Suppression of RNA recognition by Toll-like receptors: the impact of nucleoside modification and the evolutionary origin of RNA

DNA and RNA stimulate the mammalian innate immune system through activation of Toll-like receptors (TLRs). DNA containing methylated CpG motifs, however, is not stimulatory. Selected nucleosides in naturally occurring RNA are also methylated or otherwise modified, but the immunomodulatory effects of these alterations remain untested. We show that RNA signals through human TLR3, TLR7, and TLR8, but incorporation of modified nucleosides m5C, m6A, m5U, s2U, or pseudouridine ablates activity. Dendritic cells (DCs) exposed to such modified RNA express significantly less cytokines and activation markers than those treated with unmodified RNA. DCs and TLR-expressing cells are potently activated by bacterial and mitochondrial RNA, but not by mammalian total RNA, which is abundant in modified nucleosides. We conclude that nucleoside modifications suppress the potential of RNA to activate DCs. The innate immune system may therefore detect RNA lacking nucleoside modification as a means of selectively responding to bacteria or necrotic tissue.

A novel cationic liposome reagent for efficient transfection of mammalian cells

A novel cationic derivative of cholesterol, 3 beta [N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol), has been synthesized and used to prepare sonicated liposomes with dioleoylphosphatidylethanolamine. This novel cationic liposome reagent facilitates efficient DNA mediated transfection in A431 human epidermoid carcinoma cells, A549 human lung carcinoma cells, L929 mouse fibroblast cells, and YPT minipig primary endothelial cells. The activity was greater than that of a commercial reagent, Lipofectin, and was approximately 4-fold less toxic than Lipofectin when assayed with A431 cells. The reagent is easy to synthesize and stable for at least 6 weeks.

Activation of the complement system by synthetic DNA complexes: a potential barrier for intravenous gene delivery

We have examined the complement-activating properties of synthetic cationic molecules and their complexes with DNA. Commonly used gene delivery vehicles include complexes of DNA with polylysine of various chain lengths, transferrin-polylysine, a fifth-generation poly(amidoamine) (PAMAM) dendrimer, poly(ethyleneimine), and several cationic lipids (DOTAP, DC-Chol/DOPE, DOGS/DOPE, and DOTMA/DOPE). These agents activate the complement system to varying extents. Strong complement activation is seen with long-chain polylysines, the dendrimer, poly(ethyleneimine), and DOGS (half-maximal at about 3 microM amine content in the assay used). Compared to these compounds, the other cationic lipids (in liposome formulations) are weak activators of the complement system (half-maximal approximately 50-100 microM positive charge in assay). Complement activation by polylysine is strongly dependent on the chain length. Short-chain oligolysines are comparable to cationic lipids in their activation of complement. Incubation of these compounds with DNA to form complexes reduces complement activation in virtually all cases. The degree of complement activation by DNA complexes is strongly dependent on the ratio of polycation and DNA (expressed as the charge ratio) for polylysine, dendrimer, poly(ethyleneimine), and DOGS. To a lesser degree, charge ratio also influences complement activation by monovalent cationic lipid-DNA complexes. For polylysine-DNA complexes, complement activation can be considerably reduced by modifying the surface of preformed DNA complexes with polyethyleneglycol (half-maximal approximately 20 microM amine content). The data suggests that, by appropriate formulation of DNA complexes, complement activation can be minimized or even avoided. These findings should facilitate the search for DNA complex formulations appropriate for reproducible intravenous gene delivery.

DNA transfection mediated by cationic liposomes containing lipopolylysine: characterization and mechanism of action

