Electropermeabilization of Mammalian Cells Visualized with Fluorescent Semiconductor Nanocrystals (Quantum Dots)
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Electropermeabilization of Mammalian Cells Visualized with Fluorescent Semiconductor Nanocrystals (Quantum Dots) Yinghua Sun1, P. Thomas Vernier2,4, Jingjing Wang3, Andras Kuthi2, Laura Marcu3,5, Martin A. Gundersen2,1 1 Department of Materials Science; 2 Department of Electrical Engineering-Electrophysics; 3 Department of Biomedical Engineering, University of Southern California, Los Angeles, CA 90089, U.S.A 4 MOSIS, Information Sciences Institute, University of Southern California, Marina del Rey, CA 90292, U.S.A 5 Biophotonics Research and Technology Development, Cedars-Sinai Medical Center Los Angeles, CA 90048, U.S.A ABSTRACT Electroporation/electropermeabilization is a non-viral technique for gene transfection and drug delivery. Here, the transfer mechanisms were studied with fluorescent nanocrystals (quantum dots, QDs) in mammalian cells. Interactions of the cell membrane and nanoscale particles were visualized after electric pulse treatment. Responses of human multiple myeloma cells to nanocrystals were tracked for periods up to 7 days. Large particles do not cross the membrane directly after pulsing, even if the membrane is permeabilized to small molecules. Large QDs were trapped on the cell membrane for hours after electroporation and were gradually either excluded or internalized by cells. QD uptake efficiency depended on both particle size and membrane transport activity. These results, consistent with an electropermeabilization model, suggest that enhancing the interactions between the cell membrane and macromolecules may improve the transfer efficiency. INTRODUCTION Electroporation/electropermeabilization has been intensively studied for gene transfection and drug delivery, but transfection mechanisms are still unclear and the improvement of the efficiency of delivery of macromolecules to mammalian cells remains a challenge [1,2]. Pulsed electric fields can induce reversible membrane breakdown and make cells permeable to certain molecules without lethal consequences. Studies on membrane permeability have been conducted with fluorescent probes such as propidium iodide (PI) and trypan blue [2], and with green fluorescence protein (GFP) plasmids [3]. However, most organic dyes are not good for long term cell tracking because of their high toxicity and low photostability. And although GFP plasmids provide an effective endpoint for transfection efficiency, interactions between the cell membrane and the plasmid cannot be observed. Quantum dots, as fluorescent nanocrystals, are suitable for both cell permeability observations and long-term tracking because of their excellent photostability, high brightness, nanometer (macromolecular) dimensions, and low toxicity to cells and organisms [4,5,6]. They have been used recently for studies on cell mobility and cancer diagnosis [7,8,9]. In this paper, QDs were used to study the mechanisms of electroporation/ electropermeabilization. The transfer of small molecules such as PI and YO-PRO-1 into cells after electrical pulse treatment is rapid and efficient, but
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