Mechanism of interaction of different classes of cationic antimicrobial peptides with planar bilayers and with the cytoplasmic membrane of Escherichia coli

Biochemical property and membrane-peptide interactions of de novo antimicrobial peptides designed by helix-forming units

Typical peptides composed of Phe, Ile, and Arg residues have not been reported, and the effect of the helix-forming unit (HFU) composed of the tripeptide core on biological activity remains unclear. In this study, multimers of the 3-residue HFU were designed to investigate the structure-function relationships. The in vitro biological activities of the peptides were determined. We used synthetic lipid vesicles and intact bacteria to assess the interactions of the peptides with cell membranes. The well-studied peptide melittin was chosen as a control peptide. The results showed that the antimicrobial and hemolytic activities of the peptides increased with the number of HFUs. HFU3 had optimal cell selectivity as determined by the therapeutic index. HFU3 and HFU4 exhibited strong resistance to salts, pH, and heat. CD spectra revealed that the peptides except HFU2 displayed α-helix-rich secondary structures in the presence of SDS or trifluoroethanol (TFE). The peptides interacted weakly with zwitterionic phospholipids (mimicking mammalian membranes) but strongly with negatively charged phospholipids (mimicking bacterial membranes), which corresponds well with the data for the biological activities. There was a correlation between the cell selectivity of the peptides and their high binding affinity with negatively charged phospholipids. Cell membrane permeability experiments suggest that the peptides targeted the cell membrane, and HFU3 showed higher permeabilization of the inner membrane but lower permeabilization of the outer membrane than melittin. These findings provide the new insights to design antimicrobial peptides with antimicrobial potency by trimers.

Vibrio cholerae autoinducer CAI-1 interferes with Pseudomonas aeruginosa quorum sensing and inhibits its growth

The human pathogen Vibrio cholerae uses several small molecules to coordinate gene expression in a process termed quorum sensing (QS), and its main autoinducer is CAI-1. We have examined the activity of this signaling molecule in three other species of bacteria. Interestingly, while showing an inhibitory effect on QS in the opportunistic pathogen P. aeruginosa at low micromolar concentrations, it caused also growth inhibition at higher concentrations. In contrast, the two other bacteria were unaffected, and we suggest a possible mechanism for these effects, based on membrane perturbation studies.

Lipo-γ-AApeptides as a new class of potent and broad-spectrum antimicrobial agents

There is increasing demand to develop antimicrobial peptides (AMPs) as next generation antibiotic agents, as they have the potential to circumvent emerging drug resistance against conventional antibiotic treatments. Non-natural antimicrobial peptidomimetics are an ideal example of this, as they have significant potency and in vivo stability. Here we report for the first time the design of lipidated γ-AApeptides as antimicrobial agents. These lipo-γ-AApeptides show potent broad-spectrum activities against fungi and a series of Gram-positive and Gram-negative bacteria, including clinically relevant pathogens that are resistant to most antibiotics. We have analyzed their structure-function relationship and antimicrobial mechanisms using membrane depolarization and fluorescent microscopy assays. Introduction of unsaturated lipid chain significantly decreases hemolytic activity and thereby increases the selectivity. Furthermore, a representative lipo-γ-AApeptide did not induce drug resistance in S. aureus, even after 17 rounds of passaging. These results suggest that the lipo-γ-AApeptides have bactericidal mechanisms analogous to those of AMPs and have strong potential as a new class of novel antibiotic therapeutics.

Efficacy of OH-CATH30 and its analogs against drug-resistant bacteria in vitro and in mouse models

