Structure and structure-function studies of lipid/plasmid DNA complexes

Cationic lipids for gene delivery: many players, one goal

Lipid-based carriers represent the most widely used alternative to viral vectors for gene expression and gene silencing purposes. This class of non-viral vectors is particularly attractive for their ease of synthesis and chemical modifications to endow them with desirable properties. Despite combinatorial approaches have led to the generation of a large number of cationic lipids displaying different supramolecular structures and improved behavior, additional effort is needed towards the development of more and more effective cationic lipids for transfection purposes. With this review, we seek to highlight the great progress made in the design of each and every constituent domain of cationic lipids, that is, the chemical structure of the headgroup, linker and hydrophobic moieties, and on the specific effect on the assembly with nucleic acids. Since the complexity of such systems is known to affect their performances, the role of formulation, stability and phase behavior on the transfection efficiency of such assemblies will be thoroughly discussed. Our objective is to provide a conceptual framework for the development of ever more performing lipid gene delivery vectors.

Current Applications of Nanoemulsions in Cancer Therapeutics

Nanoemulsions are pharmaceutical formulations composed of particles within a nanometer range. They possess the capacity to encapsulate drugs that are poorly water soluble due to their hydrophobic core nature. Additionally, they are also composed of safe gradient excipients, which makes them a stable and safe option to deliver drugs. Cancer therapy has been an issue for several decades. Drugs developed to treat this disease are not always successful or end up failing, mainly due to low solubility, multidrug resistance (MDR), and unspecific toxicity. Nanoemulsions might be the solution to achieve efficient and safe tumor treatment. These formulations not only solve water-solubility problems but also provide specific targeting to cancer cells and might even be designed to overcome MDR. Nanoemulsions can be modified using ligands of different natures to target components present in tumor cells surface or to escape MDR mechanisms. Multifunctional nanoemulsions are being studied by a wide variety of researchers in different research areas mainly for the treatment of different types of cancer. All of these studies demonstrate that nanoemulsions are efficiently taken by the tumoral cells, reduce tumor growth, eliminate toxicity to healthy cells, and decrease migration of cancer cells to other organs.

Competition of charge-mediated and specific binding by peptide-tagged cationic liposome-DNA nanoparticles in vitro and in vivo

Cationic liposome-nucleic acid (CL-NA) complexes, which form spontaneously, are a highly modular gene delivery system. These complexes can be sterically stabilized via PEGylation [PEG: poly (ethylene glycol)] into nanoparticles (NPs) and targeted to specific tissues and cell types via the conjugation of an affinity ligand. However, there are currently no guidelines on how to effectively navigate the large space of compositional parameters that modulate the specific and nonspecific binding interactions of peptide-targeted NPs with cells. Such guidelines are desirable to accelerate the optimization of formulations with novel peptides. Using PEG-lipids functionalized with a library of prototypical tumor-homing peptides, we varied the peptide density and other parameters (binding motif, peptide charge, CL/DNA charge ratio) to study their effect on the binding and uptake of the corresponding NPs. We used flow cytometry to quantitatively assess binding as well as internalization of NPs by cultured cancer cells. Surprisingly, full peptide coverage resulted in less binding and internalization than intermediate coverage, with the optimum coverage varying between cell lines. In, addition, our data revealed that great care must be taken to prevent nonspecific electrostatic interactions from interfering with the desired specific binding and internalization. Importantly, such considerations must take into account the charge of the peptide ligand as well as the membrane charge density and the CL/DNA charge ratio. To test our guidelines, we evaluated the in vivo tumor selectivity of selected NP formulations in a mouse model of peritoneally disseminated human gastric cancer. Intraperitoneally administered peptide-tagged CL-DNA NPs showed tumor binding, minimal accumulation in healthy control tissues, and preferential penetration of smaller tumor nodules, a highly clinically relevant target known to drive recurrence of the peritoneal cancer.

What the cell surface does not see: The gene vector under the protein corona

The fate of lipid-based nanovectors, used in genetic targeting inside cells, depends on their behavior in biological media. In fact, during both in vitro and in vivo transfection, nanovectors come in contact with proteins that compete for their surface and build the protein corona, their true biological identity while engaging the cell membrane. Nonetheless, after cell internalization, the efficacy of transfection may depend also on structural modifications that occurred under the protein cover, following interaction with biological fluids. Here, based on previous in vivo experiments, two widely used lipid mixtures, namely DOTAP/DOPC and DC-Chol/DOPE, were identified as paradigms to investigate the impact of the inner structure of nanovectors on the transfection efficiency, all being proficiently internalized. The evolution of the inner structure of cationic lipoplexes and nanoparticles based on such lipid mixtures, following interaction with human plasma, could be unraveled. Particles were investigated in high dilution, approaching the biosimilar conditions. Data have demonstrated that the modulation of their inner structure depends on their lipid composition and the plasma concentration, still preserving the genetic payload. Interestingly, protein contact induces a variety of inner structures with different perviousness, including reshaping into cubic phases of different porosity, sometimes observed upon interaction between carrier-lipids and cell-lipids. Cubic reshaping is of biological relevance, as lipid cubic phases have been recently associated to both fusogenicity and to the readiness in releasing the payload to the final target via endosomal escape.

