorescence change were identical to those of the integrated delayed fluorescence emission induced by the pulse. The S-state dependence of the stimulated emission was very similar to that of the normal luminescence. We assume that the membrane potential generated by the pulse changes the activation energy for the back reaction in Photosystem II. On this basis, and making use of data we obtained earlier from electrochromic absorbance changes induced by the pulse, the kinetics of the field-induced prompt and delayed fluorescence changes, and also the amplitude of the fluorescence decrease, which was about 12% for a nearly saturating pulse, are explained. Our results indicate that in those reaction centers where a decrease of the activation energy occurs the effect of a pulse can be quite spectacular: the back reaction, which normally takes seconds, is completed in a few hundred microseconds when a sufficiently strong pulse is applied. Measurements of the polarization of the stimulated luminescence supported the interpretation given above.Only 2.8% of the back reaction was found to proceed via transition of reexcited chlorophyll to the ground state, both during the field pulse and in the absence of the field.     

三尖杉酯碱诱导HL-60细胞程序性死亡过程中的细胞质和细胞核出泡与染色质凝集

本文利用视频显微影像反差增强技术(VideoEnhancement Contrast,VEC)对三尖杉酯碱诱导的单个HL-60活细胞程序死亡(Apo-ptosis,Apo)全过程进行了观察,结果表明每个Apo细胞在染色质凝集前都要发生细胞核的出泡,而每一个核出泡又都是由相应的质出泡所诱导的,但并不是每个质出泡都能诱导核出泡,质出泡的次数远远高于核出泡,提示核、质出泡可能与染色质凝集有关,并且核、质出泡是程序死亡细胞形成Apo小体所必需的。进一步研究则说明核、质出泡与微丝解聚和重组有关。核、质出泡虽可加速细胞程序死亡过程中的染色质凝集,但并不是程序死亡细胞染色质凝集所必需的,提示HL-60细胞程序死亡过程中的核变化和质变化可能是相对独立的。     

Identification of the cardiac ryanodine receptor channel in membrane blebs of sarcoplasmic reticulum

Blebs of the sarcoplasmic reticulum (SR) membrane of heart muscle cells were generated after saponin perforation of the plasma membrane followed by complete hypercontraction of the cell. Although characteristic proteins of the plasma membrane, namely the beta1-adrenoreceptor and Galphai, were stained by monoclonal antibodies in the hypercontracted cells, these proteins could not be detected in the adjacent blebs. Monoclonal antibodies to the cardiac ryanodine receptor (RyR2), calsequestrin and SERCA2 bound at different amounts to surface components of the blebs and to components of the hypercontracted cells. From the immunofluorescence signals we conclude that the blebs are mainly constituted of corbular and junctional SR membrane, and only to a lesser extent of network SR membrane. Deconvolution microscopy revealed that the membrane location of RyR2, calsequestrin and SERCA2 in the bleb is comparable to native SR membrane. At the bleb membrane giga-ohm seals could be obtained and patches could be excised in a way that single-channel currents could be measured, although these are not completely identified.     

Fluorescence imaging in the millisecond time range of membrane electropermeabilisation of single cells using a rapid ultra-low-light intensifying detection system

A fast and sensitive fluorescence image acquisition system is described which uses an ultra-low-light intensifying camera able to acquire digitised fluorescence images with a time resolution of 3.33 ms/image. Two modes of recording were employed. The synchronisation mode allowed acquisition of six successive 3.33 ms-images synchronised with an external trigger, while the memorisation mode allowed acquisition of twelve successive 3.33 ms images starting after a 20 ms-time lag from the external trigger. Interaction of ethidium bromide (EB) with the membrane of electropermeabilised living cells was studied using this imaging system. We observed enhanced fluorescence of the dye when associated with electropermeabilised cells. Using single cells, 3.33 ms-images of the fluorescence interaction patterns of ethidium bromide showed well-defined membrane labelling. The enhanced fluorescence patterns were shown to represent the electropermeabilised area of the cell membrane. The average level of fluorescence associated with the labelled part of the cell membrane increased linearly during and immediately (less than 7 ms) after the electropermeabilisation pulse. Steady-state EB interaction with the membrane was achieved in a maximum 20 ms-time lag after electropermeabilisation. The membrane labelled parts were always observed in the cell regions facing the electrodes. They were present only when the electric field strength was higher than a threshold value which was different for the two cell sides. An increase in electric field intensity led to an increase in the dimensions of the labelled cell region. Received: 7 August 1997 / Revised version: 14 November 1997 / Accepted: 15 January 1998     

