Intracellular Trafficking Of Lipoplex

Fig. 2. Internalization pathway and fate of cationic liposome-DNA complexes. (1) Binding of cationic lipoplexes to cell surface by charge interaction. (II) Transport from plasma membrane to endosome. (III) Lysosome becomes endosome by maturation and a release of lipoplex to cytosol. (IV) DNA moves to exterior of the cell. (V) DNA moves toward nucleus. (VI) Transcription of DNA to mRNA. (VII) Transcripted mRNA exported to cytoplasm. (VIII) Translation of mRNA to protein.

Fig. 2. Internalization pathway and fate of cationic liposome-DNA complexes. (1) Binding of cationic lipoplexes to cell surface by charge interaction. (II) Transport from plasma membrane to endosome. (III) Lysosome becomes endosome by maturation and a release of lipoplex to cytosol. (IV) DNA moves to exterior of the cell. (V) DNA moves toward nucleus. (VI) Transcription of DNA to mRNA. (VII) Transcripted mRNA exported to cytoplasm. (VIII) Translation of mRNA to protein.

lipoplex (41). Sakuri et al. formed a complex with different ratio of plasmid DNA (labeled with fluorescein isothiocyanate [FITC]) to cationic liposomes. At higher charge ratios, a higher intensity of the green fluorescence was observed in the endo-somes/lysosomes, indicating more lipoplexes had moved into these compartments. Thus, the efficiency of gene transfer is highly dependent on the charge ratio between the cationic liposome and DNA.

However, the drawback of a high charge ratio is, that it results in an increased serum sensitivity. Serum proteins cause aggregation of the positively charged lipoplex, as well as a decrease of the positive charge of the complex and its interaction with the cell membrane. Thus, serum induced aggregation compromises gene transfection efficency (42,43). Another concern is that cationic liposome formulations interact nonspecifically with the majority of negatively charged glycoproteins on the cell surface. To resolve this issue, a lipoplex formulation has been developed using anionic liposomes (44). However, the lipoplex gene transfection activity observed was lower than with cationic liposomes. It was proposed that the reason for this less effective transfection resulted from the degradation of anionic lipoplex formulations in the lysosomes, whereas cationic liposomes can bypass this endosomal-lysosomal route and escape early degradation in these compartments.

To reduce this lysosomal-endosomal degradation of transgenes, Yang and Huang developed a pH-sensitive anionic liposome formulation. This pH-sensitive liposome was stable in physiological pH, but became destabilized in an acidic environment. Typically, these liposomes were prepared at pH 8.0. Under the acidic environment in the endosomes (~ pH 5.0), the pH-sensitive liposomes were protonated and destabilized, resulting in the release of DNA. A subsequent study hasshown that a pH-sensitive liposome formulation perform DNA release prior to the endosomal-lysosomal degradation, actually, suggesting that the pH-sensitive liposome could also bypasse the endosome-lysosome route (45).

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