Structural and fusogenic properties of cationic liposomes in the presence of plasmid DNA

Enhanced intercellular retention activity of novel pH-sensitive polymeric micelles in wild and multidrug resistant MCF-7 cells

PURPOSE

The purpose of this work was to demonstrate the advantage of using pH-sensitive polymeric mixed micelles (PHSM) composed of poly(L: -histidine) (polyHis)/poly(ethylene glycol) (PEG) and poly(L: -lactic acid) (pLLA)/PEG block copolymers with folate conjugation to increase drug retention in wild-type and MDR tumor cells.

MATERIALS AND METHODS

Both wild-type and multidrug resistant (MDR) human breast adenocarcinoma (MCF-7) cell lines were used to investigate the accumulation and elimination of doxorubicin (DOX), PHSM with folate (PHSM/f), and pH-insensitive micelles composed of pLLA/PEG block copolymer with folate (PHIM/f).

RESULTS

Cells treated with PHSM/f showed decelerated elimination kinetics compared to cells treated with PHIM/f. MDR cells treated with drug-containing PHSM/f for 30 min retained 80% of doxorubicin (DOX) even after incubation for 24 h in the absence of drug. On the other hand, cells treated with drug-containing PHIM/f retained only 40% of DOX within the same period of time. Flow cytometry and confocal microscopy confirmed these results.

CONCLUSIONS

Cellular entry of the micelles occurred via receptor-mediated endocytosis using folate receptors. The pH-induced destabilization of PHSM/f led to rapid distribution of drug and polymer throughout the cells, most likely due to polyHis-mediated endosomal disruption. This reduced the likelihood of drug efflux via exocytosis from resistant tumor cells.

Conductometric evidence for intact polyion-induced liposome clusters

In this note, we present a set of electrical conductivity measurements of polyion-induced liposome aggregate aqueous suspensions that supports evidence for the existence of a cluster phase in low-density colloidal systems. Heavily NaCl-loaded liposomes, dispersed in a low-conductivity aqueous solution, are forced by electrostatic interactions with oppositely charged polyions to build up into individual aggregates, where the single vesicles maintain their integrity and, upon an external force, are able to release their ionic content. The conductivity data, within the effective medium approximation theory for heterogeneous systems, are in agreement with the picture of a suspension built up by clusters of vesicles which are able to preserve their content from the external medium. This finding opens new possibilities in multicompartment drug delivery techniques.

Enhancement of transfection activity of lipoplexes by complexation with transferrin-bearing fusogenic polymer-modified liposomes

We previously developed complexes of lipoplexes containing 3beta-(N-(N',N'-dimethylaminoethane)carbamoyl)cholesterol (DC-chol) and succinylated poly(glycidol)-modified liposome, which becomes fusogenic under weakly acidic condition, for use as a novel gene delivery system. This study explored the effect of lipoplex structures--the type of cationic lipid and cationic lipid/DNA charge ratio--on the transfection activity of those complexes. Three types of cationic lipid with different polar groups were used for the preparation of lipoplexes: DC-chol, N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methylsulfate (DOTAP), and 3,5-dipentadecyloxybenzamidine (TRX-20) with dimethylamino group, trimethylammonium group, and benzamidine group, respectively. Complexation with the SucPG-modified transferrin-bearing liposomes affected transfection activity of these lipoplexes differently. The TRX-20 lipoplexes exhibited the most marked enhancement of transfection activity upon complexation with the SucPG-modified liposomes among these lipoplexes. The cationic lipid/DNA charge ratio of the lipoplex and the amount of the transferrin-bearing SucPG-modified liposomes associated to the lipoplex also affected the transfection activity of the resultant complexes. Highly potent gene vectors were obtained by adjusting these factors.

Gene delivery by cationic lipid vectors: overcoming cellular barriers

Non-viral vectors such as cationic lipids are capable of delivering nucleic acids, including genes, siRNA or antisense RNA into cells, thus potentially resulting in their functional expression. These vectors are considered as an attractive alternative for virus-based delivery systems, which may suffer from immunological and mutational hazards. However, the efficiency of cationic-mediated gene delivery, although often sufficient for cell biological purposes, runs seriously short from a therapeutics point of view, as realizing this objective requires a higher level of transfection than attained thus far. To develop strategies for improvement, there is not so much a need for novel delivery systems. Rather, better insight is needed into the mechanism of delivery, including lipoplex-cell surface interaction, route of internalization and concomitant escape of DNA/RNA into the cytosol, and transport into the nucleus. Current work indicates that a major obstacle involves the relative inefficient destabilization of membrane-bounded compartments in which lipoplexes reside after their internalization by the cell. Such an activity requires the capacity of lipoplexes of undergoing polymorphic transitions such as a membrane destabilizing hexagonal phase, while cellular components may aid in this process. A consequence of the latter notion is that for development of a novel generation of delivery devices, entry pathways have to be triggered by specific targeting to select delivery into intracellular compartments which are most susceptible to lipoplex-induced destabilization, thereby allowing the most efficient release of DNA, a minimal requirement for optimizing non-viral vector-mediated transfection.

Cationic liposomes for gene delivery

Cationic liposome-DNA complexes (lipoplexes) constitute a potentially viable alternative to viral vectors for the delivery of therapeutic genes. This review will focus on various parameters governing lipoplex biological activity, from their mode of formation to in vivo behaviour. Particular emphasis is given to the mechanism of interaction of lipoplexes with cells, in an attempt to dissect the different barriers that need to be surpassed for efficient gene expression to occur. Aspects related to new trends in the formulation of lipid-based gene delivery systems aiming at overcoming some of their limitations will be covered. Finally, examples illustrating the potential of cationic liposomes in clinical applications will be provided.

Fusion and endocytosis of anionic liposomes with Ehrlich ascitic carcinoma cells

Polyethylene glycol 300 and low pH of the incubation medium initiate fusion of anionic lipid liposome membranes with Ehrlich ascitic carcinoma cell membranes. Some liposomes are endocytosed in cells and are distributed near the inner surface of the membrane. Liposome membranes spontaneously fuse with the cells under certain conditions.

Specific lipoplex-mediated antisense against Bcl-2 in breast cancer cells: a comparison between different formulations

G3139 is an antisense oligonucleotide (ODN) that can down-regulate bcl-2, thus potentially acting as a potent anticancer drug. However, effective therapy requires efficient ODN delivery, which may be achieved by employing G3139 lipoplexes. Yet, lipofection is a complex, multifactorial process that is still poorly understood. In order to shed more light on this issue, we prepared 18 different G3139 lipoplex formulations and compared them in terms of their capability to transfect MCF-7 breast cancer cells. Each formulation was composed of a cationic lipid and sometimes a helper lipid. The cationic lipid was either DOTAP (N-(1-(2,3-dioleoyloxy)propyl)-N,N,N-trimethylammonium chloride), DC-CHOL (3ss[N-(N',N'-dimethylaminoethane)carbamoyl]-cholesterol), or CCS (ceramide carbomoyl spermine). The helper lipid was either DOPC, DOPE, or cholesterol. Each lipid combination existed in two different structural forms--either large unilamellar vesicles (approximately 100 nm LUV) or unsized heterolamellar vesicles (UHV). Cell proliferation assays were used to evaluate the cytotoxicity of G3139 lipoplexes, control cationic lipid assemblies, and free G3139. Western blots were used to confirm the specific activity of G3139 as an anti-bcl-2 antisense agent. We determined that treatment of MCF-7 cells with G3139:CCS lipoplexes (UHV-derived) produced a maximal 50-fold improvement in antisense efficacy compared to treatment with free G3139. The other G3139 lipoplexes were not superior to free G3139. Thus, successful lipofection requires precise optimization of lipoplex lipid composition, structure, and concentration.

