Membrane interaction and antibacterial properties of two mildly cationic peptide diastereomers, bombinins H2 and H4, isolated from Bombina skin

Biodegradable Polymers and Polymer Composites with Antibacterial Properties

Antibiotic resistance is one of the greatest threats to global health and food security today. It becomes increasingly difficult to treat infectious disorders because antibiotics, even the newest ones, are becoming less and less effective. One of the ways taken in the Global Plan of Action announced at the World Health Assembly in May 2015 is to ensure the prevention and treatment of infectious diseases. In order to do so, attempts are made to develop new antimicrobial therapeutics, including biomaterials with antibacterial activity, such as polycationic polymers, polypeptides, and polymeric systems, to provide non-antibiotic therapeutic agents, such as selected biologically active nanoparticles and chemical compounds. Another key issue is preventing food from contamination by developing antibacterial packaging materials, particularly based on degradable polymers and biocomposites. This review, in a cross-sectional way, describes the most significant research activities conducted in recent years in the field of the development of polymeric materials and polymer composites with antibacterial properties. We particularly focus on natural polymers, i.e., polysaccharides and polypeptides, which present a mechanism for combating many highly pathogenic microorganisms. We also attempt to use this knowledge to obtain synthetic polymers with similar antibacterial activity.

Antimicrobial Bombinin-like Peptide 3 Selectively Recognizes and Inserts into Bacterial Biomimetic Bilayers in Multiple Steps

Bombinins are a wide family of antimicrobial peptides from Xenopus skin. By sequence clustering, we highlighted at least three families named A, B, and H, which might exert antibacterial activity by different modes of action. In this work, we study bombinin-like peptide 3 (BLP-3) as a nonhemolytic representative of the quite unexplored class A due to its appealing activity toward WHO-priority-list bacteria such as Neisseria, Pseudomonas aeruginosa, and Staphylococcus aureus. A marked preference for cardiolipin and phosphatidylglycerol head groups, typically found in bacteria, is proven with biomimetic membranes studied by liquid and solid NMR and MD simulations. BLP-3 gets structured upon interaction and penetrates deeply into the bilayer in two steps involving a superficial insertion of key side chains and subsequent internalization. All along the pathway, a fundamental role is played by lysine residues in the conserved region 11-19, which act in synergy with other key residues.

Antimicrobial activity, membrane interaction and stability of the D-amino acid substituted analogs of antimicrobial peptide W3R6

Antimicrobial peptide W3R6 was derived from chensinin-1b and showed potential as a novel antibiotics. However, W3R6 was susceptible to protease cleavage, which limited its therapeutic application. To improve the proteolytic resistance of W3R6, D-amino acids were incorporated into its sequence by specific amino acid substitution or whole sequence substitution according to the specificity of the cleavage site. In this study, partially substituted analog _D-Arg-W3R6 and completely substituted D-enantiomer D-W3R6 were synthesized. The resistance of _D-Arg-W3R6 and D-W3R6 to cleavage by the tested protease increased, particularly of D-W3R6. The antimicrobial activity of _D-Arg-W3R6 was almost the same as that of the parent peptide W3R6, but the antimicrobial activity of D-W3R6 was slightly decreased. The hemolytic activity of both _D-Arg-W3R6 and D-W3R6 was negligible. The CD spectrum of D-W3R6 exhibited symmetry with that of W3R6 in a membrane-mimetic environment. The membrane interaction between the D-amino acid substituted analogs and a real/mimic bacterial cell membrane was examined. The outer membrane depolarization, inner membrane permeability and dye leakage in three types of liposomes treated with _D-Arg-W3R6 and D-W3R6 were not obviously different from W3R6, which could be due to the similar physical and chemical properties. In addition, these three peptides showed the binding ability with LPS micelles detected by ITC, and their ability to disrupt the LPS micelles was examined by DLS experiment and even neutralize the surface negative charge of E. coli cells. These results suggest that _D-Arg-W3R6 is a promising antibiotic molecule.