The role of the helper lipid on the DNA transfection efficiency of lipopolyplex formulations

Multifunctional, lipopolyplex formulations comprising a mixture of cationic liposomes and cationic, receptor-targeting peptides have potential use in gene therapy applications. Lipopolyplex formulations described here are typically far more efficient transfection agents than binary lipoplex or polyplex formulations. It has been shown previously that the peptide component mediates both DNA packaging and targeting of the nanoparticle while in this report we investigate the contribution of the lipid component. We hypothesised that the lipid components synergise with the peptides in the transfection process by promoting endosomal escape after lipid bilayer fusion. Lipopolyplexes were prepared with cationic liposomes comprising DOTAP with either neutral lipid DOPE or DOPC. DOPE promotes fusogenic, inverted hexagonal lipid structures while DOPC promotes more stable laminar structures. Lipopolyplexes containing DOPE showed substantially higher transfection efficiency than those formulated with DOPC, both in vitro and in vivo. DOPE-containing lipopolyplexes showed rapid endosomal trafficking and nuclear accumulation of DNA while DOPC-containing formulations remained within the late endo-lysosomal compartments. These findings are consistent with previous finding for the role of DOPE in lipoplexes and support the hypothesis regarding the function of the lipid components in lipopolyplexes. These findings will help to inform future lipopolyplex design, strategies and clinical development processes.

Probing DNA-lipid membrane interactions with a lipopeptide nanopore

Association of DNA molecules with lipid bilayer membranes is of considerable interest for a large variety of applications in biotechnology. Here we introduce syringomycin E (SRE), a small pore-forming lipopeptide produced by the bacterium Pseudomonas syringae, as a facile sensor for the detection of DNA interactions with lipid membranes. SRE forms highly reproducible pores in cellular and artificial membranes. The pore structure involves bilayer lipids, which have a pronounced influence on open channel conductance and gating. SRE channels act as ionic diodes that serve as current rectifiers sensitive to the charge of the bilayer. We employ this intrinsic property to electronically monitor the association of DNA molecules with the membrane in a variety of different settings. We show that SRE can be used for quantitatively probing electrostatic interactions of DNA and DNA-cholesterol conjugates with a lipid membrane. Furthermore, we demonstrate that SRE channels allow monitoring of hybridization reactions between lipid-anchored probe strands and complementary strands in solution. In the presence of double-stranded DNA, SRE channels display a particularly high degree of rectification. Finally, the formation of multilayered structures assembled from poly-(L)-lysine and DNA oligonucleotides on the membrane was precisely monitored with SRE.

Synthesis and characterization of degradable multivalent cationic lipids with disulfide-bond spacers for gene delivery

Gene therapy provides powerful new approaches to curing a large variety of diseases, which are being explored in ongoing worldwide clinical trials. To overcome the limitations of viral gene delivery systems, synthetic nonviral vectors such as cationic liposomes (CLs) are desirable. However, improvements of their efficiency at reduced toxicity and a better understanding of their mechanism of action are required. We present the efficient synthesis of a series of degradable multivalent cationic lipids (CMVLn, n=2 to 5) containing a disulfide bond spacer between headgroup and lipophilic tails. This spacer is designed to be cleaved in the reducing milieu of the cytoplasm and thus decrease lipid toxicity. Small angle X-ray scattering demonstrates that the initially formed lamellar phase of CMVLn-DNA complexes completely disappears when reducing agents such as DTT or the biologically relevant reducing peptide glutathione are added to mimic the intracellular milieu. The CMVLs (n=3 to 5) exhibit reduced cytotoxicity and transfect mammalian cells with efficiencies comparable to those of highly efficient non-degradable analogs and benchmark commercial reagents such as Lipofectamine 2000. Thus, our results demonstrate that degradable disulfide spacers may be used to reduce the cytotoxicity of synthetic nonviral gene delivery carriers without compromising their transfection efficiency.

DNA lipoplexes: formation of the inverse hexagonal phase observed by coarse-grained molecular dynamics simulation

Mixtures of dsDNA and lipids, so-called lipoplexes, are widely used as less toxic alternatives to viral vectors in transfection studies. However, the transfection efficiency achieved by lipoplexes is significantly lower than that of viral vectors and is a barrier to their use in the clinic. There is now significant evidence suggesting that the molecular organization and structure (nanoarchitecture) of lipoplexes might correlate with biological activity. As a consequence, the ability to predict quantitatively the nanoarchitecture of new systems, and how these might change intracellularly, would be a major tool in the development of rational discovery strategies for more efficient lipoplex formulations. Here we report the use of a coarse-grain molecular dynamics simulation to predict the phases formed by two lipoplex systems: dsDNA-DOPE and dsDNA-DOPE-DOTAP. The predictions of the simulations show excellent agreement with experimental data from polarized light microscopy and small-angle X-ray diffraction (SAXS); the simulations predicted the formation of phases with d-spacings that were comparable to those measured by SAXS. More significantly, the simulations were able to reproduce for the first time the experimentally observed change from a fluid lamellar to an inverse hexagonal phase in the dsDNA-DOPE-DOTAP system as a function of changes in lipid composition. Our studies indicate that coarse-grain MD simulations could provide a powerful tool to understand, and hence design, new lipoplex systems.

