Structural studies of the monolayers and bilayers formed by a novel cholesterol-phospholipid chimera.

Langmuir isotherm, neutron reflectivity, and small angle neutron scattering studies have been conducted to characterize the monolayers and vesicular bilayers formed by a novel chimeric phospholipid, ChemPPC, that incorporates a cholesteryl moeity and a C-16 aliphatic chain, each covalently linked via a glycerol backbone to phosphatidylcholine. The structures of the ChemPPC monolayers and bilayers are compared against those formed from pure dipalmitoylphoshatidylcholine (DPPC) and those formed from a 60:40 mol % mixture of DPPC and cholesterol. In accord with previous findings showing that very similar macroscopic properties were exhibited by ChemPPC and 60:40 mol % DPPC/cholesterol vesicles, it is found here that the chimeric lipid and lipid/sterol mixture have very similar monolayer structures (each having a monolayer thickness of ∼26 Å), and they also form vesicles with similar lamellar structure, each having a bilayer thickness of ∼50 Å and exhibiting a repeat spacing of ∼65 Å. The interfacial area of ChemPPC, however, is around 10 Å(2) greater than that of the combined DPPC/cholesterol unit in the mixed lipid monolayer (viz., 57 ± 1 vs 46 ± 1 Å(2), at 35 mN·m(-1)), and this difference in area is attributed to the succinyl linkage which joins the ChemPPC steroid and glyceryl moieties. The larger area of the ChemPPC is reflected in a slightly thicker monolayer solvent distribution width (9.5 vs 9 Å for the DPPC/cholesterol system) and by a marginal increase in the level of lipid headgroup hydration (16 vs 13 H(2)O per lipid, at 35 mN·m(-1)).

Robust and prolonged gene expression from injectable polymeric implants.

We introduce an injectable system for the formation of a biodegradable DNA-containing implant that releases DNA over a 2-month period to provide a robust and prolonged gene expression at the site. Sustained delivery of the appropriate plasmid DNA resulted in sustained expression of luciferase, the persistent appearance of secreted alkaline phosphatase in the serum and small blood vessel formation in the vicinity of the implant from the delivery of the development endothelial locus-1 gene. Local expression of development endothelial locus-1 protein promotes the development of blood vessels to meet the metabolic demands of new tissue and is a paradigm for the delivery of other growth factors that act locally to aid tissue regeneration. This delivery system involves simple preparation procedures and can be injected directly into the site, hence should be a useful approach to plasmid-based gene transfer for vaccination and tissue engineering.

Mechanism of lipoplex gene delivery in mouse lung: binding and internalization of fluorescent lipid and DNA components.

We introduce a lung inflation-fixation protocol to examine the distribution and gene transfer efficiency of fluorescently tagged lipoplexes using fluorescence confocal microscopy within thick lung tissue sections. Using this technique, we tested the hypothesis that factors related to lipoplex distribution were the predominant reason that intravenous (i.v.) administration of lipoplex was superior to intratracheal (i.t.) administration for gene transfer in the murine lung. Lipoplex distribution was analyzed using digitized images of overlapping fields, reconstructed to view an entire lung lobe. Intravenously administered lipoplexes were confined to the capillary network and homogenously distributed throughout the lung lobe. In contrast, i.t. administration resulted in regional distribution of lipoplex, concentrated around bronchioles and distal airways. Not all the bronchioles were stained with lipoplex, suggesting that the airway-administered solution became channeled through certain bronchiolar pathways. A fluorescent oligonucleotide was used as a marker for cytoplasmic release of nucleic acids. Quantification of the resulting fluorescent nuclei was used to define the relationship between cytoplasmic release of nucleic acids and gene expression. Endothelial cells were stained after i.v. administration, and epithelial cells were stained after i.t. administration. The delivery of nucleic acids was also more homogeneous with i.v. administration of lipoplex than with i.t. administration. After i.t. administration, it was notable that high concentrations of fluorescent nuclei correlated with low GFP expression. This suggested that toxicity was associated with high local concentrations of cationic lipoplexes. The ratio of GFP-expressing cells to fluorescent nuclei indicated that capillary endothelial cells were more efficient in gene expression per delivery event than were pulmonary epithelial cells. Thus, the greater gene expression efficiency of i.v. administered lipoplexes was due not only to the initial distribution but also to the greater efficiency of the vascular endothelial cells to appropriately traffic and express the foreign gene.

