pH-dependent membrane fusion and vesiculation of phospholipid large unilamellar vesicles induced by amphiphilic anionic and cationic peptides

Elucidating the Influence of Lipid Composition on Bilayer Perturbations Induced by the N-terminal Region of the Huntingtin Protein

Understanding the membrane interactions of the N-terminal 17 residues of the huntingtin protein (HttN) is essential for unraveling its role in cellular processes and its impact on huntingtin misfolding. In this study, we used atomic force microscopy (AFM) to examine the effects of lipid specificity in mediating bilayer perturbations induced by HttN. Across various lipid environments, the peptide consistently induced bilayer disruptions in the form of holes. Notably, our results unveiled that cholesterol enhanced bilayer perturbation induced by HttN, while phosphatidylethanolamine (PE) lipids suppressed hole formation. Furthermore, anionic phosphatidylglycerol (PG) and cardiolipin lipids, along with cholesterol at high concentrations, promoted the formation of double-bilayer patches. This unique structure suggests that the synergy among HttN, anionic lipids, and cholesterol can enhance bilayer fusion, potentially by facilitating lipid intermixing between adjacent bilayers. Additionally, our AFM-based force spectroscopy revealed that HttN enhanced the mechanical stability of lipid bilayers, as evidenced by an elevated bilayer puncture force. These findings illuminate the complex interplay between HttN and lipid membranes and provide useful insights into the role of lipid composition in modulating membrane interactions with the huntingtin protein.

Spatially regulated activation of membrane fusogenic peptides with chaperone-like ionic copolymers

Controlled or targeted membrane lysis induced by cascades of assembly and activation of biomolecules on membrane surfaces is important in programmed cell death and host defense systems. In a previous study, we reported that an ionic graft copolymer with a polycation backbone and water-soluble graft chains, poly(allylamine)-graft-dextran (PAA-g-Dex) chaperoned folding and assembly of E5, a membrane-destructive peptide derived from influenza hemagglutinin, to its increase membrane-disruptive activity. In this study, we modified the copolymer with long acyl chains, which resulted in delivery of the copolymer to membrane surfaces of liposomes and living cells. The liposomes with PAA-g-Dex functionalized with stearic acid (PAA-g-Dex-SA) on their surfaces underwent vesicle-to-sheet conversion upon addition of E5, whereas control liposomes did not. E5 also induced selective lysis of cells incubated with PAA-g-Dex-SA. The spatially specific activation of E5 on target membrane surfaces driven by self-assembly of copolymer and activation of E5 should find application in lipid-based delivery devices and cell-based therapeutics.

A structural perspective of plant antimicrobial peptides

Among the numerous strategies plants have developed to fend off enemy attack, antimicrobial peptides (AMPs) stand out as one of the most prominent defensive barriers that grant direct and durable resistance against a wide range of pests and pathogens. These small proteins are characterized by a compact structure and an overall positive charge. AMPs have an ancient origin and widespread occurrence in the plant kingdom but show an unusually high degree of variation in their amino acid sequences. Interestingly, there is a strikingly conserved topology among the plant AMP families, suggesting that the defensive properties of these peptides are not determined by their primary sequences but rather by their tridimensional structure. To explore and expand this idea, we here discuss the role of AMPs for plant defense from a structural perspective. We show how specific structural properties, such as length, charge, hydrophobicity, polar angle and conformation, are essential for plant AMPs to act as a chemical shield that hinders enemy attack. Knowledge on the topology of these peptides is facilitating the isolation, classification and even structural redesign of AMPs, thus allowing scientists to develop new peptides with multiple agronomical and pharmacological potential.

Lipid-dependence of target membrane stability during influenza viral fusion

Although influenza kills about a half million people each year, even after excluding pandemics, there is only one set of antiviral drugs: neuraminidase inhibitors. By using a new approach utilizing giant unilamellar vesicles and infectious X-31 influenza virus, and testing for the newly identified pore intermediate of membrane fusion, we observed ∼30-87% poration, depending upon lipid composition. Testing the hypothesis that spontaneous curvature (SC) of the lipid monolayer controls membrane poration, our Poisson model and Boltzmann energetic considerations suggest a transition from a leaky to a non-leaky fusion pathway depending on the SC of the target membrane. When the target membrane SC is below approximately -0.20 nm^-1 fusion between influenza virus and target membrane is predominantly non-leaky while above that fusion is predominantly leaky, suggesting that influenza hemagglutinin (HA)-catalyzed topological conversion of target membranes during fusion is associated with a loss of membrane integrity.

Membrane Disruption Mechanism of a Prion Peptide (106-126) Investigated by Atomic Force Microscopy, Raman and Electron Paramagnetic Resonance Spectroscopy

A fragment of the human prion protein spanning residues 106-126 (PrP106-126) recapitulates many essential properties of the disease-causing protein such as amyloidogenicity and cytotoxicity. PrP106-126 has an amphipathic characteristic that resembles many antimicrobial peptides (AMPs). Therefore, the toxic effect of PrP106-126 could arise from a direct association of monomeric peptides with the membrane matrix. Several experimental approaches are employed to scrutinize the impacts of monomeric PrP106-126 on model lipid membranes. Porous defects in planar bilayers are observed by using solution atomic force microscopy. Adding cholesterol does not impede defect formation. A force spectroscopy experiment shows that PrP106-126 reduces Young's modulus of planar lipid bilayers. We use Raman microspectroscopy to study the effect of PrP106-126 on lipid atomic vibrational dynamics. For phosphatidylcholine lipids, PrP106-126 disorders the intrachain conformation, while the interchain interaction is not altered; for phosphatidylethanolamine lipids, PrP106-126 increases the interchain interaction, while the intrachain conformational order remains similar. We explain the observed differences by considering different modes of peptide insertion. Finally, electron paramagnetic resonance spectroscopy shows that PrP106-126 progressively decreases the orientational order of lipid acyl chains in magnetically aligned bicelles. Together, our experimental data support the proposition that monomeric PrP106-126 can disrupt lipid membranes by using similar mechanisms found in AMPs.

