Alkyl chain moieties of polyamidoamine dendron-bearing lipids influence their function as a nonviral gene vector

We recently developed a novel family of cationic lipids consisting of a polyamidoamine (PAMAM) dendron and two dodecyl chains. Their transfection activity increases with increasing generation of the dendron moiety [Takahashi et al. (2003) Bioconjugate Chem. 14, 764-773]. In the present study, to elucidate the effect of hydrophobic tail moieties of the dendron-bearing lipids, two kinds of PAMAM G3 dendron-bearing lipids were synthesized with different alkyl lengths, DL-G3-2C18 and DL-G3-2C12. Their functions as gene vectors were compared. Irrespective of their different alkyl chain lengths, these dendron-bearing lipids formed complexes with plasmid DNA with similar efficiency. However, their complex sizes differed markedly: DL-G3-2C18 lipoplexes exhibited much smaller diameters than DL-G3-2C12 lipoplexes. Interaction of the lipoplexes with heparin revealed that the DL-G3-2C18 lipoplexes required more heparin than DL-G3-2C12 lipoplexes to cause dissociation of plasmid DNA from the lipoplexes. Although the DL-G3-2C12 lipoplexes and DL-G3-2C18 lipoplexes transfected CV1 cells with similar efficiency in the absence of serum, only the latter retained high transfection activity in the presence of serum. These results indicate that hydrophobic interaction of alkyl chain moieties plays an important role in the increment of stability and the serum-resistant transfection activity for dendron-bearing lipid lipoplexes.     

Synthesis of novel cationic lipids having polyamidoamine dendrons and their transfection activity

We designed a novel type of cationic lipid, lipids with a cationic polar group in the polyamidoamine dendron, because these dendron-bearing lipids are expected to form complexes with plasmid DNA and achieve efficient transfection of cells by synergy of endosome buffering and membrane fusion with the endosome, both of which are useful for the promotion of the transfer of plasmid DNA from endosome to cytosol. Four kinds of lipids with polyamidoamine dendrons of first to fourth generations, DL-G1, DL-G2, DL-G3, and DL-G4, were synthesized. The lipid with a dendron of a higher generation exhibited greater ability to form lipoplexes with plasmid DNA, as estimated by agarose gel electrophoresis. While the DL-G1 lipoplex did not transfect CV1 cells, the lipoplexes containing the DL-G2, DL-G3, or DL-G4 could induce transfection of the cells, and their activity was elevated with increasing generation of the dendron. Addition of dioleoylphosphatidylethanolamine (DOPE), which is known to increase fusion ability of a lipid membrane, into the lipoplexes greatly enhanced their transfection activity. In addition, the comparison with DC-Chol-containing lipoplex, which is widely used as a nonviral vector, showed that the DL-G3-DOPE lipoplex exhibits more efficient transfections. These findings imply that these dendron-bearing lipids may form the basis for a novel family of cationic lipids for efficient gene delivery.     

Polyamidoamine dendron-bearing lipids as a nonviral vector: influence of dendron generation

Recently, we demonstrated that octadecyl chains are important as alkyl chain moieties of polyamidoamine (PAMAM) dendron-bearing lipids for their serum-resistant transfection activity [Bioconjugate Chem.2007, 18, 1349-1354]. Toward production of highly potent vectors, we examined the influence of the generation of dendron moiety on transfection activity of PAMAM dendron-bearing lipids having two octadecyl chains. We synthesized dendron-bearing lipids with PAMAM G1, G2, and G3 dendrons, designated respectively as DL-G1-2C(18), DL-G2-2C(18), and DL-G3-2C(18). The DL-G2-2C(18) and DL-G3-2C(18) interacted with plasmid DNA effectively and formed stable lipoplexes with small sizes and spherical shape. However, DL-G1-2C(18) interacted with plasmid DNA less effectively and formed tubular-shaped lipoplexes with lower stability and larger size. Cells took up DL-G2-2C(18) and DL-G3-2C(18) lipoplexes efficiently, but cellular uptake of the DL-G1-2C(18) lipoplexes was less efficient. Nevertheless, DL-G1-2C(18) lipoplexes achieved 100-10?000 times higher levels of transgene expression, which was evaluated using luciferase gene as a reporter gene. Confocal scanning laser microscopic analysis of intracellular behaviors of the lipoplexes revealed that DL-G1-2C(18) lipoplexes generated free plasmid DNA molecules in the cytosol more effectively than other lipoplexes did. Moderate binding ability of DL-G1-2C(18) might be responsible for generation of lipoplexes which deliver plasmid DNA into cells, liberate it in the cytoplasm, and induce efficient transgene expression.     