Antimicrobial activity and self-assembly behavior of antimicrobial peptide chensinin-1b with lipophilic alkyl tails

The threshold hydrophobicity and amphipathic structure of the peptidic chain are important for the biological function of antimicrobial peptides. Chensinin-1b exhibits broad-spectrum bactericidal activity with no hemolytic activity but has almost no anticancer ability against the selected cancer cell lines. In this study, the conjugation of aliphatic acid was designed with different lengths of N-terminal of chensinin-1b, the antimicrobial activity of the resulting lipo-chensinin-1b was examined, in which OA-C1b showed much stronger activity than those of cheninin-1b and the other two lipopeptides. The membrane interaction between the lipo-chensinin-1b and real/mimetic bacterial cell membrane was investigated. Electrostatic interactions between the lipo-chensinin-1b and lipopolysaccharides were detected by isothermal titration calorimetry and the binding affinities were 10.83 μM, 8.77 μM and 7.35 μM for OA-C1b, LA-C1b and PA-C1b, respectively. The antimicrobial activity and membrane interaction ability of the lipo-chensinin-1b followed this order: OA-C1b > chensinin-1b > LA-C1b > PA-C1b. In addition, the lipo-chensinin-1b also exhibited lytic activity against various cancer cells and demonstrated the ability to inhibit LPS-stimulated cytokine release from human U937 cells. The CD spectra indicated that the helical or β-strands contents were existed as the main components in TFE or LPS solution, respectively. The self-assembly behavior was trigged by the solution pH and affected by the length of carbon chain, in which chensinin-1b, OA-C1b, LA-C1b and PA-C1b formed micelles at neutral pH and the micelle size increased for chensinin-1b, OA-C1b and LA-C1b. PA-C1b formed nanofibers in an acidic environment indicated by TEM experiments, and the peptides formed aggregates in an acidic environment and re-dissociated when the pH was adjusted to neutral.

The role of d-allo-isoleucine in the deposition of the anti-Leishmania peptide bombinin H4 as revealed by 31P solid-state NMR, VCD spectroscopy, and MD simulation

Bombinin H4 is an antimicrobial peptide that was isolated from the toad Bombina variegata. Bombinin H family peptides are active against gram-positive, gram-negative bacteria, and fungi as well as the parasite Leishmania. Among them, bombinin H4 (H4), which contains d-allo-isoleucine (d-allo-Ile) as the second residue in its sequence, is the most active, and its l-isomer is bombinin H2 (H2). H4 has a significantly lower LC50 than H2 against Leishmania. However, the atomic-level mechanism of the membrane interaction and higher activity of H4 has not been clarified. In this work, we investigated the behavior of the conformations and interactions of H2 and H4 with the Leishmania membrane using ³¹P solid-state nuclear magnetic resonance (NMR), vibrational circular dichroism (VCD) spectroscopy, and molecular dynamics (MD) simulations. The generation of isotropic ³¹P NMR signals depending on the peptide concentration indicated the abilities of H2 and H4 to exert antimicrobial activity via membrane disruption. The VCD experiment and density functional theory calculation confirmed the different stability and conformations of the N-termini of H2 and H4. MD simulations revealed that the N-terminus of H4 is more stable than that of H2 in the membrane, in line with the VCD experiment data. VCD and MD analyses demonstrated that the first l-Ile and second d-allo-Ile of H4 tend to take a cis conformation. These residues function as an anchor and facilitate the easy winding of the helical conformation of H4 in the membrane. It may assist to quickly reach to the threshold concentration of H4 on the Leishmania membrane. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

The synergistic antimicrobial effects of novel bombinin and bombinin H peptides from the skin secretion of Bombina orientalis

Bombinin and bombinin H are two antimicrobial peptide (AMP) families initially discovered from the skin secretion of Bombina that share the same biosynthetic precursor-encoding cDNAs, but have different structures and physicochemical properties. Insight into their possible existing relationship lead us to perform the combination investigations into their anti-infectious activities. In this work, we report the molecular cloning and functional characterization of two novel AMPs belonging to bombinin and bombinin H families from secretions of Bombina orientalis Their mature peptides (BHL-bombinin and bombinin HL), coded by single ORF, were chemically synthesized along with an analogue peptide that replaced L-leucine with D-leucine from the second position of the N-terminus (bombinin HD). CD analysis revealed that all of them displayed well-defined α-helical structures in membrane mimicking environments. Furthermore, BHL-bombinin displayed broad-spectrum bactericidal activities on a wide range of microorganisms, while bombinin H only exhibited a mildly bacteriostatic effect on the Gram-positive bacteria Staphylococcus aureus The combination potency of BHL-bombinin with either bombinin HL or bombinin HD showed the synergistic inhibition activities against S. aureus (fractional inhibitory concentration index (FICI): 0.375). A synergistic effect has also been observed between bombinin H and ampicillin, which was further systematically evaluated and confirmed by in vitro time-killing investigations. Haemolytic and cytotoxic examinations exhibited a highly synergistic selectivity and low cytotoxicity on mammalian cells of these three peptides. Taken together, the discovery of the potent synergistic effect of AMPs in a single biosynthetic precursor with superior functional selectivity provides a promising strategy to combat multidrug-resistant pathogens in clinical therapy.

