Sunfish Cationic Amphiphiles: Toward an Adaptative Lipoplex Morphology

A detailed physicochemical study is presented on a new class of cationic amphiphiles, Sunfish amphiphiles, recently designed, synthesized, and tested for gene delivery. These materials have two hydrophobic tails, connected to the cationic pyridinium headgroup at the 1- and 4-positions. Two extreme morphologies can be visualized, i.e. one by back-folding involving association of both tails at one side of the pyridinium ring and one by independent unfolding of the tails, the two molecular geometries leading to considerable differences in the aggregate morphology. The behavior of six members of the Sunfish family in mixtures with DOPE, applying different conditions relevant for transfection, has been studied by a combination of techniques (DLS, DSC, NMR, SAXS, Cryo-TEM, fluorescence, etc.). The effects of structural parameters such as the presence of unsaturation in the tails and length of the alkyl chains on the properties of the aggregates have been assessed. A correlation of these structural data with cellular transfection efficiencies reveals that the highest transfection efficiency is obtained with those amphiphiles that are easily hydrated, form fluid aggregates, and undergo a transition to the inverted hexagonal phase in the presence of plasmid DNA (p-DNA) at physiological ionic strength.

Stabilized lipid coated lipoplexes for the delivery of antisense oligonucleotides to liver endothelial cells in vitro and in vivo

We report on the preparation and in vivo/in vitro disposition of antisense ODN encapsulating coated cationic lipoplexes (CCLs), prepared by a procedure essentially developed by Stuart and Allen (Stuart, D.D. and Allen, T.M. (2000) "A new liposomal formulation for antisense oligodeoxynucleotides with small size, high incorporation efficiency and good stability", Biochim. Biophys. Acta 1463, pp. 219-229). The behavior of untargeted CCLs was compared with CCLs that were targeted to scavenger receptors on liver endothelial cells by covalent coupling of the poly-anion aconitylated human serum albumin (Aco-HSA) to the particle surface. By means of cryo transmission electron microscopy (cryo-TEM) particles of high electron density could be distinguished from electron-translucent particles, representing high and low ODN encapsulation, respectively. The two populations were separated by sucrose density gradient centrifugation. Upon injection into rats, the untargeted particles showed long circulating properties with a half-life of >10 h. These untargeted CCLs barely bound to liver endothelial cells in vitro while Aco-HSA CCLs massively and specifically interacted with scavenger receptors on these cells. With J774 cells, a macrophage cell line expressing scavenger receptors, downregulation of ICAM-1 mRNA levels was achieved when the ODN was specifically delivered by Aco-HSA targeted CCLs.

Nonbilayer phase of lipoplex–membrane mixture determines endosomal escape of genetic cargo and transfection efficiency

Cationic lipids are widely used for gene delivery, and inclusion of dioleoylphosphatidylethanolamine (DOPE) as a helper lipid in cationic lipid-DNA formulations often promotes transfection efficacy. To investigate the significance of DOPE's preference to adopt a hexagonal phase in the mechanism of transfection, the properties and transfection efficiencies of SAINT-2/DOPE lipoplexes were compared to those of lipoplexes containing lamellar-phase-forming dipalmitoylphosphatidylethanolamine (DPPE). After interaction with anionic vesicles, to simulate lipoplex-endosomal membrane interaction, SAINT-2/DOPE lipoplexes show a perfect hexagonal phase, whereas SAINT-2/DPPE lipoplexes form a mixed lamellar-hexagonal phase. The transition to the hexagonal phase is crucial for dissociation of DNA or oligonucleotides (ODN) from the lipoplexes. However, while the efficiencies of nucleic acid release from either complex were similar, SAINT-2/DOPE lipoplexes displayed a two- to threefold higher transfection efficiency or nuclear ODN delivery. Interestingly, rupture of endosomes following a cellular incubation with ODN-containing SAINT-2/DPPE complexes dramatically improved nuclear ODN delivery to a level that was similar to that observed for SAINT-2/DOPE complexes. Our data demonstrate that although hexagonal phase formation in lipoplexes is a prerequisite for nucleic acid release from the complex, it appears highly critical for accomplishing efficient translocation of nucleic acids across the endosomal membrane into the cytosol for transport to the nucleus.

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.

