Detecting Subtle Plasma Membrane Perturbation in Living Cells Using Second Harmonic Generation Imaging

The requirement of center asymmetry for the creation of second harmonic generation (SHG) signals makes it an attractive technique for visualizing changes in interfacial layers such as the plasma membrane of biological cells. In this article, we explore the use of lipophilic SHG probes to detect minute perturbations in the plasma membrane. Three candidate probes, Di-4-ANEPPDHQ (Di-4), FM4-64, and all-trans-retinol, were evaluated for SHG effectiveness in Jurkat cells. Di-4 proved superior with both strong SHG signal and limited bleaching artifacts. To test whether rapid changes in membrane symmetry could be detected using SHG, we exposed cells to nanosecond-pulsed electric fields, which are believed to cause formation of nanopores in the plasma membrane. Upon nanosecond-pulsed electric fields exposure, we observed an instantaneous drop of ∼50% in SHG signal from the anodic pole of the cell. When compared to the simultaneously acquired fluorescence signals, it appears that the signal change was not due to the probe diffusing out of the membrane or changes in membrane potential or fluidity. We hypothesize that this loss in SHG signal is due to disruption in the interfacial nature of the membrane. The results show that SHG imaging has great potential as a tool for measuring rapid and subtle plasma membrane disturbance in living cells.As the epicenter for many cellular functions, understanding the dynamics of the plasma membrane is important to monitoring biological phenomena. External forces acting upon the plasma membrane (e.g., electric, mechanical) have been shown to cause rapid disturbances, often resulting in dramatic changes in cell physiology (1–3). To understand this interaction, a minimally invasive, highly sensitive imaging technique that enables monitoring the structure of the plasma membrane is needed. Lipophilic dyes, which embed themselves into lipid membranes, are sensitive to the surrounding electric field and, therefore, report changes in membrane fluidity as well as voltage due to the capacitive nature of the membranes (4,5). This sensitivity is typically detected as a shift in the fluorescence emission spectrum. Localization of the fluorescence signal to only the plasma membrane is difficult because the probes also label internal membrane structures. Thus, to overcome this lack of spatial selectivity, second harmonic generation (SHG) has been used as an alternative to fluorescence for membrane imaging (6,7).In SHG, a second-order nonlinear polarization is induced by electronic disruption of a probe molecule from the electromagnetic field of the incident laser beam. This polarization generates oscillating dipole moments that reradiate light at twice the energy of the excitation beam. The induction of this dipole is sensitive to the static electric field surrounding the probe and the steady-state molecular polarization of the probe molecule. These properties make SHG probes useful for monitoring changes in biological membranes.First, as the voltage potential across the membrane changes, the static electric field around the probe also shifts, making the probe sensitive to these variations (7). Several SHG probes have, therefore, been employed to monitor plasma membrane potential (7,8).Second, because the dipole is affected by the steady-state molecular polarization of the probe itself, a SHG signal is only produced in materials that lack a center of inversion symmetry. In the centrosymmetric case, any emitted radiation is cancelled out by destructive interference. The properties of an interfacial environment, such as a cellular plasma membrane, not only provide the necessary asymmetry, but cause the polarized lipophilic dyes to be aligned in respect to the interface, instead of being randomly distributed as they would in a bulk environment. This alignment allows the generation of a coherent SHG signal from the plasma membrane while the rest of the cell remains nearly signal-free (6,7).We investigated whether the alignment sensitivity of the SHG response could be used to detect minute changes in the organization of the plasma membrane. Jurkat clone E6-1 human T-lymphocytes with a spherical morphology were selected for optimum signal clarity and cultured as directed by American Type Culture Collection (ATCC, Manassas, VA) with 1 I.U./mL penicillin and 0.1 μg/mL streptomycin. Cells were added to 35-mm poly-L-lysine-coated glass-bottomed dishes (MatTek, Ashland, MA) and incubated for 1 h in growth media to allow adherence. Before loading, the cells were rinsed with a buffer consisting of 135 mM NaCl, 5 mM KCl, 2 mM MgCl2, 10 mM HEPES, 10 mM glucose, 2 mM CaCl2, pH 7.4, 290–310 mOsm. SHG probes, Di-4-ANEPPDHQ (Di-4) (5 μM final concentration), FM4-64 (15 μM) or ATR (100 μM, 1 mg/mL BSA) were added to the buffer solution and incubated for 1 h. Cellular imaging was performed in the labeling buffer to limit diffusion of the probe molecules out of the cell membranes.A Ti:sapphire oscillator at 980 nm (Coherent Chameleon, 130 fs, 80 MHz, ∼15 mW at the sample; Coherent Laser, Santa Clara, CA) was coupled through the scan head of a modified model No. TCS SP5 II (Leica Geosystems, Norcross, GA) for SHG and multiphoton-excited fluorescence imaging (40×, water, 1.1 NA) using resonant scanning. SHG signal was collected in transmission by a photomultiplier tube after 680-nm shortpass and 485/25-nm bandpass filters; simultaneous fluorescence signal was collected in the epi-direction by two non-descanned photomultiplier tubes with 540/60-nm and 650/60-nm bandpass filters.Three SHG probes previously used to monitor voltage or membrane order in living cells were tested (8–10). Although ATR is reported to be effective in monitoring membrane voltage, we obtained nearly no SHG signal, despite successful loading as indicated by the fluorescence signal (Fig. 1). When FM4-64 and Di-4 were loaded to similar fluorescence intensities, nearly equivalent SHG signal was collected. Di-4 did appear to have a greater internalization of the dye. However, after the first frame, the FM4-64 signal dropped considerably (Fig. 1 b), an observation reported as a membrane voltage-independent bleaching effect (8). This drop in signal recovered after excitation was blocked for several seconds, but quantification of the response was