Discovery of two bombinin peptides with antimicrobial and anticancer activities from the skin secretion of Oriental fire-bellied toad, Bombina orientalis

Amphibian skin secretions are known to contain numerous peptides with a large array of biological activities. Bombinins are a group of amphibian-derived peptides with broad spectrum antimicrobial activities that have been only identified from the ancient toad species, Bombina. In this study, we described the identification and characterization of a novel bombinin precursor which encoded a bombinin-like peptide (BLP-7) and a novel bombinin H-type peptide (named as Bombinin H-BO) from the skin secretion of Oriental fire-bellied toad, Bombina orientalis. The primary structures of both mature peptides were determined by combinations of molecular cloning of peptide precursor-encoding cDNAs and mass spectrometry techniques. Secondary structure prediction revealed that both peptides had cationic amphipathic α-helical structural features. The synthetic replicate of BLP-7 displayed more potent antimicrobial activity than Bombinin H-BO against Gram-positive and Gram-negative bacteria and yeast. Also, in vitro antitumour assay showed that both peptides possessed obvious antiproliferative activity on three human hepatoma cells (Hep G2/SK-HEP-1/Huh7) at the non-toxic doses. These results indicate the peptide family of bombinins could be a potential source of drug candidates for anti-infection and anticancer therapy.

Crabrolin, a natural antimicrobial peptide: structural properties

A joint application of experimental and computational approaches has revealed the exceptionally high attitude of crabrolin, a 13-residue peptide with sequence FLPLILRKIVTAL-NH₂ , to adopt alpha-helix conformation not only in membrane-mimicking solvents but also in the presence of a not negligible amount of water. Our study shows that this propensity essentially resides in the intrinsic thermodynamic stability of alpha-helix conformation whose kinetic stability is drastically reduced in water solvent. Our analysis suggests that this is due to two effects enhanced by water: a more local effect consisting of the demolition of intra-peptide H-bonds, essential for the alpha-helix formation, and a bulk - electrostatic - effect favoring conformational states more polar than alpha-helix. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Promising Approaches to Optimize the Biological Properties of the Antimicrobial Peptide Esculentin-1a(1-21)NH2: Amino Acids Substitution and Conjugation to Nanoparticles

Antimicrobial peptides (AMPs) represent an interesting class of molecules with expanding biological properties which make them a viable alternative for the development of future antibiotic drugs. However, for this purpose, some limitations must be overcome: (i) the poor biostability due to enzymatic degradation; (ii) the cytotoxicity at concentrations slightly higher than the therapeutic dosages; and (iii) the inefficient delivery to the target site at effective concentrations. Recently, a derivative of the frog skin AMP esculentin-1a, named esculentin-1a(1–21)NH₂, [Esc(1–21): GIFSKLAGKKIKNLLISGLKG-NH₂] has been found to have a potent activity against the Gram-negative bacterium Pseudomonas aeruginosa; a slightly weaker activity against Gram-positive bacteria and interesting immunomodulatory properties. With the aim to optimize the antimicrobial features of Esc(1–21) and to circumvent the limitations described above, two different approaches were followed: (i) substitutions by non-coded amino acids, i.e., α-aminoisobutyric acid or d-amino acids; and (ii) peptide conjugation to gold nanoparticles. In this mini-review, we summarized the structural and functional properties of the resulting Esc(1–21)-derived compounds. Overall, our data may assist researchers in the rational design and optimization of AMPs for the development of future drugs to fight the worldwide problem of antibiotic resistance.