The ability of a polycationic lipid, lipopoly(L-lysine) (LPLL), to mediate efficient DNA transfection depended on scraping of the treated cells (Zhou et al. (1991) Biochim. Biophys. Acta 1065, 8-14). It was found that the mechanical treatment could be avoided by including a helper lipid to the liposome composition. Among the helper lipids tested, a hexagonal phase forming lipid, dioleoylphosphatidylethanolamine (DOPE), gave rise to the highest activity. The transfection efficiency was further optimized by varying the lipophilicity of the LPLL and the ratio of the cationic liposome to DNA. Transfection activity of the optimal DNA-liposome complexes was enhanced by up to 6-fold if cells were pretreated with agents interfering with the process of endocytosis. Meanwhile, pretreatment of cells with a peptide which inhibits membrane fusion decreased the activity by about 60%. These results indicated that DNA-liposome complexes are taken up by an endocytosis mechanism and that cytoplasmic delivery of DNA involves a fusion-related event probably in the endosome compartment. The transfection process was visualized by thin-section electron microscopy. It was found that the complexes entered the cytoplasm mainly by destabilizing endosomes and occasionally by penetrating through the plasma membrane. Therefore, our findings differ from a previous hypothesis which suggests that transfection is mediated by fusion of the liposomes with the plasma membrane of the treated cells.

Gating kinetics of Ca2+-activated K+ channels from rat muscle incorporated into planar lipid bilayers. Evidence for two voltage-dependent Ca2+ binding reactions

The gating kinetics of a Ca2+-activated K+ channel from adult rat muscle plasma membrane are studied in artificial planar bilayers. Analysis of single-channel fluctuations distinguishes two Ca2+- and voltage-dependent processes: (a) short-lived channel closure (less than 1 ms) events appearing in a bursting pattern; (b) opening and closing events ranging from one to several hundred milliseconds in duration. The latter process is studied independently of the first and is denoted as the primary gating mode. At constant voltage, the mean open time of the primary gating mode is a linear function of the [Ca2+], whereas the mean closed time is a linear function of the reciprocal [Ca2+]. In the limits of zero and infinite [Ca2+], the mean open and the mean closed times are, respectively, independent of voltage. These results are predicted by a kinetic scheme consisting of the following reaction steps: (a) binding of Ca2+ to a closed state; (b) channel opening; (c) binding of a second Ca2+ ion. In this scheme, the two Ca2+ binding reactions are voltage dependent, whereas the open-closed transition is voltage independent. The kinetic constant derived for this scheme gives an accurate theoretical fit to the observed equilibrium open-state probability. The results provide evidence for a novel regulatory mechanism for the activity of an ion channel: modulation by voltage of the binding of an agonist molecule, in this case, Ca2+ ion.

Influence of the steric barrier activity of amphipathic poly(ethyleneglycol) and ganglioside GM1 on the circulation time of liposomes and on the target binding of immunoliposomes in vivo

A series of dioleoyl N-(monomethoxy polyethyleneglycol succinyl)phosphatidylethanolamine (PEG-PE) of different polymer chain length was used in this study. Both the activity of PEG-PE in prolonging the circulation time of liposomes and the relative steric barrier activity of amphipathic polymer, measured by a liposome agglutination assay, were found to be directly proportional to the chain length of PEG-PE (PEG5000-PE greater than PEG2000-PE greater than PEG750-PE). However, PEG5000-PE caused a reduced target binding of immunoliposomes in mice due to its overly strong steric barrier activity. The best PEG-PE species supporting the target binding of immunoliposomes was PEG2000-PE, the activity of which was compatible to that of ganglioside GM1. However, GM1 only showed a weak steric barrier activity, suggesting a different mechanism for this glycolipid.

Reactive oxygen species affect mitochondrial electron transport complex I activity through oxidative cardiolipin damage

The aim of this study was to investigate the influence of reactive oxygen species (ROS) on the activity of complex I and on the cardiolipin content in bovine heart submitochondrial particles (SMP). ROS were generated through the use of xanthine/xanthine oxidase (X/XO) system. Treatment of SMP with X/XO resulted in a large production of superoxide anion, detected by acetylated cytochrome c method, which was blocked by superoxide dismutase (SOD). Exposure of SMP to ROS generation resulted in a marked loss of complex I activity and to parallel loss of mitochondrial cardiolipin content. Both these effects were completely abolished by SOD+catalase. Exogenous added cardiolipin was able to almost completely restore the ROS-induced loss of complex I activity. No restoration was obtained with other major phospholipid components of the mitochondrial membrane such as phosphatidylcholine and phosphatidylethanolamine, nor with peroxidized cardiolipin. These results demonstrate that ROS affect the mitochondrial complex I activity via oxidative damage of cardiolipin which is required for the functioning of this multisubunit enzyme complex. These results may prove useful in probing molecular mechanisms of ROS-induced peroxidative damage to mitochondria, which have been proposed to contribute to those pathophysiological conditions characterized by an increase in the basal production of reactive oxygen species such as aging, ischemia/reperfusion and chronic degenerative diseases.

