Release of aqueous contents from phospholipid vesicles induced by cecropin A (1-8)-magainin 2 (1-12) hybrid and its analogues

Influence of lipid composition on membrane activity of antimicrobial phenylene ethynylene oligomers

Host defense peptides (HDPs), part of the innate immune system, selectively target the membranes of bacterial cells over that of host cells. As a result, their antimicrobial properties have been under intense study. Their selectivity strongly depends on the chemical and mostly structural properties of the lipids that make up different cell membranes. The ability to synthesize HDP mimics has recently been demonstrated. To better understand how these HDP mimics interact with bilayer membranes, three homologous antimicrobial oligomers (AMOs) 1-3 with an m-phenylene ethynylene backbone and alkyl amine side chains were studied. Among them, AMO 1 is nonactive, AMO 2 is specifically active, and AMO 3 is nonspecifically active against bacteria over human red blood cells, a standard model for mammalian cells. The interactions of these three AMOs with liposomes having different lipid compositions are characterized in detail using a fluorescent dye leakage assay. AMO 2 and AMO 3 caused more leakage than AMO 1 from bacteria membrane mimic liposomes composed of PE/PG lipids. The use of E. coli lipid vesicles gave the same results. Further changes of the lipid compositions revealed that AMO 2 has selectively higher affinity toward PE/PG and E. coli lipids than PC, PC/PG or PC/PS lipids, the major components of mammalian cell membranes. In contrast, AMO 3 is devoid of this lipid selectivity and interacts with all liposomes with equal ease; AMO 1 remains inactive. These observations suggest that lipid type and structure are more important in determining membrane selectivity than lipid headgroup charges for this series of HDP mimics.

Identification and rational design of novel antimicrobial peptides for plant protection

Peptides and small proteins exhibiting antimicrobial activity have been isolated from many organisms ranging from insects to humans, including plants. Their role in defense is established, and their use in agriculture was already being proposed shortly after their discovery. However, some natural peptides have undesirable properties that complicate their application. Advances in peptide synthesis and high-throughput activity screening have made possible the de novo and rational design of novel peptides with improved properties. This review summarizes findings in the identification and design of short antimicrobial peptides with activity against plant pathogens, and will discuss alternatives for their heterologous production suited to plant disease control. Recent studies suggest that peptide antimicrobial action is not due solely to microbe permeation as previously described, but that more subtle factors might account for the specificity and absence of toxicity of some peptides. The elucidation of the mode of action and interaction with microbes will assist the improvement of peptide design with a view to targeting specific problems in agriculture and providing new tools for plant protection.

The lipid dependence of melittin action investigated by dual-color fluorescence burst analysis

Dual-color fluorescence-burst analysis was used to study melittin-induced leakage of macromolecules from liposomes of various lipid compositions. To perform dual-color fluorescence-burst analysis, fluorescently labeled size-marker molecules were encapsulated into liposomes, labeled with a second lipid-attached fluorophore. By correlating the fluorescence bursts, resulting from the liposomes diffusing through the detection volume of a dual-color confocal microscope, the distribution of size-marker molecules over the liposomes was determined. It was found that melittin causes leakage via two different mechanisms: 1), For liposomes composed of neutral bilayer-forming lipids, low melittin concentrations induced pore formation with the pore size depending on the melittin concentration. 2), For liposomes containing anionic and/or nonbilayer forming lipids, melittin induced fusion or aggregation of liposomes accompanied by a-specific leakage. Experiments with liposomes prepared from Escherichia coli lipid extracts and intact cells of Lactococcus lactis indicate that both mechanisms are physiologically relevant.

