Dielectrophoresis study of temporal change in internal conductivity of single CHO cells after electroporation by pulsed electric fields

Applying sufficiently strong pulsed electric fields to a cell can permeabilize the membrane and subsequently affect its dielectric properties. In this study, we employ a microfluidic dielectrophoresis cytometry technique to simultaneously electroporate and measure the time-dependent dielectric response of single Chinese hamster ovary cells. Using experimental measurements along with numerical simulations, we present quantitative results for the changes in the cytoplasm conductivity of single cells within seconds after exposure to 100 μs duration pulsed electric fields with various intensities. It is shown that, for electroporation in a medium with conductivity lower than that of the cell's cytoplasm, the internal conductivity of the cell decreases after the electroporation on a time scale of seconds and stronger pulses cause a larger and more rapid decrease. We also observe that, after the electroporation, the cell's internal conductivity is constrained to a threshold. This implies that the cell prevents some of the ions in its cytoplasm from diffusing through the created pores to the external medium. The temporal change in the dielectric response of each individual cell is continuously monitored over minutes after exposure to pulsed electric fields. A time constant associated with the cell's internal conductivity change is observed, which ranges from seconds to tens of seconds depending on the applied pulse intensity. This experimental observation supports the results of numerical models reported in the literature.

Dielectric model for Chinese hamster ovary cells obtained by dielectrophoresis cytometry

We present a dielectric model and its parameters for Chinese hamster ovary (CHO) cells based on a double-shell structure which includes the cell membrane, cytoplasm, nuclear envelope, and nucleoplasm. Employing a dielectrophoresis (DEP) based technique and a microfluidic system, the DEP response of many single CHO cells is measured and the spectrum of the Clausius-Mossotti factor is obtained. The dielectric parameters of the model are then extracted by curve-fitting to the measured spectral data. Using this approach over the 0.6-10 MHz frequency range, we report the values for CHO cells' membrane permittivity, membrane thickness, cytoplasm conductivity, nuclear envelope permittivity, and nucleoplasm conductivity. The size of the cell and its nuclei are obtained using optical techniques.

Monitoring the dielectric response of single cells following mitochondrial adenosine triphosphate synthase inhibition by oligomycin using a dielectrophoretic cytometer

One of the main uses of adenosine triphosphate (ATP) within mammalian cells is powering the Na(+)/K(+) ATPase pumps used to maintain ion concentrations within the cell. Since ion concentrations determine the cytoplasm conductivity, ATP concentration is expected to play a key role in controlling the cytoplasm conductivity. The two major ATP production pathways within cells are via glycolysis within the cytoplasm and via the electron transport chain within the mitochondria. In this work, a differential detector combined with dielectrophoretic (DEP) translation in a microfluidic channel was employed to observe single cell changes in the cytoplasm conductivity. The DEP response was made sensitive to changes in cytoplasm conductivity by measuring DEP response versus media conductivity and using double shell models to choose appropriate frequencies and media conductivity. Dielectric response of Chinese hamster ovary (CHO) cells was monitored following inhibition of the mitochondria ATP production by treatment with oligomycin. We show that in CHO cells following exposure to oligomycin (8 μg/ml) the cytoplasm conductivity drops, with the majority of the change occurring within 50 min. This work demonstrates that dielectric effects due to changes in ATP production can be observed at the single cell level.

Chloroacetaldehyde: mode of antitumor action of the ifosfamide metabolite

BACKGROUND

The ifosfamide metabolite chloroacetaldehyde had been made responsible for side effects only. We found in previous studies a strong cytotoxicity on human MX-1 tumor cells and xenografts in nude mice. Chloroacetaldehyde is supposed to act via alkylation or by inhibition of mitochondrial oxidative phosphorylation with decrease of ATP. The aim of this study was to further elucidate chloroacetaldehyde's mode of action.

METHODS

MX-1 breast carcinoma cells were measured for ATP-content after exposure to chloroacetaldehyde. Further, the effect of chloroacetaldehyde on DNA-synthesis and its potency of causing strand-breaks or cross-links were investigated by bromodeoxyuridine-incorporation, comet-assay and a DNA interstrand cross-linking-assay.

RESULTS

Chloroacetaldehyde in high concentrations induces a reduction of ATP-levels when anaerobic glycolysis is blocked by oxamate and reduces the bromodeoxyuridine-incorporation to 46.3% after 4 h when used in IC(50) concentrations (7.49 mumol/l). In addition we observed DNA single strand-breaks in MX-1 cells treated with chloroacetaldehyde visible in the Comet assay, but no DNA-cross-linking by comet assay and cross-linking assay.

CONCLUSION

In summary, our results show that chloroacetaldehyde influences the oxidative phosphorylation in mitochondria, however, this is observed only in high concentrations and is not of clinical relevance because the tumor cells regenerate ATP by anaerobic glycolysis. Nevertheless, chloroacetaldehyde causes DNA-strand-breaks and strong inhibition of DNA-synthesis.