Antimicrobial peptides (AMPs) have been considered alternatives to conventional antibiotics for drug-resistant bacterial infections. However, their comparatively high toxicity toward eukaryotic cells and poor efficacy in vivo hamper their clinical application. OH-CATH30, a novel cathelicidin peptide deduced from the king cobra, possesses potent antibacterial activity in vitro. The objective of this study is to evaluate the efficacy of OH-CATH30 and its analog OH-CM6 against drug-resistant bacteria in vitro and in vivo. The MICs of OH-CATH30 and OH-CM6 ranged from 1.56 to 12.5 μg/ml against drug-resistant clinical isolates of several pathogenic species, including Escherichia coli, Pseudomonas aeruginosa, and methicillin-resistant Staphylococcus aureus. The MICs of OH-CATH30 and OH-CM6 were slightly altered in the presence of 25% human serum. OH-CATH30 and OH-CM6 killed E. coli quickly (within 60 min) by disrupting the bacterial cytoplasmic membrane. Importantly, the 50% lethal doses (LD(50)) of OH-CATH30 and OH-CM6 in mice following intraperitoneal (i.p.) injection were 120 mg/kg of body weight and 100 mg/kg, respectively, and no death was observed at any dose up to 160 mg/kg following subcutaneous (s.c.) injection. Moreover, 10 mg/kg OH-CATH30 or OH-CM6 significantly decreased the bacterial counts as well as the inflammatory response in a mouse thigh infection model and rescued infected mice in a bacteremia model induced by drug-resistant E. coli. Taken together, our findings demonstrate that the natural cathelicidin peptide OH-CATH30 and its analogs exhibit relatively low toxicity and potent efficacy in mouse models, indicating that they may have therapeutic potential against the systemic infections caused by drug-resistant bacteria.

Design of potent, non-toxic antimicrobial agents based upon the structure of the frog skin peptide, temporin-1CEb from Chinese brown frog, Rana chensinensis

Temporin-1CEb shows antimicrobial activity against Gram-positive bacteria, but its therapeutic potential is limited by its haemolysis. In this study, eight temporin-1CEb analogues with altered cationicities and hydrophobicities were synthesized. Increasing cationicity and amphipathicity by substituting neutral and non-polar amino acid residues on the hydrophilic face of the α-helix by five or six lysines increased antimicrobial potency approximately 10-fold to 40-fold, although when the number of positive charges was increased from +6 to +7, the antimicrobial potency was not additionally enhanced. The substitution of an l-lysine with a d-lysine, meanwhile maintaining the net charge and the mean hydrophobicity values, had only a minor effect on its antimicrobial activity, whereas significantly led a decrease in its haemolytic activity. Of all the peptides, l-K6 has the best potential as an antimicrobial agent because its antimicrobial activity against both Gram-positive and Gram-negative bacteria is substantial, and its haemolytic activity is negligible. l-K6 adopts an α-helix in 50% trifluoroethanol/water and 30 mm SDS solutions. l-K6 killed 99.9% of E. coli and S. aureus at 4× MIC in 60 min, and its postantibiotic effect was >5 h. l-K6 affects the integrity of E. coli and S. aureus plasma membranes by rapidly inducing membrane depolarization.

Bioactive Peptides: Synthesis, Properties, and Applications in the Packaging and Preservation of Food

  Bioactive peptides are protein fragments which have a positive impact on the functions and conditions of living beings. Peptides have shown several useful properties for human health, including antimicrobial, antifungal, antiviral, and antitumor activities. These compounds are produced by almost all species of life. However, they are produced in limited quantities in nature. As a result, researchers have tried to synthesize bioactive peptides to study their properties and applications in various areas. Among their applications in food preservation, peptides have been incorporated into packaging materials. This review begins with a brief description of the methods used for the synthesis, purification, and characterization of peptides. Also, the main bioproperties and mechanisms of action of peptides are discussed. Finally, some applications of peptides are presented, especially their use in active packaging, their effects on the polymeric matrix, and peptide migration.

From a marine neuropeptide to antimicrobial pseudopeptides containing aza-β(3)-amino acids: structure and activity

Incorporation of aza-β(3)-amino acids into an endogenous neuropeptide from mollusks (ALSGDAFLRF-NH(2)) with weak antimicrobial activity allows the design of new AMPs sequences. Depending on the nature of the substitution, this can render the pseudopeptides inactive or lead to a drastic enhancement of the antimicrobial activity without high cytotoxicity. Structural studies of the pseudopeptides carried out by NMR and circular dichroism show the impact of aza-β(3)-amino acids on peptide structure. The first three-dimensional structures of pseudopeptides containing aza-β(3)-amino acids in aqueous micellar SDS were determined and demonstrate that the hydrazino turn can be formed in aqueous solution. Thus, AMP activity can be modulated through structural modifications induced by the nature and the position of such amino acid analogues in the peptide sequences.

