Calcium Ion Flow Permeates Cells through SOCs to Promote Cathode-Directed Galvanotaxis

New Opportunities for Electric Fields in Promoting Wound Healing: Collective Electrotaxis

SIGNIFICANCE

It has long been hypothesized that naturally occurring electric fields (EFs) aid wound healing by guiding cell migration. Consequently, the application of EFs has significant potential for promoting wound healing. However, the mechanisms underlying the cellular response to EFs remain unclear. Recent Advances： Although the directed migration of isolated single cells under EFs has been studied for decades, only recently has experimental evidence demonstrated the distinct collective migration of large sheets of keratinocytes and corneal epithelial cells in response to applied EFs. Accumulating evidence suggests that the emergent properties of cell groups in response to EF guidance offer new opportunities for EF-assisted directional migration.

CRITICAL ISSUES

In this review, we provide an overview of the field of collective electrotaxis, highlighting key advances made in recent years. We also discuss advanced engineering strategies utilized to manipulate collective electrotaxis.

FUTURE DIRECTIONS

We outline a series of unanswered questions in this field and propose potential applications of collective electrotaxis in developing electrical stimulation technologies for wound healing.

The dynamics of the electrotactic reaction of mouse 3T3 fibroblasts

The molecular mechanisms behind electrotaxis remain largely unknown, with no identified primary direct current electric field (dcEF) sensor. Two leading hypotheses propose mechanisms involving the redistribution of charged components in the cell membrane (driven by electrophoresis or electroosmosis) and the asymmetric activation of ion channels. To investigate these mechanisms, we studied the dynamics of electrotactic behaviour of mouse 3T3 fibroblasts. We observed that 3T3 fibroblasts exhibit cathodal migration within just 1 min when exposed to physiological dcEF. This rapid response suggests the involvement of ion channels in the cell membrane. Our large-scale screening method identified several ion channel genes as potential key players, including the inwardly rectifying potassium channel Kir4.2. Blocking the Kir channel family with Ba2+ or silencing the Kcnj15 gene, encoding Kir4.2, significantly reduced the directional migration of 3T3 cells. Additionally, the levels of the intracellular regulators of Kir channels, spermine (SPM) and spermidine (SPD), had a significant impact on cell directionality. Interestingly, inhibiting Kir4.2 resulted in the temporary cessation of electrotaxis for approximately 1-2 h before its return. This observation suggests a two-phase mechanism for the electrotaxis of mouse 3T3 fibroblasts, where ion channel activation triggers the initial rapid response to dcEF, and the subsequent redistribution of membrane receptors sustains long-term directional movement. In summary, our study unveils the involvement of Kir channels and proposes a biphasic mechanism to explain the electrotactic behaviour of mouse 3T3 fibroblasts, shedding light on the molecular underpinnings of electrotaxis.

Bronchial Fibroblasts from Asthmatic Patients Display Impaired Responsiveness to Direct Current Electric Fields (dcEFs)

Accumulating evidence suggests that an important role is played by electric signals in modifying cell behaviour during developmental, regenerative and pathological processes. However, their role in asthma has not yet been addressed. Bronchial fibroblasts have recently been identified having important roles in asthma development. Therefore, we adapted an experimental approach based on the lineages of human bronchial fibroblasts (HBF) derived from non-asthmatic (NA) donors and asthmatic (AS) patients to elucidate whether their reactivity to direct current electric fields (dcEF) could participate in the asthmatic process. The efficient responsiveness of NA HBF to an electric field in the range of 2-4 V/cm was illustrated based on the perpendicular orientation of long axes of the cells to the field lines and their directional movement towards the anode. These responses were related to the activity of TGF-β signalling, as the electrotaxis and re-orientation of NA HBF polarity was impaired by the inhibitors of canonical and non-canonical TGF-β-dependent pathways. A similar tendency towards perpendicular cell-dcEF orientation was observed for AS HBF. However, their motility remained insensitive to the electric field applied at 2-4 V/cm. Collectively, these observations demonstrate the sensitivity of NA HBF to dcEF, as well as the inter-relations between this parameter and the canonical and non-canonical TGF-β pathways, and the differences between the electrotactic responses of NA and AS HBF point to the possible role of their dcEFs in desensitisation in the asthmatic process. This process may impair the physiologic behaviour of AS HBF functions, including cell motility, ECM deposition, and contractility, thus promoting bronchial wall remodelling, which is a characteristic of bronchial asthma.

