Abstract
BackgroundApplications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment. A unified understanding of the underlying mechanisms of membrane electropermeabilization, however, has not been achieved. Protocols are empirical, and models are descriptive rather than predictive, which hampers the optimization and expansion of electroporation-based technologies. A common feature of existing models is the assumption that the permeabilized membrane is passive, and that transport through it is entirely diffusive. To demonstrate the necessity to go beyond that assumption, we present here a quantitative analysis of the post-permeabilization transport of three small molecules commonly used in electroporation research — YO-PRO-1, propidium, and calcein — after exposure of cells to minimally perturbing, 6 ns electric pulses.ResultsInflux of YO-PRO-1 from the external medium into the cell exceeds that of propidium, consistent with many published studies. Both are much greater than the influx of calcein. In contrast, the normalized molar efflux of calcein from pre-loaded cells into the medium after electropermeabilization is roughly equivalent to the influx of YO-PRO-1 and propidium. These relative transport rates are correlated not with molecular size or cross-section, but rather with molecular charge polarity.ConclusionsThis comparison of the kinetics of molecular transport of three small, charged molecules across electropermeabilized cell membranes reveals a component of the mechanism of electroporation that is customarily taken into account only for the time during electric pulse delivery. The large differences between the influx rates of propidium and YO-PRO-1 (cations) and calcein (anion), and between the influx and efflux of calcein, suggest a significant role for the post-pulse transmembrane potential in the migration of ions and charged small molecules across permeabilized cell membranes, which has been largely neglected in models of electroporation.
Highlights
Applications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment
Previous studies reporting quantitative measurements of electroporative transport have in general looked at only one small-molecule species, typically propidium, calcein, or more recently YO-PRO-1, so there has been no direct comparison of the electroporative transport of cationic, nucleic acid-binding dyes with anionic, natively fluorescent dyes
Our results show that the influx of YO-PRO-1 and propidium into electropermeabilized cells is an order of magnitude greater than that of calcein after the same pulse exposure
Summary
Applications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment. To demonstrate the necessity to go beyond that assumption, we present here a quantitative analysis of the post-permeabilization transport of three small molecules commonly used in electroporation research — YO-PRO-1, propidium, and calcein — after exposure of cells to minimally perturbing, 6 ns electric pulses. Experimental studies and models of electroporative small molecule transport rarely mention, the large quantitative differences between the influx of propidium (and YO-PRO-1) and that of calcein into cells after similar pulsed electric field exposures, and (with one exception [18])) the large difference between calcein influx from the medium into a permeabilized cell and calcein efflux from a pre-loaded cell that is permeabilized. The (mostly overlooked) clue in the literature that there is something very different about the transport of these two classes of compounds is that protocols call for much higher concentrations of calcein (or Lucifer yellow) than for propidium (or YO-PRO-1) [11, 21]
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