A wide variety of lipid molecules used as gene carriers has been reported and compared over the last twenty years. This review highlights a few examples of mechanistic analysis applied to the study of lipid carriers. The modular nature of the lipid structure offers itself up to a controlled, systematic analysis. Key to exploring structural variants is the understanding of the role each component and module plays in the formation of the lipoplex structure itself and their roles in the transfection pathway. Firstly, the lipid carrier must be able to package, and release into the cell its nucleic acid cargo. Uptake of lipoplexes into cells involves endocytic processes that lead inevitably to endosomal/liposomal degradation of the nucleic acid contents, unless lipid structures and designs are optimised to facilitate their release into the cytoplasm. Testing of possible endosomal escape mechanisms has led to improved lipid designs. For example, it was predicted that mixing of cationic lipids with shorter alkyl tails ( < C18) from liposomes with the endosomal bilayer during the transfection process within the endosome should promote membrane fluidity, enhancing plasmid release and transfection efficiency. This hypothesis has been borne out numerous times. "Smart" molecules that respond to cellular cues such as endosomal pH have now been developed that appear to offer exciting opportunities for the future role of lipoplexes in clinical applications of gene therapy. The variability of cultured cell and model biological systems remains a challenge in designing improved lipids of general utility.
Keywords: liposomes, lipoplex, gene therapy, endocytosis, lipid bilayer, smart molecules, triggered release
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