Physico-chemical characterization and transfection efficacy of cationic liposomes containing the pEGFP plasmid

Cationic liposomes-DNA complexes (lipoplexes) are largely used in gene delivery. Deciphering specific chemical and physical properties of lipoplexes is a necessary step to unravel the mechanisms underlying transfection and to improve transfection efficacy in each experimental model. In the present paper we investigated the physico-chemical features of lipoplexes containing a plasmid encoding for the GFP protein, in order to correlate these results with transfection efficacy. Cationic unilamellar vesicles (mean diameter 100 nm) were prepared, from the cationic DC-Chol lipid and the zwitterionic phospholipid DOPE. The two components of the liposome bilayer were used at molar ratio close to unity. ESR spectra were recorded and zeta potential zeta was measured on liposomes complexed with the plasmid. One of the main points of interest in this paper resided in the fact that both kinds of measurements were carried out in the same conditions (i.e. lipid concentration, medium composition, and pH) employed for cell transfection experiments. Transfection was performed on CHO cells; the percentage of fluorescent cells was evaluated and compared with the above physico-chemical features. It emerged that the composition and pH of the medium, the lipoplex/cell ratio, as well as the amount of lipoplex added to the cell culture were critical parameters for transfection efficacy. Finally, lipoplex surface charge played a fundamental role to achieve a high transfection level.

Intracellular trafficking of nonviral vectors

Nonviral vectors continue to be attractive alternatives to viruses due to their low toxicity and immunogenicity, lack of pathogenicity, and ease of pharmacologic production. However, nonviral vectors also continue to suffer from relatively low levels of gene transfer compared to viruses, thus the drive to improve these vectors continues. Many studies on vector-cell interactions have reported that nonviral vectors bind and enter cells efficiently, but yield low gene expression, thus directing our attention to the intracellular trafficking of these vectors to understand where the obstacles occur. Here, we will review nonviral vector trafficking pathways, which will be considered here as the steps from cell binding to nuclear delivery. Studies on the intracellular trafficking of nonviral vectors has given us valuable insights into the barriers these vectors must overcome to mediate efficient gene transfer. Importantly, we will highlight the different approaches used by researchers to overcome certain trafficking barriers to gene transfer, many of which incorporate components from biological systems that have naturally evolved the capacity to overcome such obstacles. The tools used to study trafficking pathways will also be discussed.

Tf-lipoplex-mediated NGF gene transfer to the CNS: neuronal protection and recovery in an excitotoxic model of brain injury

The development of efficient systems for in vivo gene transfer to the central nervous system (CNS) may provide a useful therapeutic strategy for the alleviation of several neurological disorders. In this study, we evaluated the feasibility of nonviral gene therapy to the CNS mediated by cationic liposomes. We present evidence of the successful delivery and expression of both a reporter and a therapeutic gene in the rodent brain, as evaluated by immunohistochemical assays. Our results indicate that transferrin-associated cationic liposome/DNA complexes (Tf-lipoplexes) allow a significant enhancement of transfection activity as compared to plain complexes, and that 8/1 (+/-) Tf-lipoplexes constitute the best formulation to mediate in vivo gene transfer. We demonstrated that Tf-lipoplex-mediated nerve growth factor transgene expression attenuates the morphological damages of the kainic acid-induced lesion as assessed by 2,3,5-triphenyltetrazolium chloride (TTC) vital staining. These findings suggest the usefulness of these lipid-based vectors in mediating the delivery of therapeutic genes to the CNS.

Formation and intracellular trafficking of lipoplexes and polyplexes

Cationic lipid/DNA lipoplexes and cationic polymer/DNA polyplexes represent an attractive alternative to viral vectors for cell transfection in vitro and in vivo but still suffer from a relatively low efficiency. Optimization of their transfection efficiency may be attempted by using a trial and error approach consisting of synthesizing and testing a large number of derivatives. On the other hand, rational design of highly efficient cationic lipids and polymers requires a deeper understanding of the interactions between the vector and the DNA as well as the cellular pathways and mechanisms involved in DNA entry into the cell and ultimately the nucleus. In the present review, the pathways and mechanisms involved in lipoplex- and polyplex-mediated transfection are comparatively addressed and unresolved questions are highlighted.

Analysis of the structure and composition of individual lipoplex particles by flow fluorometry

A flow fluorometric approach to study cationic lipoid-DNA complexes is presented. The approach uses standard flow cytometry equipment and common fluorescent dyes (BODIPY and ethidium homodimer-2) to detect both lipoid and DNA content in individual particles. In addition, a procedure that allows determination of whether or not liposomes remain intact is described. The procedure is based on monitoring the retention of a polar tracer that has been preloaded into its aqueous compartment. Sample preparation, instrument setup, data analysis, and methodological limitations are described. Applications of the procedure to cationic lipoid-DNA complexes are described, and illustrations are given for the determination of how the lipoid content, composition, and structure of individual lipoplexes in a population evolve over time, starting at about 1 min after DNA and vesicles are mixed. Analogous procedures can be applied to other heterogeneous particles and supramolecular structures.

Structure/Function Relationships of Polyamidoamine/DNA Dendrimers as Gene Delivery Vehicles

PAMAM dendrimers are members of a class of polyamine polymers that demonstrate significant gene delivery ability. In this study, a selection of PAMAM dendrimers, spanning a range of sizes (generations 2, 4, 7, and 9) and transfection efficiencies, are characterized by various biophysical methods to search for structural properties that correlate with transfection. Measurements of colloidal properties (size and zeta potential) as a function of charge ratio reveal that highly transfecting dendrimer/DNA complexes have size/zeta potential values between 4 and 8. Circular dichroism (CD) and FTIR spectroscopy of complexes confirm the DNA component remains in B form when associated with all dendrimer generations up to a 5:1 charge ratio (+/-). Isothermal titration calorimetry and differential scanning calorimetry detect changes that are related to polymer structure and charge ratio but do not directly correlate with transfection efficiency. Despite DNA structural and stability changes detected by CD, FTIR, DSC, and ITC that are similar to those seen with other cationic delivery vehicles [e.g., cationic lipids, peptoids/lipitoids, peptides, polyethyleneimines (PEIs), etc.], clear correlations with transfection activity are not readily apparent. This may be due, at least in part, to the heterogeneity of the complexes.