Phage display selection of a peptide DNase II inhibitor that enhances gene delivery.

BACKGROUND: Nuclease activity is thought to be a significant barrier to effective gene delivery employing synthetic vectors. In particular, the lysosomal DNase, DNase II, has significant access to plasmid DNA, when the protective condensing agent has been shed. Here, we present the identification of a peptide DNase II inhibitor, enabling enhanced levels of gene delivery. METHODS: A DNase II inhibitor was identified by phage display from a cyclic, random 12-amino acid library. Activity was assayed by inhibition of DNase II degradation of DNA. Transfection enhancement levels were measured over a range of DNA doses with CV-1 and MDBK cell types using PEI and cationic lipoplexes as vectors. RESULTS: We postulated that a DNase II inhibitor would enhance transfection by enabling a larger fraction of plasmid DNA to traffic through the cell and enter the nucleus. Peptides based on the selected sequence (SLRLLQWFLWAC) [ID2] were shown to inhibit DNase II with an observed KI,app of 0.2-2 microM. Lipoplex-mediated transfection in vitro was found to be enhanced by ID2-3 across the entire range of plasmid DNA doses examined (0.10-3.0 microg/mL). Transfection with PEI/DNA complexes was found to be specifically enhanced in the presence of ID2 peptides, with a saturable DNA-dose curve as would be expected for a competitive inhibitor. Transfection enhancements as high as 270-fold were found in the presence of ID2-3. CONCLUSIONS: A novel peptide DNase II inhibitor has been used to increase transfection. The level of enhancement was found to be significant in multiple cell types with multiple synthetic vectors.

Liposome-encapsulated doxorubicin targeted to CD44: a strategy to kill CD44-overexpressing tumor cells.

Certain tumors, including many that are found in the lung, overexpress the CD44 cell-surface marker. CD44 is a receptor that binds to hyaluronan (HA), a carbohydrate consisting of beta1,3 N-acetyl glucosaminyl-beta1,4 glucuronide. We hypothesized that the incorporation of phosphatidylethanolamine lipid derivatives-containing HA oligosaccharides (HA-PE) into liposomes could target drug-containing liposomes to tumor cells that express CD44. HA-PE containing palmitoyl oleoyl phosphatidylethanolamine or dipalmitoyl phosphatidylethanolamine (HAn-PE) were incorporated into the lipid bilayer at various mole percentages of the total lipids; and the physicochemical properties (diameter, surface charge, and stability) of the resulting liposome preparations were characterized. HA-targeted liposomes (HALs) avidly bound to the CD44-high-expressing B16F10 murine melanoma cell line but not to the CV-1 African green monkey kidney cells, which express CD44 at low levels. Binding of the HALs to the B16F10 cells was rapid, concentration dependent, and saturated at a lipid concentration of about 250 microM. HAL binding to B16F10 was inhibited by HA with high Mr and by an anti-CD44 monoclonal antibody. Binding to the B16 melanoma cells occurred at a lipid composition that contained a > or =0.1 mol % of the HAn-PE lipid. The bound liposomes were internalized by a temperature-dependent process. The IC50s of doxorubicin (DOX) encapsulated in either HALs or nontargeted liposomes and of nonencapsulated DOX were compared in two protocols: continuous exposure of the cells to treatment for 24 h and transient exposure in which the treatment was applied for a 3-h period, and in which non-cell-associated drug was replaced with drug-free medium for the duration of the experiment. The IC50s of free DOX, DOX-loaded nontargeted liposomes, and DOX-loaded HAL (HAL-DOX) for the transient exposure were 6.4 microM, > 172 microM, and 0.78 microM, respectively. For the continuous exposure protocol, the IC50s were 0.60 microm, 25.0 microl, and 0.14 microm, respectively. Thus, in both protocols, delivered DOX was significantly more potent than the nonencapsulated DOX in cells expressing high levels of CD44, which suggests that HALs may be a useful targeted drug carrier to treat CD44-expressing tumors.

