Electrostatically mediated interactions between cationic lipid-DNA particles and an anionic surface

Phosphatidylethanolamine mediated destabilization of lipid-based pDNA delivery systems

We have previously reported the development of lipid-DNA particles (LDPs) formed, via a hydrophobic cationic lipid-DNA complex intermediate, when detergent-solubilized cationic lipids are mixed with DNA. This study investigates the influence of zwitterionic co-lipid headgroups on the formation and stability of this intermediate and the subsequent DNA protection and transfection properties afforded by the resultant LDPs. We report that inclusion of diacylphosphatidylethanolamines (diacylPE), but not diacylphosphatidylcholines (diacylPC), as co-lipids destabilizes and prevents the formation of the cationic lipid-DNA intermediate to an extent dependent on the concentration of diacylPE and its acyl chain characteristics. DNA formulated in LDPs containing cationic:zwitterionic lipids at a 1:1 ratio is not readily accessible to the intercalating fluorescent dye, TO-PRO-1. At a lipid ratio 1:4, diacylPC LDPs are associated with significantly greater TO-PRO-1 fluorescence than equivalent diacylPE formulations, a result believed to reflect lipid-dependent penetration of TO-PRO-1 through the supramolecular LDP assembly, rather than condensation and protection of the DNA per se. Transfection studies utilizing the in vitro murine B16/BL6 melanoma cell line and the in vivo intraperitoneal B16/BL6 mouse tumor model demonstrated that only diacylPE LDPs mediated gene transfer. This was found not to be a consequence of differences in DNA delivery or cell toxicity.

The potential role of proteoglycans in cationic lipid-mediated gene delivery. Studies of the interaction of cationic lipid-DNA complexes with model glycosaminoglycans

Recent evidence supports a role for proteoglycans in polycation-mediated gene delivery. Therefore, the interaction of glycosaminoglycans with cationic lipid-DNA complexes (CLDCs) has been characterized using a combination of biophysical approaches. At low ionic strength, CLDCs bind to heparin-derivatized Sepharose particles, with the ratio of cationic lipid to DNA controlling the binding. Incorporation of the helper lipids cholesterol or 1,2-dioleoyl-phosphatidylethanolamine increases the amount of bound CLDC. Heparin also induces the aggregation of CLDCs, with cholesterol reducing this effect. Isothermal titration calorimetry demonstrates an endothermic heat for the binding of heparin to CLDCs at low ionic strength, whereas circular dichroism studies suggest a heparin-stimulated release of DNA from CLDCs at a greater than 20-fold charge excess. Increasing the ionic strength to 0.11 reduces CLDC binding to heparin beads, and greatly enhances the release of DNA from CLDCs by heparin. The ability of the cell surface glycosaminoglycan heparan sulfate to release DNA from CLDCs is more sensitive than heparin to the incorporation of the cholesterol or 1,2-dioleoyl-phosphatidylethanolamine. Titration calorimetry reveals an exothermic heat for the interaction glycosaminoglycans with CLDCs at higher ionic strength. These results are consistent with the direct involvement of proteoglycans in transfection.

Oxazole yellow homodimer YOYO-1-labeled DNA: a fluorescent complex that can be used to assess structural changes in DNA following formation and cellular delivery of cationic lipid DNA complexes

To improve transfection efficiency following delivery of plasmid expression vectors using lipid-based carriers, it is crucial to define structural characteristics of the lipid/DNA complexes that optimize transgene expression. Due to its strong affinity for DNA and high quantum yield, the fluorescent DNA intercalator YOYO-1 was used as a tool to assess changes in DNA that occur following lipid binding and cell delivery. In this study, the stability of the dye/DNA complex following binding of poly-L-lysine or monocationic lipids is characterized. More than 98% of the fluorescence measured for a defined DNA/YOYO-1 complex was lost when DNA was condensed using poly-L-lysine. This loss in fluorescence could be attributed to displacement of bound dye. In contrast, more than 30% of the fluorescence of the dye-labeled DNA was retained after formation of cationic lipid/DNA complexes. Significantly, the results illustrate differences in structural changes cationic lipids and PLL exert on plasmid DNA. The fluorescent lipid/DNA complex was used to assess DNA delivery to murine B16/BL6 cells in vitro. An assay relying on fluorescence resonance energy transfer between bound YOYO-1 and propidium iodide was used to distinguish between DNA attached to the cell surface and internalized DNA.

Use of poly(ethylene glycol)-lipid conjugates to regulate the surface attributes and transfection activity of lipid-DNA particles

We evaluated the use of poly(ethylene glycol) (PEG)-modified lipids to control the surface properties of a lipid-based gene transfer system. The lipid-DNA particles (LDPs) used form spontaneously when plasmid DNA is added to mixed detergent lipid micelles consisting of the non-ionic detergent n-octyl-D-glucopyranoside, the cationic lipid dioleyldimethylammonium chloride (DODAC), the zwitterionic lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and selected PEG-modified phosphatidylethanolamines. The inclusion of DODAC is required to form the hydrophobic lipid-DNA complex. DOPE is included to facilitate dissociation of DNA from the cationic lipid and the PEG-modified lipids are added in an effort to stabilize the surface attributes of the resulting lipid-DNA particles. We used PEG-lipids that varied in acyl chain composition because of recent results demonstrating acyl chain dependent transfer of PEG-lipids from lipid vesicles, providing the potential to allow a transformation of the surface properties due to loss of surface grafted PEG. The addition of PEG-modified lipids does not interfere in LDP formation and its presence favors formation of smaller particles (75 nm in contrast to 130 nm in the absence of the PEG-modified lipid). PEG-lipid incorporation causes a concentration dependent reduction in LDP-mediated transfection of B16/BL6 melanoma cells, a result that can be partially attributed to a reduction in particle binding to cells. However, significant LDP binding to B16/BL6 cells was still observed under conditions where LDP transfection activity was reduced by more than 85%. The potential for PEG to interfere with LDP processing following cell binding is discussed.