A Review of Scanning Probe Microscopy Investigations of Liposome-DNA Complexes

Liposome-DNA complexes are one of the most promising systems for the protection and delivery of nucleic acids to combat neoplastic, viral, and genetic diseases. In addition, they are being used as models in the elucidation of many biological phenomena such as viral infection and transduction. In order to understand these phenomena and to realize the mechanism of nucleic acid transfer by liposome-DNA complexes, studies at the molecular level are required. To this end, scanning probe microscopy (SPM) is increasingly being used in the characterization of lipid layers, lipid aggregates, liposomes, and their complexes with nucleic acid molecules. The most attractive attributes of SPM are the potential to image samples with subnanometer spatial resolution under physiological conditions and provide information on their physical and mechanical properties. This review describes the application of scanning tunneling microscopy and atomic force microscopy, the two most commonly applied SPM techniques, in the characterisation of liposome-DNA complexes.

Modifications in the Head Group and in the Spacer of Cholesterol-based Cationic Lipids Promote Transfection in Melanoma B16-F10 Cells and Tumours

A series of four cationic lipids derived from cholesterol was synthesised and their efficiencies to vectorise nucleic acids were compared. The investigation concerns the effects of systematic chemical modifications in the polar head and in the spacer. The cationic lipid molecules used are in the same family of 3beta[N-(N',N',N'-trimethylaminoethane)-carbamoyl] cholesterol iodide (TMAEC-Chol), presenting a spacer of two or three carbons and a quaternary ammonium polar head ramified with methyl or ethyl groups. These lipids formed stable liposomes sizing from 100 to 200 nm when prepared with the colipid dioleoyl phosphatidylethanolamine (DOPE). The goal of this work was to investigate the effect of the chemical structure of these cationic lipids on lipofection. Their ability to form complexes with DNA, their cytotoxicity and their transfection efficiency in vitro and in vivo were studied. Results were compared with those obtained from the well known cholesterol-based cationic lipid DC-Chol. In a melanoma cell line (B16-F10), results showed that either the polar head or the spacer affected the cytotoxicity. Cationic lipids with three ethyl groups in the head are more toxic than those with three methyl groups while cationic lipids with three carbons in the spacer are less toxic than those with two carbons in the spacer. The best transfection level was obtained in vitro and in vivo with cationic lipids having 3C in the spacer. Data indicated that among these lipids, in vivo gene transfer is advantaged by the methylated polar head while in vitro the best level was obtained with the ethylated one. Finally, it was observed that the chemical structure influences the transfection in the presence of serum while the complex charge and the DOPE ratios in liposomes preferentially affect the interaction with erythrocytes. Argumentations are proposed to explain the discrepancies between in vitro and in vivo transfection results concerning the optimal charge ratio and the chemical nature of the cationic lipid head group.

Non-viral gene delivery with cationic liposome-DNA complexes

A large amount of research activity worldwide is currently directed towards developing lipid- or polymer-based, non-viral gene vectors for therapeutic applications. This strong interest is motivated by their low toxicity, ease of production, ability to transfer large pieces of DNA into cells, and lack of immunogenic protein components. Cationic liposomes (CLs) are one of the most powerful non-viral vectors. In fact, CL-based vectors are among the prevalent synthetic carriers of nucleic acids currently used in human clinical gene therapy trials as well as in cell transfection applications for biological research. Our understanding of the mechanisms of action of CL-DNA complexes is still in its infancy. However, the relevance of a few crucial parameters, such as the lipid/DNA charge ratio (rho(chg)) and the membrane charge density of lamellar complexes (sigma(M)), is well established. To arrive at true comparisons of lipid performance, one must optimize both these parameters using a reproducible, reliable transfection assay. In this chapter, we aim to provide the reader with detailed procedures for liposome formation and transfection. It is our hope that the use of such optimized protocols will improve the comparability of transfection data obtained with novel lipids.

Surface polyethylene glycol enhances substrate-mediated gene delivery by nonspecifically immobilized complexes

Substrate-mediated gene delivery describes the immobilization of gene therapy vectors to a biomaterial, which enhances gene transfer by exposing adhered cells to elevated DNA concentrations within the local microenvironment. Surface chemistry has been shown to affect transfection by nonspecifically immobilized complexes using self-assembled monolayers (SAMs) of alkanethiols on gold. In this report, SAMs were again used to provide a controlled surface to investigate whether the presence of oligo(ethylene glycol) (EG) groups in a SAM could affect complex morphology and enhance transfection. EG groups were included at percentages that did not affect cell adhesion. Nonspecific complex immobilization to SAMs containing combinations of EG- and carboxylic acid-terminated alkanethiols resulted in substantially greater transfection than surfaces containing no EG groups or SAMs composed of EG groups combined with other functional groups. Enhancement in transfection levels could not be attributed to complex binding densities or release profiles. Atomic force microscopy imaging of immobilized complexes revealed that EG groups within SAMs affected complex size and appearance and could indicate the ability of these surfaces to preserve complex morphology upon binding. The ability to control the morphology of the immobilized complexes and influence transfection levels through surface chemistry could be translated to scaffolds for gene delivery in tissue engineering and diagnostic applications.

