Generation of functionally competent single bovine adrenal chromaffin cells from cell aggregates using the neutral protease dispase

Modulating Ca2+ influx into adrenal chromaffin cells with short-duration nanosecond electric pulses

Isolated bovine adrenal chromaffin cells exposed to single 2-, 4-, or 5-ns pulses undergo a rapid, transient rise in intracellular Ca2+ mediated by Ca2+ entry via voltage-gated Ca2+ channels (VGCCs), mimicking the activation of these cells in vivo by acetylcholine. However, pulse durations 150 ns or longer elicit larger amplitude and longer-lived Ca2+ responses due to Ca2+ influx via both VGCCs and a yet to be identified plasma membrane pathway(s). To further our understanding of the differential effects of ultrashort versus longer pulse durations on Ca2+ influx, chromaffin cells were loaded with calcium green-1 and exposed to single 3-, 5-, 11-, 25-, or 50-ns pulses applied at their respective Ca2+ activation threshold electric fields. Increasing pulse duration from 3 or 5 ns to only 11 ns was sufficient to elicit increased amplitude and longer-lived Ca2+ responses in the majority of cells, a trend that continued as pulse duration increased to 50 ns. The amplification of Ca2+ responses was not the result of Ca2+ release from intracellular stores and was accompanied by a decreased effectiveness of VGCC inhibitors to block the responses and a reduced reliance on extracellular Na+ and membrane depolarization to evoke the responses. Inhibitors of pannexin channels, P2X receptors, or non-selective cation channels failed to attenuate 50-ns-elicited Ca2+ responses, ruling out these Ca2+-permeable channels as secondary Ca2+ entry pathways. Analytical calculations and numerical modeling suggest that the parameter that best determines the response of chromaffin cells to increasing pulse durations is the time the membrane charges to its peak voltage. These results highlight the pronounced sensitivity of a neuroendocrine cell to pulse durations differing by only tens of nanoseconds, which has important implications for the future development of nanosecond pulse technologies enabling electrostimulation applications for spatially focused and graded in vivo neuromodulation.

5 ns electric pulses induce Ca2+-dependent exocytotic release of catecholamine from adrenal chromaffin cells

Previously we reported that adrenal chromaffin cells exposed to a 5 ns, 5 MV/m pulse release the catecholamines norepinephrine (NE) and epinephrine (EPI) in a Ca²⁺-dependent manner. Here we determined that NE and EPI release increased with pulse number (one versus five and ten pulses at 1 Hz), established that release occurs by exocytosis, and characterized the exocytotic response in real-time. Evidence of an exocytotic mechanism was the appearance of dopamine-β-hydroxylase on the plasma membrane, and the demonstration by total internal reflection fluorescence microscopy studies that a train of five or ten pulses at 1 Hz triggered the release of the fluorescent dye acridine orange from secretory granules. Release events were Ca²⁺-dependent, longer-lived relative to those evoked by nicotinic receptor stimulation, and occurred with a delay of several seconds despite an immediate rise in Ca²⁺. In complementary studies, cells labeled with the plasma membrane fluorescent dye FM 1-43 and exposed to a train of ten pulses at 1 Hz underwent Ca²⁺-dependent increases in FM 1-43 fluorescence indicative of granule fusion with the plasma membrane due to exocytosis. These results demonstrate the effectiveness of ultrashort electric pulses for stimulating catecholamine release, signifying their promise as a novel electrostimulation modality for neurosecretion.

2-ns Electrostimulation of Ca2+ Influx into Chromaffin Cells: Rapid Modulation by Field Reversal

Cellular effects of nanosecond-pulsed electric field exposures can be attenuated by an electric field reversal, a phenomenon called bipolar pulse cancellation. Our investigations of this phenomenon in neuroendocrine adrenal chromaffin cells show that a single 2-ns, 16 MV/m unipolar pulse elicited a rapid, transient rise in intracellular Ca²⁺ levels due to Ca²⁺ influx through voltage-gated calcium channels. The response was eliminated by a 2-ns bipolar pulse with positive and negative phases of equal duration and amplitude and fully restored (unipolar-equivalent response) when the delay between each phase of the bipolar pulse was 30 ns. Longer interphase intervals evoked Ca²⁺ responses that were greater in magnitude than those evoked by a unipolar pulse (stimulation). Cancellation was also observed when the amplitude of the second (negative) phase of the bipolar pulse was half that of the first (positive) phase but progressively lost as the amplitude of the second phase was incrementally increased above that of the first phase. When the amplitude of the second phase was twice that of the first phase, there was stimulation. By comparing the experimental results for each manipulation of the bipolar pulse waveform with analytical calculations of capacitive membrane charging/discharging, also known as accelerated membrane discharge mechanism, we show that the transition from cancellation to unipolar-equivalent stimulation broadly agrees with this model. Taken as a whole, our results demonstrate that electrostimulation of adrenal chromaffin cells with ultrashort pulses can be modulated with interphase intervals of tens of nanoseconds, a prediction of the accelerated membrane discharge mechanism not previously observed in other bipolar pulse cancellation studies. Such modulation of Ca²⁺ responses in a neural-type cell is promising for the potential use of nanosecond bipolar pulse technologies for remote electrostimulation applications for neuromodulation.