Antimicrobial and anti-inflammatory activities of three chensinin-1 peptides containing mutation of glycine and histidine residues

The natural peptide chensinin-1 doesnot exhibit its desired biological properties. In this study, the mutant MC1-1 was designed by replacing Gly in the chensinin-1 sequence with Trp. Mutants MC1-2 and MC1-3 were designed based on the MC1-1 sequence to investigate the specific role of His residues. The mutated peptides presented α-helicity in a membrane-mimetic environment and exhibited broad-spectrum antimicrobial activities; in contrast to Trp residues, His residues were dispensable for interacting with the cell membrane. The interactions between the mutant peptides and lipopolysaccharide (LPS) facilitated the ingestion of peptides by Gram-negative bacteria. The binding affinities of the peptides were similar, at approximately 10 μM, but ΔH for MC1-2 was −7.3 kcal.mol⁻¹, which was 6-9 folds higher than those of MC1-1 and MC1-3, probably due to the conformational changes. All mutant peptides demonstrated the ability to inhibit LPS-induced tumour-necrosis factor-α (TNF-α) and interleukin-6 (IL-6) release from murine RAW264.7 cells. In addition, the representative peptide MC1-1showed better inhibition of serum TNF-α and IL-6 levels compared to polymyxin B (PMB), a potent binder and neutralizer of LPS as positive control in LPS-challenged mice model. These data suggest that the mutant peptides could be promising molecules for development as chensinin-based therapeutic agents against sepsis.

Rational modification of a dendrimeric peptide with antimicrobial activity: consequences on membrane-binding and biological properties

Peptide-based antibiotics might help containing the rising tide of antimicrobial resistance. We developed SB056, a semi-synthetic peptide with a dimeric dendrimer scaffold, active against both Gram-negative and Gram-positive bacteria. Being the mechanism of SB056 attributed to disruption of bacterial membranes, we enhanced the amphiphilic profile of the original, empirically derived sequence [WKKIRVRLSA-NH2] by interchanging the first two residues [KWKIRVRLSA-NH2], and explored the effects of this modification on the interaction of peptide, both in linear and dimeric forms, with model membranes and on antimicrobial activity. Results obtained against Escherichia coli and Staphylococcus aureus planktonic strains, with or without salts at physiological concentrations, confirmed the added value of dendrimeric structure over the linear one, especially at physiological ionic strength, and the impact of the higher amphipathicity obtained through sequence modification on enhancing peptide performances. SB056 peptides also displayed intriguing antibiofilm properties. Staphylococcus epidermidis was the most susceptible strain in sessile form, notably to optimized linear analog lin-SB056-1 and the wild-type dendrimer den-SB056. Membrane affinity of all peptides increased with the percentage of negatively charged lipids and was less influenced by the presence of salt in the case of dendrimeric peptides. The analog lin-SB056-1 displayed the highest overall affinity, even for zwitterionic PC bilayers. Thus, in addition to electrostatics, distribution of charged/polar and hydrophobic residues along the sequence might have a significant role in driving peptide-lipid interaction. Supporting this view, dendrimeric analog den-SB056-1 retained greater membrane affinity in the presence of salt than den-SB056, despite the fact that they bear exactly the same net positive charge.

Native and dry-heated lysozyme interactions with membrane lipid monolayers: Lipid packing modifications of a phospholipid mixture, model of the Escherichia coli cytoplasmic membrane

Antimicrobial resistance is currently an important public health issue. The need for innovative antimicrobials is therefore growing. The ideal antimicrobial compound should limit antimicrobial resistance. Antimicrobial peptides or proteins such as hen egg white lysozyme are promising molecules that act on bacterial membranes. Hen egg white lysozyme has recently been identified as active on Gram-negative bacteria due to disruption of the outer and cytoplasmic membrane integrity. Furthermore, dry-heating (7 days and 80 °C) improves the membrane activity of lysozyme, resulting in higher antimicrobial activity. These in vivo findings suggest interactions between lysozyme and membrane lipids. This is consistent with the findings of several other authors who have shown lysozyme interaction with bacterial phospholipids such as phosphatidylglycerol and cardiolipin. However, until now, the interaction between lysozyme and bacterial cytoplasmic phospholipids has been in need of clarification. This study proposes the use of monolayer models with a realistic bacterial phospholipid composition in physiological conditions. The lysozyme/phospholipid interactions have been studied by surface pressure measurements, ellipsometry and atomic force microscopy. Native lysozyme has proved able to absorb and insert into a bacterial phospholipid monolayer, resulting in lipid packing reorganization, which in turn has lead to lateral cohesion modifications between phospholipids. Dry-heating of lysozyme has increased insertion capacity and ability to induce lipid packing modifications. These in vitro findings are then consistent with the increased membrane disruption potential of dry heated lysozyme in vivo compared to native lysozyme. Moreover, an eggPC monolayer study suggested that lysozyme/phospholipid interactions are specific to bacterial cytoplasmic membranes.