Synthesis, in vitro transfection activity and physicochemical characterization of novel N,N'-diacyl-1,2-diaminopropyl-3-carbamoyl-(dimethylaminoethane) amphiphilic derivatives

A novel series of N,N'-diacyl-1,2-diaminopropyl-3-carbamoyl-(dimethylaminoethane) cationic derivatives was synthesized and screened for in vitro transfection activity at different charge ratios in the presence and absence of the helper lipids DOPE and cholesterol. Physicochemical properties of lipid-DNA complexes were studied by gel electrophoresis, fluorescence spectroscopy and dynamic light scattering. The interfacial properties of the lipids in isolation were studied using the Langmuir film balance technique at 23 degrees C. It was found that only lipoplexes formulated with the dioleoyl derivative, 1,2lmt[5], mediated significant in vitro transfection activity. Optimum activity was obtained with 1,2lmt[5]/DOPE mixture at a +/-charge ratio of 2. In agreement with the transfection results, 1,2lmt[5] was the only lipid found to complex and retard DNA migration as verified by gel electrophoresis. Despite the efficient complexation, no significant condensation of plasmid DNA was observed as indicated by fluorescence spectroscopy measurements. Monolayer studies showed that the dioleoyl derivative 1,2lmt[5] was the only lipid that existed in an all liquid-expanded state with a collapse area and collapse pressure of 59.5 A2 and 38.7 mN/m, respectively. This lipid was also found to have the highest elasticity with a compressibility modulus at monolayer collapse of 80.4 mN/m. In conclusion, increased acyl chain fluidity and high molecular elasticity of cationic lipids were found to correlate with improved transfection activity.     

In vitro lipofection with novel asymmetric series of 1,2-dialkoylamidopropane-based cytofectins containing single symmetric bis-(2-dimethylaminoethane) polar headgroups

Novel N,N'-diacyl-1,2-diaminopropyl-3-carbamoyl[bis-(2-dimethylaminoethane)] bivalent cationic lipids were synthesized and evaluated for in vitro transfection activity against a murine melanoma cell line. In the absence of the helper lipid DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine), only the dioleoyl derivative 22 (1,2lb5) elicited transfection activity. The transfection activity of this lipid was reduced when formulated with DOPE. Contrary to that, the dimyristoyl derivative 19 (1,2lb2) mediated no activity when used alone but induced the highest levels of marker gene expression in the presence of DOPE. In an effort to correlate the transfection activity with cationic lipid structures, the physicochemical properties of cationic lipids in isolation and of lipoplexes were studied with surface tensiometry, photon correlation spectroscopy, gel electrophoresis mobility shift assay, and fluorescence techniques. In regard to the lipoplex properties, gel electrophoresis mobility shift assay and EtBr exclusion fluorescence assay revealed that the 1,2lb5 was the only lipid to associate and condense plasmid DNA, respectively. Photon correlation spectroscopy analysis found that 1,2lb5/DNA complexes were of relatively small size compared to all other lipoplexes. With respect to the properties of isolated lipids, Langmuir monolayer studies and fluorescence anisotropy on cationic lipid dispersions verified high two-plane elasticity and increased fluidity of the transfection competent dioleoyl derivative 1,2lb5, respectively. The results indicate that high transfection activity is mediated by cationic lipids characterized by an expanded mean molecular area, high molecular elasticity, and increased fluidity.     

