MODE OF FORMATION AND STRUCTURAL FEATURES OF DNA CATIONIC LIPOSOME COMPLEXES USED FOR TRANSFECTION

Complexes formed between cationic liposomes and nucleic acids represen t a highly efficient vehicle for delivery of DNA and RNA molecules int o a large variety of eukaryotic cells. By using fluorescence, gel elec trophoresis, and metal-shadowing electron microscopy techniques, the f actors that affect the, yet unclear, interactions between DNA and cati onic liposomes as well as the structural features of the resulting com plexes have been elucidated. A model is suggested according to which c ationic liposomes bind initially to DNA molecules to form clusters of aggregated vesicles along the nucleic acids. At a critical liposome de nsity, two processes occur, namely, DNA-induced membrane fusion, indic ated by lipid mixing studies, and liposome-induced DNA collapse, point ed out by the marked cooperativity of the encapsulation processes, by their modulations by DNA-condensing agents, and also by their conspicu ous independence upon DNA length. The DNA collapse leads to the format ion of condensed structures which can be completely encapsulated withi n the fused lipid bilayers in a fast, highly cooperative process since their exposed surface is substantially smaller than that of extended DNA molecules. The formation of the transfecting DNA-liposome complexe s in which the nucleic acids are fully encapsulated within a positivel y-charged lipid bilayer is proposed, consequently, to be dominated by mutual effects exerted by the DNA and the cationic liposomes, leading to interrelated lipid fusion and DNA collapse.