difficult. Di-4 did not suffer as dramatic a drop in signal upon excitation, and still had sufficient SHG signal/noise after several seconds, so it was used in all further experiments.Open in a separate windowFigure 1(a) Fluorescence (top) and SHG (bottom) images for the three probes. (b) Signal/noise for the fluorescence and SHG for the initial frame and shortly after beginning acquisition. Error bars represent the mean ± SE (n = 10). Scale bar is 10 μm.To test whether Di-4 would report a rapid change in membrane organization, we applied a single nanosecond-duration pulsed electric field (nsPEF) to the labeled cell. These ultrabrief, high-intensity (MV/m) pulses differ from longer (μs-ms), lower-intensity (kV/m) pulses traditionally associated with electroporation in induced cellular response (3,11,12). Through selective uptake of small ions (Ca²⁺, Ti⁺) with limited uptake of propidium iodide, nsPEF have been previously postulated to cause nanopores (<2 nm diameter) in the plasma membrane. In contrast with a previous study observing poration resulting from traditional electroporation (13), the brevity of this apparently novel cellular insult allows for the decoupling of the mechanical effects of the pulse on the membrane from the electrical effects of the pulse itself. A single pulse, generated by a custom pulse generator, was delivered to the cells using a pair of 125-μm diameter tungsten electrodes, separated edge-to-edge by 150 μm, as previously described in Ibey et al. (14). For maximum visualization of changes in the SHG signal, a half-wave plate was placed before the scan head to align the polarization of the laser such that the brightest signals from the plasma membrane were at the poles facing the electrodes.The Di-4 SHG signal in response to a single 16.7 kV/cm, 600-ns nsPEF is shown in Fig. 2. Before the pulse, the intensity of the SHG signal is high at each of these poles. Immediately after the pulse, the SHG intensity drops by ∼50% on the side of the cell facing the anodic electrode, whereas little intensity is lost at the other pole. This response is plotted in Fig. 2 (pulse applied at 2 s), where it is apparent that the response is near instantaneous with little recovery in signal in the 5 s postexposure. The SHG response matches the previously observed effect of this stimulus, where ion uptake displayed a polar dependence and persisted for a number of minutes (11,12). Images taken 5 min after an nsPEF exposure are also shown in Fig. 2. These images confirm the eventual recovery of the cell and the corresponding return of SHG signal to preexposure levels.Open in a separate windowFigure 2(a) SHG images showing drop in signal on the anodic (or A-pole) of the cell. (b) Time trace of SHG response with the electrical pulse applied at 2 s that shows a near-instantaneous drop in the SHG signal at the anodic pole of the cell. (c) SHG image preexposure, immediately postexposure, and then 5-min postexposure showing recovery of the SHG signal.To decouple membrane disturbance from environmental changes around the membrane, we compared the SHG response to the simultaneously acquired fluorescence signal. Because fluorescence is not subject to the strict orientation requirement of SHG, the plasma membrane fluorescence signal provides an indication of the membrane fluidity and/or potential. Despite the dramatic shift in SHG intensity on the anodic pole upon the electrical pulse exposure (Fig. 3 a), the fluorescence channels display little response from the equivalent membrane sections with the exception of photobleaching and a slight increase in signal in both emission bands on the anodic side (Fig. 3, b and c). The shading in these graphs represents the mean ± SE for six cells. Although this slight increase may indicate that a small amount of dye is simply diffusing in or out of the membrane upon exposure, the fluorescence response is not as rapid or as lasting as the SHG response. Change in membrane fluidity or voltage can also be quantized using these fluorescence signals and a value known as the generalized polarization (GP) (4),GP=I515−570−I620−680I515−570+I620−680.(1)As with the raw intensity of the individual signals, the GP value for the membrane (Fig. 3 d) shows no significant shift, indicating that the membrane is likely not transitioning between a more raft- and fluidlike state. Thus, it seems likely that the dye was initially aligned in the tightly-packed ordered membrane so that the probes were able to generate a SHG photon. As shown in Fig. 3 e, we postulate that upon electrical pulse exposure, the membrane was disrupted by the formation with nanopores giving the probe molecules the flexibility to disorient within the membrane. The resulting alignment of the probes is more isotropic in nature, thereby limiting the probes probability of producing a SHG photon. The fluorescence signal remained, however, indicating that the probes remained active in the membrane.Open in a separate windowFigure 3(a) Average SHG signal showing the dramatic drop in signal on the anodic pole at the pulse application (2 s). (b and c) Simultaneous TPF signals showing nearly no instantaneous change at the pulse application. (d) GP showing no observable changes in the membrane potential or fluidity after the pulse. (Shaded areas) Fit to the mean ± SE for each trace (n = 6). (e) Conceptualization of the hypothesized membrane disruption underlying the observed change in SHG response.Thus, by taking advantage of the selection criteria of SHG, we were able to successfully use the SHG probe, Di-4, to monitor rapid disruption of the plasma membrane. Because SHG can only be generated when the probes are aligned in the plasma membrane, the SHG signal diminishes significantly upon disruption. The simultaneous collection of the multiphoton-excited fluorescence signal was advantageous in that it demonstrated that the probes did not simply diffuse out of the membrane, did not appear to be energetically disrupted by the electric pulse, and showed that the membrane changes were not simply a change in lipid order. We believe that this technique holds tremendous potential for use in the study of how external stimuli interact with and change the orientation of biological membranes. Such knowledge may allow for further understanding of how manipulation of cells and biological systems can be achieved using external stimuli.     