Direct-Current Electrical Field Stimulation of Patient-Derived Colorectal Cancer Cells

Several cues for a directional migration of colorectal cancer cells were identified as being crucial in tumor progression. However, galvanotaxis, the directional migration in direct-current electrical fields, has not been investigated so far. Therefore, we asked whether direct-current electrical fields could be used to mobilize colorectal cancer cells along field vectors. For this purpose, five patient-derived low-passage cell lines were exposed to field strengths of 150-250 V/m in vitro, and migration along the field vectors was investigated. To further study the role of voltage-gated calcium channels on galvanotaxis and intracellular signaling pathways that are associated with migration of colorectal cancer cells, the cultures were exposed to selective inhibitors. In three out of five colorectal cancer cell lines, we found a preferred cathodal migration. The cellular integrity of the cells was not impaired by exposure of the cells to the selected field strengths. Galvanotaxis was sensitive to inhibition of voltage-gated calcium channels. Furthermore, signaling pathways such as AKT and MEK, but not STAT3, were also found to contribute to galvanotaxis in our in vitro model system. Overall, we identify electrical fields as an important contributor to the directional migration of colorectal cancer cells.

Galvanotactic Migration of Glioblastoma and Brain Metastases Cells

Galvanotaxis, the migration along direct current electrical fields, may contribute to the invasion of brain cancer cells in the tumor-surrounding tissue. We hypothesized that pharmacological perturbation of the epidermal growth factor (EGF) receptor and downstream phosphatidylinositol 3-kinase (PI3K)/AKT pathway prevent galvanotactic migration. In our study, patient-derived glioblastoma and brain metastases cells were exposed to direct current electrical field conditions. Velocity and direction of migration were estimated. To determine the effects of EGF receptor antagonist afatinib and AKT inhibitor capivasertib, assays of cell proliferation, apoptosis and immunoblot analyses were performed. Both inhibitors attenuated cell proliferation in a dose-dependent manner and induced apoptosis. We found that most of the glioblastoma cells migrated preferentially in an anodal direction, while brain metastases cells were unaffected by direct current stimulations. Afatinib presented only a mild attenuation of galvanotaxis. In contrast, capivasertib abolished the migration of glioblastoma cells without genetic alterations in the PI3K/AKT pathway, but not in cells harboring PTEN mutation. In these cells, an increase in the activation of ERK1/2 may in part substitute the inhibition of the AKT pathway. Overall, our data demonstrate that glioblastoma cells migrate in the electrical field and the PI3K/AKT pathway was found to be highly involved in galvanotaxis.

Electrically synchronizing and modulating the dynamics of ERK activation to regulate cell fate

Intracellular signaling dynamics play fundamental roles in cell biology. Precise modulation of the amplitude, duration, and frequency of signaling activation will be a powerful approach to investigate molecular mechanisms as well as to engineer signaling to control cell behaviors. Here, we showed a practical approach to achieve precise amplitude modulation (AM), frequency modulation (FM), and duration modulation (DM) of MAP kinase activation. Alternating current (AC) electrical stimulation induced synchronized ERK activation. Amplitude and duration of ERK activation were controlled by varying stimulation strength and duration. ERK activation frequencies were arbitrarily modulated with trains of short AC applications with accurately defined intervals. Significantly, ERK dynamics coded by well-designed AC can rewire PC12 cell fate independent of growth factors. This technique can be used to synchronize and modulate ERK activation dynamics, thus would offer a practical way to control cell behaviors in vivo without the use of biochemical agents or genetic manipulation.

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Alternating-current (AC) electric field activates ERK independently of growth factors

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AC stimulation length modulates the amplitude and duration of ERK activation

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On-off time interval of AC modulates the frequency of ERK activation peaks

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Electrical modulation of ERK dynamics promotes neuronal differentiation of PC12 cells

Vascular endothelial and smooth muscle cell galvanotactic response and differential migratory behavior

Chronic disease or injury of the vasculature impairs the functionality of vascular wall cells particularly in their ability to migrate and repair vascular surfaces. Under pathologic conditions, vascular endothelial cells (ECs) lose their non-thrombogenic properties and decrease their motility. Alternatively, vascular smooth muscle cells (SMCs) may increase motility and proliferation, leading to blood vessel luminal invasion. Current therapies to prevent subsequent blood vessel occlusion commonly mechanically injure vascular cells leading to endothelial denudation and smooth muscle cell luminal migration. Due to this dichotomous migratory behavior, a need exists for modulating vascular cell growth and migration in a more targeted manner. Here, we examine the efficacy of utilizing small direct current electric fields to influence vascular cell-specific migration ("galvanotaxis"). We designed, fabricated, and implemented an in vitro chamber for tracking vascular cell migration direction, distance, and displacement under galvanotactic influence of varying magnitude. Our results indicate that vascular ECs and SMCs have differing responses to galvanotaxis; ECs exhibit a positive correlation of anodal migration while SMCs exhibit minimal change in directional migration in relation to the electric field direction. SMCs exhibit less motility response (i.e. distance traveled in 4 h) compared to ECs, but SMCs show a significantly higher motility at low electric potentials (80 mV/cm). With further investigation and translation, galvanotaxis may be an effective solution for modulation of vascular cell-specific migration, leading to enhanced endothelialization, with coordinate reduced smooth muscle in-migration.