Effect of polypeptides in bee venom on growth inhibition and apoptosis induction of the human hepatoma cell line SMMC-7721 in-vitro and Balb/c nude mice in-vivo

Polypeptides in bee venom (PBV) produced a significant growth inhibition against SMMC-7721 human hepatoma cell line. Analysis of the mechanisms of cell death indicated that PBV induced an apoptotic cell death. SMMC-7721 cells exposed to PBV (10.0 microg mL(-1)) produced an insignificant morphological change. Analysis of the cytotoxicity with the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium) assay confirmed that the cytotoxic effects of PBV were dose- and timedependent. The result of Ki67 immunohistochemistry demonstrated that the proliferation of SMMC-7721 cells treated with PBV (10.0 mug mL(-1)) was inhibited. The apoptotic cell death was then confirmed by annexin V, propidium iodide staining and DNA fragmentation analysis. In in-vivo experiments, treatment with PBV (1.5 or 3 mg kg(-1)) resulted in a significant retardation of SMMC-7721 cell growth in Balb/c nude mice. These findings suggested that PBV could be used as a chemotherapeutic agent against tumours.

Target ability and therapy efficacy of immunoliposomes using a humanized antihepatoma disulfide-stabilized Fv fragment on tumor cells

Recently the use of peptides in bee venom (PBV) for cancer therapy has attracted considerable attention. However, PBV's extensive use is prohibited by its intense hemolytic activity. In this study, the sterically stabilized liposomal PBV (PBV-SL) was prepared using soybean phosphatidylcholine, cholesterol, and cholesterol-PEG-COOH. The humanized antihepatoma disulfide-stabilized Fv (hdsFv25) was reengineered, expressed, and coupled to sterically stabilized liposomes using the N-hydroxysuccinimide ester method. The hdsFv25-immunoliposomes (SIL [hdscFv25]) were immunoreactive as determined by ELISA assay. PBV-SIL [hdscFv25] can kill SMMC-7721 cells in vitro with higher efficiency than nontargeted liposomes. PBV-SIL [hdsFv25] displayed high antitumor activity and resulted in a significant reduction in tumor size compared to nontargeted liposomes and PBV. These results indicated that this strategy should be applicable to applicable in the treatment of other cancers.

Synergism of Leu-Lys rich antimicrobial peptides and chloramphenicol against bacterial cells

To investigate the antibiotic activity and synergistic effect, analogues were designed to increase not only net positive charge by Lys-substitution but also hydrophobic helix region by Leu-substitution from CA (1-8)-MA (1-12) hybrid peptide (CA-MA). In particular, CA-MA analogue P5 (P5), designed by flexible region (GIG-->P)-substitution, Lys- (positions 4, 8, 14, 15) and Leu- (positions 5, 6, 12, 13, 16, 17, 20) substitutions, showed potent antibacterial activity in minimal inhibition concentration (MIC) and minimal bactericidal concentration (MBC) without having hemolytic activity. In addition, P5 and chloramphenicol has potent synergistic effect against tested cell lines. As determined by propidium iodide (PI) staining, flow cytometry showed that P5 plus chloramphenicol-treated cells had higher fluorescence intensity than untreated, P5- and chloramphenicol-treated cells. The effect on plasma membrane was examined by investigating the transmembrane potential depolarizing experiments of S. aureus with P5 and chloramphenicol. The result showed that the peptide exerts its antibacterial activity by acting on the plasma membrane. Furthermore, P5 caused significant morphological alterations of S. aureus, as shown by scanning electron microscopy. Our results suggest that peptide P5 is an excellent candidate as a lead compound for the development of novel anti-infective agents and synergistic effects with conventional antibiotic agents but lack hemolytic activity.

Antimicrobial peptides: pore formers or metabolic inhibitors in bacteria?

Antimicrobial peptides are an abundant and diverse group of molecules that are produced by many tissues and cell types in a variety of invertebrate, plant and animal species. Their amino acid composition, amphipathicity, cationic charge and size allow them to attach to and insert into membrane bilayers to form pores by 'barrel-stave', 'carpet' or 'toroidal-pore' mechanisms. Although these models are helpful for defining mechanisms of antimicrobial peptide activity, their relevance to how peptides damage and kill microorganisms still need to be clarified. Recently, there has been speculation that transmembrane pore formation is not the only mechanism of microbial killing. In fact several observations suggest that translocated peptides can alter cytoplasmic membrane septum formation, inhibit cell-wall synthesis, inhibit nucleic-acid synthesis, inhibit protein synthesis or inhibit enzymatic activity. In this review the different models of antimicrobial-peptide-induced pore formation and cell killing are presented.