Two-dimensional lipid mixing entropy regulates the formation of multicomponent lipoplexes

The mechanism of formation of multicomponent lipoplexes was investigated by means of synchrotron Small-Angle X-ray Diffraction (SAXD). Mixed lipid dispersions were prepared by mixing different populations of binary cationic liposomes. When adding DNA to mixed lipid dispersions, multicomponent lipoplexes spontaneously formed exhibiting structural properties, i.e., membrane thickness, surface charge density, and one-dimensional DNA packing density, intermediate between those of binary lipoplexes. These results suggested that DNA lets liposomes come into contact and fuse and that a complete lipid mixing at the molecular level occurs. The equilibrium structure of multicomponent lipoplexes was found to be unique and did not depend on the number and kind of populations composing lipid dispersion but only on the lipid species involved and on their relative molar ratio. According to recent theoretical models we identified two-dimensional lipid mixing entropy as the key factor regulating the existence of only multicomponent lipoplexes with ideally mixed lipid species.

Structural and transfection properties of amine-substituted gemini surfactant-based nanoparticles

BACKGROUND

Increases in DNA transfection efficiencies for non-viral vectors can be achieved through rational design of novel cationic building blocks. Based on previous results examining DNA condensation by polyamines, novel gemini surfactants have been designed that incorporate aza or imino substituents within the spacer group in order to increase interactions with DNA and potentially improve their DNA transfection ability.

METHODS

Transfection efficiencies and cell toxicity of gemini nanoparticles constructed from plasmid DNA, gemini surfactant, and a neutral lipid were measured in COS7 cells using a luciferase assay. Structural properties of nanoparticles were examined by using circular dichroism, particle size, zeta potential, and small-angle X-ray scattering (SAXS) measurements.

RESULTS

The incorporation of aza and imino substituents within the spacer group was observed to enhance the transfection ability of gemini surfactants. Incorporation of an imino group in the structure of the 1,9-bis(dodecyl)-1,1,9,9-tetramethyl-5-imino-1,9-nonanediammonium dibromide surfactant (12-7NH-12) resulted in a statistically significant (p < 0.01) 9-fold increase in transfection compared to an unsubstituted gemini surfactant and a 3-fold increase compared to the corresponding aza-substituted compound. A pH-dependent transition in size and zeta potential was observed to occur at pH 5.5 for complexes formed from the 12-7NH-12 compound. SAXS results show weakly ordered structures and the presence of multiple phases.

CONCLUSIONS

The incorporation of a pH-active imino group within the spacer of the gemini surfactant results in a significant increase in transfection efficiency that can be related to both pH-induced changes in nanoparticle structure and the formation of multiple phases that more readily allow for membrane fusion that may facilitate DNA release.

Cationic liposome-DNA complexes: from liquid crystal science to gene delivery applications

At present, there is an unprecedented level of interest in the properties and structures of complexes consisting of DNA mixed with oppositely charged cationic liposomes (CLs). The interest arises because the complexes mimic natural viruses as chemical carriers of DNA into cells in worldwide human gene therapy clinical trials. However, since our understanding of the mechanisms of action of CL-DNA complexes interacting with cells remains poor, significant additional insights and discoveries will be required before the development of efficient chemical carriers suitable for long-term therapeutic applications. Recent studies describe synchrotron X-ray diffraction, which has revealed the liquid crystalline nature of CL-DNA complexes, and three-dimensional laser-scanning confocal microscopy, which reveals CL-DNA pathways and interactions with cells. The importance of the liquid crystalline structures in biological function is revealed in the application of these modern techniques in combination with functional transfection efficiency measurements, which shows that the mechanism of gene release from complexes in the cell cytoplasm is dependent on their precise liquid crystalline nature and the physical and chemical parameters (for example, the membrane charge density) of the complexes. In [section sign] 5, we describe some recent new results aimed at developing bionanotube vectors for gene delivery.