Steric stabilization of fusogenic liposomes by a low-pH sensitive PEG--diortho ester--lipid conjugate.

We describe the synthesis and characterization of a pH-sensitive poly(ethylene glycol)-diortho ester-distearoyl glycerol conjugate (POD). POD was prepared by a one-step synthesis, and its acid sensitivity characterized by TLC. The conjugate was found to be stable at neutral pH for greater than 3 h but degraded completely within 1 h at pH 5. Liposomes composed of 10% of POD and 90% of a fusogenic lipid, dioleoyl phosphatidylethanolamine (DOPE) were readily prepared and remained stable for up to 12 h in neutral buffer as shown by photon correlation spectrometry and a liposome contents leakage assay. However, when POD/DOPE liposomes were incubated in acidic pH as mild as 5.5, they aggregated and released most of their contents within 30 min. The kinetics of content release from POD/DOPE liposomes consisted of two phases, a lag phase, and a burst phase. The lag phase is inversely correlated with pH and the logarithm of the length of lag phase showed a linear relationship with the buffer pH. When the POD/DOPE liposomes were incubated in 75% of fetal bovine serum at 37 degrees C, they remained as stable as traditional PEG-grafted liposomes for 12 h but released 84% of the encapsulated ANTS in the following 4 h. Upon intravenous administration into mice, liposomes composed of 10% POD and 90% DOPE were cleared from circulation by a one-compartment kinetics with a half-life of about 200 min. POD is an example for the design of a novel category of pH sensitive lipids composed of a headgroup, an acid-labile diortho ester linker and a hydrophobic tail. The uniquely fast degradation kinetics of POD at pH 5-6 and its ability to stabilize liposomes in serum make the conjugate suitable for applications for triggered drug release systems targeted to mildly acidic bio-environments such as endosomes, solid tumors, and inflammatory tissues.

Branched polyamines cure prion-infected neuroblastoma cells.

Branched polyamines, including polyamidoamine and polypropyleneimine (PPI) dendrimers, are able to purge PrP(Sc), the disease-causing isoform of the prion protein, from scrapie-infected neuroblastoma (ScN2a) cells in culture (S. Supattapone, H.-O. B. Nguyen, F. E. Cohen, S. B. Prusiner, and M. R. Scott, Proc. Natl. Acad. Sci. USA 96:14529-14534, 1999). We now demonstrate that exposure of ScN2a cells to 3 microg of PPI generation 4.0/ml for 4 weeks not only reduced PrP(Sc) to a level undetectable by Western blot but also eradicated prion infectivity as determined by a bioassay in mice. Exposure of purified RML prions to branched polyamines in vitro disaggregated the prion rods, reduced the beta-sheet content of PrP 27-30, and rendered PrP 27-30 susceptible to proteolysis. The susceptibility of PrP(Sc) to proteolytic digestion induced by branched polyamines in vitro was strain dependent. Notably, PrP(Sc) from bovine spongiform encephalopathy-infected brain was susceptible to PPI-mediated denaturation in vitro, whereas PrP(Sc) from natural sheep scrapie-infected brain was resistant. Fluorescein-labeled PPI accumulated specifically in lysosomes, suggesting that branched polyamines act within this acidic compartment to mediate PrP(Sc) clearance. Branched polyamines are the first class of compounds shown to cure prion infection in living cells and may prove useful as therapeutic, disinfecting, and strain-typing reagents for prion diseases.

Evidence for the prion hypothesis: induction of the yeast [PSI+] factor by in vitro- converted Sup35 protein.