A Leu-Lys-rich antimicrobial peptide: activity and mechanism

To develop novel antibiotic peptides useful as therapeutic drugs, the analogues were designed to increase not only net positive charge by Lys substitution but also hydrophobic helix region by Leu substitution from cecropin A (1-8)-magainin 2 (1-12) hybrid peptide (CA-MA). In particular, CA-MA analogue P5 (P5), designed by flexible region (GIG-->P) substitution, Lys (positions 4, 8, 14, 15) and Leu (positions 5, 6, 12, 13, 16, 17, 20) substitutions, showed an enhanced antimicrobial and antitumor activity without hemolysis. Confocal microscopy showed that P5 was located in the plasma membrane. The antibacterial effects of analogues were further confirmed by using 1,6-diphenyl-1,3,5-hexatriene as a plasma membrane probe. Flow cytometric analysis revealed that P5 acted in an energy-independent manner. This interaction is also independent of the ionic environment. Furthermore, P5 causes significant morphological alterations of the bacterial surfaces as shown by scanning electron microscopy and showed strong membrane disrupting activity when examined using liposomes (phosphatidyl choline/cholesterol; 10:1, w/w). Its potent antibiotic activity suggests that P5 is an excellent candidate as a lead compound for the development of novel antiinfective agents.

The increased bactericidal activity of a fatty acid-modified synthetic antimicrobial peptide of human cathepsin G correlates with its enhanced capacity to interact with model membranes

The bactericidal potency of a synthetic peptide (CG 117-136) of human lysosomal cathepsin G (cat G) can be substantially increased by covalent attachment to its N- or C-termini, of saturated, linear fatty acids (FAs), namely those with C-8, C-10 and C-12 hydrocarbon chains. In order to understand better the mechanism by which FA moieties increase the bactericidal activity of CG 117-136, the interaction of N-terminally FA-modified peptides with artificial membranes was studied. First, the content of secondary structure motifs in the modified and unmodified peptides was determined by circular dichroism (CD). A marked increase in the propensity of FA-modified CG 117-136 to form an alpha-helix structure was observed for the C-8, C-10 and C-12 derivatives compared with unmodified/short-chain and long-chain (C-14, C-16, C-18) derivatives. These effects were observed both in the presence of large unilamellar liposomes or in trifuluoroethanol, a membrane-stimulating agent. Second, the capacity of peptides to insert into large unilamellar liposomes as a function of FA length was determined by their ability to release a trapped fluorescent dye. FA derivatives with the highest alpha-helical content were found to be the most effective in releasing a fluorescent dye, compared with an unmodified peptide and/or derivatives having a low alpha-helical content. The ability of the peptides to attain alpha-helical structure in the membrane-like environment and the ability to disrupt the liposomal membrane, therefore correlate remarkably well with their increased ability to kill bacteria. A plausible explanation for improved bactericidal action of the modified peptide is that the FA moiety facilitates formation of the peptide with an alpha-helical structure formation in membranes, which is essential for disrupting the integrity of the bacterial cytoplasmic membrane.