Physicochemical characterization and purification of cationic lipoplexes.

Cationic lipid-nucleic acid complexes (lipoplexes) consisting of dioleoyltrimethylammoniumpropane (DOTAP) liposomes and plasmid DNA were prepared at various charge ratios (cationic group to nucleotide phosphate), and the excess component was separated from the lipoplex. We measured the stoichiometry of the lipoplex, noted its colloidal properties, and observed its morphology and structure by electron microscopy. The colloidal properties of the lipoplexes were principally determined by the cationic lipid/DNA charge ratio and were independent of the lipid composition. In lipoplexes, the lipid membranes as observed in freeze-fracture electron microscopy were deformed into high-radius-of-curvature features whose characteristics depended on the lipid composition. Lipoplexes prepared at a threefold or greater excess of either DOTAP or DNA could be resolved into complexes with a defined stoichiometry and the excess component by sedimentation to equilibrium on sucrose gradients. The separated, positively charged complex retained high transfection activity and had reduced toxicity. The negatively charged lipoplex showed increased transfection activity compared to the starting mixture. In cryoelectron micrographs the positively charged complex was spherical and contained a condensed but indistinct interior structure. In contrast, the separated negatively charged lipoplexes had a prominent internal 5.9 +/- 0.1-nm periodic feature with material projecting as spikes from the spherical structure into the solution. It is likely that these two lipoplexes represent structures with different lipid and DNA packing.     

In vitro lipofection with novel series of symmetric 1,3-dialkoylamidopropane-based cationic surfactants containing single primary and tertiary amine polar head groups

A novel series of symmetric double-chained primary and tertiary 1,3-dialkoylamido monovalent cationic lipids were synthesized and evaluated for their transfection activities. In the absence of the helper lipid DOPE (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine), only the primary and tertiary dioleoyl derivatives 1,3lmp5 and 1,3lmt5, respectively elicited transfection activity. This is a striking difference between symmetrical 1,2-diacyl glycerol-based monovalent cationic lipids that always found both dioleoyl and dimyristoyl analogues being efficient transfection reagents. In the presence of helper lipid, all cationic derivatives induced marker gene expression, except the dilauroyl analogues 1,3lmp1 and 1,3lmt1 that elicited no transfection activity. Combining electrophoretic mobility data of the lipoplexes at different charge ratios with transfection activity suggested two requirements for high transfection activity with monovalent double-chained cationic lipids, that is, binding/association of the lipid to the plasmid DNA and membrane fusion properties of the lipid layers surrounding the DNA.     

Molecular shape of the cationic lipid controls the structure of cationic lipid/dioleylphosphatidylethanolamine-DNA complexes and the efficiency of gene delivery

Pyridinium amphiphiles, abbreviated as SAINT, are highly efficient vectors for delivery of DNA into cells. Within a group of structurally related compounds that differ in transfection capacity, we have investigated the role of the shape and structure of the pyridinium molecule on the stability of bilayers formed from a given SAINT and dioleoylphosphatidylethanolamine (DOPE) and on the polymorphism of SAINT/DOPE-DNA complexes. Using electron microscopy and small angle x-ray scattering, a relationship was established between the structure, stability, and morphology of the lipoplexes and their transfection efficiency. The structure with the lowest ratio of the cross-sectional area occupied by polar over hydrophobic domains (SAINT-2) formed the most unstable bilayers when mixed with DOPE and tended to convert into the hexagonal structure. In SAINT-2-containing lipoplexes, a hexagonal topology was apparent, provided that DOPE was present and complex assembly occurred in 150 mm NaCl. If not, a lamellar phase was obtained, as for lipoplexes prepared from geometrically more balanced SAINT structures. The hexagonal topology strongly promotes transfection efficiency, whereas a strongly reduced activity is seen for complexes displaying the lamellar topology. We conclude that in the DOPE-containing complexes the molecular shape and the nonbilayer preferences of the cationic lipid control the topology of the lipoplex and thereby the transfection efficiency.     