Paradoxical effects on voltage-gated Na+ conductance in adrenal chromaffin cells by twin vs single high intensity nanosecond electric pulses

We previously reported that a single 5 ns high intensity electric pulse (NEP) caused an E-field-dependent decrease in peak inward voltage-gated Na+ current (INa) in isolated bovine adrenal chromaffin cells. This study explored the effects of a pair of 5 ns pulses on INa recorded in the same cell type, and how varying the E-field amplitude and interval between the pulses altered its response. Regardless of the E-field strength (5 to 10 MV/m), twin NEPs having interpulse intervals ≥ than 5 s caused the inhibition of TTX-sensitive INa to approximately double relative to that produced by a single pulse. However, reducing the interval from 1 s to 10 ms between twin NEPs at E-fields of 5 and 8 MV/m but not 10 MV/m decreased the magnitude of the additive inhibitory effect by the second pulse in a pair on INa. The enhanced inhibitory effects of twin vs single NEPs on INa were not due to a shift in the voltage-dependence of steady-state activation and inactivation but were associated with a reduction in maximal Na+ conductance. Paradoxically, reducing the interval between twin NEPs at 5 or 8 MV/m but not 10 MV/m led to a progressive interval-dependent recovery of INa, which after 9 min exceeded the level of INa reached following the application of a single NEP. Disrupting lipid rafts by depleting membrane cholesterol with methyl-β-cyclodextrin enhanced the inhibitory effects of twin NEPs on INa and ablated the progressive recovery of this current at short twin pulse intervals, suggesting a complete dissociation of the inhibitory effects of twin NEPs on this current from their ability to stimulate its recovery. Our results suggest that in contrast to a single NEP, twin NEPs may influence membrane lipid rafts in a manner that enhances the trafficking of newly synthesized and/or recycling of endocytosed voltage-gated Na+ channels, thereby pointing to novel means to regulate ion channels in excitable cells.

Stimulation or Cancellation of Ca2+ Influx by Bipolar Nanosecond Pulsed Electric Fields in Adrenal Chromaffin Cells Can Be Achieved by Tuning Pulse Waveform

Exposing adrenal chromaffin cells to single 150 to 400 ns electric pulses triggers a rise in intracellular Ca²⁺ ([Ca²⁺]_i) that is due to Ca²⁺ influx through voltage-gated Ca²⁺ channels (VGCC) and plasma membrane electropores. Immediate delivery of a second pulse of the opposite polarity in which the duration and amplitude were the same as the first pulse (a symmetrical bipolar pulse) or greater than the first pulse (an asymmetrical bipolar pulse) had a stimulatory effect, evoking larger Ca²⁺ responses than the corresponding unipolar pulse. Progressively decreasing the amplitude of the opposite polarity pulse while also increasing its duration converted stimulation to attenuation, which reached a maximum of 43% when the positive phase was 150 ns at 3.1 kV/cm, and the negative phase was 800 ns at 0.2 kV/cm. When VGCCs were blocked, Ca²⁺ responses evoked by asymmetrical and even symmetrical bipolar pulses were significantly reduced relative to those evoked by the corresponding unipolar pulse under the same conditions, indicating that attenuation involved mainly the portion of Ca²⁺ influx attributable to membrane electropermeabilization. Thus, by tuning the shape of the bipolar pulse, Ca²⁺ entry into chromaffin cells through electropores could be attenuated while preserving Ca²⁺ influx through VGCCs.