Direct visualization of bactericidal action of cationic conjugated polyelectrolytes and oligomers

The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation.

The effect of the placement and total charge of the basic amino acid clusters on antibacterial organism selectivity and potency

Extensive circular dichroism, isothermal titration calorimetry and induced calcein leakage studies were conducted on a series of antimicrobial peptides (AMPs), with a varying number of Lys residues located at either the C-terminus or the N-terminus to gain insight into their effect on the mechanisms of binding with zwitterionic and anionic membrane model systems. Different CD spectra were observed for these AMPs in the presence of zwitterionic DPC and anionic SDS micelles indicating that they adopt different conformations on binding to the surfaces of zwitterionic and anionic membrane models. Different CD spectra were observed for these AMPs in the presence of zwitterionic POPC and anionic mixed 4:1 POPC/POPG LUVs and SUVs, indicating that they adopt very different conformations on interaction with these two types of LUVs and SUVs. In addition, ITC and calcein leakage data indicated that all the AMPs studied interact via very different mechanisms with anionic and zwitterionic LUVs. ITC data suggest these peptides interact primarily with the surface of zwitterionic LUVs while they insert into and form pores in anionic LUVs. CD studies indicated that these compounds adopt different conformations depending on the ratio of POPC to POPG lipids present in the liposome. There are detectable spectroscopic and thermodynamic differences between how each of these AMPs interacts with membranes, that is position and total charge density defines how these AMPs interact with specific membrane models and thus partially explain the resulting diversity of antibacterial activity of these compounds.

Mapping residue-specific contacts of polymyxin B with lipopolysaccharide by saturation transfer difference NMR: insights into outer-membrane disruption and endotoxin neutralization

High-resolution interactions studies of molecules with lipopolysaccharide (LPS) or endotoxin are important for health, diseases and environment. LPS is the major constituent of the outer layer of the outer membrane of the gram-negative bacteria. LPS provides an efficient barrier against permeation of variety of compounds including antibacterial agents and antimicrobial peptides. In the intensive care units, LPS is known for the fatal septic shock syndromes. Because of LPS toxicity, high affinity LPS sensors are sought-after for the assessment of the quality of water and pharmaceutical products. Therefore, elucidation of binding epitopes of LPS interacting molecules would be vital for the development of antimicrobial, antiendotoxic molecules. Polymyxin B (PMB), an antibacterial cyclic lipo-peptide, is well known for its LPS sequestering and neutralizing activities. Here, we have used saturation transfer difference (STD) NMR methods for characterizing interactions of PMB with LPS from E. coli 0111:B4 and P. aeruginosa. The dissociation constants of the LPS-PMB complexes were obtained from concentration dependent STD studies. Further a detailed epitope mapping of PMB has been carried out in E. coli 0111:B4 LPS micelles. Experiments including one-dimensional 1H STD, two-dimensional 1H-1H STD-TOCSY and naturalabundance 13C-1H STD-HSQC, are performed to determine the site(s) of interactions of PMB with endotoxin at atomic resolution. Our studies reveal that the hydrophobic sidechains of PMB including a part of the N-terminus lipidic tail demonstrate close contacts with LPS. In contrast, cyclic backbone structure of PMB has the lowest STD effects suggesting a rather loose association with endotoxin.

Examination of a synthetic benzophenone membrane-targeted antibiotic

The enormous success of antibiotics is seriously threatened by the development of resistance to most of the drugs available on the market. Thus, novel antibiotics are needed that are less prone to bacterial resistance and are directed toward novel biological targets. Antimicrobial peptides (AMPs) have attracted considerable attention due to their unique mode of action and broad spectrum activity. However, these agents suffer from liability to proteases and the high cost of manufacturing has impeded their development. Previously, we have reported on a novel class of benzophenone-based antibiotics and early studies suggested that these agents might target the bacterial membrane. In this study, we present our work on the mechanism of action of these novel membrane targeted antibiotics. These compounds have good affinities to polyanionic components of the cell wall such as lipoteichoic acid (LTA) and lipopolysaccharide (LPS). We found that these agents release potassium ions from treated bacteria; thus, resulting in disruption of the bacterial membrane potential. Benzophenone-based membrane targeted antibiotics (BPMTAs) cause membrane disruption in synthetic lipid vesicles that mimic Gram-positive or Gram-negative bacteria. The compounds display no hemolytic activity up to a concentration that is 100 times the MIC values and they are capable of curing mice of a lethal MRSA infection. Repeated attempts to develop a mutant resistant to these agents has failed. Taken together, BPMTAs represent a promising new class of membrane-targeted antibacterial agents.