Polymers for DNA delivery

Nucleic acid delivery has many applications in basic science, biotechnology, agriculture, and medicine. One of the main applications is DNA or RNA delivery for gene therapy purposes. Gene therapy, an approach for treatment or prevention of diseases associated with defective gene expression, involves the insertion of a therapeutic gene into cells, followed by expression and production of the required proteins. This approach enables replacement of damaged genes or expression inhibition of undesired genes. Following two decades of research, there are two major methods for delivery of genes. The first method, considered the dominant approach, utilizes viral vectors and is generally an efficient tool of transfection. Attempts, however, to resolve drawbacks related with viral vectors (e.g., high risk of mutagenicity, immunogenicity, low production yield, limited gene size, etc.), led to the development of an alternative method, which makes use of non-viral vectors. This review describes non-viral gene delivery vectors, termed "self-assembled" systems, and are based on cationic molecules, which form spontaneous complexes with negatively charged nucleic acids. It introduces the most important cationic polymers used for gene delivery. A transition from in vitro to in vivo gene delivery is also presented, with an emphasis on the obstacles to achieve successful transfection in vivo.

Polymorphism of pyridinium amphiphiles for gene delivery: influence of ionic strength, helper lipid content, and plasmid DNA complexation

Two double-tailed pyridinium cationic amphiphiles, differing only in the degree of unsaturation of the alkyl chains, have been selected for a detailed study of their aggregation behavior, under conditions employed for transfection experiments. The transfection efficiencies of the two molecules are remarkably different, especially when combined with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) as helper lipid. The phase behavior of the cationic amphiphile/DOPE mixtures have been studied using (31)P- and (2)H-NMR (on deuterated cationic amphiphiles) as main techniques, to monitor independently the behavior of the two components. In water, the lamellar organization is dominant for both the surfactants in their mixtures with the helper lipid. In HEPES saline buffer (HBS), the mixtures of the unsaturated surfactant form inverted phases and, in particular, stable H(II) phases for DOPE contents > or =30 mol %. By contrast, the saturated surfactant does not form homogeneously mixed inverted phases in mixtures with DOPE at room temperature. However, mixed inverted phases are observed for this system at higher temperatures and, after mixing has been achieved by heating, the metastable mixed phases remain present for several hours at 5 degrees C. At 35 degrees C the dominant phase is the cubic phase. The lipoplex composed of equimolar mixtures of the unsaturated surfactant with DOPE and plasmid DNA was found to be organized in highly curved bilayers.

The effect of liposome size on the final lipid/DNA ratio of cationic lipoplexes

Several studies have demonstrated that lipoplexes are two-phase systems over most mixing lipid/DNA charge ratios. Because these studies have focused on small unilamellar vesicles (SUV), they leave open the question as to whether a similar pattern is followed by other liposome types. The main purpose of this work is to examine the question further by characterizing the assembly of cationic lipoplexes prepared from 1-[2-(oleoyloxy)ethyl]-2-oleyl-3-(2-hydroxyethyl)imidazolinium chloride (DOTIM)/dioleoylphosphatidylethanolamine (DOPE) (1:1) liposomes of various types. Sedimentation in sucrose density gradients reveals that large unilamellar vesicles (LUV) and sedimented multilamellar vesicles (sMLV), as opposed to SUV, form lipoplexes that exist as a single phase over a relatively broad range of mixing (+/-) ratios. This is indicated by observing that most of the LUV and sMLV become involved in the assembly reaction up to mixing (+/-) ratios of 4 and 9, respectively, while only a small and constant fraction of SUV associates with DNA at all mixing (+/-) ratios tested. Consequently, while maximal (+/-) ratios of approximately 4.5 and 9 are found in LUV and sMLV lipoplexes, respectively, a final (+/-) ratio of only approximately 2 is determined in SUV lipoplexes. Isothermal titration calorimetry shows that this is the lowest possible charge ratio achieved when liposomes are titrated with DNA. Based on these observations and on the size differences of the liposomes used, a model of lipoplex formation is proposed.

Correlation between structure and transfection efficiency: a study of DC-Chol−DOPE/DNA complexes

The supramolecular structural nature of some cationic liposomes-DNA complexes, currently used as vehicles in non-viral gene delivery, has been elucidated by recent X-ray diffraction experiments. The relationship between the chemico-physical properties of these self-assembled structures and their transfection efficiency is extensively studied. Here we report a first comprehensive structural study by using energy dispersive X-ray diffraction, of the complex DC-Chol--DOPE/DNA (3beta[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol dioleoylphosphatidylethanolamine/DNA), which has been classified as one of the most effective in in-vivo experiments. Our results show that DC-Chol--DOPE/DNA lipoplexes have a columnar inverted hexagonal structure, which is not influenced by the cationic liposome/DNA charge ratio. The transfection efficiency of C6 rat glioma cells by DC-Chol--DOPE/DNA lipoplexes and the toxicity of lipoplexes to cells are dramatically affected by cationic liposome/DNA weight ratio. It seems therefore that the lipoplex structures have not any influence on transfection efficiency and toxicity in our experimental system.

Structure/function analysis of peptoid/lipitoid:DNA complexes

Previous transfection studies of cationic peptoid polymers (N-substituted polyglycines) and cationic lipitoid polymers (peptoid-phospholipid conjugates) have shown that only the polymers which possessed a repeating (cationic, hydrophobic, hydrophobic) substituent sequence are efficient in gene transfer in vitro. To determine if there is a physical attribute of peptoid and lipitoid complexes that correlates with efficient gene transfection, biophysical, and transfection measurements were performed with polymer:DNA complexes containing each of seven structurally diverse peptoid polymers and two lipitoids that possess different hydrophobic substituents. These measurements revealed that the biophysical properties of these complexes (size, zeta-potential, ethidium bromide exclusion) varied with polymer structure and complex (+/-) charge ratio but were not directly predictive of transfection efficiency. Unique alterations in the circular dichroism spectra of DNA were observed in complexes containing several of the peptoids and both lipitoids, although FTIR spectroscopy demonstrated that the DNA remained in the B-form. The lack of correlations between the physical properties and the transfection activities of these polyplexes suggests that a further subpopulation examination of these complexes by these methods may reveal hidden structure-activity relationships.

Growth of cationic lipid toward bilayer lipid membrane by solution spreading: scanning probe microscopy study

The growth of cationic lipid dioctadecyldimethylammonium bromide (DODAB) toward bilayer lipid membrane (BLM) by solution spreading on cleaved mica surface was studied by atomic force microscopy (AFM). Bilayer of DODAB was formed by exposing mica to a solution of DODAB in chloroform and subsequently immersing into potassium chloride solution for film developing. AFM studies showed that at the initial stage of the growth, the adsorbed molecules exhibited the small fractal-like aggregates. These aggregates grew up and expanded laterally into larger patches with time and experienced from monolayer to bilayer, finally a close-packed bilayer film (5.4+/-0.2 nm) was approached. AFM results of the film growth process indicated a growth mechanism of nucleation, growth and coalescence of dense submonolayer, it revealed the direct information about the film morphology and confirmed that solution spreading was an effective technique to prepare a cationic bilayer in a short time.