Cationic copolymer augments membrane permeabilizing activity of an amphiphilic peptide

Membrane disruptive peptides (also called membrane fusogenic peptides) have been employed for cytosolic delivery of macromolecules such as nucleic acids and proteins. We reported previously that the cationic graft copolymer, poly(allylamine)-graft-dextran (PAA-g-Dex), augments membrane disruptive activity of the negatively charged E5 peptide. Strong membrane disruptive activity was observed in the presence of the copolymer at both acidic and neutral pH. In this paper, activities of E5/PAA-g-Dex mixture were further explored. Membrane permeabilization activity of E5/PAA-g-Dex was dependent on concentrations of both E5 and PAA-g-Dex, indicating that a complex between E5 and PAA-g-Dex produced the activity. Since the activity of peptide/PAA-g-Dex was peptide sequence-specific, we reasoned that PAA-g-Dex activated membrane-permeabilization activity by facilitating folding of E5 into its active conformation. The membrane permeabilization activity of E5/PAA-g-Dex resulted in transportation of bovine serum albumin into HL-60 cells with less cellular toxicity than digitonin, a naturally occurring surfactant used for delivery of macromolecules into cells.

The fusogenic peptide HA2 impairs selectivity of CXCR4-targeted protein nanoparticles

We demonstrate here that the genetic incorporation of the fusogenic peptide HA2 to a CXCR4-targeted protein nanoparticle dramatically reduces the specificity of the interaction between nanoparticles and cell receptor.

A Novel p19 Fusion Protein as a Delivery Agent for Short-interfering RNAs

RNA interference (RNAi) is the biological mechanism that allows targeted gene knockdown through the addition of exogenous short-interfering RNAs (siRNAs) to cells and organisms. RNAi has revolutionized cell biology and holds enormous potential for human therapy. One of the major challenges facing RNAi as a therapy is achieving efficient and nontoxic delivery of siRNAs into the cell cytoplasm, since their highly anionic character precludes their passage across the cell membrane unaided. Herein, we report a novel fusion protein between the tombusviral p19 protein, which binds siRNAs with picomolar affinity, and the “TAT” peptide (RKKRRQRRRR), which is derived from the transactivator of transcription (TAT) protein of the human immunodeficiency virus and acts as a cell-penetrating peptide. We demonstrate that this fusion protein, 2x-p19-TAT, delivers siRNAs into the cytoplasm of human hepatoma cells where they elicit potent and sustained gene knockdown activity without toxic effects.

Inter-polyelectrolyte nano-assembly induces folding and activation of functional peptides

Insufficient solubility, fragile folding structure and short half-life frequently hamper use of peptides as biological reagents or therapies. To enhance the peptide function, the effect of complexation of the peptides with ionic graft copolymers with water-soluble graft chains was tested in this study. Amphiphilic anionic peptide E5 acquires membrane disrupting activity at acidic pH due to folding from the random coil state to an ordered α-helical structure. Aggregation and imprecise folding of the peptide limited membrane disrupting activity of the peptide. In the presence of a cationic graft copolymer, E5 and its analogs adopted an ordered conformation without aggregation. The mixture of the peptides and the copolymer functioned more efficiently than peptide alone at not only acidic pH but also neutral pH at which the peptide alone had no activity. Similarly, a cationic peptide was successfully folded and activated by an anionic graft copolymer. Thus, our analysis indicated that spontaneous nano-assembly of ionic peptides with graft copolymers having opposite ionic charges triggers the folding of peptides without loss of solubility, leading to enhanced bioactivity.

Fusion of lipid vesicles with planar lipid bilayers induced by a combination of peptides

We studied the peptide-induced membrane fusion process between small unilamellar vesicles (SUVs) and supported planar bilayers (SPBs) with the aim of developing a method for incorporating membrane components into SPBs. As fusogenic peptides, two analogues of the N-terminal region of an influenza membrane fusion protein hemaggulutinin, anionic E5 and cationic K5, were synthesized, and the membrane fusion was investigated using SPB and SUVs composed of phosphatidylcholine from egg yolk (EggPC). We directly visualized the process of lipid transfer from SUVs to SPB by total internal reflection fluorescence (TIRF) microscopy. The transfer of fluorescent lipids was effectively induced only by the combination of two peptides. The TIRF microscopy observations of single SUV fusion events also revealed that lipid membranes from SUV could completely fuse into the SPB. However, the presence of single peptide (either E5 or K5) rather inhibited the lipid transfer, presumably due to the electrostatic repulsion between SUVs and SPB. The opposite effects induced by the peptides indicate the possibility for a designed application of two peptides as a means to control the membrane fusion spatially and temporally.

A HA2-Fusion tag limits the endosomal release of its protein cargo despite causing endosomal lysis

BACKGROUND

Protein transduction domains (PTDs) can be fused to a protein to render it cell-permeable. The delivery efficiencies of PTDs are, however, often poor because PTD-protein conjugates cannot escape from endosomes. A potential solution to this problem consists in adding HA2 analogs to the PTD-protein construct as these peptides can cause endosomal lysis upon acidification of the endosomal lumen. To date, however, the utility of HA2-based PTDs has not been clearly established.

METHODS

We investigate the biophysical and cellular properties of the glutamate-rich HA2 analog E5 fused to the model protein TAT-mCherry.