Circadian and Social Cues Regulate Ion Channel Trafficking

Electric fish generate and sense electric fields for navigation and communication. These signals can be energetically costly to produce and can attract electroreceptive predators. To minimize costs, some nocturnally active electric fish rapidly boost the power of their signals only at times of high social activity, either as night approaches or in response to social encounters. Here we show that the gymnotiform electric fish Sternopygus macrurus rapidly boosts signal amplitude by 40% at night and during social encounters. S. macrurus increases signal magnitude through the rapid and selective trafficking of voltage-gated sodium channels into the excitable membranes of its electrogenic cells, a process under the control of pituitary peptide hormones and intracellular second-messenger pathways. S. macrurus thus maintains a circadian rhythm in signal amplitude and adapts within minutes to environmental events by increasing signal amplitude through the rapid trafficking of ion channels, a process that directly modifies an ongoing behavior in real time.     

Second-harmonic generation imaging of membrane potential with retinal analogues

Second-harmonic generation (SHG) by membrane-incorporated probes is a nonlinear optical signal that is voltage-sensitive and the basis of a sensitive method for imaging membrane potential. The voltage dependence of SHG by four different probes, three retinoids (all-trans retinal), and two new retinal analogs, 3-methyl-7-(4′-dimethylamino-phenyl)-2,4,6-heptatrienal (AR-3) and 3,7-dimethyl-9-(4′-dimethylamino-phenyl)-2,4,6,8-nonatetraenal (AR-4), and a styryl dye (FM4-64), were compared in HEK-293 cells. Results were analyzed by fitting data with an expression based on an electrooptic mechanism for SHG, which depends on the complex-valued first- and second-order nonlinear electric susceptibilities (χ₂ and χ₃) of the probe. This gave values for the voltage sensitivity at the cell's resting potential, the voltage where the SHG is minimal, and the amplitude of the signal at that voltage for each of the four compounds. These measures show that χ₂ and χ₃ are complex numbers for all compounds except all-trans retinal, consistent with the proximities of excitation and/or emission wavelengths to molecular resonances. Estimates of probe orientation and location in the membrane electric field show that, for the far-from-resonance case, the shot noise-limited signal/noise ratio depends on the location of the probe in the membrane, and on χ₃ but not on χ₂.     

