Protamine is an FDA-approved compound with a documented safety profile that facilitates efficient plasmid condensation for gene delivery by various types of cationic liposomes. It also improves adenoviral vector-mediated gene transfer as a transfection accelerator. However, there is no consensus as to the mechanism of protamine on gene delivery into cells. To analyze the uptake and subcellular distribution, plasmid and protamine were labeled with FITC and Texas-Red, respectively. Although the uptake of FITC-labeled plasmid/protamine complexes into the cells was the same as that of free FITC-labeled plasmid in HeLa, SOJ and A549 cells, they improved the transfection efficiency by several orders of magnitude. Moreover, we found that protamine derived from different sources (salmon, herring and trout sperm) had different transfection efficiencies; however, the gene transfer efficiency with protamine was lower than with optimized poly(L-lysine) and DEAE-Dextran. There were likely two main reasons: firstly, the uptake of plasmid mediated by protamine was complete within the first 10 min because the particle size increased as time passed, and secondly, the plasmid/protamine complexes were not released from endosomal membrane. These results indicate that as a transfection accelerator from an appropriate protamine source, with controlled particle size and facile release from endosomes would lead to successful gene delivery with protamine.
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