RESULTS

E5-TAT-mCherry causes the release of fluorescent dextrans trapped with the protein inside endosomes. Yet, E5-TAT-mCherry itself is not released in the cytosol of cells, indicating that the protein remained trapped inside endosomes even after endosomal lysis takes place. Cytosolic delivery of the protein could be achieved, however, by insertion of a disulfide bond between E5 and its cargo.

CONCLUSIONS

These results show that E5 causes the retention of its fused protein inside endosomes even after lysis takes place.

GENERAL SIGNIFICANCE

These data establish that HA2 analogs might not be useful PTDs unless cleavable linkers are engineered between PTD and protein cargo.

Magnetic nanoparticles as targeted delivery systems in oncology

BACKGROUND

Many different types of nanoparticles, magnetic nanoparticles being just a category among them, offer exciting opportunities for technologies at the interfaces between chemistry, physics and biology. Some magnetic nanoparticles have already been utilized in clinical practice as contrast enhancing agents for magnetic resonance imaging (MRI). However, their physicochemical properties are constantly being improved upon also for other biological applications, such as magnetically-guided delivery systems for different therapeutics. By exposure of magnetic nanoparticles with attached therapeutics to an external magnetic field with appropriate characteristics, they are concentrated and retained at the preferred site which enables the targeted delivery of therapeutics to the desired spot.

CONCLUSIONS

The idea of binding chemotherapeutics to magnetic nanoparticles has been around for 30 years, however, no magnetic nanoparticles as delivery systems have yet been approved for clinical practice. Recently, binding of nucleic acids to magnetic nanoparticles has been demonstrated as a successful non-viral transfection method of different cell lines in vitro. With the optimization of this method called magnetofection, it will hopefully become another form of gene delivery for the treatment of cancer.

Modeling of the endosomolytic activity of HA2-TAT peptides with red blood cells and ghosts

HA2-TAT is a peptide-based delivery agent that combines the pH-sensitive HA2 fusion peptide from influenza and the cell-penetrating peptide TAT from HIV. This chimeric peptide is engineered to induce the cellular uptake of macromolecules into endosomes via the TAT moiety and to respond to the acidifying lumen of endosomes to cause membrane leakage and release of macromolecules into cells via the HA2 moiety. The question of how HA2 and TAT affect the properties of one another remains, however, unanswered, and the behavior of the peptide inside endosomes is mostly uncharacterized. To address these issues, the binding and membrane leakage activity of a glutamic acid-enriched analogue E5-TAT was assessed with red blood cells and giant unilamellar vesicles as membrane models for endosomes. Hemolysis and microscopy assays reveal that E5-TAT binds to membranes in a pH-dependent manner and causes membrane leakage by inducing the formation of pores through which macromolecules can escape. The TAT moiety contributes to this activity by causing a shift in the pH response of E5 and by binding to negatively charged phospholipids. On the other hand, TAT binding to glycosaminoglycans reduces the lytic activity of E5-TAT. Addition of TAT to the C-terminus of E5 can therefore either increase or inhibit the activity of E5 depending on the cellular components present at the membrane. Taken together, these results suggest a model for the endosomolytic activity of the peptide and provide the basis for the molecular design of future delivery agents.

Alkylated derivatives of poly(ethylacrylic acid) can be inserted into preformed liposomes and trigger pH-dependent intracellular delivery of liposomal contents

Poly(ethylacrylic acid) (PEAA) is a pH-sensitive polymer that undergoes a transition from a hydrophilic to a hydrophobic form as the pH is lowered from neutral to acidic values. In this work we show that pH sensitive liposomes capable of intracellular delivery can be constructed by inserting a lipid derivative of PEAA into preformed large unilamellar vesicles (LUV) using a simple one step incubation procedure. The lipid derivatives of PEAA were synthesized by reacting a small proportion (3%) of the carboxylic groups of PEAA with C10 alkylamines to produce C10-PEAA. Incubation of C10-PEAA with preformed LUV resulted in the association of up to 8% by weight of derivatized polymer with the LUV without inducing aggregation. The resulting C10-PEAA-LUV exhibited pH-dependent fusion and leakage of LUV contents on reduction of the external pH below pH 6.0 as demonstrated by lipid mixing and release of calcein encapsulated in the LUV. In addition, C10-PEAA-LUV exhibited pH dependent intracellular delivery properties following uptake into COS-7 cells with appreciable delivery to the cell cytoplasm as evidenced by the appearance of diffuse intracellular calcein fluorescence. It is demonstrated that the cytoplasmic delivery of calcein by C10-PEAA-LUV could be inhibited by agents (bafilomycin or chloroquine) that inhibit acidification of endosomal compartments, indicating that this intracellular delivery resulted from the pH-dependent destabilization of LUV and endosomal membranes by the PEAA component of the C10-PEAA-LUV. It is concluded that C10-PEAA-LUV represents a promising intracellular delivery system for in vitro and in vivo applications.

Histidine-rich peptide: evidence for a single zinc-binding site on H5WYG peptide that promotes membrane fusion at neutral pH

The histidine-rich peptide H5WYG (GLFHAIAHFIHGGWHGLIHGWYG) was found to induce membrane fusion at physiologic pH in the presence of zinc chloride. In this study, we examined the ion selectivity of the interaction of Zn(2+) with H5WYG. This investigation was conducted by using adsorption at air/water interface and mass spectrometry. We found that a peptide-metal complex is formed with Zn(2+) ions. Electrospray ionisation-mass spectrometry (ESI-MS) reveals that the [H5WYG + Zn + 2H](4+), [H5WYG + Zn + H](3+) and [H5WYG + Zn](2+) ions, appearing by increasing the amount of Zn(2+) equivalent, correspond to a monomolecular H5WYG - Zn(2+) complex. Tandem mass spectrometry (MS/MS) provides evidence for the binding of the single Zn(2+) ion to the H(11) and H(19) and probably H(15) residues.