In vitro cationic lipid-mediated gene delivery with fluorinated glycerophosphoethanolamine helper lipids.

There is a need for the development of nonviral gene transfer systems with improved and original properties. "Fluorinated" lipoplexes are such candidates, as supported by the remarkably higher in vitro and in vivo transfection potency found for such fluorinated lipoplexes as compared with conventional ones or even with PEI-based polyplexes (Boussif, O., Gaucheron, J., Boulanger, C., Santaella, C., Kolbe, H. V. J., Vierling, P. (2001) Enhanced in vitro and in vivo cationic lipid-mediated gene delivery with a fluorinated glycerophosphoethanolamine helper lipid. J. Gene Med. 3, 109-114). Here, we describe the synthesis of fluorinated glycerophosphoethanolamines (F-PEs), close analogues of dioleoylphosphatidylethanolamine (DOPE), and report on their lipid helper properties vs that of DOPE, as in vitro gene transfer components of fluorinated lipoplexes based on pcTG90, DOGS (Transfectam), or DOTAP. To evaluate the contribution of the F-PEs to in vitro lipoplex-mediated gene transfer, we examined the effect of including the F-PEs in lipoplexes formulated with these cationic lipids (CL) for various CL:DOPE:F-PE molar ratios [1:(1 - x):x with x = 0, 0.5 and 1; 1:(2 - y):y with y = 0, 1, 1.5, and 2], and various N/P ratios (from 10 to 0.8, N = number of CL amines, P = number of DNA phosphates). Irrespective of the F-PE chemical structure, of the colipid F-PE:DOPE composition, and of the N/P ratio, comparable transfection levels to those of their respective control DOPE lipoplexes were most frequently obtained when using one of the F-PEs as colipid of DOGS, pcTG90, or DOTAP in place of part of or of all DOPE. However, a large proportion of DOGS-based lipoplexes were found to display a higher transfection efficiency when formulated with the F-PEs rather than with DOPE alone while the opposite tendency was evidenced for the DOTAP-based lipoplexes. The present work indicates that "fluorinated" lipoplexes formulated with fluorinated helper lipids and conventional cationic lipids are very attractive candidates for gene delivery. It confirms further that lipophobicity and restricted miscibility of the lipoplex lipids with the endogenous lipids does not preclude efficient gene transfer and expression. Their transfection potency is rather attributable to their unique lipophobic and hydrophobic character (resulting from the formulation of DNA with fluorinated lipids), thus preventing to some extent DNA from interactions with lipophilic and hydrophilic biocompounds, and from degradation.     

Liposome Vector Containing Biosurfactant-Complexed DNA as Herpes Simplex Virus Thymidine Kinase Gene Delivery System

For injectable-sized liposome complexed with DNA (lipoplexes) with high transfection efficiency of genes, we initially prepared small-sized liposomes by addition of biosurfactant. For selectivity of gene expression, the thymidine kinase (MK-tk) gene controlled by midkine was used for herpes simplex virus thymidine kinase (HSV-tk) gene therapy. Liposomes composed of 3([N-(N′,N′–dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol), L-dioleoylphosphatidylethanolamine (DOPE), and a biosurfactant, such as β-sitosterol β-D-glucoside (Sit-G) for Sit-G-liposomes and mannosylerythrytol lipid A (MEL) for MEL-liposomes, produced about 300-nm-sized lipoplexes. Sit-G- and MEL-liposomes showed higher transfection efficiency of the luciferase marker gene and thymidine kinase activity in the presence of serum in the cells. The treatment with transfection of MK-tk gene by Sit-G-liposome and injection of ganciclovir significantly reduced tumor growth in a solid tumor model, compared with that by Sit-G-liposome alone. This finding suggested that Sit-G-liposome is a potential vector for HSV-tk gene therapy.     