Different Membrane Pathways Mediate Ca2+ Influx in Adrenal Chromaffin Cells Exposed to 150-400 ns Electric Pulses

Exposing adrenal chromaffin cells to 5 ns electric pulses (nsPEF) causes a rapid rise in intracellular Ca²⁺ ([Ca²⁺]_i) that is solely the result of Ca²⁺ influx through voltage-gated Ca²⁺ channels (VGCCs). This study explored the effect of longer duration nsPEF on [Ca²⁺]_i. Single 150, 200, or 400 ns pulses at 3.1 kV/cm evoked rapid increases in [Ca²⁺]_i, the magnitude of which increased linearly with pulse width and electric field amplitude. Recovery of [Ca²⁺]_i to prestimulus levels was faster for 150 ns exposures. Regardless of pulse width, no rise in [Ca²⁺]_i occurred in the absence of extracellular Ca²⁺, indicating that the source of Ca²⁺ was from outside the cell. Ca²⁺ responses evoked by a 150 ns pulse were inhibited to varying degrees by ω-agatoxin IVA, ω-conotoxin GVIA, nitrendipine or nimodipine, antagonists of P/Q-, N-, and L-type VGCCs, respectively, and by 67% when all four types of VGCCs were blocked simultaneously. The remaining Ca²⁺ influx insensitive to VGCC inhibitors was attributed to plasma membrane nanoporation, which comprised the E-field sensitive component of the response. Both pathways of Ca²⁺ entry were inhibited by 200 μM Cd²⁺. These results demonstrate that, in excitable chromaffin cells, single 150-400 ns pulses increased the permeability of the plasma membrane to Ca²⁺ in addition to causing Ca²⁺ influx via VGCCs.

Dielectric properties of isolated adrenal chromaffin cells determined by microfluidic impedance spectroscopy

Knowledge of the dielectric properties of biological cells plays an important role in numerical models aimed at understanding how high intensity ultrashort nanosecond electric pulses affect the plasma membrane and the membranes of intracellular organelles. To this end, using electrical impedance spectroscopy, the dielectric properties of isolated, neuroendocrine adrenal chromaffin cells were obtained. Measured impedance data of the cell suspension, acquired between 1kHz and 20MHz, were fit into a combination of constant phase element and Cole-Cole models from which the effect of electrode polarization was extracted. The dielectric spectrum of each cell suspension was fit into a Maxwell-Wagner mixture model and the Clausius-Mossotti factor was obtained. Lastly, to extract the cellular dielectric parameters, the cell dielectric data were fit into a granular cell model representative of a chromaffin cell, which was based on the inclusion of secretory granules in the cytoplasm. Chromaffin cell parameters determined from this study were the cell and secretory granule membrane specific capacitance (1.22 and 7.10μF/cm², respectively), the cytoplasmic conductivity, which excludes and includes the effect of intracellular membranous structures (1.14 and 0.49S/m, respectively), and the secretory granule milieu conductivity (0.35S/m). These measurements will be crucial for incorporating into numerical models aimed at understanding the differential poration effect of nanosecond electric pulses on chromaffin cell membranes.

Adrenal Chromaffin Cells Exposed to 5-ns Pulses Require Higher Electric Fields to Porate Intracellular Membranes than the Plasma Membrane: An Experimental and Modeling Study

Nanosecond-duration electric pulses (NEPs) can permeabilize the endoplasmic reticulum (ER), causing release of Ca²⁺ into the cytoplasm. This study used experimentation coupled with numerical modeling to understand the lack of Ca²⁺ mobilization from Ca²⁺-storing organelles in catecholamine-secreting adrenal chromaffin cells exposed to 5-ns pulses. Fluorescence imaging determined a threshold electric (E) field of 8 MV/m for mobilizing intracellular Ca²⁺ whereas whole-cell recordings of membrane conductance determined a threshold E-field of 3 MV/m for causing plasma membrane permeabilization. In contrast, a 2D numerical model of a chromaffin cell, which was constructed with internal structures representing a nucleus, mitochondrion, ER, and secretory granule, predicted that exposing the cell to the same 5-ns pulse electroporated the plasma and ER membranes at the same E-field amplitude, 3-4 MV/m. Agreement of the numerical simulations with the experimental results was obtained only when the ER interior conductivity was 30-fold lower than that of the cytoplasm and the ER membrane permittivity was twice that of the plasma membrane. A more realistic intracellular geometry for chromaffin cells in which structures representing multiple secretory granules and an ER showed slight differences in the thresholds necessary to porate the membranes of the secretory granules. We conclude that more sophisticated cell models together with knowledge of accurate dielectric properties are needed to understand the effects of NEPs on intracellular membranes in chromaffin cells, information that will be important for elucidating how NEPs porate organelle membranes in other cell types having a similarly complex cytoplasmic ultrastructure.