Dendritic Cationic Lipids with Highly Charged Headgroups for Efficient Gene Delivery

Gene therapy is expected to lead to powerful new approaches for curing many diseases, a potential that is currently explored in worldwide clinical trials. Nonviral DNA delivery systems are desirable to overcome the inherent problems of viral vectors, but their current efficiency requires improvement and the understanding of their mechanism of action is incomplete. We have synthesized new multivalent cationic lipids with highly charged dendritic headgroups to probe the structure-transfection efficiency relationships of cationic liposome (CL)-DNA complexes, a prevalent nonviral vector. The lipid headgroups are constructed from ornithine cores and ornithine or carboxyspermine endgroups. The dendritic lipids were prepared on a gram scale, using a synthetic scheme that permits facile variation of the lipid building blocks headgroup, spacer, and hydrophobic moiety. They carry four to sixteen positive charges in their headgroups. Complexes of DNA with mixtures of the dendritic lipids and neutral 1,2-dioleoyl-sn-glycero phosphatidylcholine (DOPC) exhibit novel structures at high contents of the highly charged lipids, while the well-known lamellar phase is formed at high contents of DOPC. DNA complexes of the new dendritic lipids efficiently transfect mammalian cells in culture without cytotoxicity and, in contrast to lamellar complexes, maintain high transfection efficiency over a broad range of composition.

Multicomponent cationic lipid-DNA complex formation: role of lipid mixing

Multicomponent cationic lipid-DNA complexes (lipoplexes) were prepared by adding linear DNA to mixed lipid dispersions containing two populations of binary cationic liposomes and characterized by means of small angle X-ray scattering (SAXS). Four kinds of cationic liposomes were used. The first binary lipid mixture was made of the cationic lipid (3'[N-(N',N'-dimethylaminoethane)-carbamoyl]cholesterol (DC-Chol) and the neutral helper lipid dioleoylphosphocholine (DOPC) (DC-Chol/DOPC liposomes), the second one of the cationic 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and the neutral dioleoylphosphatidylethanolamine (DOPE) (DOTAP/DOPE liposomes), the third one of DC-Chol and DOPE (DC-Chol/DOPE liposomes), and the fourth one of DOTAP and DOPC (DOTAP/DOPC liposomes). Upon DNA-induced fusion of liposomes, large lipid mixing at the molecular level occurs. As a result, highly organized mixed lipoplexes spontaneously form with membrane properties intermediate between those of starting liposomes. By varying the composition of lipid dispersions, different DNA packing density regimes can also be achieved. Furthermore, occurring lipid mixing was found to induce hexagonal to lamellar phase transition in DOTAP/DOPE membranes. Molecular mechanisms underlying experimental findings are discussed.

Polymorphism of DNA-anionic liposome complexes reveals hierarchy of ion-mediated interactions

Self-assembled DNA delivery systems based on anionic lipids (ALs) complexed with DNA mediated by divalent cations have been recently introduced as an alternative to cationic lipid-DNA complexes because of their low cytotoxicity. We investigate AL-DNA complexes induced by different cations by using synchrotron small angle x-ray scattering and confocal microscopy to show how different ion-mediated interactions are expressed in the self-assembled structures and phase behavior of AL-DNA complexes. The governing interactions in AL-DNA systems are complex: divalent ions can mediate strong attractions between different combinations of the components (such as DNA-DNA and membrane-membrane). Moreover, divalent cations can coordinate non-electrostatically with lipids and modify the resultant membrane structure. We find that at low membrane charge densities AL-DNA complexes organize into a lamellar structure of alternating DNA and membrane layers crosslinked by ions. At high membrane charge densities, a new phase with no analog in cationic lipid-DNA systems is observed: DNA is expelled from the complex, and a lamellar stack of membranes and intercalated ions is formed. For a subset of the ionic species, high ion concentrations generate an inverted hexagonal phase comprised of DNA strands wrapped by ion-coated lipid tubes. A simple theoretical model that takes into account the electrostatic and membrane elastic contributions to the free energy shows that this transition is consistent with an ion-induced change in the membrane spontaneous curvature, c0. Moreover, the crossover between the lamellar and inverted hexagonal phases occurs at a critical c0 that agrees well with experimental values.

Cationic lipid–DNA complexes for non-viral gene therapy: relating supramolecular structures to cellular pathways

Cationic liposomes (CLs) are used as nonviral vectors in worldwide human clinical trials of gene therapy. Among other advantages, lipid-DNA complexes have the ability to transfer very large genes into cells, but their efficiency is much lower than that of viruses. Recent studies combining structural and biological techniques are beginning to unravel the relationship between the distinctly structured CL-DNA complexes and their transfection efficiency. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, LalphaC). On rare occasions, an inverted hexagonal structure (HIIC) is observed. An important recent insight is that the membrane charge density (sigmaM) of the CL-vector is a universal parameter governing the transfection efficiency of LalphaC (but not HIIC) complexes. This has led to a new model of the cellular uptake of LalphaC complexes through activated fusion with endosomal membranes. Surface-functionalised complexes with poly(ethylene glycol)-lipids, potentially suitable for transfection invivo, have also been investigated, and the novel aspects of these complexes are discussed.