Folded structure and insertion depth of the frog-skin antimicrobial Peptide esculentin-1b(1-18) in the presence of differently charged membrane-mimicking micelles

Antimicrobial peptides (AMPs) are effectors of the innate immunity of most organisms. Their role in the defense against pathogen attack and their high selectivity for bacterial cells make them attractive for the development of a new class of antimicrobial drugs. The N-terminal fragment of the frog-skin peptide esculentin-1b (Esc(1-18)) has shown broad-spectrum antimicrobial activity. Similarly to most cationic AMPs, it is supposed to act by binding to and damaging the negatively charged plasma membrane of bacteria. Differently from many other AMPs, Esc(1-18) activity is preserved in biological fluids such as serum. In this work, a structural investigation was performed through NMR spectroscopy. The 3D structure was obtained in the presence of either zwitterionic or negatively charged micelles as membrane models for eukaryotic and prokaryotic membranes, respectively. Esc(1-18) showed a higher affinity for and deeper insertion into the latter and adopted an amphipathic helical structure characterized by a kink at the residue G8. These findings were confirmed by measuring penetration into lipid monolayers. The presence of negatively charged lipids in the bilayer appears to be necessary for Esc(1-18) to bind, to fold in the right three-dimensional structure, and, ultimately, to exert its biological role as an AMP.

Insights into the membrane interaction mechanism and antibacterial properties of chensinin-1b

Antimicrobial peptides (AMPs) with non-specific membrane disrupting activities are thought to exert their antimicrobial activity as a result of their cationicity, hydrophobicity and α-helical or β-sheet structures. Chensinin-1, a native peptide from skin secretions of Rana chensinensis, fails to manifest its desired biological properties because its low hydrophobic nature and an adopted random coil structure in a membrane-mimetic environment. In this study, chensinin-1b was designed by rearranging the amino acid sequence of its hydrophilic/polar residues on one face and its hydrophobic/nonpolar residues on the opposite face according to its helical diagram, and by replacing three Gly residues with three Trp residues. Introduction of Trp residues significantly promoted the binding of the peptide to the bacterial outer membrane and exerted bactericidal activity through cytoplasmic membrane damage. Chensinin-1b demonstrates higher antimicrobial activity and greater cell selectivity than its parent peptide, chensinin-1. The electrostatic interactions between chensinin-1b and lipopolysaccharide (LPS) may have facilitated the uptake of the peptide into Gram-negative cells and be also helpful to disrupt the bacterial cytoplasmic membrane, as evidenced by depolarisation of the membrane potential and leakage of calceins from the liposomes of Escherichia coli and Staphylococcus aureus. Chensinin-1b was also found to penetrate mouse skin and was also effective in vivo, as measured by hydroxyproline levels in a wound infection mouse model, and could therefore act as an anti-infective agent for wound healing.

Feleucins: novel bombinin precursor-encoded nonapeptide amides from the skin secretion of Bombina variegata

The first amphibian skin antimicrobial peptide (AMP) to be identified was named bombinin, reflecting its origin from the skin of the European yellow-bellied toad (Bombina variegata). Bombinins and their related peptides, the bombinin Hs, were subsequently reported from other bombinid toads. Molecular cloning of bombinin-encoding cDNAs from skin found that bombinins and bombinin Hs were coencoded on the same precursor proteins. Here, we report the molecular cloning of two novel cDNAs from a skin secretion-derived cDNA library of B. variegata whose open-reading frames each encode a novel bombinin (GIGGALLNVGKVALKGLAKGLAEHFANamide) and a C-terminally located single copy of a novel nonapeptide (FLGLLGGLLamide or FLGLIGSLLamide). These novel nonapeptides were named feleucin-BV1 and feleucin-BV2, respectively. The novel bombinin exhibited 89% identity to homologues from the toads, B. microdeladigitora and B. maxima. The feleucins exhibited no identity with any amphibian AMP archived in databases. Synthetic feleucins exhibited a weak activity against Staphylococcus aureus (128–256 mg/L) but feleucin-BV1 exhibited a synergistic action with the novel bombinin. The present report clearly demonstrates that the skin secretions of bombinid toads continue to represent a source of peptides of novel structure that could provide templates for the design of therapeutics.