Starting with purified, bacterially produced protein, we have created a [PSI(+)]-inducing agent based on an altered (prion) conformation of the yeast Sup35 protein. After converting Sup35p to its prion conformation in vitro, we introduced it into the cytoplasm of living yeast using a liposome transformation protocol. Introduction of substoichiometric quantities of converted Sup35p greatly increased the rate of appearance of the well-characterized epigenetic factor [PSI+], which results from self-propagating aggregates of cellular Sup35p. Thus, as predicted by the prion hypothesis, proteins can act as infectious agents by causing self-propagating conformational changes.

Fluorometric determination of deoxyribonuclease I activity with PicoGreen.

A rapid and sensitive assay for the detection of deoxyribonuclease I (DNase I) activity is described. This method is based on the ability of PicoGreen dye to enhance its fluorescence when bound to double-stranded DNA. In the standard assay, reaction mixtures containing the DNase I sample and 0.2 microg of the substrate DNA were prepared in a fluorescence microtiter plate and incubated at 37 degrees C. At the end of the reaction, the diluted PicoGreen reagent was added to each well and fluorescence intensity was measured with a fluorescence plate reader. By this assay, it was possible to determine precisely as little as 5 pg of DNase I within an hour. Moreover, using a small amount of the substrate DNA, the method was shown to be suitable for the sensitive detection of DNase I inhibitor activity.

Gene delivery and expression in human retinal pigment epithelial cells: effects of synthetic carriers, serum, extracellular matrix and viral promoters.

Non-viral gene therapy is a potential treatment to many incurable retinal diseases. To fulfill this promise, plasmid DNA must be delivered to the retinal target cells. We evaluated the efficacy of synthetic DNA complexing compounds in transfecting primary human retinal pigment epithelial (RPE) cells in vitro. Fetal human RPE cells were cultured with or without extracellular matrix (ECM), produced using calf corneal endothelial cells. Plasmids encoding nuclear localizing beta galactosidase or luciferase (pRSVLuc, pCLuc4, pSV2Luc) were complexed in water at various +/- charge ratios using cationic lipids (Lipofectin, DOTAP, DOGS), polyethylene imines (25 and 750 kDa), and with degraded 6th generation starburst polyamidoamine dendrimers. Luciferase was quantified using a luminometric assay and beta galactosidase with X-gal staining. Toxicities of transfections were evaluated with the MTT-assay. Using beta galactosidase as the reporter gene naked DNA did not transfect RPE cells at measurable levels whereas 1-5% of the cells expressed histochemically detectable amounts of the gene after transfection with cationic lipid DNA complexes. In RPE cells, Rous sarcoma virus and cytomegalovirus (CMV) were more efficient promoters than SV40 in driving luciferase expression, and CMV was chosen for further experiments. At optimal complex charge ratios, expression levels of luciferase were > 10(9) light units/mg protein after transfection using dendrimers and PEI25, while transfection mediated with the other carriers resulted in luciferase expression levels of 10(7)-10(9) light units/mg protein or less. In general, dendrimers and large molecular weight PEI were less toxic than cationic lipids or PEI25 to RPE cells. Serum and ECM decreased gene expression to the RPE cells with all carriers. Despite low percentage of transfected cells the transgene expression per RPE cell is high, important feature in the retinal tissue with small dimensions, in particular in the case of secreted gene products. Degraded dendrimers and high molecular weight PEI exhibited the best combination of high activity and low toxicity in RPE cell transfection.

Effect of phospholipid composition on an amphipathic peptide-mediated pore formation in bilayer vesicles.