Interaction and fusion dynamics between cellular blebs

Membrane blebbing, as a mechanism for cells to regulate their internal pressure and membrane tension, is believed to play important roles in processes such as cell migration, spreading and apoptosis. However, the fundamental question of how different blebs interact with each other during their life cycles remains largely unclear. Here, we report a combined theoretical and experimental investigation to examine how the growth and retraction of a cellular bleb are influenced by neighboring blebs as well as the fusion dynamics between them. Specifically, a boundary integral model was developed to describe the shape evolution of cell membrane during the blebbing/retracting process. We showed that a drop in the intracellular pressure will be induced by the formation of a bleb whose retraction then restores the pressure level. Consequently, the volume that a second bleb can reach was predicted to heavily depend on its initial weakened size and the time lag with respect to the first bleb, all in quantitative agreement with our experimental observations. In addition, it was found that as the strength of membrane-cortex adhesion increases, the possible coalescence of two neighboring blebs changes from smooth fusion to abrupt coalescence and eventually to no fusion at all. Phase diagrams summarizing the dependence of such transition on key physical factors, such as the intracellular pressure and bleb separation, were also obtained.     

Reversible large-scale deformations in the membranes of electrically-treated cells: electroinduced bleb formation

Morphological changes in electrically-treated cells have been investigated by light and scanning electron microscopy. The application of 100-microseconds rectangular pulses of 1.3 kV/cm electric field to different types of cells (FBT, MEF, RAT-1, L-cells) in the physiological medium leads to the formation and growth of spherical and hemispherical protuberances of the cell membrane. The formation of such electroinduced blebs is not associated with the cells' death and is reversible. The electroinduced blebs are mainly formed at those sites of the cell membrane which are subjected to the highest voltage during the electric pulses. Increasing the tonicity of the medium by introducing 20 mM of inulin prevents the bleb formation, indicating the osmotically-dependent nature of the processes involved. When electric pulses are applied to the cells pre-treated with cytochalasin B, the formation of electroinduced blebs occurs independently from cytochalasin-induced ones originally present on such cells. Speculations are presented concerning the nature of the membrane structural changes underlying the electroinduced blebbing and their possible role in some electrically-induced processes.     

Electric Field Exposure Triggers and Guides Formation of Pseudopod-Like Blebs in U937 Monocytes

We describe a new phenomenon of anodotropic pseudopod-like blebbing in U937 cells stimulated by nanosecond pulsed electric field (nsPEF). In contrast to “regular,” round-shaped blebs, which are often seen in response to cell damage, pseudopod-like blebs (PLBs) formed as longitudinal membrane protrusions toward anode. PLB length could exceed the cell diameter in 2 min of exposure to 60-ns, 10-kV/cm pulses delivered at 10–20 Hz. Both PLBs and round-shaped nsPEF-induced blebs could be efficiently inhibited by partial isosmotic replacement of bath NaCl for a larger solute (sucrose), thereby pointing to the colloid-osmotic water uptake as the principal driving force for bleb formation. In contrast to round-shaped blebs, PLBs retracted within several minutes after exposure. Cells treated with 1 nM of the actin polymerization blocker cytochalasin D were unable to form PLBs and instead produced stationary, spherical blebs with no elongation or retraction capacity. Live cell fluorescent actin tagging showed that during elongation actin promptly entered the PLB interior, forming bleb cortex and scaffold, which was not seen in stationary blebs. Overall, PLB formation was governed by both passive (physicochemical) effects of membrane permeabilization and active cytoskeleton assembly in the living cell. To a certain extent, PLB mimics the membrane extension in the process of cell migration and can be employed as a nonchemical model for studies of cytomechanics, membrane–cytoskeleton interaction and cell motility.     