The effect of endosomal escape peptides on in vitro gene delivery of polyethylene glycol-based vehicles

BACKGROUND

With recent progress in gene therapy clinical trials, there is an even greater demand to advance the development of nonviral gene delivery vehicles. We have previously developed poly(ethylene glycol) (PEG)-based vehicles with transfection efficiency similar to polyethyleneimine and low cytotoxicity. It was hypothesized that conjugating endosomal escape peptides (EEPs) to PEG-based vehicles would further increase their transfection efficiency. The present study aimed to determine how two different EEPs, INF7 and H5WYG, which destabilize the endosomal membrane at different pHs, affect the efficiency of PEG-based vehicles.

METHODS

INF7 and H5WYG were conjugated to PEG-tetraacrylate (PEG-TA) via a Michael-type addition at the desired molar ratios. The pH-dependent membrane lytic activity, transfection efficiency, particle size, zeta potential, and endosomal escape kinetic rate constants were determined.

RESULTS

Fusogenic peptides, INF7 and H5WYG, showed pH-dependent membrane lytic activity when conjugated to PEG-TA. The highest membrane lytic activity of PEG-INF7 and PEG-H5WYG conjugates occurred at pH 5 and 5.5, respectively. Coupling one INF7 peptide to PEG-DNA binding peptide (DBP) vehicles increased the transfection efficiency ten-fold and showed greater transfection efficiency than PEG-DBP vehicles coupled with H5WYG peptide. Fitting a first-order kinetic model to the average intracellular pH of the vehicle/DNA particles over time determined that coupling EEPs to PEG-DBP vehicles increased the endosomal escape rate constant by two orders of magnitude.

CONCLUSIONS

Endosomal escape is a key step in nonviral cellular trafficking and thus the transfection efficiency of nonviral vehicles can be increased by targeting release of DNA from the endosome with EEPs.

Triggering and visualizing the aggregation and fusion of lipid membranes in microfluidic chambers

We present a method that makes it possible to trigger, observe, and quantify membrane aggregation and fusion of giant liposomes in microfluidic chambers. Using electroformation from spin-coated films of lipids on transparent indium tin oxide electrodes, we formed two-dimensional networks of closely packed, surface-attached giant liposomes. We investigated the effects of fusogenic agents by simply flowing these molecules into the chambers and analyzing the resulting shape changes of more than 100 liposomes in parallel. We used this setup to quantify membrane fusion by several well-studied mechanisms, including fusion triggered by Ca2+, polyethylene glycol, and biospecific tethering. Directly observing many liposomes simultaneously proved particularly useful for studying fusion events in the presence of low concentrations of fusogenic agents, when fusion was rare and probabilistic. We applied this microfluidic fusion assay to investigate a novel 30-mer peptide derived from a recently identified human receptor protein, B5, that is important for membrane fusion during the entry of herpes simplex virus into host cells. This peptide triggered fusion of liposomes at an approximately 6 times higher probability than control peptides and caused irreversible interactions between adjacent membranes; it was, however, less fusogenic than Ca2+ at comparable concentrations. Closely packed, surface-attached giant liposomes in microfluidic chambers offer a method to observe membrane aggregation and fusion in parallel without requiring the use of micromanipulators. This technique makes it possible to characterize rapidly novel fusogenic agents under well-defined conditions.

Characterization of the family of Mistic homologues

Background

Mistic is a unique Bacillus subtilis protein with virtually no detectable homologues in GenBank, which appears to integrate into the bacterial membrane despite an overall hydrophilic composition. These unusual properties have been shown to be useful for high-yield recombinant expression of other membrane proteins through fusion to the C-terminus of Mistic. To better understand the structure and function of Mistic, we systematically searched for and characterized homologous proteins among closely related bacteria.

Results

Three homologues of Mistic were found with 62% to 93% residue identity, all only 84 residues in length, corresponding to the C-terminal residues of B. subtilis Mistic. In every case, the Mistic gene was found partially overlapping a downstream gene for a K⁺ channel protein. Residue variation amongst these sequences is restricted to loop regions of the protein's structure, suggesting that secondary structure elements and overall fold have been conserved. Additionally, all three homologues retain the functional ability to chaperone fusion partners to the membrane.

Conclusion

The functional core of Mistic consists of 84 moderately conserved residues that are sufficient for membrane targeting and integration. Understanding the minimal structural and chemical complexity of Mistic will lead to insights into the mechanistic underpinnings of Mistic-chaperoned membrane integration, as well as how to optimize its use for the recombinant heterologous expression of other integral membrane proteins of interest.

Properties and structures of the influenza and HIV fusion peptides on lipid membranes: implications for a role in fusion

The fusion peptides of HIV and influenza virus are crucial for viral entry into a host cell. We report the membrane-perturbing and structural properties of fusion peptides from the HA fusion protein of influenza virus and the gp41 fusion protein of HIV. Our goals were to determine: 1), how fusion peptides alter structure within the bilayers of fusogenic and nonfusogenic lipid vesicles and 2), how fusion peptide structure is related to the ability to promote fusion. Fluorescent probes revealed that neither peptide had a significant effect on bilayer packing at the water-membrane interface, but both increased acyl chain order in both fusogenic and nonfusogenic vesicles. Both also reduced free volume within the bilayer as indicated by partitioning of a lipophilic fluorophore into membranes. These membrane ordering effects were smaller for the gp41 peptide than for the HA peptide at low peptide/lipid ratio, suggesting that the two peptides assume different structures on membranes. The influenza peptide was predominantly helical, and the gp41 peptide was predominantly antiparallel beta-sheet when membrane bound, however, the depths of penetration of Trps of both peptides into neutral membranes were similar and independent of membrane composition. We previously demonstrated: 1), the abilities of both peptides to promote fusion but not initial intermediate formation during PEG-mediated fusion and 2), the ability of hexadecane to compete with this effect of the fusion peptides. Taken together, our current and past results suggest a hypothesis for a common mechanism by which these two viral fusion peptides promote fusion.