Short antibacterial peptides with significantly reduced hemolytic activity can be identified by a systematic L-to-D exchange scan of their amino acid residues

High systemic toxicity of antimicrobial peptides (AMPs) limits their clinical application to the treatment of topical infections; in parenteral systemic application of AMPs the problem of hemolysis is one of the first to be tackled. We now show that the selectivity of lipidated short synthetic AMPs can be optimized substantially by reducing their hemolytic activity without affecting their activity against methicillin resistant Staphylococcus aureus (MRSA). In order to identify the optimized peptides, two sets of 32 diastereomeric H-(D)Arg-WRWRW-(L)Lys(C(O)CnH2n+1)-NH2 (n = 7 or 9) peptides were prepared using a split-split procedure to perform a systematic L-to-D exchange scan on the central WRWRW-fragment. Compared to the all-L C8-lipidated lead sequence, diastereomeric peptides had very similar antibacterial properties, but were over 30 times less hemolytic. We show that the observed hemolysis and antibacterial activity is affected by both differences in lipophilicity of the different peptides and specific combinations of L- and D-amino acid residues. This study identified several peptides that can be used as tools to precisely unravel the origin of hemolysis and thus help to design even further optimized nontoxic very active short antibacterial peptides.

Investigation of the role of tryptophan residues in cationic antimicrobial peptides to determine the mechanism of antimicrobial action

AIMS

To understand the effects of Trp residues in linear antimicrobial peptides with α-helical conformations on cell permeation ability and membrane transduction efficacy.

METHODS AND RESULTS

A series of L-K6 analogues were designed and synthesized by replacing Ile or Leu with Trp at different positions on the hydrophobic face of L-K6. The antimicrobial and haemolytic activity and secondary structure of the designed Trp-containing peptides were assessed. In addition, the role of Trp in membrane disruption for these designed peptides was investigated. I1W, I4W and L5W demonstrated stronger activity than the other peptides against both Gram-positive and Gram-negative bacteria. All of the tested peptides preferentially interacted with negatively charged vesicles composed of phosphatidylglycerol (PG)/cardiolipin (CL) or PG/CL/phosphatidylethanolamine, and, to a lesser extent, with zwitterionic vesicles. I1W, I4W and L5W caused calcein release at 2·5 μmol l(-1) .

CONCLUSIONS

The position of Trp, rather than the number of Trp residues, in these peptides was an important factor in the antimicrobial activity. Trp residues were deeply inserted into negatively charged membranes but were largely exposed in aqueous buffer solution.

SIGNIFICANCE AND IMPACT OF THE STUDY

These Trp-containing peptides may represent good candidates for new antibiotic agents and for use in new therapeutic approaches.

Isomerization of an antimicrobial peptide broadens antimicrobial spectrum to gram-positive bacterial pathogens

The branched M33 antimicrobial peptide was previously shown to be very active against Gram-negative bacterial pathogens, including multidrug-resistant strains. In an attempt to produce back-up molecules, we synthesized an M33 peptide isomer consisting of D-aminoacids (M33-D). This isomeric version showed 4 to 16-fold higher activity against Gram-positive pathogens, including Staphylococcus aureus and Staphylococcus epidermidis, than the original peptide, while retaining strong activity against Gram-negative bacteria. The antimicrobial activity of both peptides was influenced by their differential sensitivity to bacterial proteases. The better activity shown by M33-D against S. aureus compared to M33-L was confirmed in biofilm eradication experiments where M33-L showed 12% activity with respect to M33-D, and in vivo models where Balb-c mice infected with S. aureus showed 100% and 0% survival when treated with M33-D and M33-L, respectively. M33-D appears to be an interesting candidate for the development of novel broad-spectrum antimicrobials active against bacterial pathogens of clinical importance.

Phosphatidylethanolamine binding is a conserved feature of cyclotide-membrane interactions

Cyclotides are bioactive cyclic peptides isolated from plants that are characterized by a topologically complex structure and exceptional resistance to enzymatic or thermal degradation. With their sequence diversity, ultra-stable core structural motif, and range of bioactivities, cyclotides are regarded as a combinatorial peptide template with potential applications in drug design. The mode of action of cyclotides remains elusive, but all reported biological activities are consistent with a mechanism involving membrane interactions. In this study, a diverse set of cyclotides from the two major subfamilies, Möbius and bracelet, and an all-d mirror image form, were examined to determine their mode of action. Their lipid selectivity and membrane affinity were determined, as were their toxicities against a range of targets (red blood cells, bacteria, and HIV particles). Although they had different membrane-binding affinities, all of the tested cyclotides targeted membranes through binding to phospholipids containing phosphatidylethanolamine headgroups. Furthermore, the biological potency of the tested cyclotides broadly correlated with their ability to target and disrupt cell membranes. The finding that a broad range of cyclotides target a specific lipid suggests their categorization as a new lipid-binding protein family. Knowledge of their membrane specificity has the potential to assist in the design of novel drugs based on the cyclotide framework, perhaps allowing the targeting of peptide drugs to specific cell types.