DNA-binding proteins in cells and membrane blebs of Neisseria gonorrhoeae.

Naturally elaborated membrane bleb fractions BI and BII of Neisseria gonorrhoeae contain both linear and circular DNAs. Because little is known about the interactions between DNA and blebs, studies were initiated to identify specific proteins that bind DNA in elaborated membrane blebs. Western immunoblots of whole-cell and bleb proteins from transformation-competent and DNA-uptake-deficient (dud) mutants were probed with single- or double-stranded gonococcal DNA, pBR322, or synthetic DNA oligomers containing intact or altered gonococcal transformation uptake sequences. The specificity and sensitivity of a nonradioactive DNA-binding protein assay was evaluated, and the assay was used to visualize DNA-protein complexes on the blots. The complexes were then characterized by molecular mass, DNA-binding specificity, and expression in bleb fractions. The assay effectively detected blotted DNA-binding proteins. At least 17 gonococcal DNA-binding proteins were identified; unique subsets occurred in BI and BII. Certain DNA-binding proteins had varied affinities for single- and double-stranded DNA, and the intact transformation uptake sequence competitively displaced the altered sequence from a BI protein at 11 kilodaltons (kDa). A dud mutant, strain FA660, lacked DNA-binding activity at the 11-kDa protein in BI. The segregation of DNA-binding proteins within BI and BII correlates with their distinct protein profiles and suggests that these vesicles may play different roles. Although the DNA-binding proteins expressed in BII may influence the nuclease-resistant export of plasmids within BII vesicles, the BI 11-kDa protein may bind transforming DNA.     

Direct observation in the millisecond time range of fluorescent molecule asymmetrical interaction with the electropermeabilized cell membrane.

Interaction of two stains (propidium iodide and ethidium bromide) with electropermeabilized living Chinese hamster ovary cells is observed using an ultrafast fluorescence image acquisition system. The computing process is linked to an ultra-low-light intensifying camera working with a very short time resolution (3.33 ms per image). Altered parts of the cell membrane were identified via the enhancement in fluorescence intensity of the dyes. They reflect the electropermeabilized part of the membrane in which free flow of dye occurred. Images of the fluorescence interaction patterns of the two dyes, in a maximum 20-ms time lag after pulsation, reveal asymmetrical permeabilization of the cell membrane. For electric field intensities higher than a first threshold value, permeabilization is always observed on the anode-facing side of the cell. For electric field intensities over a second higher threshold value, the two electrode-facing hemispheres of the cell are permeabilized, the hemisphere facing the anode being most permeable. These data support the conclusion that electropermeabilization of living cell membrane is affected by its resting potential. The asymmetrical pattern of the dye interaction is not dependent on the nature or concentration of the dye, the ionic strength of the pulsing buffer, or the duration of the pulse. The field intensity determines the fraction of the membrane in which molecular alterations can occur. The extent of alteration in this localized region is determined by the duration of the pulse when a single pulse in the millisecond time range is applied.     

Irreversible injury in anoxic hepatocytes precipitated by an abrupt increase in plasma membrane permeability

Using low-light digitized video microscopy, the onset, progression, and reversibility of anoxic injury were assessed in single hepatocytes isolated from fasted rats. Cell-surface bleb formation occurred in three stages over 1-3 h after anoxia. Stage I was characterized by formation of numerous small blebs. In stage II, small blebs enlarged by coalescence and fusion to form a few large terminal blebs. Near the end of stage II, cells began to swell rapidly, ending with the apparent breakdown of one of the terminal blebs. Breakdown of the bleb membrane initiated stage III of injury and was coincident with a rapid increase of nonspecific permeability to organic cationic and anionic molecules. On reoxygenation, stages I and II were fully reversible, and plasma membrane blebs were resorbed completely within 6 min of reoxygenation without loss of viability. Stage III, however, was not reversible, and no morphological changes occurred on reoxygenation. The results indicate that onset of cell death owing to anoxia is a rapid event initiated by a sudden increase of nonspecific plasma membrane permeability caused by rupture of a terminal bleb. Anoxic injury is reversible until this event occurs.     