Slow non-specific accumulation of 2′-deoxy and 2′-O-methyl oligonucleotide probes at mitochondria in live cells

Molecular beacons (MBs) have the potential to provide a powerful tool for rapid RNA detection in living cells, as well as monitoring the dynamics of RNA expression in response to external stimuli. To exploit this potential, it is necessary to distinguish true signal from background signal due to non-specific interactions. Here, we show that, when cyanine-dye labeled 2′-deoxy and 2′-O-methyl oligonucleotide probes are inside living cells for >5 h, most of their signals co-localize with mitochondrial staining. These probes include random-sequence MB, dye-labeled single-strand linear oligonucleotide and dye-labeled double-stranded oligonucleotide. Using carbonyl cyanide m-chlorophenyl hydrazone treatment, we found that the non-specific accumulation of oligonucleotide probes at mitochondria was driven by mitochondrial membrane potential. We further demonstrated that the dye-labeled oligonucleotide probes were likely on/near the surface of mitochondria but not inside mitochondrial inner membrane. Interestingly, oligonucleotides probes labeled respectively with Alexa Fluor 488 and Alexa Fluor 546 did not accumulate at mitochondria, suggesting that the non-specific interaction between dye-labeled ODN probes and mitochondria is dye-specific. These results may help design and optimize fluorescence imaging probes for long-time RNA detection and monitoring in living cells.     

Applications of second harmonic generation (SHG)/sum-frequency generation (SFG) imaging for biophysical characterization of the plasma membrane

The plasma membrane is a lipid bilayer of < 10 nm width that separates intra- and extra-cellular environments and serves as the site of cell-cell communication, as well as communication between cells and the extracellular environment. As such, biophysical phenomena at and around the plasma membrane play key roles in determining cellular physiology and pathophysiology. Thus, the selective visualization and characterization of the plasma membrane are crucial aspects of research in wide areas of biology and medicine. However, the specific characterization of the plasma membrane has been a challenge using conventional imaging techniques, which are unable to effectively distinguish between signals arising from the plasma membrane and those from intracellular lipid structures. In this regard, interface-specific second harmonic generation (SHG) and sum-frequency generation (SFG) imaging demonstrate great potential. When combined with exogenous SHG/SFG active dyes, SHG/SFG can specifically highlight the plasma membrane as the most prominent interface associated with cells. Furthermore, SHG/SFG imaging can be readily extended to multimodal multiphoton microscopy with simultaneous occurrence of other multiphoton phenomena, including multiphoton excitation and coherent Raman scattering, which shed light on the biophysical properties of the plasma membrane from different perspectives. Here, we review traditional and current applications, as well as the prospects of long-known but unexplored SHG/SFG imaging techniques in biophysics, with special focus on their use in the biophysical characterization of the plasma membrane.     

Rapid evaluation of a protein-based voltage probe using a field-induced membrane potential change

The development of a high performance protein probe for the measurement of membrane potential will allow elucidation of spatiotemporal regulation of electrical signals within a network of excitable cells. Engineering such a probe requires a functional screen of many candidates. Although the glass-microelectrode technique generally provides an accurate measure of a given test probe, throughputs are limited. In this study, we focused on an approach that uses the membrane potential changes induced by an external electric field in a geometrically simple mammalian cell. For quantitative evaluation of membrane voltage probes that rely on the structural transition of the S1–S4 voltage sensor domain and hence have non-linear voltage dependencies, it was crucial to introduce exogenous inwardly rectifying potassium conductance to reduce cell-to-cell variability in resting membrane potentials. Importantly, the addition of the exogenous conductance drastically altered the profile of the field-induced potential. Following a site-directed random mutagenesis and the rapid screen, we identified a mutant of a voltage probe Mermaid, exhibiting positively shifted voltage sensitivity. Due to its simplicity, the current approach will be applicable under a microfluidic configuration to carry out an efficient screen. Additionally, we demonstrate another interesting aspect of the field-induced optical signals, ability to visualize electrical couplings between cells.     