Rational design of anti-microbial peptides with enhanced activity and low cytotoxicity based on the structure of the arginine/histidine-rich peptide, chensinin-1

AIMS

To understand the structure-activity relationship of chensinin-1, a anti-microbial peptide (AMP) with an unusual structure, and to develop novel AMPs as therapeutic agents.

METHODS AND RESULTS

A series of chensinin-1 analogues were designed and synthesized by one to three replacement of glycines with leucines at the hydrophilic face of chensinin-1 or rearrangement of some of the residues in its sequence. Circular dichroism spectroscopy showed that the analogues adopted α-helical-type conformations in 50% trifluoroethanol/water but adopted β-strand-type conformations in 30 mmol l(-1) sodium dodecyl sulphate. The anti-microbial activities of the peptides against Gram-positive bacteria increased 5- to 30-fold, and these increases paralleled the increases in the peptides' hydrophobicities. Their haemolytic activities also increased. Amphipathicities had little influence on the bactericidal activity of chensinin-1. All peptides caused leakage of calcein entrapped in negatively charged liposomes although with different efficiencies. The peptides did not induce leakage of calcein from uncharged liposomes.

CONCLUSIONS

Peptide adopted an aperiodic structure can improve the anti-microbial potency by increasing peptide hydrophobicity. Its target is bacteria plasma membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

Chensinin-1 can act as a new lead molecule for the study of AMPs with atypical structures.

A novel dendrimeric peptide with antimicrobial properties: structure-function analysis of SB056

The novel antimicrobial peptide with a dimeric dendrimer scaffold, SB056, was empirically optimized by high-throughput screening. This procedure produced an intriguing primary sequence whose structure-function analysis is described here. The alternating pattern of hydrophilic and hydrophobic amino acids suggests the possibility that SB056 is a membrane-active peptide that forms amphiphilic β-strands in a lipid environment. Circular dichroism confirmed that the cationic SB056 folds as β-sheets in the presence of anionic vesicles. Lipid monolayer surface pressure experiments revealed unusual kinetics of monolayer penetration, which suggest lipid-induced aggregation as a membranolytic mechanism. NMR analyses of the linear monomer and the dendrimeric SB056 in water and in 30% trifluoroethanol, on the other hand, yielded essentially unstructured conformations, supporting the excellent solubility and storage properties of this compound. However, simulated annealing showed that many residues lie in the β-region of the Ramachandran plot, and molecular-dynamics simulations confirmed the propensity of this peptide to fold as a β-type conformation. The excellent solubility in water and the lipid-induced oligomerization characteristics of this peptide thus shed light on its mechanism of antimicrobial action, which may also be relevant for systems that can form toxic β-amyloid fibrils when in contact with cellular membranes. Functionally, SB056 showed high activity against Gram-negative bacteria and some limited activity against Gram-positive bacteria. Its potency against Gram-negative strains was comparable (on a molar basis) to that of colistin and polymyxin B, with an even broader spectrum of activity than numerous other reference compounds.

Short native antimicrobial peptides and engineered ultrashort lipopeptides: similarities and differences in cell specificities and modes of action

Due to the rapid emergence of resistant microbes to the currently available antibiotics, cationic antimicrobial peptides have attracted considerable interest as a possible new generation of anti-infective compounds. However, low cost development for therapeutic or industrial purposes requires, among other properties, that the peptides will be small and with simple structure. Therefore, considerable research has been devoted to optimizing peptide length combined with a simple design. This review focuses on the similarities and differences in the mode of action and target cell specificity of two families of small peptides: the naturally occurring temporins from the skin of amphibia and the engineered ultrashort lipopeptides. We will also discuss the finding that acylation of cationic peptides results in molecules with a more potent spectrum of activity and a higher resistance to proteolytic degradation. Conjugation of fatty acids to linear native peptide sequences is a powerful strategy to engineer novel successful anti-infective drugs.