Highly fluorinated lipospermines for gene transfer: synthesis and evaluation of their in vitro transfection efficiency

Fluorinated double-chain lipospermines (one or both of these chains being ended by a highly fluorinated tail of various length) which are close analogues of DOGS (Transfectam) were designed as synthetic vectors for gene delivery. For N/P ratios (N = number of amine functions of the lipid; P = number of DNA phosphates) from 0.8 to 10, these lipospermines condensed DNA, with or without the use of DOPE, to form fluorinated lipoplexes. The efficiency of the fluorinated lipoplexes to transfect lung epithelial A549 cells was significantly higher than that of the DOGS lipoplexes. No specific cell toxicity was evidenced for the fluorinated lipoplexes as compared to that of the DOGS ones. The palette of structural elements explored allowed to determine those required for efficient transfection, highlighting the importance of highly fluorinated chains, the unique properties of unsaturated double-chain lipids and of the use of DOPE as helper lipid on transfection.     

Hydration of lipoplexes commonly used in gene delivery: follow-up by laurdan fluorescence changes and quantification by differential scanning calorimetry.

Lipoplexes, which are formed spontaneously between cationic liposomes and negatively charged nucleic acids, are commonly used for gene and oligonucleotide delivery in vitro and in vivo. Being assemblies, lipoplexes can be characterized by various physicochemical parameters, including size distribution, shape, physical state (lamellar, hexagonal type II and/or other phases), sign and magnitude of electrical surface potential, and level of hydration at the lipid-DNA interface. Only after all these variables will be characterized for lipoplexes with a broad spectrum of lipid compositions and DNA/cationic lipid (L(+)) mole (or charge) ratios can their relevance to transfection efficiency be understood. Of all these physicochemical parameters, hydration is the most neglected, and therefore the focus of this study. Cationic liposomes composed of DOTAP without and with helper lipids (DOPC, DOPE, or cholesterol) or of DC-Chol/DOPE were complexed with pDNA (S16 human growth hormone) at various DNA(-)/L(+) charge ratios (0.1-3.2). (DOTAP=N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride; DC-Chol=(3beta-[N-(N',N'-dimethylaminoethane)-carbamoyl]-cholester ol; DOPC=1, 2-dioleoyl-sn-glycero-3-phosphocholine; DOPE=1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine). The hydration levels of the different cationic liposomes and the DNA separately are compared with the hydration levels of the lipoplexes. Two independent approaches were applied to study hydration. First, we used a semi-quantitative approach of determining changes in the 'generalized polarization' (GP) of laurdan (6-dodecanoyl-2-dimethylaminonaphthalene). This method was recently used extensively and successfully to characterize changes of hydration at lipid-water interfaces. Laurdan excitation GP at 340 nm (GP(340)DOTAP. The GP(340) of lipoplexes of all lipid compositions (except those based on DC-Chol/DOPE) was higher than the GP(340) of the cationic liposomes alone and increased with increasing DNA(-)/L(+) charge ratio, reaching a plateau at a charge ratio of 1. 0, suggesting an increase in dehydration at the lipid-water interface with increasing DNA(-)/L(+) charge ratio. Confirmation was obtained from the second method, differential scanning calorimetry (DSC). DOTAP/DOPE lipoplexes with charge ratio 0.44 had 16.5% dehydration and with charge ratio 1.5, 46.4% dehydration. For DOTAP/Chol lipoplexes with these charge ratios, there was 17.9% and 49% dehydration, respectively. These data are in good agreement with the laurdan data described above. They suggest that the dehydration occurs during lipoplex formation and that this is a prerequisite for the intimate contact between cationic lipids and DNA.     

Mechanistic evaluation of the transfection barriers involved in lipid-mediated gene delivery: Interplay between nanostructure and composition