Comparison of Two Voltage-Sensitive Dyes and Their Suitability for Long-Term Imaging of Neuronal Activity

One of the key approaches for studying neural network function is the simultaneous measurement of the activity of many neurons. Voltage-sensitive dyes (VSDs) simultaneously report the membrane potential of multiple neurons, but often have pharmacological and phototoxic effects on neuronal cells. Yet, to study the homeostatic processes that regulate neural network function long-term recordings of neuronal activities are required. This study aims to test the suitability of the VSDs RH795 and Di-4-ANEPPS for optically recording pattern generating neurons in the stomatogastric nervous system of crustaceans with an emphasis on long-term recordings of the pyloric central pattern generator. We demonstrate that both dyes stain pyloric neurons and determined an optimal concentration and light intensity for optical imaging. Although both dyes provided sufficient signal-to-noise ratio for measuring membrane potentials, Di-4-ANEPPS displayed a higher signal quality indicating an advantage of this dye over RH795 when small neuronal signals need to be recorded. For Di-4-ANEPPS, higher dye concentrations resulted in faster and brighter staining. Signal quality, however, only depended on excitation light strength, but not on dye concentration. RH795 showed weak and slowly developing phototoxic effects on the pyloric motor pattern as well as slow bleaching of the staining and is thus the better choice for long-term experiments. Low concentrations and low excitation intensities can be used as, in contrast to Di-4-ANEPPS, the signal-to-noise ratio was independent of excitation light strength. In summary, RH795 and Di-4-ANEPPS are suitable for optical imaging in the stomatogastric nervous system of crustaceans. They allow simultaneous recording of the membrane potential of multiple neurons with high signal quality. While Di-4-ANEPPS is better suited for short-term experiments that require high signal quality, RH795 is a better candidate for long-term experiments since it has only minor effects on the motor pattern.     

Second harmonic generation in neurons: electro-optic mechanism of membrane potential sensitivity

Second harmonic generation (SHG) from membrane-bound chromophores can be used to image membrane potential in neurons. We investigate the biophysical mechanism responsible for the SHG voltage sensitivity of the styryl dye FM 4-64 in pyramidal neurons from mouse neocortical slices. SHG signals are exquisitely sensitive to the polarization of the incident laser light. Using this polarization sensitivity in two complementary approaches, we estimate a approximately 36 degrees tilt angle of the chromophore to the membrane normal. Changes in membrane potential do not affect the polarization of the SHG signal. The voltage response of FM 4-64 is faster than 1 ms and does not reverse sign when imaged at either side of its absorption peak. We conclude that FM 4-64 senses membrane potential through an electro-optic mechanism, without significant chromophore membrane reorientation, redistribution, or spectral shift.     

Cells immersed in collagen matrices show a decrease in plasma membrane fluidity as the matrix stiffness increases

Cells are constantly adapting to maintain their identity in response to the surrounding media's temporal and spatial heterogeneity. The plasma membrane, which participates in the transduction of external signals, plays a crucial role in this adaptation. Studies suggest that nano and micrometer areas with different fluidities at the plasma membrane change their distribution in response to external mechanical signals. However, investigations linking fluidity domains with mechanical stimuli, specifically matrix stiffness, are still in progress. This report tests the hypothesis that the stiffness of the extracellular matrix can modify the equilibrium of areas with different order in the plasma membrane, resulting in changes in overall membrane fluidity distribution. We studied the effect of matrix stiffness on the distribution of membrane lipid domains in NIH-3 T3 cells immersed in matrices of varying concentrations of collagen type I, for 24 or 72 h. The stiffness and viscoelastic properties of the collagen matrices were characterized by rheometry, fiber sizes were measured by Scanning Electron Microscopy (SEM) and the volume occupied by the fibers by second harmonic generation imaging (SHG). Membrane fluidity was measured using the fluorescent dye LAURDAN and spectral phasor analysis. The results demonstrate that an increase in collagen stiffness alters the distribution of membrane fluidity, leading to an increasing amount of the LAURDAN fraction with a high degree of packing. These findings suggest that changes in the equilibrium of fluidity domains could represent a versatile and refined component of the signal transduction mechanism for cells to respond to the highly heterogeneous matrix structural composition. Overall, this study sheds light on the importance of the plasma membrane's role in adapting to the extracellular matrix's mechanical cues.     