Lipoplexes with alkylphospholipid as new helper lipid for efficient in vitro and in vivo gene transfer in tumor therapy

To improve liposomal gene transfer we investigated the influence of membrane-interacting alkylphospholipids (APLs) on gene transfer efficiency in vitro and in vivo using the LacZ reporter gene and the cytosine deaminase (CD) suicide gene. Liposomes were first optimized concerning the kind and amount of APL and the additional liposome components. Thus, an up to 270% increase in the transfer efficiency of the LacZ gene into HCT15 and HCT116 human colon carcinoma cells could be obtained in vitro compared to lipofectin-mediated transfection by using a lipoplex consisting of tetradecylphosphocholine/dimethyldioctadecylamine/cholesterol/dioleylphosphoethanolamine-liposomes and the pSV40-betaGal-plasmid. The in vivo experiments revealed that alkylphospholipid-lipoplexes (APL-LPs) were similarly effective in the transfer of the LacZ gene into colon carcinoma as formulations consisting of lipofectin. Using the CD-gene in combination with APL-LPs resulted in a significantly stronger inhibition of C26 colon carcinoma growth compared to lipofectin-mediated gene transfer following treatment of mice with the prodrug 5-fluorocytosine. The results of this study demonstrate for the first time that the utilization of membrane-active APLs as component of the liposomal part of lipoplexes enhances the efficacy of gene therapy in vitro and in vivo.

Cationic liposome-mediated gene delivery: biophysical study and mechanism of internalization

To identify factors affecting cationic liposome-mediated gene delivery efficiency, we studied the relationship between the biophysical characteristics of liposome/DNA complexes (lipoplexes) at different (+/-) charge ratios, their structures as monitored by atomic force microscopy (AFM), and their mechanism(s) of internalization into the cells. Significant changes were observed in the particle size and zeta potential of liposomes and their structures assessed by AFM upon addition of DNA, which depended on (+/-) charge ratios. AFM images showed that lipoplexes were formed from extensively fused and apparently homogeneous lipid particles encapsulating DNA. Lipoplexes were found to internalize the cells through the endocytosis pathway. Lipoplex-cell fusion was found to occur mainly at the plasma membrane level; however, this lipoplex-cell membrane fusion was found to be essential for the uptake of the large particles. A new perspective for the internalization of large lipoplex particles into cytoplasm is discussed.

Surface charge density determines the efficiency of cationic gemini surfactant based lipofection

The efficiencies of the binary liposomes composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and cationic gemini surfactant, (2S,3R)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide as transfection vectors, were measured using the enhanced green fluorescent protein coding plasmid and COS-1 cells. Strong correlation between the transfection efficiency and lipid stoichiometry was observed. Accordingly, liposomes with X(SR-1) > or = 0.50 conveyed the enhanced green fluorescent protein coding plasmid effectively into cells. The condensation of DNA by liposomes with X(SR-1) > 0.50 was indicated by static light scattering and ethidium bromide intercalation assay, whereas differential scanning calorimetry and fluorescence anisotropy of diphenylhexatriene revealed stoichiometry dependent reorganization in the headgroup region of the liposome bilayer, in alignment with our previous Langmuir-balance study. Surface charge density and the organization of positive charges appear to determine the mode of interaction of DNA with (2S,3R)-2,3-dimethoxy-1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide/1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomes, only resulting in DNA condensation when X(SR-1) > 0.50. Condensation of DNA in turn seems to be required for efficient transfection.

An experimental approach for direct observation of the interaction of polyanions with sphingosine-containing giant vesicles

A new approach for direct optical microscopy observation of polyanion interactions with bilayers of giant cationic liposomes (GUVs) was suggested. Polyanions as DNA, dextran sulfate (DS), heparin (H) and polyacrylic acids (PA) were locally delivered by a micropipette to a part of a giant unilamellar vesicle membrane. The phenomena were directly observed under optical microscope. GUVs, about 100 micro m in diameter, formed of phosphatidylcholines and up to 33 mol% of the natural bioactive cationic amphiphile sphingosine (Sph), were prepared by electroformation. The effects of water-soluble molecules with high negative linear charge density as dextran sulfate (DS), heparin (H) polyacrylic acids (PA) and adenosine-5'-triphosphoric acid (ATP) were compared with those of DNAs. The resulting membrane topology transformations were monitored in phase contrast, while the DNA distribution was followed in fluorescence. DNA-induced endocytosis-like membrane morphology transformation due to the DNA/lipid membrane local interactions was observed. The DS, H and PA induced membrane topology transformations similar to those of the DNAs, while ATP did not cause any detectable ones. The endocytosis mechanism involves the formation of ordered domains in the GUV membrane where some surface and charge asymmetries between the two membrane monolayers were created. The sizes of created polyanionic/cationic membrane domains depend on the form, length and elasticity of the adsorbed highly charged molecules. Endosome-including capacities of polyanionic molecules depend heavily on the high linear negative charge at a certain length. An original method for direct studying of the DNA/membrane interactions in autoadaptable giant liposome system imitating biological membrane interactions was forwarded. The model observations could also help for understanding events associated with cationic liposome/DNA complex formation in gene transfer processes.

The Structural Organization of Cationic Lipid-DNA Complexes

The interaction of cationic liposomes with supercoiled plasmid DNA results in a major rearrangement of each component to form compact multilamellar structures comprised of alternating layers of two-dimensional arrays of DNA sandwiched between lipid bilayers. Fluorescence resonance energy transfer was used to estimate the distance of closest approach of DNA to the lipid bilayers in these complexes. The effect of several compositional variables on this distance, including the ratio of cationic lipid to DNA, and the charge density, intrinsic curvature, and fluidity of the lipid bilayer were examined. Additionally, the effect of ionic strength was studied. For complexes prepared at or above a 3:1 charge ratio (+/-), the observed distance of closest approach was found to be in agreement with the intercalation of DNA between lipid bilayers. As the charge ratio was decreased, a monotonic increase in the distance was observed with a maximum observed at 0.5:1. Correlations between differences in the proximity of DNA to the lipid bilayer and the hydrodynamic size of the complexes were also found. A model based on these observations and previous reports suggests the formation of discrete populations of complexes below a charge ratio of 0.5:1 and above 3:1. The structure of the negatively charged complexes is consistent with DNA extending from the surface of the particles, whereas those possessing excess positive charge were multilamellar aggregates with the DNA effectively condensed between lipid bilayers. Complexes between these two states consist of weighted fractions of these two species.