Here we present a quantitative mechanism-based investigation aimed at comparing the cell uptake, intracellular trafficking, endosomal escape and final fate of lipoplexes and lipid–protamine/deoxyribonucleic acid (DNA) (LPD) nanoparticles (NPs) in living Chinese hamster ovary (CHO) cells. As a model, two lipid formulations were used for comparison. The first formulation is made of the cationic lipid 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the zwitterionic lipid dioleoylphosphocholine (DOPC), while the second mixture is made of the cationic 3β-[N-(N,N-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol) and the zwitterionic helper lipid dioleoylphosphatidylethanolamine (DOPE). Our findings indicate that lipoplexes are efficiently taken up through fluid-phase macropinocytosis, while a less efficient uptake of LPD NPs occurs through a combination of both macropinocytosis and clathrin-dependent pathways. Inside the cell, both lipoplexes and LPD NPs are actively transported towards the cell nucleus, as quantitatively addressed by spatio-temporal image correlation spectroscopy (STICS). For each lipid formulation, LPD NPs escape from endosomes more efficiently than lipoplexes. When cells were treated with DOTAP–DOPC-containing systems the majority of the DNA was trapped in the lysosome compartment, suggesting that extensive lysosomal degradation was the rate-limiting factors in DOTAP–DOPC-mediated transfection. On the other side, escape from endosomes is large for DC-Chol–DOPE-containing systems most likely due to DOPE and cholesterol-like molecules, which are able to destabilize the endosomal membrane. The lipid-dependent and structure-dependent enhancement of transfection activity suggests that DNA is delivered to the nucleus synergistically: the process requires both the membrane-fusogenic activity of the nanocarrier envelope and the employment of lipid species with intrinsic endosomal rupture ability.     

The use of fluorescence resonance energy transfer to monitor dynamic changes of lipid-DNA interactions during lipoplex formation

Fluorescence resonance energy transfer (FRET) was used to monitor interactions between Cy3-labeled plasmid DNA and NBD-labeled cationic liposomes. FRET data show that binding of cationic liposomes to DNA occurs immediately upon mixing (within 1 min), but FRET efficiencies do not stabilize for 1-5 h. The time allowed for complex formation has effects on in vitro luciferase transfection efficiencies of DOPE-based lipoplexes; i.e., lipoplexes prepared with a 1-h incubation have much higher transfection efficiencies than samples with 1-min or 5-h incubations. The molar charge ratio of DOTAP to negatively charged phosphates in the DNA (DOTAP+/DNA-) also affected the interaction between liposomes and plasmid DNA, and interactions stabilized more rapidly at higher charge ratios. Lipoplexes formulated with DOPE were more resistant to high ionic strength than complexes formulated with cholesterol. Taken together, our data demonstrate that lipid-DNA interactions and in vitro transfection efficiencies are strongly affected by the time allowed for complex formation. This effect is especially evident in DOPE-based lipoplexes, and suggests that the time allowed for lipoplex formation is a parameter that should be carefully controlled in future studies.     

Evaluation of lipid-based reagents to mediate intracellular gene delivery

We characterized different cationic lipid-based gene delivery systems consisting of both liposomes and nonliposomal structures, in terms of their in vitro transfection activity, resistance to the presence of serum, protective effect against nuclease degradation and stability under different storage conditions. The effect of lipid/DNA charge ratio of the resulting complexes on these properties was also evaluated. Our results indicate that the highest levels of transfection activity were observed for complexes prepared from nonliposomal structures composed of FuGENE 6. However, their DNA protective effect was shown to be lower than that observed for cationic liposome formulations when prepared at the optimal (+/-) charge ratio. Our results suggest that lipoplexes are resistant to serum up to 30% when prepared at a 2:1 lipid/DNA charge ratio. However, when they were prepared at higher (+/-) charge ratios, they become sensitive to serum for even lower concentrations (10%). Replacement of dioleoyl-phosphatidylethanolamine (DOPE) by cholesterol enhanced the resistance of the complexes to the inhibitory effect of serum. This different biological activity in the presence of serum was attributed to different extents of binding of serum proteins to the complexes, as evaluated by the immunoblotting assay. Studies on the stability under storage show that lipoplexes maintain most of their biological activity when stored at -80 degrees C, following their fast freezing in liquid nitrogen.     

