Formation of supramolecular structures by negatively charged liposomes in the presence of nucleic acids and divalent cations

Abstract

Cationic liposomes are being increasingly studied as delivery vehicles for bioactive agents such as DNA and other polynucleotides. The mechanism of interaction of DNA with liposomes and the organization of these interacting structures during and after the interaction are still poorly understood. Nucleic acids are known to induce aggregation and size enlargement of liposomes. In the case of phosphatidylcholine (PC) vesicles, these processes depend on the presence and concentration of divalent metal cations and the amount of cholesterol in the liposomes. In this study, anionic small unilamellar vesicles (SUV) and multilamellar vesicles (MLV) composed of dicetylphosphate (DCP):PC:cholesterol at 2:7:1 molar ratios were prepared and incubated with the DNA (from wheat) and Ca2+ (50 mM) at 25°C with the aim of transferring the genetic material into the liposomes by inducing fusion of liposome-liposome aggregates created in the presence, and with the help, of DNA. The organization and the nature of the resultant liposome-DNA-Ca2+ complexes were investigated by scanning tunneling microscopy (STM) and fluorescence microscopy. Observations of complexes with similar appearances with both SUV and MLV, as shown by two quite different microscopic approaches, prove that the resultant forms are real and not artifacts of the methodology used. At this stage it is not clear whether the detected complexes represent an intermediate state before fusion of liposomes which will lead to engulfing of the genomic material by the fused liposomes, or the final form. In either case the structures consisting of some adhered or semifused liposomes bearing the nucleic acid seem to be candidates as vehicles for in-vitro and in-vivo transfection.