Lipoplex-mediated transfection of mammalian cells occurs through the cholesterol-dependent clathrin-mediated pathway of endocytosis

Synthetic amphiphiles are widely used as a carrier system. However, to match transfection efficiencies as obtained for viral vectors, further insight is required into the properties of lipoplexes that dictate transfection efficiency, including the mechanism of delivery. Although endocytosis is often referred to as the pathway of lipoplex entry and transfection, its precise nature has been poorly defined. Here, we demonstrate that lipoplex-mediated transfection is inhibited by more than 80%, when plasma membrane cholesterol is depleted with methyl-beta-cyclodextrin. Cholesterol replenishment restores the transfection capacity. Investigation of the cellular distribution of lipoplexes after cholesterol depletion revealed an exclusive inhibition of internalization, whereas cell-association remained unaffected. These data strongly support the notion that complex internalization, rather than the direct translocation of plasmid across the plasma membrane, is a prerequisite for accomplishing effective lipoplex-mediated transfection. We demonstrate that internalized lipoplexes colocalize with transferrin in early endocytic compartments and that lipoplex internalization is inhibited in potassium-depleted cells and in cells overexpressing dominant negative Eps15 mutants. In conjunction with the notion that caveolae-mediated internalization can be excluded, we conclude that efficient lipoplex-mediated transfection requires complex internalization via the cholesterol-dependent clathrin-mediated pathway of endocytosis.

Grafted poly-(ethylene glycol) on lipid surfaces inhibits protein adsorption and cell adhesion

Monolayers of dipalmitoyl-phosphatidylethanolamine (DPPE) mixing with various mole percentages of distearoyl-phosphatidylethanolamine (DSPE)-conjugated poly-(ethylene glycol) (PEG m.w. 750-5000) were deposited on DPPE-coated glass surfaces by the Langmuir-Blodgett method. Increasing percentages of grafted PEG in these supported lipid surfaces increasingly inhibit the adsorption of bovine serum albumin (BSA), laminin, and fibronectin. Increasing percentages of grafted PEG also inhibit the adhesion of erythrocytes, lymphocytes, and macrophages to these supported lipid surfaces. The adsorption of proteins on lipid coated glass surfaces were assayed by the fluorescence of FITC-labelled proteins. Cell adhesion was measured mainly by microscopic counting. The concentration of PEG-grafted lipids required for the inhibition of erythrocyte adhesion decreases with increasing molecular weight of the grafted PEG. The inhibitory effects are strongly dependent on the graft density of PEG at low concentrations, but weakly dependent on graft density at higher concentrations. For DSPE-PEG5000, the change of graft density dependency occurs approximately at the complete coverage of the lipid surface by the grafted polymer in the mushroom conformation (0.7 mol%), and the transition to partial brush conformation. The change-overs become less distinctive for grafted PEG of lower molecular weights, probably due to the failure of strictly mushroom and brush models of the polymer. The relative inhibitory efficiency is protein or cell dependent. The implication on the function of stealth liposomes is discussed.

Inhibition of neuronal apoptosis by docosahexaenoic acid (22:6n-3). Role of phosphatidylserine in antiapoptotic effect