Drug delivery of oligonucleotides by peptides

Oligonucleotides are promising tools for in vitro studies where specific downregulation of proteins is required. In addition, antisense oligonucleotides have been studied in vivo and have entered clinical trials as new chemical entities with various therapeutic targets such as antiviral drugs or for tumour treatments. The formulation of these substances were widely studied in the past. With this review we will focus on peptides used as drug delivery vehicles for oligonucleotides. Different strategies are summarised. Cationically charged peptides from different origins were used e.g. as cellular penetration enhancers or nuclear localisation tool. Examples are given for Poly-L-lysine alone or in combination with receptor specific targeting ligands such as asialoglycoprotein, galactose, growth factors or transferrin. Another large group of peptides are those with membrane translocating properties. Fusogenic peptides rich in lysine or arginine are reviewed. They have been used for DNA complexation and condensation to form transport vehicles. Some of them, additionally, have so called nuclear localisation properties. Here, DNA sequences, which facilitate intracellular trafficking of macromolecules to the nucleus were explored. Summarizing the present literature, peptides are interesting pharmaceutical excipients and it seems to be feasible to combine the specific properties of peptides to improve drug delivery devices for oligonucleotides in the future.

Self-association of glutamic acid-rich fusion peptide analogs of influenza hemagglutinin in the membrane-mimic environments: Effects of positional difference of glutamic acids on side chain ionization constant and intra- and inter-peptide interactions deduced from NMR and gel electrophoresis measurements

Two glutamic acid-rich fusion peptide analogs of influenza hemagglutinin were synthesized to study the organization of the charged peptides in the membranous media. Fluorescence and gel electrophoresis experiments suggested a loose association between the monomers in the vesicles. A model was built which showed that a positional difference of 3, 7 and 4, 8 results in the exposure of Glu3 and Glu7 side chains to the apolar lipidic core. Supportive results include: first, pK(a) values of two pH units higher than reference value in aqueous medium for Glu3 and Glu7 CgammaH, whereas the deviation of pK(a) from the reference value for Glu4 and Glu8 CgammaH is substantially smaller; second, Hill coefficients of titration shift of these protons indicate anti-cooperativity for Glu3 and Glu7 side chain protons but less so for Glu4 and Glu8, implying a strong electrostatic interaction between Glu3 and Glu7 possibly resulting from their localization in an apolar environment; third, positive and larger titration shift for NH of Glu3 is observed compared to that of Glu4, suggesting stronger hydrogen bond between the NH and the carboxylic group of Glu3 than that of Glu4, consistent with higher degree of exposure to hydrophobic medium for the side chain of Glu3.

A novel gene, msa1, inhibits sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation in the fission yeast Schizosaccharomyces pombe is triggered by nutrient starvation or by the presence of mating pheromones. We identified a novel gene, msa1, which encodes a 533-aa putative RNA-binding protein that inhibits sexual differentiation. Disruption of the msa1 gene caused cells to hypersporulate. Intracellular levels of msa1 RNA and Msa1 protein diminished after several hours of nitrogen starvation. Genetic analysis suggested that the function of msa1 is independent of the cAMP pathway and stress-responsive pathway. Deletion of the ras1 gene in diploid cells inhibited sporulation and in haploid cells decreased expression of mating-pheromone-induced genes such as mei2, mam2, ste11, and rep1; simultaneous deletion of msa1 reversed both phenotypes. Overexpression of msa1 decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both msa1 and rad24, but simultaneous deletion of msa1 and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of Msa1 is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the pheromone-signaling pathway.

Secondary structure, phospholipid membrane interactions, and fusion activity of two glutamate-rich analogs of influenza hemagglutinin fusion peptide

Two synthetic mutants of influenza HA2 fusion peptide (residues 1-25), containing Glu on the polar (residues 4,8-E5(4,8)) or the hydrophobic (residues 3,7-E5(3,7)) face of the amphipathic helix, were synthesized and labeled with NBD at the N-terminus. Introduction of Glu residues into the fusion peptide leads to increased sensitivity of various biochemical properties to pH compared to the wild type. The E5 peptides showed a decrease of alpha-helix content and increase of beta-sheet structure. Lipid binding was diminished, but not abolished even at high pH. The E5 analogs penetrate the lipid bilayer less deeply than the wild type, especially at high pH. The N-terminal half of the peptide showed significant variation of the depth of the penetration into the lipid bilayer. Both E5 peptides were fusion active. The properties of E5(3,7) were more affected by the Glu substitution and showed greater variation with pH than E5(4,8).

Microscopic observations reveal that fusogenic peptides induce liposome shrinkage prior to membrane fusion

To study the mechanisms involved in membrane fusion, we visualized the fusion process of giant liposomes in real time by optical dark-field microscopy. To induce membrane fusion, we used (i) influenza hemagglutinin peptide (HA), a 20-aa peptide derived from the N-terminal fusion peptide region of the HA2 subunit, and (ii) two synthetic analogue peptides of HA, a negatively (E5) and positively (K5) charged analogue. We were able to visualize membrane fusion caused by E5 or by K5 alone, as well as by the mixture of these two peptides. The HA peptide however, did not induce membrane fusion, even at an acidic pH, which has been described as the optimal condition for the fusion of large unilamellar vesicles. Surprisingly, before membrane fusion, the shrinkage of liposomes was always observed. Our results suggest that a perturbation of lipid bilayers, which probably resulted from alterations in the bending folds of membranes, is a critical factor in fusion efficiency.