Easy strategy to increase salt resistance of antimicrobial peptides

The efficacies of many antimicrobial peptides are greatly reduced under high salt concentrations, limiting their development as pharmaceutical compounds. Here, we describe an easy strategy to increase salt resistance of antimicrobial peptides by replacing tryptophan or histidine residues with the bulky amino acids β-naphthylalanine and β-(4,4'-biphenyl)alanine. The activities of the salt-sensitive peptide P-113 were diminished at high salt concentrations, whereas the activities of its β-naphthylalanine and β-(4,4'-biphenyl)alanine-substituted variant were less affected.

Selective membrane disruption: mode of action of C16G2, a specifically targeted antimicrobial peptide

The specifically targeted antimicrobial peptide (STAMP) C16G2 was developed to target the cariogenic oral pathogen Streptococcus mutans. Because the design of this peptide was novel, we sought to better understand the mechanism through which it functioned. Compared to antimicrobial peptides (AMPs) with wide spectra of activity, the STAMP C16G2 has demonstrated specificity for S. mutans in a mixed-culture environment, resulting in the complete killing of S. mutans while having minimal effect on the other streptococci. In the current study, we sought to further confirm the selectivity of C16G2 and also compare its membrane activity to that of melittin B, a classical toxic AMP, in order to determine the STAMP's mechanism of cell killing. Disruption of S. mutans cell membranes by C16G2 was demonstrated by increased SYTOX green uptake and ATP efflux from the cells similar to those of melittin B. Treatment with C16G2 also resulted in a loss of membrane potential as measured by DiSC(3)5 fluorescence. In comparison, the individual moieties of C16G2 demonstrated no specificity and limited antimicrobial activity compared to those of the STAMP C16G2. The data suggest that C16G2 has a mechanism of action similar to that of traditional AMPs and kills S. mutans through disruption of the cell membrane, allowing small molecules to leak out of the cell, which is followed by a loss of membrane potential and cell death. Interestingly, this membrane activity is rapid and potent against S. mutans, but not other noncariogenic oral streptococci.

Multifunctional cationic host defence peptides and their clinical applications

With the rapid rise in the emergence of bacterial strains resistant to multiple classes of antimicrobial agents, there is an urgent need to develop novel antimicrobial therapies to combat these pathogens. Cationic host defence peptides (HDPs) and synthetic derivatives termed innate defence regulators (IDRs) represent a promising alternative approach in the treatment of microbial-related diseases. Cationic HDPs (also termed antimicrobial peptides) have emerged from their origins as nature's antibiotics and are widely distributed in organisms from insects to plants to mammals and non-mammalian vertebrates. Although their original and primary function was proposed to be direct antimicrobial activity against bacteria, fungi, parasites and/or viruses, cationic HDPs are becoming increasingly recognized as multifunctional mediators, with both antimicrobial activity and diverse immunomodulatory properties. Here we provide an overview of the antimicrobial and immunomodulatory activities of cationic HDPs, and discuss their potential application as beneficial therapeutics in overcoming infectious diseases.