Evidence for a transition in the cortical membranes of Paramecium

Ever since the pioneering studies in the 1960s and 70s, the importance of order transitions for cell membrane functions has remained a matter of debate. Recently, it has been proposed that the nonlinear stimulus-response curve of excitable cells, which manifests in all-or-none pulses (action potentials (AP)), is due to a transition in the cell membrane. Indeed, evidence for transitions has accumulated in plant cells and neurons, but studies with other excitable cells are expedient in order to show if this finding is of a general nature. Herein, we investigated intact, motile specimens of the “swimming neuron” Paramecium. The cellular membranes were labelled with the solvatochromic fluorophores LAURDAN or Di-4-ANEPPDHQ. Subsequently, a cell was trapped in a microfluidic channel and investigated by fluorescence spectroscopy. The generalized polarization (GP) of the fluorescence emission from cell cortical membranes (probably plasma and alveolar membranes) was extracted by an edge-finding algorithm. The thermo-optical state diagram, i.e. the dependence of GP on temperature, exhibited clear indications for a reversible transition. This transition had a width of ~10–15 °C and a midpoint that was located ~4 °C below the growth temperature. The state diagrams with LAURDAN and Di-4-ANEPPDHQ had widely identical characteristics. These results suggested that the cortical membranes of Paramecium reside in an order transition regime under physiological growth conditions. Based on these findings, membrane potential fluctuations, spontaneous depolarizing spikes, and thermal excitation of Paramecium was interpreted.     

Protein Conformational Changes Are Detected and Resolved Site Specifically by Second-Harmonic Generation

We present here a straightforward, broadly applicable technique for real-time detection and measurement of protein conformational changes in solution. This method is based on tethering proteins labeled with a second-harmonic generation (SHG) active dye to supported lipid bilayers. We demonstrate our method by measuring the conformational changes that occur upon ligand binding with three well-characterized proteins labeled at lysine residues: calmodulin (CaM), maltose-binding protein (MBP), and dihydrofolate reductase (DHFR). We also create a single-site cysteine mutant of DHFR engineered within the Met20 catalytic loop region and study the protein’s structural motion at this site. Using published x-ray crystal structures, we show that the changes in the SHG signals upon ligand binding are the result of structural motions that occur at the labeled sites between the apo and ligand-bound forms of the proteins, which are easily distinguished from each other. In addition, we demonstrate that different magnitudes of the SHG signal changes are due to different and specific ligand-induced conformational changes. Taken together, these data illustrate the potential of the SHG approach for detecting and measuring protein conformational changes for a wide range of biological applications.     

Fluorescent Protein Voltage Probes Derived from ArcLight that Respond to Membrane Voltage Changes with Fast Kinetics

We previously reported the discovery of a fluorescent protein voltage probe, ArcLight, and its derivatives that exhibit large changes in fluorescence intensity in response to changes of plasma membrane voltage. ArcLight allows the reliable detection of single action potentials and sub-threshold activities in individual neurons and dendrites. The response kinetics of ArcLight (τ1-on ~10 ms, τ2-on ~ 50 ms) are comparable with most published genetically-encoded voltage probes. However, probes using voltage-sensing domains other than that from the Ciona intestinalis voltage sensitive phosphatase exhibit faster kinetics. Here we report new versions of ArcLight, in which the Ciona voltage-sensing domain was replaced with those from chicken, zebrafish, frog, mouse or human. We found that the chicken and zebrafish-based ArcLight exhibit faster kinetics, with a time constant (τ) less than 6ms for a 100 mV depolarization. Although the response amplitude of these two probes (8-9%) is not as large as the Ciona-based ArcLight (~35%), they are better at reporting action potentials from cultured neurons at higher frequency. In contrast, probes based on frog, mouse and human voltage sensing domains were either slower than the Ciona-based ArcLight or had very small signals.     

Diverse effects of nanosecond pulsed electric fields on cells and tissues

The application of pulsed electric fields to cells is extended to include nonthermal pulses with shorter durations (10-300 ns), higher electric fields (< or =350 kV/cm), higher power (gigawatts), and distinct effects (nsPEF) compared to classical electroporation. Here we define effects and explore potential application for nsPEF in biology and medicine. As the pulse duration is decreased below the plasma membrane charging time constant, plasma membrane effects decrease and intracellular effects predominate. NsPEFs induced apoptosis and caspase activation that was calcium-dependent (Jurkat cells) and calcium-independent (HL-60 and Jurkat cells). In mouse B10-2 fibrosarcoma tumors, nsPEFs induced caspase activation and DNA fragmentation ex vivo, and reduced tumor size in vivo. With conditions below thresholds for classical electroporation and apoptosis, nsPEF induced calcium release from intracellular stores and subsequent calcium influx through store-operated channels in the plasma membrane that mimicked purinergic receptor-mediated calcium mobilization. When nsPEF were applied after classical electroporation pulses, GFP reporter gene expression was enhanced above that observed for classical electroporation. These findings indicate that nsPEF extend classical electroporation to include events that primarily affect intracellular structures and functions. Potential applications for nsPEF include inducing apoptosis in cells and tumors, probing signal transduction mechanisms that determine cell fate, and enhancing gene expression.     