Lipoplex Structures and Their Distinct Cellular Pathways

Cationic liposomes (CLs) are used as non-viral vectors in worldwide clinical trials of gene therapy. Among other advantages, CL-DNA complexes have the ability to transfer very large genes into cells. However, since the understanding of their mechanisms of action is still incomplete, their transfection efficiencies remain low compared to those of viruses. We describe recent studies which have started to unravel the relationship between the distinct structures and physicochemical properties of CL-DNA complexes and their transfection efficiency by combining several techniques: synchrotron X-ray diffraction for structure determination, laser-scanning confocal microscopy to probe the interactions of CL-DNA particles with cells, and luciferase reporter-gene expression assays to measure transfection efficiencies in mammalian cells. Most CL-DNA complexes form a multilayered structure with DNA sandwiched between the cationic lipids (lamellar complexes, L(alpha)(C)). Much more rarely, an inverted hexagonal structure (H(II)(C)) with single DNA strands encapsulated in lipid tubules is observed. An important recent insight is that the membrane charge density sigma(M) of the CL-vector, rather than, for example, the charge of the cationic lipid, is a universal parameter governing the transfection efficiency of L(alpha)(C) complexes. This has led to a new model of the intracellular release of L(alpha)(C) complexes, through activated fusion with endosomal membranes. In contrast to L(alpha)(C) complexes, H(II)(C) complexes exhibit no dependence on sigma(M), since their structure leads to a distinctly different mechanism of cell entry. Surface-functionalized complexes with poly(ethyleneglycol)-lipids (PEG-lipids), potentially suitable for transfection in vivo, have also been investigated, and the novel aspects of these complexes are discussed.

New multivalent cationic lipids reveal bell curve for transfection efficiency versus membrane charge density: lipid-DNA complexes for gene delivery

BACKGROUND

Gene carriers based on lipids or polymers-rather than on engineered viruses-constitute the latest technique for delivering genes into cells for gene therapy. Cationic liposome-DNA (CL-DNA) complexes have emerged as leading nonviral vectors in worldwide gene therapy clinical trials. To arrive at therapeutic dosages, however, their efficiency requires substantial further improvement.

METHODS

Newly synthesized multivalent lipids (MVLs) enable control of headgroup charge and size. Complexes comprised of MVLs and DNA have been characterized by X-ray diffraction and ethidium bromide displacement assays. Their transfection efficiency (TE) in L-cells was measured with a luciferase assay.

RESULTS

Plots of TE versus the membrane charge density (sigmaM, average charge/unit area of membrane) for the MVLs and monovalent 2,3-dioleyloxypropyltrimethylammonium chloride (DOTAP) merge onto a universal, bell-shaped curve. This bell curve leads to the identification of three distinct regimes, related to interactions between complexes and cells: at low sigmaM, TE increases with increasing sigmaM; at intermediate sigmaM, TE exhibits saturated behavior; and unexpectedly, at high sigmaM, TE decreases with increasing sigmaM.

CONCLUSIONS

Complexes with low sigmaM remain trapped in the endosome. In the high sigmaM regime, accessible for the first time with the new MVLs, complexes escape by overcoming a kinetic barrier to fusion with the endosomal membrane (activated fusion), yet they exhibit a reduced level of efficiency, presumably due to the inability of the DNA to dissociate from the highly charged membranes in the cytosol. The intermediate, optimal regime reflects a compromise between the opposing demands on sigmaM for endosomal escape and dissociation in the cytosol.

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.

Surface functionalized cationic lipid-DNA complexes for gene delivery: PEGylated lamellar complexes exhibit distinct DNA-DNA interaction regimes

Cationic lipid-DNA (CL-DNA) complexes are abundantly used in nonviral gene therapy clinical applications. Surface functionality is the next step in developing these complexes as competent, target-specific gene carriers. Poly(ethylene glycol) (PEG) is the natural choice to serve as a protective coat or act as a tether for a specific ligand on the surface of these complexes due to its biocompatibility and ability to convey stealth-like properties. Understanding the effect of PEG on the internal structure and surface properties of CL-DNA complexes is essential in developing vectors with more complex derivatives of PEG, such as Arg-Gly-Asp (RGD)-based peptide-PEG-lipids. We report on x-ray diffraction studies to probe the internal structure of CL-DNA complexes consisting of a ternary mixture of cationic lipids, neutral lipids, and PEG-lipids. The PEG-coated complexes are found to exhibit a structure consistent with the lamellar phase. In addition, three distinct DNA interchain interaction regimes were found to exist, due to a), repulsive long-range electrostatic forces; b), short-range repulsive hydration forces; and c), novel polymer-induced depletion attraction forces in two dimensions. Optical microscopy and reporter gene assays further demonstrate the incorporation of the PEG-lipids into the lamellar CL-DNA complexes under biologically relevant conditions, revealing surface modification. Both techniques show that PEG-lipids with a polymer chain of molecular weight 400 do not provide adequate shielding of the PEGylated CL-DNA complexes, whereas PEG-lipids with a polymer chain of molecular weight 2000 confer stealth-like properties. This surface functionalization is a crucial initial step in the development of competent vectors for in vivo systemic gene delivery and suggests that a second type of surface functionality can be added specifically for targeting by the incorporation of peptide-PEG-lipids.