Liposomes possess drastic capabilities for topological transformation

Morphological and topological changes of biological membranes play essential roles in cellular activities. It has been thought that these transformations are made possible through interactions with proteins. However, direct observation of giant liposomes by optical dark-field microscopy reveals that the lipid bilayer itself possesses the ability to undergo topological transformation.

Structural characterizations of fusion peptide analogs of influenza virus hemagglutinin. Implication of the necessity of a helix-hinge-helix motif in fusion activity

Infection by enveloped viruses initially involves membrane fusion between viral and host cell membranes. The fusion peptide plays a crucial role in triggering this reaction. To clarify how the fusion peptide exerts this specific function, we carried out biophysical studies of three fusion peptide analogs of influenza virus hemagglutinin HA2, namely E5, G13L, and L17A. E5 exhibits an activity similar to the native fusion peptide, whereas G13L and L17A, which are two point mutants of the E5 analog, possess much less fusion activity. Our CD data showed that the conformations of these three analogs in SDS micelles are pH-dependent, with higher alpha-helical contents at acidic pH. Tryptophan fluorescence emission experiments indicated that these three analogs insert deeper into lipid bilayers at acidic pH. The three-dimensional structure of the E5 analog in SDS micelles at pH 4.0 revealed that two segments, Leu(2)-Glu(11) and Trp(14)-Ile(18), form amphipathic helical conformations, with Gly(12)-Gly(13) forming a hinge. The hydrophobic residues in the N- and C-terminal helices form a hydrophobic cluster. At neutral pH, however, the C-terminal helix of Trp(14)-Ile(18) reduces dramatically, and the hydrophobic core observed at acidic pH is severely disrupted. We suggest that the disruption of the C-terminal helix renders the E5 analog fusion-inactive at neutral pH. Furthermore, the decrease of the hinge and the reduction of fusion activity in G13L reveal the importance of the hinge in fusion activity. Also, the decrease in the C-terminal helix and the reduction of fusion activity in L17A demonstrates the importance of the C-terminal helix in fusion activity. Based on these biophysical studies, we propose a model that illustrates the structural change of the HA2 fusion peptide analog and explains how the analog interacts with the lipid bilayer at different pH values.

Histidine-rich peptides and polymers for nucleic acids delivery

Nucleic acids transfer into mammalian cells requires devices to improve their escape from endocytic vesicles where they are mainly confined following cellular uptake. In this review, we describe histidine-rich molecules that enable the transfer of plasmid and oligonucleotides (ODN) in human and non-human cultured cells. An histidine-rich peptide which permeabilizes biological membrane at pH 6.4, favored the transfection mediated by lactosylated polylysine/pDNA complexes. Histidylated polylysine forms cationic particles of 100 nm with a plasmid and yielded a transfection of 3-4.5 orders of magnitude higher than polylysine. The biological activity of antisense ODN was increased more than 20-fold when it was complexed with highly histidylated oligolysine into small cationic spherical particles of 35 nm. Evidence that imidazole protonation mediates the effect of these molecules in endosomes are provided. We also describe a disulfide-containing polylysine conjugate capable of mediating DNA unpackaging in a reductive medium and to increase the transfection efficiency. Overall, these molecules constitute interesting devices for developing non-viral gene delivery systems.

Delivery of oligonucleotides into mammalian cells by anionic peptides: comparison between monomeric and dimeric peptides

The use of antisense oligonucleotides as putative therapeutic agents is limited by their poor delivery into the cytosol and/or the nucleus because they are not able to efficiently cross lipid bilayers. To circumvent this pitfall, anionic amphipathic peptides derived from the influenza virus fusogenic peptide have been used to destabilize membranes in an acidic environment. In this paper, we compare the ability of a monomeric and a dimeric peptide to introduce oligonucleotides into the cytosol and nuclei of several types of cultured cells. Cells incubated at pH 6.2 or at a slightly lower pH in the presence of the monomeric peptide but not the dimeric peptide were efficiently permeabilized. The location of fluorescent derivatives of peptides and of oligonucleotides was assessed by confocal microscopy. Both the peptides and oligonucleotides remained entrapped in vesicular compartments at neutral pH; at acidic pH, oligonucleotides in the presence of the monomeric peptide were mainly in the nucleus, while in the presence of the dimeric peptide they co-localized with the peptide into vesicles. The data are interpreted on the basis of the spectroscopic behaviour of monomeric and dimeric peptides in relation to the environmental pH.

Common properties of fusion peptides from diverse systems

Although membrane fusion occurs ubiquitously and continuously in all eukaroytic cells, little is known about the mechanism that governs lipid bilayer fusion associated with any intracellular fusion reactions. Recent studies of the fusion of enveloped viruses with host cell membranes have helped to define the fusion process. The identification and characterization of key proteins involved in fusion reactions have mainly driven recent advances in our understanding of membrane fusion. The most important denominator among the fusion proteins is the fusion peptide. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion will be highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. Much of what is known regarding the molecular mechanism of viral membrane fusion has been gained using liposomes as model systems in which the molecular components of the membrane and the environment are strictly controlled. Many amphilphilic peptides have a high affinity for lipid bilayers, but only a few sequences are able to induce membrane fusion. The presence of alpha-helical structure in at least part of the fusion peptide is strongly correlated with activity whereas, beta-structure tends to be less prevalent, associated with non-native experimental conditions, and more related to vesicle aggregation than fusion. The specific angle of insertion of the peptides into the membrane plane is also found to be an important characteristic for the fusion process. A shallow penetration, extending only to the central aliphatic core region, is likely responsible for the destabilization of the lipids required for coalescence of the apposing membranes and fusion.