Nanosecond electric pulses differentially affect inward and outward currents in patch clamped adrenal chromaffin cells

This study examined the effect of 5 ns electric pulses on macroscopic ionic currents in whole-cell voltage-clamped adrenal chromaffin cells. Current-voltage (I-V) relationships first established that the early peak inward current was primarily composed of a fast voltage-dependent Na+ current (INa), whereas the late outward current was composed of at least three ionic currents: a voltage-gated Ca2+ current (ICa), a Ca2+-activated K+ current (IK(Ca)), and a sustained voltage-dependent delayed rectifier K+ current (IKV). A constant-voltage step protocol was next used to monitor peak inward and late outward currents before and after cell exposure to a 5 ns pulse. A single pulse applied at an electric (E)-field amplitude of 5 MV/m resulted in an instantaneous decrease of ~4% in peak INa that then declined exponentially to a level that was ~85% of the initial level after 10 min. Increasing the E-field amplitude to 8 or 10 MV/m caused a twofold greater inhibitory effect on peak INa. The decrease in INa was not due to a change in either the steady-state inactivation or activation of the Na+ channel but instead was associated with a decrease in maximal Na+ conductance. Late outward current was not affected by a pulse applied at 5 MV/m. However, for a pulse applied at the higher E-field amplitudes of 8 and 10 MV/m, late outward current in some cells underwent a progressive ~22% decline over the course of the first 20 s following pulse exposure, with no further decline. The effect was most likely concentrated on ICa and IK(Ca) as IKV was not affected. The results of this study indicate that in whole-cell patch clamped adrenal chromaffin cells, a 5 ns pulse differentially inhibits specific voltage-gated ionic currents in a manner that can be manipulated by tuning E-field amplitude.

Enhanced Monitoring of Nanosecond Electric Pulse-Evoked Membrane Conductance Changes in Whole-Cell Patch Clamp Experiments

Patch clamp electrophysiology serves as a powerful method for studying changes in plasma membrane ion conductance induced by externally applied high-intensity nanosecond electric pulses (NEPs). This paper describes an enhanced monitoring technique that minimizes the length of time between pulse exposure and data recording in a patch-clamped excitable cell. Whole-cell membrane currents were continuously recorded up to 11 ms before and resumed 8 ms after delivery of a 5-ns, 6 MV/m pulse by a pair of tungsten rod electrodes to a patched adrenal chromaffin cell maintained at a holding potential of -70 mV. This timing was achieved by two sets of relay switches. One set was used to disconnect the patch pipette electrode from the pre-amplifier and connect it to a battery to maintain membrane potential at -70 mV, and also to disconnect the reference electrode from the amplifier. The other set was used to disconnect the electrodes from the pulse generator until the time of NEP/sham exposure. The sequence and timing of both sets of relays were computer-controlled. Using this procedure, we observed that a 5-ns pulse induced an instantaneous inward current that decayed exponentially over the course of several minutes, that a second pulse induced a similar response, and that the current was carried, at least in part, by Na⁺. This approach for characterizing ion conductance changes in an excitable cell in response to NEPs will yield information essential for assessing the potential use of NEP stimulation for therapeutic applications.

Metastatic SW620 colon cancer cells are primed for death when detached and can be sensitized to anoikis by the BH3-mimetic ABT-737

Anoikis, a Bax-dependent apoptosis triggered by detachment from the extracellular matrix, is often inhibited in metastatic cancer cells. Using a couple of isogenic human colon cancer cell lines derived either from the primary tumor (SW480) or from a lymph node metastasis (SW620), we found that only SW480 cells were sensitive to anoikis. Bim upregulation but not Mcl-1 degradation was determined to be a critical factor of anoikis initiation in SW480 cells. ERK-mediated phosphorylation targets Bim for ubiquitination and proteasomal degradation. A MEK inhibitor (PD0325901) was able to increase Bim expression in SW620 cells and to sensitize these cells to anoikis. Thus, in both cell lines anoikis is under the control of proteins of the Bcl-2 family. Most interestingly, the BH3-mimetic ABT-737 was found not only to increase the level of apoptosis in suspended SW480 cells but also to sensitize SW620 cells to anoikis. Accordingly, both cell lines cultured in suspension were found to be primed for death, as determined by the detection of Bcl-2:Bim and Bcl-xL:Bim complexes. In contrast, adherent SW480 and SW620 cells were resistant to ABT-737. This indicates that, whether or not they undergo anoikis, colon cancer cells that have detached from the extracellular matrix might go through a transient state, where they are sensitive to BH3 mimetics. This would confer to compounds such as Navitoclax or ABT-199 a therapeutic window where they could have anti-metastatic potential.