Three-dimensional imaging of lipid gene-carriers: membrane charge density controls universal transfection behavior in lamellar cationic liposome-DNA complexes

Cationic liposomes (CLs) are used worldwide as gene vectors (carriers) in nonviral clinical applications of gene delivery, albeit with unacceptably low transfection efficiencies (TE). We present three-dimensional laser scanning confocal microscopy studies revealing distinct interactions between CL-DNA complexes, for both lamellar L(alpha)(C) and inverted hexagonal H(II)(C) nanostructures, and mouse fibroblast cells. Confocal images of L(alpha)(C) complexes in cells identified two regimes. For low membrane charge density (sigma(M)), DNA remained trapped in CL-vectors. By contrast, for high sigma(M), released DNA was observed in the cytoplasm, indicative of escape from endosomes through fusion. Remarkably, firefly luciferase reporter gene studies in the highly complex L(alpha)(C)-mammalian cell system revealed an unexpected simplicity where, at a constant cationic to anionic charge ratio, TE data for univalent and multivalent cationic lipids merged into a single curve as a function of sigma(M), identifying it as a key universal parameter. The universal curve for transfection by L(alpha)(C) complexes climbs exponentially over approximately four decades with increasing sigma(M) below an optimal charge density (sigma(M)(*)), and saturates for at a value rivaling the high transfection efficiency of H(II)(C) complexes. In contrast, the transfection efficiency of H(II)(C) complexes is independent of sigma(M). The exponential dependence of TE on sigma(M) for L(alpha)(C) complexes, suggests the existence of a kinetic barrier against endosomal fusion, where an increase in sigma(M) lowers the barrier. In the saturated TE regime, for both L(alpha)(C) complexes and H(II)(C), confocal microscopy reveals the dissociation of lipid and DNA. However, the lipid-released DNA is observed to be in a condensed state, most likely with oppositely charged macro-ion condensing agents from the cytoplasm, which remain to be identified. Much of the observed bulk of condensed DNA may be transcriptionally inactive and may determine the current limiting factor to transfection by cationic lipid gene vectors.

Thermodynamics of cationic lipid-DNA complex formation as studied by isothermal titration calorimetry

The detailed analysis of the cationic lipid-DNA complex formation by means of isothermal titration calorimetry is presented. Most experiments were done using 1,2-dioleyl-sn-glycero-3-ethylphosphocholine (EDOPC), but basic titrations were also done using DOTAP, DOTAP:DOPC, and DOTAP:DOPE mixtures. Complex formation was endothermic with less than 1 kcal absorbed per mole of lipid or DNA charge. This enthalpy change was attributed to DNA-DNA mutual repulsion within the lamellar complex. The exception was DOTAP:DOPE-containing lipoplex for which the enthalpy of formation was exothermic, presumably because of DOPE amine group protonation. Experimental conditions, namely, direction and titration increment as well as concentration of titrant, which dictate the structure of resulting lipoplex (whether lamellar complex or DNA-coated vesicle), were found to affect the apparent thermodynamics of complex formation. The structure, in turn, influences the biological properties of the lipoplex. If the titration of lipid into DNA was carried out in large increments, the DeltaH was larger than when the injection increments were smaller, a finding that is consistent with increased vesicle disruption under large increments and which is expected theoretically. Cationic lipid-DNA binding was weak in high ionic strength solutions, however, the effective binding constant is within micromolar range because of macromolecular nature of the interaction.

Biophysical and transfection studies of the diC(14)-amidine/DNA complex

Liposomes of the synthetic cationic lipid, N-t-butyl-N'-tetradecylamino-propionamidine (diC(14)-amidine), efficiently ports DNA into mammalian cells in the absence of other (neutral) lipids. The compositional simplicity of this transfection mix makes it attractive from a formulation perspective. We have used low- and wide-angle x-ray diffraction and polarized light microscopy to characterize the thermotropic phase behavior and microstructure of diC(14)-amidine and of the lipid/DNA (circular plasmid, 5.4 kb) complex with a view to understanding the structure of the complex and its role in transfection. Upon heating, the lipid in buffer undergoes a lamellar crystalline (L(c), d(001) = 41.7 A)-to-lamellar liquid crystal (L(c)(alpha), d(001) depends on hydration and T) transition at approximately 40 degrees C. Sonicated lipid vesicles with a reported transition temperature of approximately 23 degrees C complex with DNA. Complex formation is complete at a DNA/lipid mole ratio (rho) of 0.8. Adding DNA to the lipid causes d(001) of the multilayered complex to drop from 52 to 49 A as rho rises from 0.03 to 1.64. The minimal DNA-DNA duplex separation observed is 26 A, consistent with the close packing of B-DNA. Lipid bilayers in the complex undergo a lamellar gel (L(c)(beta))-to-L(c)(alpha) (superscript c refers to complex) transition at approximately 23 degrees C. Transfection efficiency was maximized at rho = 0.4. The structure and transfection data combined suggest that densely packaged DNA in a net positively charged complex is essential for transfection.