Using fluorine nuclear magnetic resonance to probe changes in the structure and dynamics of membrane-active peptides interacting with lipid bilayers

The antimicrobial peptide MSI-78 serves as a model system for studying interactions of bioactive peptides with membranes. Using a series of MSI-78 peptides that incorporate l-4,4,4-trifluoroethylglycine, a small and sensitive (19)F nuclear magnetic resonance probe, we investigated how the local structure and dynamics of the peptide change when it binds to the lipid bilayer. The fluorinated MSI-78 analogues exhibited position-specific changes in (19)F chemical shift ranging from 1.28 to -1.35 ppm upon binding to lipid bicelles. The largest upfield shifts are associated with the most hydrophobic positions in the peptide. Changes in solvent isotope effects (H(2)O/D(2)O) on (19)F chemical shifts were observed for the peptides that are consistent with the MSI-78 solvent-inaccessible hydrophobic core upon binding bicelles. Transverse relaxation measurements of the (19)F nucleus, using the Carr-Purcell-Meiboom-Gill pulse sequence, were used to examine changes in the local mobility of MSI-78 that occur upon binding to the lipid bilayer. Positions in the hydrophobic core of peptide-membrane complex show the greatest decrease in mobility upon binding of the lipid bilayer, whereas residues that interact with lipid headgroups are more mobile. The most mobile positions are at the N- and C-termini of the peptide. These results provide support for the proposed mechanism of membrane disruption by MSI-78 and reveal new details about the dynamic changes that accompany membrane binding.

Protein epitope mimetics as anti-infectives

There is growing interest in the design of synthetic molecules that mimic the structures and functions of epitopes found on the surface of peptides and proteins. Epitope mimetics can provide valuable tools to probe complex biological processes, as well as interesting leads for drug and vaccine discovery. One application of epitope mimetics is reviewed here, focusing on mimetics of the cationic antimicrobial peptides that form part of the innate immune response to microbial and viral infection in many organisms. Mimetics of these naturally occurring peptides and proteins may be useful to explore mechanisms of antimicrobial and immunomodulatory action, and as a potential source of new antibiotics to address one of the most pressing current threats to human health.

Rationale-based, de novo design of dehydrophenylalanine-containing antibiotic peptides and systematic modification in sequence for enhanced potency

Increased microbial drug resistance has generated a global requirement for new anti-infective agents. As part of an effort to develop new, low-molecular-mass peptide antibiotics, we used a rationale-based minimalist approach to design short, nonhemolytic, potent, and broad-spectrum antibiotic peptides with increased serum stability. These peptides were designed to attain an amphipathic structure in helical conformations. VS1 was used as the lead compound, and its properties were compared with three series of derivates obtained by (i) N-terminal amino acid addition, (ii) systematic Trp substitution, and (iii) peptide dendrimerization. The Trp substitution approach underlined the optimized sequence of VS2 in terms of potency, faster membrane permeation, and cost-effectiveness. VS2 (a variant of VS1 with two Trp substitutions) was found to exhibit good antimicrobial activity against both the Gram-negative Escherichia coli and the Gram-positive bacterium Staphylococcus aureus. It was also found to have noncytolytic activity and the ability to permeate and depolarize the bacterial membrane. Lysis of the bacterial cell wall and inner membrane by the peptide was confirmed by transmission electron microscopy. A combination of small size, the presence of unnatural amino acids, high antimicrobial activity, insignificant hemolysis, and proteolytic resistance provides fundamental information for the de novo design of an antimicrobial peptide useful for the management of infectious disease.

Importance of residue 13 and the C-terminus for the structure and activity of the antimicrobial peptide aurein 2.2

Previous studies on aurein 2.2 and 2.3 in DMPC/DMPG and POPC/POPG membranes have shown that bilayer thickness and phosphatidylglycerol content have a significant impact on the interaction of these peptides with membrane bilayers. Further examination with the DiSC(3)5 assay has indicated that aurein 2.2 induces greater membrane leakage than aurein 2.3 in Staphylococcus aureus C622. The only difference between these peptides is a Leu to Ile mutation at residue 13. To better understand the importance of this residue, the structure and activity of the L13A, L13F, and L13V mutants were investigated. In addition, we investigated a number of peptides with truncations at the C-terminus to determine whether the C-terminus, which contains residue 13, is crucial for antimicrobial activity. Solution circular dichroism results demonstrated that the L13F mutation and the truncation of the C-terminus by six residues resulted in decreased helical content, whereas the L13A or L13V mutation and the truncation of the C-terminus by three residues showed little to no effect on the structure. Oriented circular dichroism results demonstrated that only an extensive C-terminal truncation reduced the ability of the peptide to insert into lipid bilayers. (31)P NMR spectroscopy showed that all peptides disorder the headgroups. The implications of these results in terms of antimicrobial activity and the ability of these peptides to induce leakage in S. aureus are discussed. The results suggest that the presence of the 13th residue in aurein 2.2 is important for structure and activity, but the exact nature of residue 13 is less important as long as it is a hydrophobic residue.