Inhibition of voltage-gated Na(+) current by nanosecond pulsed electric field (nsPEF) is not mediated by Na(+) influx or Ca(2+) signaling

In earlier studies, we found that permeabilization of mammalian cells with nsPEF was accompanied by prolonged inhibition of voltage-gated (VG) currents through the plasma membrane. This study explored if the inhibition of VG Na(+) current (I(Na)) resulted from (i) reduction of the transmembrane Na(+) gradient due to its influx via nsPEF-opened pores, and/or (ii) downregulation of the VG channels by a Ca(2+)-dependent mechanism. We found that a single 300 ns electric pulse at 1.6-5.3 kV/cm triggered sustained Na(+) influx in exposed NG108 cells and in primary chromaffin cells, as detected by increased fluorescence of a Sodium Green Dye. In the whole-cell patch clamp configuration, this influx was efficiently buffered by the pipette solution so that the increase in the intracellular concentration of Na(+) ([Na](i)) did not exceed 2-3 mM. [Na](i) increased uniformly over the cell volume and showed no additional peaks immediately below the plasma membrane. Concurrently, nsPEF reduced VG I(Na) by 30-60% (at 4 and 5.3 kV/cm). In control experiments, even a greater increase of the pipette [Na(+)] (by 5 mM) did not attenuate VG I(Na), thereby indicating that the nsPEF-induced Na(+) influx was not the cause of VG I(Na) inhibition. Similarly, adding 20 mM of a fast Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) into the pipette solution did not prevent or attenuate the inhibition of the VG I(Na) by nsPEF. These findings point to possible Ca(2+)-independent downregulation of the VG Na(+) channels (e.g., caused by alteration of the lipid bilayer) or the direct effect of nsPEF on the channel.

Modulation of intracellular Ca2+ levels in chromaffin cells by nanoelectropulses

Exposing chromaffin cells to a single 5 ns, 5 MV/m pulse causes Ca(2+) influx and a rapid, transient rise in intracellular calcium concentration ([Ca(2+)](i)). A comparison of responses at room temperature versus 37°C revealed no effect of temperature on the magnitude of the increase in [Ca(2+)](i). The Ca(2+) transient, however, was shortened in duration almost twofold at 37°C, indicating that the rate of recovery was temperature-sensitive. Temperature also affected the interval required for a second pulse to elicit another maximal rise in [Ca(2+)](i), which was shorter at the higher temperature. In addition, a second pulse applied 5s after the first pulse was sufficient to cause cells at room temperature to become refractory to subsequent stimulation. At 37°C, cells became refractory after 5 pulses regardless of whether pulse delivery was at low (1 and 10 Hz) or high (1 kHz) rates. When refractory, cells showed no signs of swelling or uptake of the impermeant dye YO-PRO-1. These results demonstrate that temperature plays a role in determining how chromaffin cells respond to and become refractory to nanoelectropulses. They also indicate that despite the ultra-short duration of the pulses, pronounced effects on cell excitability result from the application of only very few pulses.

Isolation and culture of primary equine tracheal epithelial cells

Culture of airway epithelial cells is a useful model to investigate physiology of airway epithelia and airway disease mechanisms. In vitro models of airway epithelial cells are established for various species. However, earlier published method for isolation and culture of equine tracheal epithelial cells requires significant improvements. In this report, the development of a procedure for efficient isolation, characterization, culture, and passage of primary equine tracheal epithelial cells are described. Epithelial cells were isolated from adult equine trachea by exposing and stripping the mucosal epithelium from the adjacent connective tissue and smooth muscle. The tissue was minced and dissociated enzymatically using 0.25% trypsin-ethylenediaminetetraacetic acid (EDTA) solution for 2 h at 37 degrees C. Cells were collected by sieving and centrifugation, and contaminating fibroblasts were removed by differential adhesion. This procedure resulted in a typical yield of 1 x 10(7) cytokeratin-positive epithelial cells per gram tracheal lining tissue. Viability was 95% by trypan blue exclusion and isolates contained approximately 94% cytokeratin-positive cells of epithelial origin. Cells seeded at a density of 6.9 x 10(4) cells/cm2 in serum-free airway epithelial cell growth medium formed monolayers near confluency within a week. Confluent cells were dissociated using dispase II and first passages (P1) and second passages (P2) were successfully established in serum-free medium. Collagen coating of tissue culture flask was not required for cell adhesion, and cultures could be maintained at the level of P2 over 30 d. In the present study, we could establish a high-yield protocol for isolation and culture of equine tracheal epithelial cells that can serve for in vitro/ex vivo studies on the (patho-)physiology of equine airway disease as well as pharmacological and toxicological targets relevant to airway diseases.