Novel non-endocytic delivery of antisense oligonucleotides

Antisense oligonucleotides (ONs) have several properties that make them attractive as therapeutic agents. Hybridization of antisense ONs to their complementary nucleic acid sequences by Watson-Crick base pairing is a highly selective and efficient process. Design of therapeutic antisense agents can be made more rationally as compared to most traditional drugs, i.e., they can be designed on the basis of target RNA sequences and their secondary structures. Despite these advantages, the design and use of antisense ONs as therapeutic agents are still faced with several obstacles. One major obstacle is their inefficient cellular uptake and poor accessibility to target sites. In this article, we will discuss key barriers affecting ON delivery and approaches to overcome these barriers. Current methods of ON delivery will be reviewed with an emphasis on novel non-endocytic methods of delivery. ONs are taken up by cells via an endocytic process. The process of ON release from endosomes is a very inefficient process and, hence, ONs end up being degraded in the endosomes. Thus, ONs do not reach their intended site of action in the cytoplasm or nucleus. Delivery systems ensuring a cytoplasmic delivery of ONs have the potential to increase the amount of ON reaching the target. Here, we shall examine various ON delivery methods that bypass the endosomal pathway. The advantages and disadvantages of these methods compared to other existing methods of ON delivery will be discussed.

Delivery systems for antisense oligonucleotides

In vitro, the efficacy of the antisense approach is strongly increased by systems delivering oligodeoxyribonucleotides (ODNs) to cells. Up to now, most of the developed vectors favor ODN entrance by a mechanism based on endocytosis. Such is the case for particulate systems, including liposomes (cationic or non-cationic), cationic polyelectrolytes, and delivery systems targeted to specific receptors. Under these conditions, endosomal compartments may represent a dead end for ODNs. Current research attempts to develop conditions for escaping from these compartments. A new class of vectors acts by passive permeabilization of the plasma membrane. It includes peptides, streptolysin O, and cationic derivatives of polyene antibiotics. In vivo, the interest of a delivery system, up to now, has appeared limited. Development of vectors insensitive to the presence of serum seems to be a prerequisite for future improvements.

Effects of complexation between liposome and poly(malic acid) on aggregation and leakage behaviour

The design and development of novel pH-sensitive liposomes were investigated to improve the release of liposome-encapsulated chemicals. Stable liposomes comprising of L-alpha-dipalmitoylphosphatidylcholine (DPPC) and poly(carboxylic acid) were prepared and characterized. Poly(malic acid) (PMLA) was chosen as a fusogen, because of its excellent biodegradability in physiological regions. Octyl groups introduced in the poly(malic acid) worked as anchors at the surface of the liposomes and made a remarkable contribution to complexing. The interaction between the liposomes and the polyacids was studied in terms of the change in size of the liposomes. The influences of molecular weight and amounts of polymer upon their characteristics, especially fusion, were discussed. The influences of pH change with respect to the association behavior of the liposomes such as aggregation and fusion were estimated by the particle size of the liposomes, turbidimetry of the solution and resonance energy transfer assay. From the results of these studies, it was shown that more tightly complexed liposomes aggregated and fused more positively with increasing acidity of the solution. The leakage of calcein entrapped in the inner aqueous phase of the liposomes increased with decreasing pH. The effect of pH on the liposome aggregation in a solution qualitatively paralleled that found in the leakage behavior.

Structure of an analog of fusion peptide from hemagglutinin

A 20-residue peptide E5 containing five glutamates, an analog of the fusion peptide of influenza virus hemagglutinin (HA) exhibiting fusion activity at acidic pH lower than 6.0-6.5 was studied by circular dichroism (CD), Fourier transform infrared, and 1H-NMR spectroscopy in water, water/trifluoroethanol (TFE) mixtures, dodecylphosphocholine (DPC) micelles, and phospholipid vesicles. E5 became structurally ordered at pH < or = 6 and the helical content in the peptide increased in the row: water < water/TFE < DPC approximately = phospholipid vesicle while the amount of beta-structure was approximately reverse. 1H-NMR data and line-broadening effect of 5-, 16-doxylstearates on proton resonances of DPC bound peptide showed E5 forms amphiphilic alpha-helix in residues 2-18, which is flexible in 11-18 part. The analysis of the proton chemical shifts of DPC bound and CD intensity at 220 nm of phospholipid bound E5 showed that the pH dependence of helical content is characterized by the same pKa approximately 5.6. Only Glu11 and Glu15 in DPC bound peptide showed such elevated pKas, presumably due to transient hydrogen bond(s) Glu11 (Glu15) deltaCOO- (H+)...HN Glu15 that dispose(s) the side chain of Glu11 (Glu15) residue(s) close to the micelle/water interface. These glutamates are present in the HA-fusion peptide and the experimental half-maximal pH of fusion for HA and E5 peptides is approximately 5.6. Therefore, a specific anchorage of these peptides onto membrane necessary for fusion is likely driven by the protonation of the carboxylate group of Glu11 (Glu15) residue(s) participating in transient hydrogen bond(s).