The role of tryptophan in the antibacterial activity of a 15-residue bovine lactoferricin peptide

Bovine lactoferricin is a 25-residue antibacterial peptide isolated after gastric cleavage of the iron transporting protein lactoferrin. A 15-residue fragment, FKCRRWQWRMKKLGA of this peptide sustains most of the antibacterial activity. In this truncated sequence, the two Trp residues are found to be essential for antibacterial activity. The anchoring properties of Trp, as have been observed in membrane proteins, are believed to be important for the interaction of Trp containing antibacterial peptides with bacterial cell membranes. We have investigated the molecular properties which make Trp important for the antibacterial activity of the 15-residue peptide by replacing Trp with natural and unnatural aromatic amino acids. This series of peptides was tested for antibacterial activity against Echerichia coli and Staphylococcus aureus. We found that neither the hydrogen bonding ability nor the amphipathicity of the indole system are essential properties for the effect of Trp on the antibacterial activity of the peptides. Replacement of Trp with residues containing aromatic hydrocarbon side chains gave the most active peptides. We propose that aromatic hydrocarbon residues are able to position themselves deeper into the bacterial cell membrane, making the peptide more efficient in disrupting the bacterial cell membrane. From our results the size, shape and aromatic character of Trp seem to be the most important features for the activity of this class of Trp containing antibacterial peptides.

Effects of the hinge region of cecropin A(1-8)-magainin 2(1-12), a synthetic antimicrobial peptide, on liposomes, bacterial and tumor cells

A 20-residue hybrid peptide (CA(1-8)-MA(1-12): KWKLFKKIGIGKFLHSAKKF-NH(2)) incorporating 1-8 residues of cecropin A (CA) and 1-12 residues of magainin 2 (MA) has potent antibiotic activity without hemolytic activity. In order to investigate the effects of the flexible hinge sequence, Gly-Ile-Gly of CA(1-8)-MA(1-12) (CA-MA) on antibiotic activity, CA-MA and its three analogues, CA-MA1, CA-MA2 and CA-MA3 were synthesized. The Gly-Ile-Gly sequence of CA-MA was deleted in CA-MA1 and replaced with Pro and Gly-Pro-Gly in CA-MA2 and CA-MA3, respectively. CA-MA1 and CA-MA3 caused a significant decrease in the bactericidal rate against Escherichia coli and Bacillus subtilis and the tumoricidal activity against four different tumor cells, and the PC/PS (4:1, w/w) vesicle-aggregating and disrupting activities. However, CA-MA2 showed a similar bactericidal rate and antitumor, vesicle-aggregating and disrupting activities, as compared with CA-MA. These results suggested that the flexibility or beta-turn induced by Gly-Ile-Gly or Pro in the central part of CA-MA may be important in the electrostatic interaction of the cationic short alpha-helical region in the N-terminus with the cell membrane surface and the hydrophobic interaction of amphipathic alpha-helical region in the C-terminus with the hydrophobic acyl chains in the cell membrane. CA-MA3 exhibited lower activity in antibacterial, antitumor, and vesicle-aggregating and disrupting activities than CA-MA and CA-MA2. This result suggested that the excessive beta-turn structure by Gly-Pro-Gly in CA-MA3 seems to interrupt the ion channel/pore formation on the lipid bilayer. It was concluded that the appropriate flexibility or beta-turn structure provided by the central hinge is responsible for the effective antibiotic activity of the antimicrobial peptides with the helix-hinge-helix structure.

The structure, dynamics and orientation of antimicrobial peptides in membranes by multidimensional solid-state NMR spectroscopy

Linear peptide antibiotics have been isolated from amphibians, insects and humans and used as templates to design cheaper and more potent analogues for medical applications. Peptides such as cecropins or magainins are < or = 40 amino acids in length. Many of them have been prepared by solid-phase peptide synthesis with isotopic labels incorporated at selected sites. Structural analysis by solid-state NMR spectroscopy and other biophysical techniques indicates that these peptide antibiotics strongly interact with lipid membranes. In bilayer environments they exhibit amphipathic alpha-helical conformations and alignments of the helix axis parallel to the membrane surface. This contrasts the transmembrane orientations observed for alamethicin or gramicidin A. Models that have been proposed to explain the antibiotic and pore-forming activities of membrane-associated peptides, as well as other experimental results, include transmembrane helical bundles, wormholes, carpets, detergent-like effects or the in-plane diffusion of peptide-induced bilayer instabilities.