To better understand the influence of phospholipid acyl-chain composition on the formation of pores by cytotoxic amphipathic helices in biological membranes, the leakage of aqueous contents induced by the synthetic peptide GALA (WEAALAEALAE ALAEHLAEALAEALEALAA) from large unilamellar phospholipid vesicles of various compositions has been studied. Peptide-mediated leakage was examined at pH 5.0 from vesicles made of phosphatidylcholine (PC) and phosphatidylglycerol (PG) with the following acyl-chain compositions: 1-palmitoyl-2-oleoyl (PO), 1,2-dioleoyl (DO), 1, 2-dielaidoyl (DE), and 1,2-dipetroselinoyl (DPe). A mathematical model predicts and simulates the final extents of GALA-mediated leakage of 1-aminonaphthalene-3,6,8-trisulfonic acid (ANTS) and p-xylene-bis-pyridinium bromide (DPX) from 1-palmitoyl-2-oleoyl-phosphatidylcholine/1-palmitoyl-2-oleoyl-phospha tidylglycerol (POPC/POPG) and 1, 2-dielaidoyl-sn-glycero-3-phosphocholine/1, 2-dielaidoyl-phosphatidylglycerol (DEPC/DEPG) liposomes at pH 5.0 as a function of peptide concentration in the bilayer, by considering that GALA pores responsible for this leakage have a minimum size of 10 +/- 2 monomers and are formed by quasiirreversible aggregation of the peptide. With the phospholipid acyl-chain compositions tested, GALA-induced ANTS/DPX leakage follows the rank order POPC/POPG approximately DEPC/DEPG > DPePC/DPePG > DOPC/DOPG. Results from binding experiments reveal that this reduced leakage from DOPC/DOPG vesicles cannot be explained by a reduced binding affinity of the peptide to these membranes. As shown by monitoring the leakage of a fluorescent dextran, an increase in the minimum pore size also does not explain the reduction in ANTS/DPX leakage. The data suggest that surface-associated GALA monomers or aggregates are stabilized in bilayers composed of phospholipids containing a cis unsaturation per acyl chain (DO and DPe), while transbilayer peptide insertion is reduced. GALA-induced ANTS/DPX leakage is also decreased when the vesicles contain phosphatidylethanolamine (PE). This lends further support to the suggestion that factors stabilizing the surface state of the peptide reduce its insertion and subsequent pore formation in the bilayer.

Controlled template-assisted assembly of plasmid DNA into nanometric particles with high DNA concentration.

A series of novel cationic detergents that contain cleavable hydrophilic isothiuronium headgroups was synthesized, and their utility in controlled assembly of plasmid DNA into small stable particles with high DNA concentration investigated. The detergents have alkyl chains of C(8)-C(12) and contain hydrophilic isothiuronium headgroups that give relatively high critical micelle concentration (CMC) to the detergents (>10 mM). The isothiuronium group masks a sulfhydryl group on the detergent and can be cleaved in a controlled manner under basic conditions to generate a reactive thiol group. The thiol group can undergo a further reaction after the detergents have accumulated on a DNA template to form a disulfide-linked lipid containing two alkyl chains. The pH-dependent kinetics of cleavage of the isothiuronium group, the CMC of the surfactants, the formation of the complexes, and the transfection efficiency of the DNA complexes have been investigated. Using the C(12) detergent, a approximately 6 kilo-basepair plasmid DNA was compacted into a small particle with an average diameter of around 40 nm with a approximately -13 mV zeta-potential at high DNA concentration (up to 0.3 mg/mL). The compounds were well tolerated in cell culture and showed no cytotoxicity under their CMCs. Under appropriate conditions, the small particle retained transfection activity.

Cationic lipids are essential for gene delivery mediated by intravenous administration of lipoplexes.

It was recently suggested that intravenously administered lipoplexes serve as a depot for the extracellular release of naked DNA and it is the naked DNA that mediates gene delivery in the lung. If this is the mechanism responsible for gene expression, we reasoned that continuous infusion of plasmid DNA should also result in significant lung expression in the absence of lipoplexes. Moreover, the infusion of non-coding plasmid DNA should inhibit gene delivery by lipoplexes. Infusion of plasmid DNA at a rate of 80 microg/min into the tail vein of a mouse resulted in a DNA serum concentration of 800 microg/ml. This was equivalent to a transcriptionally active DNA concentration of 120 microg/ml plasma as determined by an in vitro transfection assay. In spite of this high level of transcriptionally active DNA, there was no significant gene expression in the lung or any other organ tested. In addition, when lipoplex containing a reporter gene was injected, followed by an infusion of non-coding plasmid DNA as a potential competing molecule for DNA released from the lipoplex there was no effect on gene expression. These experiments indicate that the cationic lipid component of the lipoplex functions in an active capacity beyond that of a simple passive release matrix for plasmid DNA.