Bipolar nanosecond electric pulses are less efficient at electropermeabilization and killing cells than monopolar pulses

Multiple studies have shown that bipolar (BP) electric pulses in the microsecond range are more effective at permeabilizing cells while maintaining similar cell survival rates as compared to monopolar (MP) pulse equivalents. In this paper, we investigated whether the same advantage existed for BP nanosecond-pulsed electric fields (nsPEF) as compared to MP nsPEF. To study permeabilization effectiveness, MP or BP pulses were delivered to single Chinese hamster ovary (CHO) cells and the response of three dyes, Calcium Green-1, propidium iodide (PI), and FM1-43, was measured by confocal microscopy. Results show that BP pulses were less effective at increasing intracellular calcium concentration or PI uptake and cause less membrane reorganization (FM1-43) than MP pulses. Twenty-four hour survival was measured in three cell lines (Jurkat, U937, CHO) and over ten times more BP pulses were required to induce death as compared to MP pulses of similar magnitude and duration. Flow cytometry analysis of CHO cells after exposure (at 15 min) revealed that to achieve positive FITC-Annexin V and PI expression, ten times more BP pulses were required than MP pulses. Overall, unlike longer pulse exposures, BP nsPEF exposures proved far less effective at both membrane permeabilization and cell killing than MP nsPEF.     

Transient Features in Nanosecond Pulsed Electric Fields Differentially Modulate Mitochondria and Viability

It is hypothesized that high frequency components of nanosecond pulsed electric fields (nsPEFs), determined by transient pulse features, are important for maximizing electric field interactions with intracellular structures. For monopolar square wave pulses, these transient features are determined by the rapid rise and fall of the pulsed electric fields. To determine effects on mitochondria membranes and plasma membranes, N1-S1 hepatocellular carcinoma cells were exposed to single 600 ns pulses with varying electric fields (0–80 kV/cm) and short (15 ns) or long (150 ns) rise and fall times. Plasma membrane effects were evaluated using Fluo-4 to determine calcium influx, the only measurable source of increases in intracellular calcium. Mitochondria membrane effects were evaluated using tetramethylrhodamine ethyl ester (TMRE) to determine mitochondria membrane potentials (ΔΨm). Single pulses with short rise and fall times caused electric field-dependent increases in calcium influx, dissipation of ΔΨm and cell death. Pulses with long rise and fall times exhibited electric field-dependent increases in calcium influx, but diminished effects on dissipation of ΔΨm and viability. Results indicate that high frequency components have significant differential impact on mitochondria membranes, which determines cell death, but lesser variances on plasma membranes, which allows calcium influxes, a primary determinant for dissipation of ΔΨm and cell death.     

Evaluation of Voltage-Sensitive Fluorescence Dyes for Monitoring Neuronal Activity in the Embryonic Central Nervous System

Using an optical imaging technique with voltage-sensitive dyes (VSDs), we investigated the functional organization and architecture of the central nervous system (CNS) during embryogenesis. In the embryonic nervous system, a merocyanine-rhodanine dye, NK2761, has proved to be the most useful absorption dye for detecting neuronal activity because of its high signal-to-noise ratio (S/N), low toxicity and small dye bleaching. In the present study, we evaluated the suitability of fluorescence VSDs for optical recording in the embryonic CNS. We screened eight styryl (hemicyanine) dyes in isolated brainstem–spinal cord preparations from 7-day-old chick embryos. Measurements of voltage-related optical signals were made using a multiple-site optical recording system. The signal size, S/N, photobleaching, effects of perfusion and recovery of neural responses after staining were compared. We also evaluated optical responses with various magnifications. Although the S/N was lower than with the absorption dye, clear optical responses were detected with several fluorescence dyes, including di-2-ANEPEQ, di-4-ANEPPS, di-3-ANEPPDHQ, di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA. Di-2-ANEPEQ showed the largest S/N, whereas its photobleaching was faster and the recovery of neural responses after staining was slower. Di-4-ANEPPS and di-3-ANEPPDHQ also exhibited a large S/N but required a relatively long time for recovery of neural activity. Di-4-AN(F)EPPTEA, di-2-AN(F)EPPTEA and di-2-ANEPPTEA showed smaller S/Ns than di-2-ANEPEQ, di-4-ANEPPS and di-3-ANEPPDHQ; but the recovery of neural responses after staining was faster. This study demonstrates the potential utility of these styryl dyes in optical monitoring of voltage changes in the embryonic CNS.     