Enrichment of Neuro 2A cells with docosahexaenoic acid (22:6n-3) decreased apoptotic cell death induced by serum starvation as evidenced by the reduced DNA fragmentation and caspase-3 activity. The protective effect of 22:6n-3 became evident only after at least 24 h of enrichment before serum starvation and was potentiated as a function of the enrichment period. During enrichment 22:6n-3 incorporated into phosphatidylserine (PS) steadily, resulting in a significant increase in the total PS content. Similar treatment with oleic acid (18:1n-9) neither altered PS content nor resulted in protective effect. Hindering PS accumulation by enriching cells in a serine-free medium diminished the protective effect of 22:6n-3. Membrane translocation of Raf-1 was significantly enhanced by 22:6n-3 enrichment in Neuro 2A cells. Consistently, in vitro biomolecular interaction between PS/phosphatidylethanolamine /phosphatidylcholine liposomes, and Raf-1 increased in a PS concentration-dependent manner. Collectively, enrichment of neuronal cells with 22:6n-3 increases the PS content and Raf-1 translocation, down-regulates caspase-3 activity, and prevents apoptotic cell death. Both the antiapoptotic effect of 22:6n-3 and Raf-1 translocation are sensitive to 22:6n-3 enrichment-induced PS accumulation, strongly suggesting that the protective effect of 22:6n-3 may be mediated at least in part through the promoted accumulation of PS in neuronal membranes.

Liposomes loaded with paclitaxel and modified with novel triphenylphosphonium-PEG-PE conjugate possess low toxicity, target mitochondria and demonstrate enhanced antitumor effects in vitro and in vivo

Previously, stearyl triphenylphosphonium (STPP)-modified liposomes (STPP-L) were reported to target mitochondria. To overcome a non-specific cytotoxicity of STPP-L, we synthesized a novel polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugate with the TPP group attached to the distal end of the PEG block (TPP-PEG-PE). This conjugate was incorporated into the liposomal lipid bilayer, and the modified liposomes were studied for their toxicity, mitochondrial targeting, and efficacy in delivering paclitaxel (PTX) to cancer cells in vitro and in vivo. These TPP-PEG-PE-modified liposomes (TPP-PEG-L), surface grafted with as high as 8 mol% of the conjugate, were less cytotoxic compared to STPP-L or PEGylated STPP-L. At the same time, TPP-PEG-L demonstrated efficient mitochondrial targeting in cancer cells as shown by confocal microscopy in co-localization experiments with stained mitochondria. PTX-loaded TPP-PEG-L demonstrated enhanced PTX-induced cytotoxicity and anti-tumor efficacy in cell culture and mouse experiments compared to PTX-loaded unmodified plain liposomes (PL). Thus, TPP-PEG-PE can serve as a targeting ligand to prepare non-toxic liposomes as mitochondria-targeted drug delivery systems (DDS).

Enhancement of gene expression by polyamidoamine dendrimer conjugates with alpha-, beta-, and gamma-cyclodextrins

To improve the transfection efficiency of nonviral vector, we synthesized the starburst polyamidoamine dendrimer conjugates with alpha-, beta-, and gamma-cyclodextrins (CDE conjugates), expecting the synergistic effect of dendrimer and cyclodextrins (CyDs). The (1)H NMR spectroscopic data indicated that alpha-, beta-, and gamma-CyDs are covalently bound to dendrimer in a molar ratio of 1:1. The agarose gel electrophoretic studies revealed that CDE conjugates formed the complexes with plasmid DNA (pDNA) and protected the degradation of pDNA by DNase I in the same manner as dendrimer. CDE conjugates showed a potent luciferase gene expression, especially in the dendrimer conjugate with alpha-CyD (alpha-CDE conjugate) which provided the greatest transfection activity (approximately 100 times higher than those of dendrimer alone and of the physical mixture of dendrimer and alpha-CyD) in NIH3T3 and RAW264.7 cells. In addition, the gene transfer activity of alpha-CDE conjugate was superior to that of Lipofectin. The enhancing gene transfer effect of alpha-CDE conjugate may be attributable to not only increasing the cellular association, but also changing the intracellular trafficking of pDNA. These findings suggest that alpha-CDE conjugate could be a new preferable nonviral vector of pDNA.