Physicochemical characterization and purification of cationic lipoplexes.

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

Lipoplex-mediated gene delivery to the lung occurs within 60 minutes of intravenous administration.

We have defined the critical time period for gene delivery in the lung after intravenous administration of cationic lipoplex. We accomplished this through the displacement of intravenously injected cationic lipoplexes from the lungs by the subsequent administration of anionic liposomes. When reporter gene-bearing lipoplexes were injected intravenously and followed by anionic liposomes 5 min later, reporter gene expression was reduced up to 400-fold compared with animals into which lipoplex alone was administered. Administration of anionic liposomes 60-90 min after lipoplex injection yielded no significant reduction in lung transfection. When lipoplexes were disrupted 5 min after administration, the pulmonary distribution of the cationic lipid and DNA components was reduced by 80%. Lipids subsequently accumulated primarily in the liver, while the plasmid DNA constituent distributed into the blood and liver. As the interval between lipoplex and anionic liposome injection increased, the degree of lipoplex displacement from the lung decreased to such a point that, 60 min after lipoplex injection, the anionic liposome injection did not displace significant quantities of the lipoplex. We conclude that cationic lipid-DNA complexes can be disrupted in vivo via the administration of anionic liposomes; moreover, we have employed this phenomenon to demonstrate that transfectionally active DNA is taken up within 60 min of systemic lipoplex administration.

Surface aggregation and membrane penetration by peptides: relation to pore formation and fusion.

The peptide GALA undergoes a conformational change to an amphipathic alpha-helix when the pH is reduced, inducing leakage of contents from vesicles. Leakage from neutral or negatively-charged vesicles at pH 5.0 was similar and could be adequately explained by a mathematical model which assumed that GALA becomes incorporated into the vesicle bilayer and irreversibly aggregates to form a pore consisting of M = 10 +/- 2 peptides. Increasing cholesterol content in the membranes resulted in reduced leakage, and increased reversibility of surface aggregation of the peptide. Employing fluorescently labelled peptides confirmed that the degree of reversibility of surface aggregation of GALA was significantly larger in cholesterol containing liposomes. Orientation of the peptide GALA in bilayers was determined by a bodipy-avidin/biotin binding assay. The peptide was labelled by biotin at the N- or C-terminus and bodipy-avidin molecules were added externally or were preencapsulated in the vesicles. The peptides are arranged in the pore perpendicularly to the membrane, such that 3/4 of the N-termini are on the internal side of the membrane. The pores are stable and persist for at least 10 min. When the peptides form an aggregate of size smaller than M, the orientation of the peptide is mostly parallel to the surface and the biotinylated peptide does not translocate. When a critical size of the aggregate is attained, a rearrangement of the peptide occurs, which amounts to rapid penetration and formation of a pore structure. Induction of fusion by peptides may be antagonistic to pore formation, the outcome being dependent on vesicle aggregation.

Efficient adventitial gene delivery to rabbit carotid artery with cationic polymer-plasmid complexes.

Different lipids and cationic polymers were tested in vitro for their ability to transfect rabbit aortic smooth muscle cells and human endothelial cells with lacZ marker gene. Toxicity of the complexes was evaluated with MTT assay. Selected plasmid-polymer complexes with different charge ratios were then tested for in vivo gene transfer efficiency using adventitial gene transfer by placing a silastic gene delivery reservoir (collar) around the carotid artery. Transfection efficiency was determined by X-gal staining 3 days after the gene transfer. Based on in vitro experiments, fractured polyamidoamine dendrimers and polyethylenimines (PEI) were selected for in vivo experiments. Fractured dendrimers (generation 6, +/- charge ratio of 3) had the highest in vivo gene transfer efficiency (4.4% +/- 1.7). PEI with molecular size of 25 kDa (+/- charge ratio 4) was also effective (2.8% +/- 1.8) in this model. PEI of 800 kDa showed a constant but modest gene transfer efficiency (1.8% +/- 0.1) with all charge ratios. A low level gene transfer was also detected with naked DNA (0.5% +/- 0.3). No signs of inflammation were seen in any of the study groups. We show here that in vitro cell culture experiments can be used to identify efficient in vivo gene transfer methods for arterial gene therapy, but the charge ratios for each complex must be optimized in vivo. It is concluded that fractured dendrimer and PEI are efficient gene delivery vehicles and can be used for arterial gene therapy via adventitial gene delivery route.