Physicochemical characterization of anionic lipid-based ternary siRNA complexes

Physicochemical characterization is a useful tool in understanding lipoplex assemblies and their correlation to biological activity. Anionic lipid-based ternary siRNA complexes composed of anionic liposomes (DOPG/DOPE), calcium ions and siRNA, have recently been shown to be safe and efficient in a breast cancer cell culture model. In the present work, the effects of various formulation parameters such as liposome composition (DOPG/DOPE ratio) and anionic lipid/Ca(2+)/siRNA molar charge ratio, on the physicochemical attributes (particle size, surface charge, siRNA loading efficiency and serum stability) of these ternary anionic lipoplexes were evaluated. Particle size, siRNA loading efficiency and serum stability correlated with the in vitro silencing efficiency of these lipoplexes. For example, large lipoplex particles (5/2.5/1 anionic lipid/Ca(2+)/siRNA molar charge ratio) showed less efficient silencing while absolute serum stability and high siRNA loading (1.3/2.5/1 anionic lipid/Ca(2+)/siRNA molar charge ratio), exhibited maximum silencing in breast cancer cells. The physicochemical properties also indicated that the siRNA exists in the complexed and/or encapsulated form within the lipoplexes, depending on the anionic lipid/siRNA charge ratio. Based on these studies a model representing lipid-siRNA association within the anionic lipoplexes prepared under various formulation conditions is proposed. Physicochemical attributes can be utilized to estimate in vitro activity of lipid-siRNA complexes and understand their morphology.     

The use of fluorescence resonance energy transfer to monitor dynamic changes of lipid-DNA interactions during lipoplex formation

Fluorescence resonance energy transfer (FRET) was used to monitor interactions between Cy3-labeled plasmid DNA and NBD-labeled cationic liposomes. FRET data show that binding of cationic liposomes to DNA occurs immediately upon mixing (within 1 min), but FRET efficiencies do not stabilize for 1-5 h. The time allowed for complex formation has effects on in vitro luciferase transfection efficiencies of DOPE-based lipoplexes; i.e., lipoplexes prepared with a 1-h incubation have much higher transfection efficiencies than samples with 1-min or 5-h incubations. The molar charge ratio of DOTAP to negatively charged phosphates in the DNA (DOTAP⁺/DNA⁻) also affected the interaction between liposomes and plasmid DNA, and interactions stabilized more rapidly at higher charge ratios. Lipoplexes formulated with DOPE were more resistant to high ionic strength than complexes formulated with cholesterol. Taken together, our data demonstrate that lipid-DNA interactions and in vitro transfection efficiencies are strongly affected by the time allowed for complex formation. This effect is especially evident in DOPE-based lipoplexes, and suggests that the time allowed for lipoplex formation is a parameter that should be carefully controlled in future studies.     

DODAB:monoolein-based lipoplexes as non-viral vectors for transfection of mammalian cells

DNA/Cationic liposome complexes (lipoplexes) have been widely used as non-viral vectors for transfection. Neutral lipids in liposomal formulation are determinant for transfection efficiency using these vectors. In this work, we studied the potential of monoolein (MO) as helper lipid for cellular transfection. Lipoplexes composed of pDNA and dioctadecyldimethylammonium bromide (DODAB)/1-monooleoyl-rac-glycerol (MO) at different molar ratios (4:1, 2:1 and 1:1) and at different cationic lipid/DNA ratios were investigated. The physicochemical properties of the lipoplexes (size, charge and structure), were studied by Dynamic Light Scattering (DLS), Zeta Potential (ζ) and cryo-transmission electron microscopy (cryo-TEM). The effect of MO on pDNA condensation and the effect of heparin and heparan sulphate on the percentage of pDNA release from the lipoplexes were also studied by Ethidium Bromide (EtBr) exclusion assays and electrophoresis. Cytotoxicity and transfection efficiency of these lipoplexes were evaluated using 293T cells and compared with the golden standard helper lipids 1,2-dioleoyl-sn-glycero-3-hosphoethanolamine (DOPE) and cholesterol (Chol) as well as with a commercial transfection agent (Lipofectamine? LTX). The internalization of transfected fluorescently-labeled pDNA was also visualized using the same cell line. The results demonstrate that the presence of MO not only increases pDNA compactation efficiency, but also affects the physicochemical properties of the lipoplexes, which can interfere with lipoplex-cell interactions. The DODAB:MO formulations tested showed little toxicity and successfully mediated in vitro cell transfection. These results were supported by fluorescence microscopy studies, which illustrated that lipoplexes were able to access the cytosol and deliver pDNA to the nucleus. DODAB:MO-based lipoplexes were thus validated as non-toxic, efficient lipofection vectors for genetic modification of mammalian cells. Understanding the relation between structure and activity of MO-based lipoplexes will further strengthen the development of these novel delivery systems.     