Improvement of DNA transfection with cationic liposomes

The increasing use of cationic liposomes as vectors for DNA transfection of eukaryotic cells is due to its high efficiency and reproducibility. After the interaction of the DNA cationic-liposome complexes (DNA-CLC) with the plasma membrane, the entry into the cells delivers the DNA-CLC to the endosome-lysosome pathway where some of the DNA-CLC are degraded. The non-degraded DNA that escapes to the cytoplasm, still has to transverse the nuclear membrane to be transcribed and then translated. To improve the efficiency of the whole process, we can manipulate the DNA (sequences, promoters, enhancers, nuclear localisation signals, etc), the DNA-CLC (lipids) or the plasmatic, endosomal and/or nuclear cellular membranes (ultrasound, electroporation, Ca++, pH of the endosomes, mitosis, fusogenic peptides, nuclear localisation signals, etc). Most of these methods have been generally used individually but in combination, may greatly improve the efficiency and reproducibility of in vitro transfection. While much of this work remains yet to be done and present results further explored, the application of these efforts is essential to the future development of new gene therapy strategies.

Self-assembly of a virus-mimicking nanostructure system for efficient tumor-targeted gene delivery

Molecular therapy, including gene therapy, is a promising strategy for the treatment of human disease. However, delivery of molecular therapeutics efficiently and specifically to the target tissue remains a significant challenge. A human transferrin (Tf)-targeted cationic liposome-DNA complex, Tf-lipoplex, has shown high gene transfer efficiency and efficacy with human head and neck cancer in vitro and in vivo (Xu, L., Pirollo, K.F., Tang, W.H., Rait, A., and Chang, E.H. Hum. Gene Ther. 1999;10:2941-2952). Here we explore the structure, size, formation process, and structure-function relationships of Tf-lipoplex. We have observed Tf-lipoplex to have a highly compact structure, with a relatively uniform size of 50-90 nm. This nanostructure is novel in that it resembles a virus particle with a dense core enveloped by a membrane coated with Tf molecules spiking the surface. More importantly, compared with unliganded lipoplex, Tf-lipoplex shows enhanced stability, improved in vivo gene transfer efficiency, and long-term efficacy for systemic p53 gene therapy of human prostate cancer when used in combination with conventional radiotherapy. On the basis of our observations, we propose a multistep self-assembly process and Tf-facilitated DNA cocondensation model that may provide an explanation for the resultant small size and effectiveness of our nanostructural Tf-lipoplex system.

High temperature stabilization of DNA in complexes with cationic lipids

The influence on the melting of calf thymus and plasmid DNA of cationic lipids of the type used in gene therapy was studied by ultraviolet spectrophotometry and differential scanning calorimetry. It was found that various membrane-forming cationic lipids are able to protect calf thymus DNA against denaturation at 100 degrees C. After interaction with cationic lipids, the differential scanning calorimetry melting profile of both calf thymus and plasmid DNA revealed two major components, one corresponding to a thermolabile complex with transition temperature, T(m(labile)), close to that of free DNA and a second corresponding to a thermostable complex with a transition temperature, T(m(stable)), at 105 to 115 degrees C. The parameter T(m(stable)) did not depend on the charge ratio, R(+/-). Instead, the amount of thermostable DNA and the enthalpy ratio Delta H((stable))/Delta H((labile)) depended upon R(+/-) and conditions of complex formation. In the case of O-ethyldioleoylphosphatidylcholine, the cationic lipid that was the main subject of the investigation, the maximal stabilization of DNA exceeded 90% between R(+/-) = 1.5 and 3.0. Several other lipids gave at least 75% protection in the range R(+/-) = 1.5 to 2.0. Centrifugal separation of the thermostable and thermolabile fractions revealed that almost all the transfection activity was present at the thermostable fraction. Electron microscopy of the thermostable complex demonstrated the presence of multilamellar membranes with a periodicity 6.0 to 6.5 nm. This periodic multilamellar structure was retained at temperatures as high as 130 degrees C. It is concluded that constraint of the DNA molecules between oppositely charged membrane surfaces in the multilamellar complex is responsible for DNA stabilization.

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.

SV40 Immortalization of feline fibroblasts as targets for MHC-restricted cytotoxic T-cell assays

CTL assays in outbred cats have been difficult to perform because of a lack of a good source of syngeneic target cell. Primary fibroblasts from cats are widely used as target cells for MHC-restricted cytotoxic T-cell (CTL) assays, but their limited life-spans of 8-10 culture passages can be problematic for longitudinal studies. To circumvent the life-span limitations of primary fibroblast cultures, we developed a procedure for immortalizing feline primary fibroblast cells by transfection with a molecular clone of simian virus 40 (SV40). Fibroblast cultures from skin biopsies of 28 cats were immortalized using this procedure and have been passaged for longer than 6 months without showing any phenotypic difference from the original primary cells. Non-SV40 transfected feline fibroblasts from a selection of animals in the same group survived for only 6-8 weeks before reaching senescence. The immortalized fibroblasts expressed SV40 T-antigen and Class I MHC protein, and were successfully used as target cells in 51Cr release CTL assays in feline immunodeficiency virus (FIV)-infected cats and in vitro stimulated allogeneic T-cell cultures.