Role of endocytosis in the transfection of L929 fibroblasts by polyethylenimine/DNA complexes.

Polyethylenimine (PEI) is one of the most efficient nonviral vectors for gene therapy. The aim of this study was to investigate the role of endocytosis in the transfection of synchronized L929 fibroblasts by PEI/DNA complexes. This was performed by confocal microscopy and flow cytometry, using the endocytosis marker FM4-64 and PEI/DNA complexes labeled either with the DNA intercalator YOYO-1, or with fluorescein covalently linked to PEI. Endocytosis appeared as the major if not the sole mode of entry of the PEI/DNA complexes into the L929 cells. The complexes followed a typical fluid phase endocytosis pathway and were efficiently taken up in less than 10 min in endosomes that did not exceed 200 nm in diameter. Later, the localization of the complexes became perinuclear and fusion between late endosomes was shown to occur. Comparison with the intracellular trafficking of the same complexes in EA.hy 926 cells (W.T. Godbey, K. Wu, A.G. Mikos, Proc. Natl. Acad. Sci. USA 96 (1999)) revealed that endocytosis of PEI/DNA complexes is strongly cell-dependent. In L929 cells, escape of the complexes from the endosomes is a major barrier for transfection. This limited the number of transfected cells to a few percent, even though an internalization of PEI/DNA complexes was observed in most cells. In addition, the entry of the complexes into the nucleus apparently required a mitosis and did not involve the lipids of the endosome membrane. This entry seems to be a short-lived event that involves only a few complexes.

Gene transfer by cationic surfactants is essentially limited by the trapping of the surfactant/DNA complexes onto the cell membrane: a fluorescence investigation.

The interaction between complexes of plasmid DNA with cetyltrimethylammonium bromide (CTAB) and L929 fibroblasts was first examined using confocal microscopy. The complexes labeled with the DNA intercalator, YOYO-1, were found to be trapped onto the external face of the plasma membrane; a feature that may constitute a major limiting step in transfection. Moreover, since no cytotoxic effect appeared in these conditions, we further inferred that the CTAB molecules remained bound to the DNA. The interaction of the complexes with the membranes was best modeled with neutral vesicles. From anisotropy thermotropic curves of DPHpPC-labeled vesicles and fluorescence resonance energy transfer measurements between these vesicles and YOYO-labeled complexes, we evidenced that the binding of the complexes to the vesicle surface opened the micelle-like domains and unwound DNA. However, DNA was not released but remained stably bound via electrostatic interactions to the CTAB molecules incorporated in the external liposome leaflet. Consequently, the large diameter of the unwound plasmid DNA is likely the major factor that precludes its internalization into the cells by endocytosis. In contrast, anionic vesicles that mimic the cytoplasmic facing monolayer of the plasma membrane rapidly released DNA from the complex. This may explain the previously reported high transfection efficiency of DNA complexed with liposomes composed of neutral lipids and cationic surfactants, since the latter may destabilize the endosomal membrane and induce the release of DNA in the cytoplasm.