Voltage-gated ion channels mediate the electrotaxis of glioblastoma cells in a hybrid PMMA/PDMS microdevice

Transformed astrocytes in the most aggressive form cause glioblastoma, the most common cancer in the central nervous system with high mortality. The physiological electric field by neuronal local field potentials and tissue polarity may guide the infiltration of glioblastoma cells through the electrotaxis process. However, microenvironments with multiplex gradients are difficult to create. In this work, we have developed a hybrid microfluidic platform to study glioblastoma electrotaxis in controlled microenvironments with high throughput quantitative analysis by machine learning-powered single cell tracking software. By equalizing the hydrostatic pressure difference between inlets and outlets of the microchannel, uniform single cells can be seeded reliably inside the microdevice. The electrotaxis of two glioblastoma models, T98G and U-251MG, requires an optimal laminin-containing extracellular matrix and exhibits opposite directional and electro-alignment tendencies. Calcium signaling is a key contributor in glioblastoma pathophysiology but its role in glioblastoma electrotaxis is still an open question. Anodal T98G electrotaxis and cathodal U-251MG electrotaxis require the presence of extracellular calcium cations. U-251MG electrotaxis is dependent on the P/Q-type voltage-gated calcium channel (VGCC) and T98G is dependent on the R-type VGCC. U-251MG electrotaxis and T98G electrotaxis are also mediated by A-type (rapidly inactivating) voltage-gated potassium channels and acid-sensing sodium channels. The involvement of multiple ion channels suggests that the glioblastoma electrotaxis is complex and patient-specific ion channel expression can be critical to develop personalized therapeutics to fight against cancer metastasis. The hybrid microfluidic design and machine learning-powered single cell analysis provide a simple and flexible platform for quantitative investigation of complicated biological systems.

Electrical Circuits That Supply Constant Electric Fields In Vitro

Background: To study the molecular mechanism of galvanotropism in vitro, the strength of an electric field (EF) must be controlled precisely. In this study, I present a culture system that supplies an EF of constant strength by regulating the amount of current supplied to the culture medium.

Materials and Methods: Voltage of the medium was recorded at two fixed points along the current flow throughout the culture period. The voltage drop between the two points was maintained at the desired value by a negative feedback circuit and an operational amplifier.

Results: The field strength was defined by the voltage drop and the distance between the two points, and in this system, the EF can range from 0.0005 to 15 mV/mm.

Conclusions: This culture system may be a useful tool to determine the nature of galvanotropism.

Nanosecond pulsed current under plasma-producing conditions induces morphological alterations and stress fiber formation in human fibrosarcoma HT-1080 cells

Cold atmospheric plasma (CAP) is a promising means for various biomedical applications, including cancer therapy. Although the biological action of CAP is considered to be brought about by synergistic effects of reactive species and electrical factors of CAP, limited information is currently available on the contribution of electrical factors to CAP-induced cell responses. We have previously demonstrated that nanosecond pulsed current (nsPC) under CAP-producing conditions significantly promoted the motility of human HT-1080 cells. In this study, we explored the effects of nsPC on cell morphology associated with cell motility. We observed that nsPC stimulation caused extended cell shape, membrane protrusion formation, and increased cell surface area, but not cell death induction. nsPC stimulation also caused elevated intracellular ROS and Ca²⁺. HT-1080 cells can undergo two modes of cell motility, namely mesenchymal and ameboid motility, and we found that morphological features of mesenchymal motility was partly shared with nsPC-stimulated cells. Furthermore, nsPC-stimulated cells had extended stress fibers composed of filamentous actin. Taken together, this study provides a novel insight into the electrical aspect of CAP action, and we speculate that nsPC activates a certain mechanism involving intracellular signaling for stress fiber formation, leading to altered cell morphology and increased cell motility.