The relationship between the antioxidant and the antibacterial properties of galloylated catechins and the structure of phospholipid model membranes

The effects of four catechins, (+)-catechin (C), (-)-epicatechin (EC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG), on the physical properties of phospholipid model membranes and the correlation to their antioxidant and antibacterial capacities have been studied by using differential scanning calorimetry (DSC), fluorescence spectroscopy, infrared spectroscopy (IR), AAPH-induced oxidation, and leakage experiments. DSC data revealed that galloylated catechins, especially ECG, affected the physical properties of both the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) bilayers dramatically. Galloylated catechins showed higher phospholipid/water partition coefficients than their homologues and were immersed in the phospholipid palisade intercalating within the hydrocarbon chains, ECG being at the deepest position. In contrast, nongalloylated catechins presented a shallow location close to the phospholipid/water interface. ECG also exhibited the highest antioxidant capacity against lipid peroxidation, which correlated with its strong effect on DPH fluorescence anisotropy (as observed by the increase of the lipid order of fluid PC bilayers) and with the presence of highly cooperative transitions as seen by DSC. We propose that the high antioxidant capacity of some galloylated catechins such as ECG could be partially due to the formation of membrane structures showing resistance to detergent solubilization and in which the phospholipids have tightly packed acyl chains and highly hydrated phosphate groups. Significantly, PE was found to be essential to the promotion of carboxyfluorescein leakage from bacterial model membranes by galloylated catechins, indicating that their bactericidal activity, at least at the membrane level, could be due to the specific effect of these catechins on PE.

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.

Phosphatidylinositol-4,5-bisphosphate phosphodiesterase and phosphomonoesterase activities of rat brain. Some properties and possible control mechanisms

The phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] [and to a lesser extent, the phosphatidylinositol-4-phosphate (PtdIns4P)] phosphodiesterase and monoesterase activities of a rat brain supernatant have been studied by using 32P-labelled substrates prepared from human red blood cells. PtdIns(4,5)P2 monoesterase is maximally stimulated by Mg2+, though some activity is detectable in Ca2+/EDTA (Mg2+-free) buffers. The phosphodiesterase, however, is Ca2+-dependent, and in Ca2+/EDTA buffers with the pure lipid as substrate, shows maximal activity at 100 nM-Ca2+. If PtdIns(4,5)P2 is presented as a component of a lipid mixture of similar composition to that of the inner half of the lipid bilayer of a rat liver plasma membrane, the phosphodiesterase shows considerable activity at 1 microM-Ca2+, and is maximal at 100 microM-Ca2+. However, if it is assayed against the same substrate in Ca2+/EGTA buffers with 3mM-Mg2+ and 80 mM-KCl present (as an approximate parallel with the ionic environment in vivo), it shows no detectable activity below 100 microM-Ca2+, and is maximal at 1 mM-Ca2+. The monoesterase can hydrolyse PtdIns(4,5)P2 in such a lipid mixture at all Ca2+ concentrations with 1 or 3 mM-Mg2+ present. PtdIns(4,5)P2 phosphodiesterase can be induced to attack its substrate under ionic conditions similar to those in vivo (0.1-1 microM-Ca2+; 1 mM-Mg2+; 80 mM-KCl) by the conversion of its substrate into a non-bilayer configuration. If given such a substrate [by mixing PtdIns(4,5)P2 with an excess of phosphatidylethanolamine (PtdEtn)] it shows a shallow Ca2+-dependency curve from 0.1 to 100 microM and then a steep rise to 1 mM-Ca2+. Together these observations lead us to the suggestion that a perturbation in a membrane in vivo equivalent to a non-bilayer configuration would be sufficient to induce phosphodiesterase-catalysed PtdIns(4,5)P2 breakdown. When given substrates mixed with excess PtdEtn at pH 7.25 (or 5.5), 1 microM-Ca2+, 1 mM-Mg2+ and 80 mM-KCl, the rat brain supernatant phosphodiesterase activity hydrolysed PtdIns(4,5)P 50-100-fold faster than it hydrolysed phosphatidylinositol (PtdIns). If the supernatant was presented with such a non-bilayer mixture containing a ten-fold excess of PtdIns over PtdIns(4,5)P2, the latter phospholipid was still hydrolysed by phosphodiesterasic cleavage at nearly ten times the rate of the former. Receptor-stimulated phosphodiesterase cleavage of polyphosphoinositides is an early event in cell activation by many agonists. The properties of PtdIns(4,5)P2 phosphodiesterase in vitro suggest that a change in the presentation of its substrate would be a sensitive and sufficient control on the enzyme's activity in vivo.