Lipid mixing between lipoplexes and plasma lipoproteins is a major barrier for intravenous transfection mediated by cationic lipids

It has been previously shown that transfection activity of cationic liposome/DNA lipoplexes delivered systemically is drastically inhibited by lipoproteins (Tandia, B. M., Vandenbranden, M., Wattiez, R., Lakhdar, Z., Ruysschaert, J. M., and Elouahabi, A. (2003) Mol Ther. 8, 264-273). In this work, we have compared the binding/uptake and transfection activities of DOTAP (N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium chloride) and diC14-amidine (3-tetradecylamino-N-tert-butyl-N'-tetra-decylpropionamidine)-containing lipoplexes in the presence or absence of purified low density lipoproteins and high density lipoprotein. Binding/uptake of both lipoplexes by the mouse lung endothelial cell line was inhibited to a similar extent in the presence of lipoproteins. In contrast, transfection activity of diC14-amidine-containing lipoplexes was almost completely inhibited (approximately by 95%), whereas approximately 40% transfection activity of DOTAP-containing lipoplexes was preserved in the presence of lipoproteins. Interestingly, the ability of lipoproteins to inhibit the transfection efficiency of lipoplexes was well correlated with their ability to undergo lipid mixing with the cationic lipid bilayer as revealed by fluorescence resonance energy transfer assay. Incubation of lipoplexes with increased doses of lipoproteins resulted in enhanced lipid mixing and reduced transfection activity of the lipoplexes in mouse lung endothelial cells. The role of lipid mixing in transfection was further demonstrated using lipid-mixing inhibitor, lyso-phosphatidylcholine, or activator (dioleoylphosphatidylethanolamine). Incorporation of Lyso-PC into diC14-amidine-containing lipoplexes completely abolished their capacity to undergo lipid mixing with lipoproteins and allowed them to reach a high transfection efficiency in the presence of lipoproteins. On the other hand, the incorporation of dioleoylphosphatidylethanolamine into DOTAP/DNA lipoplex activated lipid mixing with the lipoproteins and was shown to be detrimental toward the transfection activity of these lipoplexes. Taken together, these results indicate that fusion of lipoplexes with lipoproteins is a limiting factor for in vivo transfection.     

The role of lipid charge density in the serum stability of cationic lipid/DNA complexes

To evaluate the role of lipid charge density in the serum stability of DOTAP-Chol/DNA complexes (lipoplexes), lipid-DNA interactions, extent of aggregation, supercoil content, and in vitro transfection efficiency of lipoplexes were investigated. In general, higher serum concentration destabilized, and increasing molar charge ratio of DOTAP to negatively charged phosphates in the DNA (DOTAP(+)/DNA(-)) stabilized lipoplexes in serum as assessed by the criteria used in this study. The increase of cholesterol content led to increased serum stability, and DOTAP:Chol (mol/mol 1:4)/DNA lipoplex with DOTAP(+)/DNA(-) ratio 4 was the most serum stable formulation of all the formulations examined, and maintained lipid-DNA interactions, did not aggregate and exhibited high in vitro transfection efficiency in 50% (v/v) serum. The increased stability of this formulation could not be explained by the decreased charge density of the lipid component. Furthermore, no single parameter examined in the study could be used to consistently predict the in vitro transfection efficiency of lipoplexes in serum. Surprisingly, no correlation between the maintenance of supercoiled DNA content and in vitro transfection efficiency was found in the study.