Mechanism of formation of multilayered 2D crystals of the Enzyme IIC-mannitol transporter

We have recently reported the crystallization by reconstitution into lipid bilayer structures of Enzyme IIC(mtl), the transmembrane C-domain of the mannitol transporter from E. coli. The projected structure was determined to a resolution of 0.5 nm [J. Mol. Biol. 287 5 (1999) 845]. However, further investigation proved that these crystals were multilamellar stacks instead of 2D crystals, and therefore were unsuitable for three-dimensional structural analysis by electron crystallography. Understanding the crystallogenesis of these crystals could reveal the mechanism of formation of multilayers. In the present study, cryo-electron microscopy (cryo-EM) and turbidimetry are used to study the successive steps of reconstitution of Enzyme IIC(mtl) into phospholipid-containing structures and its crystallization under different conditions. Our experimental approach enabled us to distinguish the separate steps of reconstitution and crystallization. The salt concentration especially influenced the nature of the vesicles, either half open unilamellar or aggregated multilamellar, formed during reconstitution of Enzyme IIC(mtl). The presence of DOPE and DOPC and the temperature influenced the type of lipid structures that were formed during the crystallization phase of Enzyme IIC(mtl). Cryo-EM showed that protein crystallization is closely associated with the formation of isotropic lipid (cubic) phases. We believe that DOPE is responsible for the formation of these lipid cubic phases, and that crystallization is driven by exclusion of protein from these phases and its concentration into the lamellar phases. This mechanism is inextricably associated with the formation of multilayers.

Transfection mediated by gemini surfactants: engineered escape from the endosomal compartment

The structure of the lipoplex formed from DNA and the sugar-based cationic gemini surfactant 1, which exhibits excellent transfection efficiency, has been investigated in the pH range 8.8-3.0 utilizing small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-TEM). Uniquely, three well-defined morphologies of the lipoplex were observed upon gradual acidification: a lamellar phase, a condensed lamellar phase, and an inverted hexagonal (H(II)) columnar phase. Using molecular modeling, we link the observed lipoplex morphologies and physical behavior to specific structural features in the individual surfactant, illuminating key factors in future surfactant design, viz., a spacer of six methylene groups, the presence of two nitrogens that can be protonated in the physiological pH range, two unsaturated alkyl tails, and hydrophilic sugar headgroups. Assuming that the mechanism of transfection by synthetic cationic surfactants involves endocytosis, we contend that the efficacy of gemini surfactant 1 as a gene delivery vehicle can be explained by the unprecedented observation of a pH-induced formation of the inverted hexagonal phase of the lipoplex in the endosomal pH range. This change in morphology leads to destabilization of the endosome through fusion of the lipoplex with the endosomal wall, resulting in release of DNA into the cytoplasm.

Interference of poly(ethylene glycol)-lipid analogues with cationic-lipid-mediated delivery of oligonucleotides; role of lipid exchangeability and non-lamellar transitions

Cationic liposomes are applied to transfer oligonucleotides (ODNs) into cells to regulate gene expression for gene therapeutic or cell biological purposes. In vivo, poly(ethylene glycol) (PEG)-lipid derivatives are employed to stabilize and prolong the circulation lifetime of nucleic acid-containing particles, and to improve targeting strategies. In this study, we have studied the effects of PEG-lipid analogues, i.e. PEG coupled to either phosphatidylethanolamine (PE) or ceramide, on cationic-lipid-DNA complex ('lipoplex') assembly and the mechanism of cationic-lipid-mediated delivery of ODNs in vitro. Inclusion of 10 mol% PEG-PE in ODN lipoplexes inhibited their internalization in Chinese hamster ovary cells by more than 70%. The intracellular fraction remained entrapped in the endosomal/lysosomal pathway, and no release of ODNs was apparent. Similar observations were made for complexes prepared from liposomes that contained PEG-ceramides. Interestingly, delivery resumed when lipoplexes had been externally coated with PEG-ceramides. In this case, the kinetics of delivery were dependent on the length of the ceramide acyl chain, consistent with a requirement for the PEG-lipid to dissociate from the complex. Moreover, although the chemical nature of the PEG-ceramides distinctly affected the net internalization of the complexes, impediment of delivery was largely related to an inhibitory effect of the PEG-lipid on the release of ODNs from the endosomal compartment. Cryo-electron microscopy and small-angle X-ray scattering revealed that the PEG-lipids stabilize the lamellar phase of the lipoplexes, while their acyl-chain-length-dependent transfer from the complex enables adaptation of the hexagonal phase. Within the endosomal compartment, this transition appears to be instrumental in causing the dissociation and cytosolic release of the ODNs for their nuclear homing.