Intracellular distribution and mechanism of delivery of oligonucleotides mediated by cationic lipids

PURPOSE

To study the parameters influencing the intracellular trafficking of oligonucleotides delivered by cationic 1,2-dioleoyl-3-trimethy- lammonium-propane (DOTAP) lipids and to elucidate the mechanism of uptake.

METHODS

We have studied the intracellular localization of fluorescently labeled oligonucleotide (F-ODN) delivered by DOTAP using confocal microscopy and measured inhibition of luciferase synthesis. The delivery mechanism of ODN/DOTAP complexes was investigated using inhibitors of the endocytosis pathway.

RESULTS

F-ODN delivered by DOTAP liposomes redistribute from punctate cytoplasmic regions into the nucleus. The nuclear uptake of F-ODN depends on: charge ratio (+/-), time of incubation, temperature and presence of serum. A positively charged complex is required for enhanced uptake. The association of neutral lipids with DOTAP reduced the optimum charge ratio without altering the delivery efficiency. DOTAP lipids increased > 100 fold the antisense activity of a specific anti-luciferase ODN. Inhibitors of the endocytosis pathway show that the majority of F-ODN are introduced through an endocytic pathway mainly involving uncoated vesicles. Nuclear accumulation of oligonucleotides can be decreased by inhibitors of actin microfilaments, energy metabolism and proteins implicated in the fusion of endosomes. Nuclear uptake is independent of acidification of the endosomal vesicles and unaffected by inhibitors of microtubules.

CONCLUSIONS

Oligonucleotides are delivered by cationic lipids into the cytoplasm at an early stage of the endocytotic pathway which leads to a marked increase in antisense activity and oligonucleotide nuclear uptake.

Novel pyridinium surfactants for efficient, nontoxic in vitro gene delivery

Novel, double-chained pyridinium compounds have been developed that display highly efficient DNA transfection properties. The transfection efficiency of several of these compounds is enhanced by an order of magnitude, when compared with the transfection efficiency accomplished with the widely used cationic lipid system, lipofectin. Most importantly, the pyridinium compounds were found to be essentially nontoxic toward cells. Using various reporter genes, such as beta-galactosidase and pNEO (a gene construct that renders cells resistent to antibiotic derivatives of neomycin like G418), we demonstrate that the enhanced efficiency relates to the fact that a relative higher number of cells in the population is transfected (approximately 50% in the case of COS cells) by the pyridinium derivatives, whereas the delivery of DNA per cell is also enhanced. Furthermore, application of the pyridinium derivatives shows little cellular preference in their ability to transfect cells. By systematically modifying the structure of the pyridinium amphiphile, i.e., by changing either the headgroup structure or the alkyl chains, some insight was obtained that may lead to unraveling the mechanism of amphiphile-mediated transfection, and thus to protocols that further optimize the carrier properties of the amphiphile. Our results reveal that unsaturated alkyl chains enhance the transfection properties of the pyridinium-based amphiphiles. Preliminary experiments suggest that the structure-dependent improvement of transfection efficiency, when comparing pyridinium derivatives with lipofectin, likely relates to the mechanism of delivery rather than the packaging of the amphiphile/DNA complex.

Design, synthesis, and characterization of pH-sensitive PEG-PE conjugates for stimuli-sensitive pharmaceutical nanocarriers: the effect of substitutes at the hydrazone linkage on the ph stability of PEG-PE conjugates

A set of aliphatic and aromatic aldehyde-derived hydrazone (HZ)-based acid-sensitive polyethylene glycol-phosphatidylethanolamine (PEG-PE) conjugates was synthesized and evaluated for their hydrolytic stability at neutral and slightly acidic pH values. The micelles formed by aliphatic aldehyde-based PEG-HZ-PE conjugates were found to be highly sensitive to mildly acidic pH and reasonably stable at physiologic pH, while those derived from aromatic aldehydes were highly stable at both pH values. The pH-sensitive PEG-PE conjugates with controlled pH sensitivity may find applications in biological stimuli-mediated drug targeting for building pharmaceutical nanocarriers capable of specific release of their cargo at certain pathological sites in the body (tumors, infarcts) or intracellular compartments (endosomes, cytoplasm) demonstrating decreased pH.