Synergy between cationic lipid and co-lipid determines the macroscopic structure and transfection activity of lipoplexes

The large number of cytofectin and co-lipid combinations currently used for lipoplex-mediated gene delivery reflects the fact that the optimal cytofectin/co-lipid combination varies with the application. The effects of structural changes in both cytofectin and co-lipid were systematically examined to identify structure-activity relationships. Specifically, alkyl chain length, degree of unsaturation and the head group to which the alkyl side chain was attached were examined to determine their effect on lipoplex structure and biological activity. The macroscopic lipoplex structure was assessed using a dye-binding assay and the biological activity was examined using in vitro transfection in three diverse cell lines. Lipoplexes were formulated in three different vehicles currently in use for in vivo delivery of naked plasmid DNA (pDNA) and lipoplex formulations. The changes in dye accessibility were consistent with structural changes in the lipoplex, which correlated with alterations in the formulation. In contrast, transfection activity of different lipoplexes was cell type and vehicle dependent and did not correlate with dye accessibility. Overall, the results show a correlation between transfection and enhanced membrane fluidity in both the lipoplex and cellular membranes.

Polyelectrolyte molecular weight and electrostatically-induced domains in lipid bilayer membranes

Polyelectrolyte-induced domain formation in charged lipid bilayer membranes was investigated as a function of polyelectrolyte molecular weight using 2H nuclear magnetic resonance (NMR) spectroscopy. Lipid bilayers consisting of mixtures of alpha- or beta-choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC-alpha-d2 or POPC-beta-d2) plus the cationic amphiphile 1,2-dioleoyl-3-(dimethylamino)propane (DODAP) were exposed to the anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSSS) of various molecular weights. Regardless of molecular weight, PSSS produced dual component 2H NMR spectra, indicating two distinct POPC populations, corresponding to PSSS-bound and PSSS-free lipid, in slow exchange with one another. Analysis of the 2H NMR subspectra quadrupolar splittings and intensities showed the PSSS-bound domain to be enriched in DODAP, with the PSSS-free domain correspondingly depleted. At polyelectrolyte loadings below global charge equivalence, PSSS bound DODAP stoichiometrically for all PSSS molecular weights, indicating that the polyelectrolyte chain lies flat upon the membrane surface. At higher PSSS loadings the domains dissipated, leading to single component 2H NMR spectra. At high NaCl concentrations PSSS dissociated from the bilayer surface. Domain size on a per PSSS chain basis increased while the degree of enrichment with DODAP decreased progressively as the PSSS chain length decreased. Such molecular weight-dependent domain characteristics have not been predicted theoretically and need to be taken into account in future refinements of domain models.

Phase behavior, DNA ordering, and size instability of cationic lipoplexes. Relevance to optimal transfection activity

Mechanisms of cationic lipid-based nucleic acid delivery are receiving increasing attention, but despite this the factors that determine high or low activity of lipoplexes are poorly understood. This study is focused on the fine structure of cationic lipid-DNA complexes (lipoplexes) and its relevance to transfection efficiency. Monocationic (N-(1-(2,3-dioleoyloxy)propyl),N,N,N-trimethylammonium chloride, N-(1-(2,3-dimyristyloxypropyl)-N,N-dimethyl-(2-hydroxyethyl)ammonium bromide) and polycationic (2,3-dioleyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propanammonium trifluoroacetate) lipid-based assemblies, with or without neutral lipid (1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine, 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine, cholesterol) were used to prepare lipoplexes of different L(+)/DNA(-) charge ratios. Circular dichroism, cryogenic-transmission electron microscopy, and static light scattering were used for lipoplex characterization, whereas expression of human growth hormone or green fluorescent protein was used to quantify transfection efficiency. All monocationic lipids in the presence of inverted hexagonal phase-promoting helper lipids (1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine, cholesterol) induced appearance of Psi(-) DNA, a chiral tertiary DNA structure. The formation of Psi(-) DNA was also dependent on cationic lipid-DNA charge ratio. On the other hand, monocationic lipids either alone or with 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine as helper lipid, or polycationic 2,3-dioleyloxy-N-[2(sperminecarboxamido)ethyl]-N,N-dimethyl-1-propanammonium trifluoroacetate-based assemblies, neither of which promotes a lipid-DNA hexagonal phase, did not induce the formation of Psi(-) DNA. Parallel transfection studies reveal that the size and phase instability of the lipoplexes, and not the formation of Psi(-) DNA structure, correlate with optimal transfection.

Maintenance of nonviral vector particle size during the freezing step of the lyophilization process is insufficient for preservation of activity: insight from other structural indicators

The instability of nonviral vectors as liquid formulations has stimulated considerable interest in developing dehydrated formulations that would be resistant to shipping stresses and could be stored at room temperature. Recently, we reported that high sucrose/DNA ratios are capable of maintaining particle size during the freezing step of the lyophilization process and we suggested that the separation of individual particles within sugar matrices is responsible for the reported protection of nonviral vectors during the freezing step of a typical lyophilization protocol. The purpose of this study was to extend these observations to other nonviral vectors that incorporate different cationic components. Cationic lipid-based complexes composed of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), with helper lipid cholesterol (Chol) or dioleoylphosphatidyl-ethanolamine (DOPE), showed similar protection by sucrose. Formulations of a polyethylenimine (PEI)-based vector required much higher excipient/DNA ratios for size protection compared with protamine- and lipid-based vectors. At low sucrose/DNA ratios, zeta potentials for all complexes were significantly lowered during freezing. Similar results were obtained at high sucrose/DNA ratios, except for DOTAP-DOPE-containing vectors which maintained zeta potential values comparable to unfrozen controls. The changes in zeta potential values indicate that complexes are altered during freezing despite the maintenance of particle size as determined by light scattering. Furthermore, these changes might explain the observed reduction in transfection activity and provide new information about the effects of physicochemical changes of nonviral vectors during the freezing step of lyophilization.

On the mechanism whereby cationic lipids promote intracellular delivery of polynucleic acids

The mechanism whereby cationic lipids destabilize cell membranes to facilitate the intracellular delivery of macromolecules such as plasmid DNA or antisense oligonucleotides is not well understood. Here, we show that cationic lipids can destabilize lipid bilayers by promoting the formation of nonbilayer lipid structures. In particular, we show that mixtures of cationic lipids and anionic phospholipids preferentially adopt the inverted hexagonal (H(II)) phase. Further, the presence of 'helper' lipids such as dioleoylphosphatidylethanolamine or cholesterol, lipids that enhance cationic lipid-mediated transfection of cells also facilitate the formation of the H(II)phase. It is suggested that the ability of cationic lipids to promote nonbilayer structures in combination with anionic phospholipids leads to disruption of the endosomal membrane following uptake of nucleic acid-cationic lipid complexes into cells, thus facilitating cytoplasmic release of the plasmid or oligonucleotide.

