Design of fusogenic liposomes using a poly(ethylene glycol) derivative having amino groups

As a novel fusogenic liposome, we designed liposomes modified with poly(glycidol) having beta-alanine residues, which is a poly(ethylene glycol) derivative with positively charged groups. The polymer-modified liposomes of egg yolk phosphatidylcholine (EYPC) and dioleoylphosphatidylethanolamine (DOPE) were prepared by reverse phase evaporation. Fusion of the polymer-modified liposomes with anionic liposomes consisting of phosphatidic acid and DOPE was investigated. Fusion ability of the polymer-modified liposomes increased with increasing amount of the polymer fixed on the liposome. Also, inclusion of DOPE was necessary for the generation of the fusion ability of the polymer-modified liposomes. CV1 cells treated with the polymer-modified DOPE/EYPC liposomes containing calcein displayed diffuse fluorescence, suggesting that calcein was introduced into the cytoplasm. In contrast, only punctual fluorescence was observed in the cells treated with the polymer-modified EYPC liposomes containing calcein, indicating that calcein remained in the endosome and/or lysosome. In addition, COS1 cells were transfected efficiently by treatment with the polymer-modified EYPC/DOPE liposomes containing pSV2cat plasmid, whereas the transfection was not induced by treatment with the polymer-modified EYPC liposomes. Close correlation between fusion ability of the polymer-modified liposomes and their ability to deliver their contents to the cytoplasm implies that membrane fusion plays an important role in the liposome-mediated cytoplasmic delivery.