Alterations in phospholipid-dependent (Na+ +K+)-ATPase activity due to lipid fluidity. Effects of cholesterol and Mg2+

The (Na+ +K+)-activated, Mg2+-dependent ATPase from rabbit kidney outer medulla was prepared in a partially inactivated, soluble form depleted of endogenous phospholipids, using deoxycholate. This preparation was reactivated 10 to 50-fold by sonicated liposomes of phosphatidylserine, but not by non-sonicated phosphatidylserine liposomes or sonicated phosphatidylcholine liposomes. The reconstituted enzyme resembled native membrane preparations of (Na+ +K+)-ATPase in its pH optimum being around 7.0, showing optimal activity at Mg2+:ATP mol ratios of approximately 1 and a Km value for ATP of 0.4 mM. Arrhenius plots of this reactivated activity at a constant pH of 7.0 and an Mg2+: ATP mol ratio of 1:1 showed a discontinuity (sharp change of slope) at 17 degrees C, with activation energy (Ea) values of 13-15 kcal/mol above this temperature and 30-35 kcal below it. A further discontinuity was also found at 8.0 degrees C and the Ea below this was very high (greater than 100 kcal/mol). Increased Mg2+ concentrations at Mg2+:ATP ratios in excess of 1:1 inhibited the (Na+ +K+)-ATPase activity and also abolished the discontinuities in the Arrhenius plots. The addition of cholesterol to phosphatidylserine at a 1:1 mol ratio partially inhibited (Na+ +K+)-ATPase reactivation. Arrhenius plots under these conditions showed a single discontinuity at 20 degrees C and Ea values of 22 and 68 kcal/mol above and below this temperature respectively. The ouabain-insensitive Mg2+-ATPase normally showed a linear Arrhenius plot with an Ea of 8 kcal/mol. The cholesterol-phosphatidylserine mixed liposomes stimulated the Mg2+-ATPase activity, which now also showed a discontinuity at 20 degrees C with, however, an increased value of 14 kcal/mol above this temperature and 6 kcal/mol below. Kinetic studies showed that cholesterol had no significant effect on the Km values for ATP. Since both cholesterol and Mg2+ are known to alter the effects of temperature on the fluidity of phospholipids, the above results are discussed in this context.

The apoptotic protein tBid promotes leakage by altering membrane curvature

The apoptotic protein tBid is effective in promoting both leakage and lipid mixing in liposomes composed of cardiolipin and phosphatidylcholine at a molar ratio of 1:2 in the presence of calcium. When half of the phosphatidylcholine component of these liposomes is replaced with phosphatidylethanolamine, a lipid that promotes negative membrane curvature, the rates of both leakage and lipid mixing caused by tBid are substantially increased. Replacement of cardiolipin with phosphatidylglycerol, a lipid that is structurally similar to cardiolipin but does not promote negative membrane curvature in the presence of calcium, prevents the tBid from promoting leakage. The promotion of leakage by tBid is also inhibited by several substances that promote positive membrane curvature, including lysophosphatidylcholine, tritrpticin, a potent antimicrobial peptide, and cyclosporin A, a known inhibitor of cytochrome c release from mitochondria. We directly measured the effect of tBid on membrane curvature by (31)P NMR. We found that tBid promotes the formation of highly curved non-lamellar phases. All of these data are consistent with the hypothesis that tBid promotes negative curvature, and as a result it destabilizes bilayer membranes. Bcl-X(L) inhibits leakage and lipid mixing induced by tBid. Bcl-X(L) is anti-apoptotic. It reduces the promotion of non-bilayer phases by tBid, although by itself Bcl-X(L) is capable of promoting their formation. Bcl-X(L) has little effect on liposomal integrity. Our results suggest that the anti-apoptotic activity of Bcl-X(L) is not a consequence of its interaction with membranes, but rather with other proteins, such as tBid.