Interplay in lipoplexes between type of pDNA promoter and lipid composition determines transfection efficiency of human growth hormone in NIH3T3 cells in culture

This study was aimed to investigate if and to what extent there is an interplay between lipoplex physicochemical properties and plasmid promoter type affecting transfection efficiency in vitro. To reduce the number of variables only one cell type (NIH3T3 cells), one gene (human growth hormone), one cationic lipid (DOTAP) in a plasmid >85% in supercoiled form, and the same medium conditions were used. The variables of the physicochemical properties included presence and type of helper lipid (DOPE, DOPC, or cholesterol, all in 1:1 mole ratio with DOTAP), size and lamellarity of the liposomes used for lipoplex preparation (large unilamellar vesicles, LUV, versus multilamellar vesicles, MLV), and DNA(-)/cationic lipid(+) charge ratio, all containing the same human growth hormone but differing in their promoter enhancer region. Two of the promoters were of viral origin: (a) SV40 promoter (simian virus early promoter) and (b) CMV promoter (cytomegalovirus early promoter); two were of mammalian cell origin: (c) PABP promoter (human poly(A)-binding protein promoter) and (d) S16 promoter (mouse ribosomal protein (rp) S16 promoter). Transfection studies showed that, irrespective of promoter type, large (> or =500 nm) MLV were superior to approximately 100 nm LUV; the extent of superiority was dependent on liposome lipid composition (larger for 100% DOTAP and DOTAP/DOPE than for DOTAP/DOPC and DOTAP/cholesterol). The optimal DNA(-)/DOTAP(+) charge ratio for all types of lipoplexes used was 0.2 or 0.5 (namely, when the lipoplexes were positively charged). Scoring the six best lipoplex formulations (out of 128 studied) revealed the following order: pCMV (DOTAP/DOPE) >> pSV (DOTAP/DOPE)=pCMV(DOTAP/cholesterol)=pS16 (100% DOTAP)=pS16 DOTAP/DOPE >> pCMV (DOTAP/DOPC). The lack of trivial consistency in the transfection efficiency score, the pattern of transfection efficiency, and statistical analysis of the data suggest that there is cross-talk between promoter type and lipoplex lipid composition, which may be related to the way the promoter is associated with the lipids.

Spontaneous entrapment of polynucleotides upon electrostatic interaction with ethanol-destabilized cationic liposomes

This study describes the effect of ethanol and the presence of poly(ethylene) glycol (PEG) lipids on the interaction of nucleotide-based polyelectrolytes with cationic liposomes. It is shown that preformed large unilamellar vesicles (LUVs) containing a cationic lipid and a PEG coating can be induced to entrap polynucleotides such as antisense oligonucleotides and plasmid DNA in the presence of ethanol. The interaction of the cationic liposomes with the polynucleotides leads to the formation of multilamellar liposomes ranging in size from 70 to 120 nm, only slightly bigger than the parent LUVs from which they originated. The degree of lamellarity as well as the size and polydispersity of the liposomes formed increases with increasing polynucleotide-to-lipid ratio. A direct correlation between the entrapment efficiency and the membrane-destabilizing effect of ethanol was observed. Although the morphology of the liposomes is still preserved at the ethanol concentrations used for entrapment (25-40%, v/v), entrapped low-molecular-weight solutes leak rapidly. In addition, lipids can flip-flop across the membrane and exchange rapidly between liposomes. Furthermore, there are indications that the interaction of the polynucleotides with the cationic liposomes in ethanol leads to formation of polynucleotide-cationic lipid domains, which act as adhesion points between liposomes. It is suggested that the spreading of this contact area leads to expulsion of PEG-ceramide and triggers processes that result in the formation of multilamellar systems with internalized polynucleotides. The high entrapment efficiencies achieved at high polyelectrolyte-to-lipid ratios and the small size and neutral character of these novel liposomal systems are of utility for liposomal delivery of macromolecular drugs.

Cationic lipid-DNA complexes in gene delivery: from biophysics to biological applications

Great expectations from the application of gene therapy approaches to human disease have been impaired by the unsatisfactory clinical progress observed. Among others, the use of an efficient carrier for nucleic acid-based medicines is considered to be a determinant factor for the successful application of this promising therapeutic strategy. The drawbacks associated with the use of viral vectors, namely those related with safety problems, have prompted investigators to develop alternative methods for gene delivery, cationic lipid-based systems being the most representative. This review focuses on the various parameters that are considered to be crucial to optimize the use of cationic lipid-DNA complexes for gene therapy purposes. Particular emphasis is devoted to the analysis of the different stages involved in the transfection process, from the biophysical aspects underlying the formation of the complexes to the different biological barriers that need to be surpassed for gene expression to occur.

Roles of lipid polymorphism in intracellular delivery

Lipids, which adopt nonbilayer phases, have fascinated researchers as to the functional roles of these components in biomembranes. In particular, lipids capable of adopting the hexagonal H(II) phase have received considerable attention because of the observation that such lipids can promote membrane fusion. In the rational design of lipid-based delivery systems, H(II) phase lipids have been employed to endow systems with fusogenic, membrane-destabilizing properties. We will outline the molecular basis for the polymorphic phase behavior of lipids and highlight some of the uses of nonbilayer lipids in the preparation of lipid-based delivery systems. In addition, a distinction will be drawn between lipid-based systems which rely on the inclusion of nonbilayer lipids for activity, and systems which contain components which actively promote formation of nonbilayer structure within biological membranes.

Trends in lipoplex physical properties dependent on cationic lipid structure, vehicle and complexation procedure do not correlate with biological activity

Using a group of structurally related cytofectins, the effects of different vehicle constituents and mixing techniques on the physical properties and biological activity of lipoplexes were systematically examined. Physical properties were examined using a combination of dye accessibility assays, centrifugation, gel electrophoresis and dynamic light scattering. Biological activity was examined using in vitro transfection. Lipoplexes were formulated using two injection vehicles commonly used for in vivo delivery (PBS pH 7.2 and 0.9% saline), and a sodium phosphate vehicle previously shown to enhance the biological activity of naked pDNA and lipoplex formulations. Phosphate was found to be unique in its effect on lipoplexes. Specifically, the accessible pDNA in lipoplexes formulated with cytofectins containing a gamma-amine substitution in the headgroup was dependent on alkyl side chain length and sodium phosphate concentration, but the same effects were not observed when using cytofectins containing a beta-OH headgroup substitution. The physicochemical features of the phosphate anion, which give rise to this effect in gamma-amine cytofectins, were deduced using a series of phosphate analogs. The effects of the formulation vehicle on transfection were found to be cell type-dependent; however, of the formulation variables examined, the liposome/pDNA mixing method had the greatest effect on transgene expression in vitro. Thus, though predictive physical structure relationships involving the vehicle and cytofectin components of the lipoplex were uncovered, they did not extrapolate to trends in biological activity.