A fluorometric approach to local electric field measurements in a voltage-gated ion channel

Site-specific electrostatic measurements have been limited to soluble proteins purified for in vitro spectroscopic characterization or proteins of known structure; however, comparable measurements have not been made for functional membrane bound proteins. Here, using an electrochromic fluorophore, we describe a method to monitor localized electric field changes in a voltage-gated potassium channel. By coupling the novel probe Di-1-ANEPIA to cysteines in Shaker and tracking field-induced optical changes, in vivo electrostatic measurements were recorded with submillisecond resolution. This technique reports dynamic changes in the electric field during the gating process and elucidates the electric field profile within Shaker. The extension of this method to other membrane bound proteins, including transporters, will yield insight into the role of electrical forces on protein function.     

工业模式微生物膜有序性的活细胞定量分析

作为一种相位敏感的荧光探针,Di-4-ANEPPDHQ可以特异性标记膜的有序相和无序相,在理论上可以对细胞膜的有序性进行定量成像。通过将Di-4-ANEPPDHQ和激光扫描共聚焦显微术相结合,对多种具有代表性的工业模式微生物进行了有序相和无序相活细胞成像,结合极性归一化数值的统计比较,最终实现对上述工业模式微生物细胞膜有序性的定量分析,为细胞膜工程提供了一种直观且快速的活细胞检测方法。     

Sine wave electropermeabilization reveals the frequency-dependent response of the biological membranes

The permeabilization of biological membranes by electric fields, known as electroporation, has been traditionally performed with square electric pulses. These signals distribute the energy applied to cells in a wide frequency band. This paper investigates the use of sine waves, which are narrow band signals, to provoke electropermeabilization and the frequency dependence of this phenomenon.Single bursts of sine waves at different frequencies in the range from 8 kHz–130 kHz were applied to cells in vitro. Electroporation was studied in the plasma membrane and the internal organelles membrane using calcium as a permeabilization marker. Additionally, a double-shell electrical model was simulated to give a theoretical framework to our results.The electroporation efficiency shows a low pass filter frequency dependence for both the plasma membrane and the internal organelles membrane. The mismatch between the theoretical response and the observed behavior for the internal organelles membrane is explained by a two-step permeabilization process: first the permeabilization of the external membrane and afterwards that of the internal membranes. The simulations in the model confirm this two-step hypothesis when a variable plasma membrane conductivity is considered in the analysis.This study demonstrates how the use of narrow-band signals as sine waves is a suitable method to perform electroporation in a controlled manner. We suggest that the use of this type of signals could bring a simplification in the investigations of the very complex phenomenon of electroporation, thus representing an interesting option in future fundamental studies.     

Transmembrane calcium influx induced by ac electric fields.

Exogenous electric fields induce cellular responses including redistribution of integral membrane proteins, reorganization of microfilament structures, and changes in intracellular calcium ion concentration ([Ca2+]i). Although increases in [Ca2+]i caused by application of direct current electric fields have been documented, quantitative measurements of the effects of alternating current (ac) electric fields on [Ca2+]i are lacking and the Ca2+ pathways that mediate such effects remain to be identified. Using epifluorescence microscopy, we have examined in a model cell type the [Ca2+]i response to ac electric fields. Application of a 1 or 10 Hz electric field to human hepatoma (Hep3B) cells induces a fourfold increase in [Ca2+]i (from 50 nM to 200 nM) within 30 min of continuous field exposure. Depletion of Ca2+ in the extracellular medium prevents the electric field-induced increase in [Ca2+]i, suggesting that Ca2+ influx across the plasma membrane is responsible for the [Ca2+]i increase. Incubation of cells with the phospholipase C inhibitor U73122 does not inhibit ac electric field-induced increases in [Ca2+]i, suggesting that receptor-regulated release of intracellular Ca2+ is not important for this effect. Treatment of cells with either the stretch-activated cation channel inhibitor GdCl3 or the nonspecific calcium channel blocker CoCl2 partially inhibits the [Ca2+]i increase induced by ac electric fields, and concomitant treatment with both GdCl3 and CoCl2 completely inhibits the field-induced [Ca2+]i increase. Since neither Gd3+ nor Co2+ is efficiently transported across the plasma membrane, these data suggest that the increase in [Ca2+]i induced by ac electric fields depends entirely on Ca2+ influx from the extracellular medium.