Dissociation of the dimeric SecA ATPase during protein translocation across the bacterial membrane

The ATPase SecA mediates post-translational translocation of precursor proteins through the SecYEG channel of the bacterial inner membrane. We show that SecA, up to now considered to be a stable dimer, is actually in equilibrium with a small fraction of monomers. In the presence of membranes containing acidic phospholipids or in certain detergents, SecA completely dissociates into monomers. A synthetic signal peptide also affects dissociation into monomers. In addition, conversion into the monomeric state can be achieved by mutating a small number of residues in a dimeric and fully functional SecA fragment. This monomeric SecA fragment still maintains strong binding to SecYEG in the membrane as well as significant in vitro translocation activity. Together, the data suggest that the SecA dimer dissociates during protein translocation. Since SecA contains all characteristic motifs of a certain class of monomeric helicases, and since mutations in residues shared with the helicases abolish its translocation activity, SecA may function in a similar manner.

Lipophilic polylysines mediate efficient DNA transfection in mammalian cells

Low molecular weight (Mr approximately 3000) poly(L-lysine) (PLL) conjugated to N-glutarylphosphatidylethanolamine is an effective carrier to promote DNA-mediated transfection in cultured mammalian cells. The conjugates, named 'lipopolylysines', contained an average of two phospholipid groups per molecule of PLL. Similar conjugates of the non-degradable poly(D-lysine) also had a similar transfection activity, indicating that the degradation of the carrier is not required for the activity. Unconjugated polylysines had little activity. The transfection activity of the lipopolylysine has been optimized with respect to the DNA concentration, DNA/carrier ratio, incubation time and the presence of serum in the incubation medium. The binding of lipopolylysine with DNA was measured by the degree of retardation of DNA in agarose gel electrophoresis. It was found that at the optimal DNA/lipopolylysine ratio for transfection, all DNA were found in large complexes which did not enter the gel. The transfection activity of the lipopolylysine, under optimal conditions, was approximately 3-fold higher than that of lipofectin, a widely used commercial reagent. Moreover, lipopolylysine mediated transfection even in the presence of 10% calf serum; whereas the lipofectin lost about 70% of its activity under the same condition. However, unlike lipofectin the transfection activity of the lipopolylysine depended on scraping the treated cells. Furthermore, lipopolylysine only transfected attached monolayer cells, and not suspension cells.

Potential arrhythmogenic electrophysiological derangements in canine Purkinje fibers induced by lysophosphoglycerides

We have recently detected accumulation of lysophosphoglycerides, catabolites of phospholipids, in ischemic myocardium early after coronary occlusion. In the present study we delineated effects of selected concentrations of albumin-bound lysophosphatidyl choline (LPC) comparable to those accompanying ischemia in vivo on action potentials of isolated canine Purkinje fibers. Lysophosphoglycerides induced concentration-dependent (0.75-3.0 mM) decreases in resting membrane potential, overshoot of phase 0, maximal velocity of upstroke (Vmax) of phase 0, and action potential duration. The highest concentrations (2.0-3.0 mM) induced fractionation of the action potential into several components, unresponsiveness to external stimulation, and enhanced automaticity at normal and reduced membrane potentials. LPC induced a rightward shift in the membrane response curve, a 40-fold prolongation of conduction time, and an increase in the ratio of effective refractory period to action potential duration such that the effective refractory period persisted beyond action potential duration, resulting in postrepolarization refractoriness. These electrophysiological alterations were entirely reversible after 70 minutes of perfusion without LPC, with the exception of a persistent depression in the Vmax of phase 0. Lysophosphatidyl ethanolamine (LPE) elicited alterations in action potentials indentical to those elicited by LPC. Furthermore, LPC (3.0 mM) induced comparable alterations in action potentials recorded from isolated rabbit papillary muscles. Since lysophospholipids accumulate early after myocardial ischemia, and since concentrations equivalent to those occurring in vivo induce electrophysiological alterations resembling those seen in ischemic myocardium in vivo, lysophosphoglycerides may be of major importance as biochemical mediators of malignant dysrhythmia induced by ischemia.

Phosphatidylserine: selective enhancer of histamine release

Phosphatidylserine, in contrast with other phospholipids, markedly enhanced histamine release from rat peritoneal mast cells induced by dextran or protein antigens. This enhancing effect was selective for dextran and protein antigens and did not extend to the action of compound 48/80 or chymotrypsin. These findings suggest a role for phosphatidylserine in the response of mast cells to antigens.