Orientation of the pore-forming peptide GALA in POPC vesicles determined by a BODIPY-avidin/biotin binding assay.

We determined the orientation of a biotinylated version of the pore-forming peptide GALA (WEAALAEALAEALAEHLAEALAEALEALAA) at pH 5.0 in large unilamellar phosphatidylcholine vesicles, using the enhancement of BODIPY-avidin fluorescence subsequent to its irreversible binding to a biotin moiety. GALA and its variants were biotinylated at the N- or C-terminus. BODIPY-avidin was either added externally or was pre-encapsulated in vesicles to assess the fraction of liposome-bound biotinylated GALA that exposed its labeled terminus to the external or internal side of the bilayer, respectively. Under conditions where most of the membrane-bound peptides were involved in transmembrane aggregates and formed aqueous pores (at a lipid/bound peptide molar ratio of 2500/1), the head-to-tail (N- to C-terminus) orientation of the membrane-inserted peptides was such that 3/4 of the peptides exposed their N-terminus on the inside of the vesicle and their C-terminus on the outside. Under conditions resulting in reduced pore formation (at higher lipid/peptide molar ratios), we observed an increase in the fraction of GALA termini exposed to the outside of the vesicle. These results are consistent with a model (Parente et al., Biochemistry, 29:8720, 1990) that requires a critical number of peptides (M) in an aggregate to form a transbilayer structure. When the peptides form an aggregate of size i, with i < M = 4 to 6, the orientation of the peptides is mostly parallel to the membrane surface, such that both termini of the biotinylated peptide are exposed to external BODIPY-avidin. This BODIPY-avidin/biotin binding assay should be useful to determine the orientation of other membrane-interacting molecules.

Branched cationic peptides for gene delivery: role of type and number of cationic residues in formation and in vitro activity of DNA polyplexes.

To examine the suitability of synthetic peptides as DNA-binding and -compacting agents for receptor-mediated gene delivery, we have synthesized and characterized a series of branched oligocationic peptides that differ in the number and type (lysine, arginine, ornithine) of cationic amino acids in the DNA-binding moiety. The peptides were designed as branched molecules to provide a coupling site via a spacer for the attachment of effectors at a flexible distance from the DNA-binding moiety. This design provides torsional flexibility in the peptide backbone of the DNA-binding moiety to maximize cation-DNA phosphate interactions and also minimizes the potential for interference by the effector with DNA binding. The branched peptides bind DNA with affinities that increase with the number of cationic groups. The peptides compact DNA into microparticulate structures as judged by an ethidium bromide displacement assay, dynamic light scattering, and electron microscopy. In general, differences in DNA binding and compaction owing to variation in the cationic side chain were modest, with the rank order being arginyl > lysyl approximately ornithyl. Incorporation of tryptophans into the DNA-binding moiety had no major effect on apparent binding affinity but clearly reduced the DNA-compacting potency of the peptides. Compared with polylysine, the peptides and their DNA complexes are weak activators of the complement system. Complement activation by an octaarginyl peptide was stronger than that induced by an octalysyl peptide. The microparticulate peptide-DNA complexes are suitable for receptor-mediated gene delivery as evidenced by transferrinfection of K562 cells in the presence of chloroquine. The results obtained in gene delivery in vitro suggest that a minimum chain length of six to eight cationic amino acids is required to compact DNA into structures active in receptor-mediated gene delivery.