Dipole interactions in electrofusion. Contributions of membrane potential and effective dipole interaction pressures.

The contributions of pulse-induced dipole-dipole interaction to the total pressure acting normal to the membranes of closely positioned pronase treated human erythrocytes during electrofusion was calculated. The total pressure was modeled as the sum of pressures arising from membrane potential and dipole-dipole attraction opposed by interbilayer repulsion. The dipole-dipole interaction was derived from the experimentally obtained cell polarizability. The threshold electric field amplitude necessary for fusion of pronase-treated human erythrocytes was experimentally obtained at various combinations of pulse duration, frequency, and the conductivity of the external medium. The theoretical values of the critical electric field amplitude compared favorably to the experimentally obtained threshold field amplitudes. Fusion by dc pulses may be primarily attributed to attainment of sufficiently high membrane potentials. However, with decreasing external conductivity and increasing sinusoidal pulse frequency (100 kHz-2.5 MHz), the induced dipole-dipole interactions provide the principal driving force for membrane failure leading to fusion.     

On the origin of membrane vesicles in Gram-negative bacteria

It is proposed that the genesis of extracellular membrane vesicles in Gram-negative bacteria is a result of cell wall turnover. Peptidoglycan turnover would cause a turgor on the outer membrane, causing the outer membrane to bulge and finally bleb. Mechanical motion would then shear the blebs into the culture medium.     

Electropermeabilization of mammalian cells. Quantitative analysis of the phenomenon.

Transient membrane permeabilization by application of high electric field intensity pulses on cells (electropermeabilization) depends on several physical parameters associated with the technique (pulse intensity, number, and duration). In the present study, electropermeabilization is studied in terms of flow of diffusing molecules between cells and external medium. Direct quantification of the phenomenon shows that electric field intensity is a critical parameter in the induction of permeabilization. Electric field intensity must be higher than a critical threshold to make the membrane permeable. This critical threshold depends on the cell size. Extent of permeabilization (i.e., the flow rate across the membrane) is then controlled by both pulse number and duration. Increasing electric field intensity above the critical threshold needed for permeabilization results in an increase membrane area able to be permeabilized but not due to an increase in the specific permeability of the field alterated area. The electroinduced permeabilization is transient and disappears progressively after the application of the electric field pulses. Its life time is under the control of the electric field parameters. The rate constant of the annealing phase is shown to be dependent on both pulse duration and number, but is independent of electric field intensity which creates the permeabilization. The phenomenon is described in terms of membrane organization transition between the natural impermeable state and the electro-induced permeable state, phenomenon only locally induced for electric field intensities above a critical threshold and expanding in relation to both pulse number and duration.     

Plasma membrane charging of Jurkat cells by nanosecond pulsed electric fields

The initial effect of nanosecond pulsed electric fields (nsPEFs) on cells is a change of charge distributions along membranes. This first response is observed as a sudden shift in the plasma transmembrane potential that is faster than can be attributed to any physiological event. These immediate, yet transient, effects are only measurable if the diagnostic is faster than the exposure, i.e., on a nanosecond time scale. In this study, we monitored changes in the plasma transmembrane potential of Jurkat cells exposed to nsPEFs of 60 ns and amplitudes from 5 to 90 kV/cm with a temporal resolution of 5 ns by means of the fast voltage-sensitive dye Annine-6. The measurements suggest the contribution of both dipole effects and asymmetric conduction currents across opposite sides of the cell to the charging. With the application of higher field strengths the membrane charges until a threshold voltage value of 1.4–1.6 V is attained at the anodic pole. This indicates when the ion exchange rates exceed charging currents, thus providing strong evidence for pore formation. Prior to reaching this threshold, the time for the charging of the membrane by conductive currents is qualitatively in agreement with accepted models of membrane charging, which predict longer charging times for lower field strengths. The comparison of the data with previous studies suggests that the sub-physiological induced ionic imbalances may trigger other intracellular signaling events leading to dramatic outcomes, such as apoptosis.