Challenges and Opportunities in Gene Delivery - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 62

The cationic bilayers are observed to be stable at a physiological pH of 7.4; but at an
acidic pH of 5-6, as in the case of endosomes, the conversion of DOPE to a nonla-
mellar hexagonal-II structure enhances the endosome membrane destabilization, thus
significantly improving the chance of DNA release into the cytoplasm and therefore
gene expression. This role of DOPE has been widely exploited in pH-sensitive lipo-
somes for drug and gene delivery [123,124] .
Another factor contributing to the release of DNA from endosomes involves the
fraction of anionic lipid present in the bilayer. The interaction of cationic lipid-DNA
complexes with negatively charged liposomes has been shown to result in the release
of DNA [122] . Based on the above studies, it is suggested that mixing the cationic
bilayers and anionic bilayers of liposomes results in significant charge interaction with
reduced cationic charge, and hence reduces the affinity for DNA binding. Once the
opposite charged lipids were mixed, cubic and inverted hexagonal phases of the lipids
were observed rather than stable liposomal bilayers [125,126] . This phenomenon was
also observed in the presence of DNA [125] . Hence, lipid interaction between cat-
ionic lipid-DNA complexes and negatively charged endosomal membranes may cause
reduced affinity of cationic lipid for DNA, thus facilitating its release from the com-
plex, whereas the resulting nonbilayer phases formed by positive-negative charged
lipid interaction may enhance the DNA access to the cytosol. The interaction of cat-
ionic lipid-DNA complex with negatively charged plasma membrane does not release
the DNA, but it releases the DNA after interaction with endosomal membrane. This
may have to do with the fact that endosomal membranes possess roughly twice the
phosphatidylserine content of plasma membranes [45,127,128] . As phosphatidylserine
is a negatively charged lipid, and fusion of cationic liposomes-DNA complex with
negatively charged lipid along with release of DNA is significantly dependent on the
fraction of anionic lipid in the bilayer [45,127,128] , a significant role of the lipid com-
position of the endosome layer has been hypothesized.
2.4.1.2 Cationic Polymers
The endosomal release of DNA from the cationic polyplexes has been hypothesized
by two mechanisms. One of the hypotheses suggests possible physical disruption of
the negatively charged endosomal membrane after direct interaction of the cationic
polymer vector. This mechanism has been proposed for polyamidoamine dendrimers,
polylysine, and polyornithine [129] . The membrane disruption was dependent on the
membrane composition and the ratio of positively charged to negatively charged lipid
present in it. The hypothesis has been boosted by the results of the preferential and
selective disruption ability of PEI to disrupt the isolated lysosomes [130] and bacte-
rial cells membrane [131] , but not red blood cells, [132] suggesting a significant role
of the composition of target membrane in controlling the extent of disruption.
The second hypothesis explains endosomal disruption by the “proton sponge”
hypothesis observed by Behr et al. and is applicable for cationic polymers with ioniz-
able amine groups [133] . It has been observed that the endosomal membranes possess
a V-ATPase proton pump that transports protons against a gradient into the endosome,
resulting in acidification of the compartment [134] . PEI contains a nitrogen atom with