Kinetic analysis of the initial steps involved in lipoplex--cell interactions: effect of various factors that influence transfection activity

We investigated the mode of interaction of lipoplexes (DOTAP:DOPE/DNA) with HeLa cells, focusing on the analysis of the initial steps involved in the process of gene delivery. We evaluated the effect of different factors, namely the stoichiometry of cationic lipids and DNA, the presence of serum in the cell culture medium, and the incorporation of the ligand transferrin into the lipoplexes, on the extent of binding, association and fusion (lipid mixing) of the lipoplexes with the cells. Parallel experiments were performed upon cell treatment with inhibitors of endocytosis. Our results indicate that a decrease of the net charge of the complexes (upon addition of DNA) generally leads to a decrease in the extent of binding, cell association and fusion, except for the neutral complexes. Association of transferrin to the lipoplexes resulted in a significant enhancement of the interaction processes referred to above, which correlates well with the promotion of transfection observed under the same conditions. Besides triggering internalization of the complexes, transferrin was also shown to mediate fusion with the endosomal membrane. The extent of fusion of this type of complexes was reduced upon their incubation with cells in the presence of serum, suggesting that serum components limit the transferrin fusogenic properties. Results were analyzed by using a theoretical model which allowed to estimate the kinetic parameters involved in lipoplex--cell interactions. The deduced fusion and endocytosis rate constants are discussed and compared with those obtained for other biological systems. From the kinetic studies we found a twofold enhancement of the fusion rate constant (f) for the ternary lipoplexes. We also concluded that HeLa cells yield a relatively low rate of endocytosis. Overall, our results estimate the relative contribution of fusion of lipoplexes with the plasma membrane, endocytosis and fusion with the endosomal membrane to their interactions with cells, this information being of crucial importance for the development of gene therapy strategies.

Delivery and pathway in MCF7 cells of DNA vectorized by cationic liposomes derived from cholesterol

We have investigated the delivery and the pathway in tumoral MCF7 cells of DNA carried by liposomes prepared from (trimethyl aminoethane carbamoyl cholesterol iodide (TMAE-Chol), a cholesterol-based cationic lipid with a quaternary ammonium on the polar head. The structure of DNA-liposome complexes depends on the length of DNA and on the lipid-DNA charge ratio X. Spherical beads constitute fine structures of the observed complexes even when they appear as aggregates. For oligonucleotide transfer, dissociation from liposomes after transfection, penetration of the oligonucleotides into nuclei, and a long resident time were observed. For plasmid transfer, a correlation between the variation in the transfection level and the ultrastructure of complexes was demonstrated. The results showed a cellular route of lipid/plasmid complexes from the beginning by endocytosis, entrapped into endosomes, released by the latter until entry in the perinuclear area, and then penetration of plasmids inside the nuclei resulting in the observed expression of the beta-galactosidase gene.

Biophysical and structural properties of DNA.diC(14)-amidine complexes. Influence of the DNA/lipid ratio

Cationic liposomes are used as vectors for gene delivery both in vitro and in vivo. Comprehension of both DNA/liposome interactions on a molecular level and a description of structural modifications involved, are prerequisites to an optimization of the transfection protocol and, thus, successful application in therapy. Formation and stability of a DNA/cationic liposome complex were investigated here at different DNA:lipid molar ratios (rho). Isothermal titration calorimetry (ITC) of cationic liposomes with plasmid DNA was used to characterize the DNA-lipid interaction. Two processes were shown to be involved in the complex formation. A fast exothermic process was attributed to the electrostatic binding of DNA to the liposome surface. A subsequent slower endothermic reaction is likely to be caused by the fusion of the two components and their rearrangement into a new structure. Fluorescence and differential scanning calorimetry confirmed this interpretation. A kinetic model analyzes the ITC profile in terms of DNA/cationic liposome interactions.

Tunable pH-sensitive liposomes composed of mixtures of cationic and anionic lipids

The pH-dependent fusion properties of large unilamellar vesicles (LUVs) composed of binary mixtures of anionic and cationic lipids have been investigated. It is shown that stable LUVs can be prepared from the ionizable anionic lipid cholesteryl hemisuccinate (CHEMS) and the permanently charged cationic lipid N,N-dioleoyl-N, N-dimethylammonium chloride (DODAC) at neutral pH values and that these LUVs undergo fusion as the pH is reduced. The critical pH at which fusion was observed (pH(f)) was dependent on the cationic lipid-to-anionic lipid ratio. LUVs prepared from DODAC/CHEMS mixtures at molar ratios of 0 to 0.85 resulted in vesicles with pH(f) values that ranged from pH 4.0 to 6.7, respectively. This behavior is consistent with a model in which fusion occurs at pH values such that the DODAC/CHEMS LUV surface charge is zero. Related behavior was observed for LUVs composed of the ionizable cationic lipid 3alpha-[N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol hydrochloride (DC-Chol) and the acidic lipid dioleoylphosphatidic acid (DOPA). Freeze-fracture and (31)P NMR evidence is presented which indicates that pH-dependent fusion results from a preference of mixtures of cationic and anionic lipid for "inverted" nonbilayer lipid phases under conditions where the surface charge is zero. It is concluded that tunable pH-sensitive LUVs composed of cationic and anionic lipids may be of utility for drug delivery applications. It is also suggested that the ability of cationic lipids to adopt inverted nonbilayer structures in combination with anionic lipids may be related to the ability of cationic lipids to facilitate the intracellular delivery of macromolecules.

Lipoplex formation under equilibrium conditions reveals a three-step mechanism

Cellular transfection can be accomplished by the use of synthetic amphiphiles as gene carrier system. To understand the mechanism and hence to improve the efficiency of transfection, insight into the assembly and properties of the amphiphile/gene complex is crucial. Here, we have studied the interaction between a plasmid and cationic amphiphiles, using a monolayer technique, and have examined complex assembly by atomic force microscopy. The data reveal a three-step mechanism for complex formation. In a first step, the plasmids, interacting with the monolayer, display a strong tendency of orientational ordering. Subsequently, individual plasmids enwrap themselves with amphiphile molecules in a multilamellar fashion. The size of the complex formed is determined by the supercoiled size of the plasmid, and calculations reveal that the plasmid can be surrounded by 3 to 5 bilayers of the amphiphile. The eventual size of the transfecting complex is finally governed by fusion events between individually wrapped amphiphile/DNA complexes. In bulk phase, where complex assembly is triggered by mixing amphiphilic vesicles and plasmids, a similar wrapping process is observed. However, in this case, imperfections in this process may give rise to a partial exposure of plasmids, i.e., part of the plasmid is not covered with a layer of amphiphile. We suggest that these exposed sites may act as nucleation sites for massive lipoplex clustering, which in turn may affect transfection efficiency.

Membrane perturbation and the mechanism of lipid-mediated transfer of DNA into cells

Mixtures of cationic lipids and unsaturated phosphatidylethanolamine are used extensively for the intracellular delivery of plasmids and antisense oligodeoxynucleotides (ODN) in vitro. However, the mechanism by which cytoplasmic delivery of these large molecules is achieved remains unclear. The common hypothesis is that phosphatidylethanolamine promotes fusion of lipid/DNA particles with endosomal membranes, but this is inconsistent with several reports that have failed to correlate the fusogenic activity of a wide variety of lipid/DNA particles, measured by lipid mixing techniques, with their transfection activity. To address this issue further we have conducted a detailed analysis of the lipid mixing and DNA transfer activity of two, physically similar but functionally different, lipid/DNA particles composed of equimolar dioleyldimethylammonium chloride (DODAC) and dioleoylphosphatidylethanolamine (DOPE) or dioleoylphosphatidylcholine (DOPC). In combination with DODAC both phospholipids form almost identical lipid/DNA particles, they are endocytosed by cells to the same extent and each undergoes equivalent lipid mixing with cell membranes after uptake. Despite this, DNA transfer is 10- to 100-fold more extensive for lipid/DNA particles containing DOPE. We conclude that lipid mixing between lipid-based delivery systems and endosomal membranes must occur for DNA transfer to occur. However, the potency of different lipid/DNA particles correlates better with the ability of the exogenous lipid to disrupt membrane integrity.