Coordinate packaging of newly synthesized phosphatidylcholine and phosphatidylglycerol in lamellar bodies in alveolar type II cells

Methylamine, a weak base, inhibits packaging of newly synthesized phosphatidylcholine (PC) in lamellar bodies in 20-22 h cultured alveolar type II cells, suggesting a role for acidic pH of lamellar bodies. In this study, we tested if (i) the packaging of PC is similarly regulated in freshly isolated type II cells and (ii) methylamine also inhibits the packaging of other surfactant phospholipids, particularly, phosphatidylglycerol (PG). The latter would suggest coordinated packaging so as to maintain the phospholipid composition of lung surfactant. During the short-term metabolic labeling experiments in freshly isolated type II cells, methylamine treatment decreased the incorporation of radioactive precursors into PC, disaturated PC (DSPC), and PG of lamellar bodies but not of the microsomes, when compared with controls. The calculated packaging (the percentage of microsomal lipid packaged in lamellar bodies) of each phospholipid was similarly decreased (approximately 50%) in methylamine-treated cells, suggesting coordinated packaging of surfactant phospholipids in lamellar bodies. Equilibrium-labeling studies with freshly isolated type II cells (as is routinely done for studies on surfactant secretion) +/- methylamine showed that in methylamine-treated cells, the secretion of PC and PG was decreased (possibly due to decreased packaging), but the phospholipid composition of released surfactant (measured by radioactivity distribution) was unchanged; and the PC content (measured by mass or radioactivity) of lamellar bodies was lower, but the PC composition (as percentage of total phospholipids) was unchanged when compared with control cells. We speculate that the newly synthesized surfactant phospholipids, PC, DSPC, and PG, are coordinately transported into lamellar bodies by a mechanism requiring the acidic pH, presumably, of lamellar bodies.

Factors important for fusogenic activity of peptides: molecular modeling study of analogs of fusion peptide of influenza virus hemagglutinin

Nine analogs of fusion peptide of influenza virus hemagglutinin whose membrane perturbation activity has been thoroughly tested [Murata et al. (1992) Biochemistry 31, 1986-1992; Murata et al. (1993) Biophys. J. 64, 724-734] were characterized by molecular modeling techniques with the aim of delineating any specific structural and/or hydrophobic properties inherent in peptides with fusogenic activity. It was shown that, regardless of characteristics common to all analogs (peripheral disposition at the water-lipid interface, amphiphilic nature, alpha-helical structure, etc.), only fusion active peptides reveal a specific 'tilted oblique-oriented' pattern of hydrophobicity on their surfaces and a certain depth of penetration to the non-polar membrane core. The conclusion was reached that these factors are among the most important for the specific destabilization of a bilayer, which is followed by membrane fusion.

Role of the N-terminal peptides of viral envelope proteins in membrane fusion

Membrane fusion is an important biological process that is observed in a wide variety of intra and intercellular events. In this review, work done in the last few years on the molecular mechanism of viral membrane fusion is highlighted, focusing in particular on the role of the fusion peptide and the modification of the lipid bilayer structure. While the Influenza hemagglutinin is currently the best understand fusion protein, there is still much to be learned about the key events in enveloped virus fusion reactions. This review compares our current understanding of the membrane fusion activity of Influenza and retrovirus viruses. We shall be concerned especially with the studies that lead to interpretations at the molecular level, so we shall concentrate on model membrane systems where the molecular components of the membrane and the environment are strictly controlled.

Membrane fusion induced by a short fusogenic peptide is assessed by its insertion and orientation into target bilayers

To clarify the molecular mechanism by which an amphipathic negatively charged peptide consisting of 11 residues (WAE) induces fusion, and the relevance of these features for fusion, its mode of insertion and orientation into target bilayers were investigated. Using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) in combination with techniques based on tryptophan fluorescence, the peptide was found to form an alpha-helix, shallowly inserted into the membrane to which it is anchored. Interestingly, in the presence of target membranes, WAE inserts into the target bilayer as an alpha-helix oriented almost parallel to the lipid acyl chains. The accessibility of the peptide to either acrylamide (as an aqueous quencher of Trp fluorescence) or deuterium oxide (on the course of an FTIR deuteration kinetics) was lower in the presence than in the absence of target membranes, confirming that under those conditions, the peptide was shielded from the aqueous environment. Since fusion experiments have shown a temperature dependence, the effect of this later parameter on the structure and mode of insertion of the peptide was also analyzed. In the presence of target membrane, but not in their absence, the amount of alpha-helical structure increased with temperature, reflecting a similar temperature-dependent increase in the rate and extent of WAE-induced fusion. Also, the extent of penetration of the helix into the target membrane was greater at 37 degrees C than at lower temperatures. This temperature-dependent distinction was revealed by a decreased accessibility of the peptide to deuterium oxide and acrylamide at 37 degrees C as compared to that at lower temperatures. These data underscore the role of peptide structure, peptide penetration, and orientation in the mechanism of protein-induced membrane fusion.

Membrane destabilizing activity of influenza virus hemagglutinin-based synthetic peptide: implications of critical glycine residue in fusion peptide

Peptide III is a 20-residue synthetic model peptide based on the fusion peptide of influenza virus A/PR/8/34 strain and takes a secondary structure similar to the original peptide. While conserving the amphiphilic helical nature, 20 peptides to modify the bulkiness of side chains of peptide III were synthesized, and acid-induced membrane destabilization was assessed by aqueous content leakage from large unilamellar vesicles. Substitutions on the hydrophobic side decreased activity but showed less effect on the hydrophilic side, which confirmed the importance of the hydrophobic side for interaction with the membrane. Interestingly, substitution at the 13th Gly residue enhanced the amphiphilic helical nature but severely reduced activity. Correlation between alpha-helical content at acidic pH and the activity was not recognized, suggesting rather that the importance of this site was due to helix termination by glycine which allows N-terminal and C-terminal halves to behave as different secondary structural units.