Effects of magnesium chloride on the anaerobic digestion and the implication on forward osmosis membrane bioreactor for sludge anaerobic digestion

This work elucidates the effects of model reversed salt MgCl₂ on methane production in an anaerobic digestion bioreactor treating waste sludge. Along with MgCl₂ concentration being raised stepwise, the methane production was only slightly less than in the control when MgCl₂ was 20 g/L and under, and then suddenly reduced to only about 10 mL/(L·d) at a MgCl₂ concentration of 30 g/L, and finally stopped when the MgCl₂ concentration reached 50 g/L. However, the total relative abundance of methanogens Methanomicrobia and Methanobacteria still accounted for 84.97% of the archaeal community when MgCl₂ was 50 g/L. The high correlation between live/dead cell ratio and methane production suggests that the live/dead cell ratio instead of the inhibition of methanogen might be the major cause for the halt of methane production at a magnesium chloride concentration of 50 g/L.

Effect of pharmacological modulation of actin and myosin on collective cell electrotaxis

Electrotaxis-the directional migration of cells in response to an electric field-is most evident in multicellular collectives and plays an important role in physiological contexts. While most cell types respond to applied electric fields of the order of a Volt per centimeter, our knowledge of the factors influencing this response is limited. This is especially true for collective cell electrotaxis, in which the subcellular migration response within a cell has to be coordinated with coupled neighboring cells. Here, we investigated the effect of the level of actin cytoskeleton polymerization and myosin activity on collective cell electrotaxis of Madin-Darby Canine Kidney (MDCK) cells in response to a weak electric field of physiologically relevant magnitude. We modulated the polymerization state of the actin cytoskeleton using the depolymerizing agent cytochalasin D or the polymerizing agent jasplakinolide. We also modulated the contractility of the cell using the myosin motor inhibitor blebbistatin or the phosphatase inhibitor calyculin A. While all the above pharmacological treatments altered cell speed to various extents, we found that only increasing the contractility and a high level of increase/stabilization of polymerized actin had a strong inhibitory effect specifically on the directedness of collective cell electrotaxis. On the other hand, even as the effect of the actin modulators on collective cell migration was varied, most conditions of actin and myosin pharmacological modulation-except for high level of actin polymerization/stabilization-resulted in cell speeds that were similar in the absence or presence of the electric field. Our results led us to speculate that the applied electric field may largely impact the cellular apparatus specifying the polarity of collective cell migration, rather than the functioning of the migratory apparatus. Bioelectromagnetics. 39:289-298, 2018. © 2018 Wiley Periodicals, Inc.

Unraveling the mechanistic effects of electric field stimulation towards directing stem cell fate and function: A tissue engineering perspective

Electric field (EF) stimulation can play a vital role in eliciting appropriate stem cell response. Such an approach is recently being established to guide stem cell differentiation through osteogenesis/neurogenesis/cardiomyogenesis. Despite significant recent efforts, the biophysical mechanisms by which stem cells sense, interpret and transform electrical cues into biochemical and biological signals still remain unclear. The present review critically analyses the variety of EF stimulation approaches that can be employed to evoke appropriate stem cell response and also makes an attempt to summarize the underlying concepts of this notion, placing special emphasis on stem cell based tissue engineering and regenerative medicine. This review also discusses the major signaling pathways and cellular responses that are elicited by electric stimulation, including the participation of reactive oxygen species and heat shock proteins, modulation of intracellular calcium ion concentration, ATP production and numerous other events involving the clustering or reassembling of cell surface receptors, cytoskeletal remodeling and so on. The specific advantages of using external electric stimulation in different modalities to regulate stem cell fate processes are highlighted with explicit examples, in vitro and in vivo.

Studying Electrotaxis in Microfluidic Devices

Collective cell migration is important in various physiological processes such as morphogenesis, cancer metastasis and cell regeneration. Such migration can be induced and guided by different chemical and physical cues. Electrotaxis, referring to the directional migration of adherent cells under stimulus of electric fields, is believed to be highly involved in the wound-healing process. Electrotactic experiments are conventionally conducted in Petri dishes or cover glasses wherein cells are cultured and electric fields are applied. However, these devices suffer from evaporation of the culture medium, non-uniformity of electric fields and low throughput. To overcome these drawbacks, micro-fabricated devices composed of micro-channels and fluidic components have lately been applied to electrotactic studies. Microfluidic devices are capable of providing cells with a precise micro-environment including pH, nutrition, temperature and various stimuli. Therefore, with the advantages of reduced cell/reagent consumption, reduced Joule heating and uniform and precise electric fields, microfluidic chips are perfect platforms for observing cell migration under applied electric fields. In this paper, I review recent developments in designing and fabricating microfluidic devices for studying electrotaxis, aiming to provide critical updates in this rapidly-growing, interdisciplinary field.