Gene transfer in vitro and in vivo by cationic lipids is not significantly affected by levels of supercoiling of a reporter plasmid

PURPOSE

It is a common preconception that supercoiled plasmid DNA is more desirable for the transfection of cells that the relaxed form of the plasmid. This notion has led to the recommendation that a specification for the minimum amount of plasmid in the supercoiled form should exist in a gene therapy product. We have tested this notion by examining the effects of the degree of supercoiling on cationic lipid-mediated gene transfer in vitro and in vivo.

METHODS

An ion-exchange high performance liquid chromatography (HPLC) method was developed to accurately quantitate the relative amounts of supercoiled DNA in purified plasmid. A sample of the purified plasmid was fully relaxed using topoisomerase. Next, the ability of various levels of supercoiled plasmid to transfect mammalian cells was measured.

RESULTS

This study suggests that there is no relation between the degree of supercoiling and lipofection efficiency. Subsequent transfection using several different lipofection agents, different cell types, and an in vivo model support these results.

CONCLUSIONS

In considering a specification for the amount of supercoiled plasmid in a gene therapy product, it must be noted that the relaxed forms of the plasmid are no less efficient at gene delivery than the supercoiled forms.

Effect of DNA/liposome mixing ratio on the physicochemical characteristics, cellular uptake and intracellular trafficking of plasmid DNA/cationic liposome complexes and subsequent gene expression

In order to identify the important factors involved in cationic liposome-mediated gene transfer, in vitro transfection efficiencies by plasmid DNA complexed with DOTMA/DOPE liposomes at different DNA/liposome mixing ratios were evaluated using four types of cultured cells with respect to their physicochemical properties. Significant changes were observed in the particle size and zeta potential of the complexes as well as in their structures, assessed by atomic force microscopy, which depended on the mixing ratio. In transfection experiments, except for RAW 264.7 cells (mouse macrophages), efficient gene expression was obtained in MBT-2 cells (mouse bladder tumor), NLH3T3 cells (mouse fibroblasts) and HUVEC (human umbilical vein endothelial cells) at an optimal ratio of 1:5, 1:7.5 or 1:5, respectively. On the other hand, cellular uptake of the [32P]DNA/liposome complexes increased in all cell types with an increase in the mixing ratio, which was not reflected by the transfection efficiency. The cellular damage determined by MTT assay was minimal even at the highest DNA/liposome ratio (1:10), indicating that the lower gene expression level at the higher ratio was not due to cytotoxicity induced by the complex. An ethidium bromide intercalation assay showed that the release of plasmid DNA from the complex, following the addition of negatively charged liposomes, was restricted as the mixing ratio increased. Furthermore, confocal microscopic studies using HUVEC showed that the 1:5 complexes exhibited a dispersed distribution in the cytoplasm whereas a punctuate intracellular distribution was observed for the 1:10 complexes. This suggests that there was a significant difference in intracellular trafficking, probably release from the endosomes or lysosomes, of the plasmid DNA/cationic liposome complexes between these mixing ratios. Taken together, these findings suggest that the DNA/liposome mixing ratio significantly affects the intracellular trafficking of plasmid DNA complexed with the cationic liposomes, which is an important determinant of the optimal mixing ratio in cationic liposome-mediated transfection.

2H NMR and polyelectrolyte-induced domains in lipid bilayers

2H NMR studies of polyelectrolyte-induced domain formation in lipid bilayer membranes are reviewed. The 2H NMR spectrum of choline-deuterated phosphatidylcholine (PC) reports on any and all sources of lipid bilayer surface charge, since these produce a conformation change in the choline head group of PC, manifest as a change in the 2H NMR quadrupolar splitting. In addition, homogeneous and inhomogeneous surface charge distributions are differentiated. Adding polyelectrolytes to lipid bilayers consisting of mixtures of oppositely charged and zwitterionic lipids produces 2H NMR spectra which are superpositions of two Pake sub-spectra: one corresponding to a polyelectrolyte-bound lipid population and the other to a polyelectrolyte-free lipid population. Quantitative analysis of the quadrupolar splittings and spectral intensities of the two sub-spectra indicate that the polyelectrolyte-bound populations is enriched with oppositely charged lipid, while the polyelectrolyte-free lipid population is correspondingly depleted. The same domain-segregation effect is produced whether cationic polyelectrolytes are added to anionic lipid bilayers or anionic polyelectrolytes are added to cationic lipid bilayers. The 2H NMR spectra permit a complete characterization of domain composition and size. The anion:cation ratio within the domains is always stoichiometric, as expected for a process driven by Coulombic interactions. The zwitterionic lipid content of the domains is always statistical, reflecting the systems tendency to minimize the entropic cost of demixing charged lipids into domains. Domain formation is observed even with rather short polyelectrolytes, suggesting that individual polyelectrolyte chains aggregate at the surface to form "superdomains". Overall, the polyelectrolyte bound at the lipid bilayer surface appears to lie flat along the surface and to be essentially immobilized through its multiple electrostatic contacts.

Factors governing the assembly of cationic phospholipid-DNA complexes

The interaction of DNA with a novel cationic phospholipid transfection reagent, 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (EDOPC), was investigated by monitoring thermal effects, particle size, vesicle rupture, and lipid mixing. By isothermal titration calorimetry, the heat of interaction between large unilamellar EDOPC vesicles and plasmid DNA was endothermic at both physiological and low ionic strength, although the heat absorbed was slightly larger at the higher ionic strength. The energetic driving force for DNA-EDOPC association is thus an increase in entropy, presumably due to release of counterions and water. The estimated minimum entropy gain per released counterion was 1.4 cal/mole- degrees K (about 0.7 kT), consistent with previous theoretical predictions. All experimental approaches revealed significant differences in the DNA-lipid particle, depending upon whether complexes were formed by the addition of DNA to lipid or vice versa. When EDOPC vesicles were titrated with DNA at physiological ionic strength, particle size increased, vesicles ruptured, and membrane lipids became mixed as the amount of DNA was added up to a 1.6:1 (+:-) charge ratio. This charge ratio also corresponded to the calorimetric end point. In contrast, when lipid was added to DNA, vesicles remained separate and intact until a charge ratio of 1:1 (+:-) was exceeded. Under such conditions, the calorimetric end point was 3:1 (+:-). Thus it is clear that fundamental differences in DNA-cationic lipid complexes exist, depending upon their mode of formation. A model is proposed to explain the major differences between these two situations. Significant effects of ionic strength were observed; these are rationalized in terms of the model. The implications of the analysis are that considerable control can be exerted over the structure of the complex by exploiting vectorial preparation methods and manipulating ionic strength.