Computational studies of protegrin antimicrobial peptides: a review

Antimicrobial peptides (AMPs) are small, naturally occurring peptides that exhibit strong antibacterial properties generally believed to be a result of selective bacterial membrane disruption. As a result, there has been significant interest in the development of therapeutic antibiotics based on AMPs; however, the poor understanding of the fundamental mechanism of action of these peptides has largely hampered such efforts. We present a summary of computational and theoretical investigations of protegrin, a particularly potent peptide that is both an excellent model for the mechanism of action of AMPs and a promising therapeutic candidate. Experimental investigations have shed light on many of the key steps in the action of protegrin: protegrin monomers are known to dimerize in various lipid environments; protegrin peptides interact strongly with lipid bilayer membranes, particularly anionic lipids; protegrins have been shown to form pores in lipid bilayers, which results in uncontrolled ion transport and may be a key factor in bacterial death. In this work, we present a comprehensive review of the computational and theoretical studies that have complemented and extended the information obtained from experimental work with protegrins, as well as a brief survey of the experimental biophysical studies that are most pertinent to such computational work. We show that a consistent, mechanistic description of the bactericidal mechanism of action of protegrins is emerging, and briefly outline areas where the current understanding is deficient. We hope that the research reviewed herein offers compelling evidence of the benefits of computational investigations of protegrins and other AMPs, as well as providing a useful guide to future work in this area.

The importance of bacterial membrane composition in the structure and function of aurein 2.2 and selected variants

For cationic antimicrobial peptides to become useful therapeutic agents, it is important to understand their mechanism of action. To obtain high resolution data, this involves studying the structure and membrane interaction of these peptides in tractable model bacterial membranes rather than directly utilizing more complex bacterial surfaces. A number of lipid mixtures have been used as bacterial mimetics, including a range of lipid headgroups, and different ratios of neutral to negatively charged headgroups. Here we examine how the structure and membrane interaction of aurein 2.2 and some of its variants depend on the choice of lipids, and how these models correlate with activity data in intact bacteria (MICs, membrane depolarization). Specifically, we investigated the structure and membrane interaction of aurein 2.2 and aurein 2.3 in 1:1 cardiolipin/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (CL/POPG) (mol/mol), as an alternative to 1:1 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine(POPC)/POPG and a potential model for Gram positive bacteria such as S. aureus. The structure and membrane interaction of aurein 2.2, aurein 2.3, and five variants of aurein 2.2 were also investigated in 1:1 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE)/POPG (mol/mol) lipids as a possible model for other Gram positive bacteria, such as Bacillus cereus. Solution circular dichroism (CD) results demonstrated that the aurein peptides adopted α-helical structure in all lipid membranes examined, but demonstrated a greater helical content in the presence of POPE/POPG membranes. Oriented CD and ³¹P NMR results showed that the aurein peptides had similar membrane insertion profiles and headgroup disordering effects on POPC/POPG and CL/POPG bilayers, but demonstrated reduced membrane insertion and decreased headgroup disordering on mixing with POPE/POPG bilayers at low peptide concentrations. Since the aurein peptides behaved very differently in POPE/POPG membrane, minimal inhibitory concentrations (MICs) of the aurein peptides in B. cereus strain C737 were determined. The MIC results indicated that all aurein peptides are significantly less active against B. cereus than against S. aureus and S. epidermidis. Overall, the data suggest that it is important to use a relevant model for bacterial membranes to gain insight into the mode of action of a given antimicrobial peptide in specific bacteria.

Permeabilization of fungal hyphae by the plant defensin NaD1 occurs through a cell wall-dependent process

The antifungal activity of the plant defensin NaD1 involves specific interaction with the fungal cell wall, followed by permeabilization of the plasma membrane and entry of NaD1 into the cytoplasm. Prior to this study, the role of membrane permeabilization in the activity of NaD1, as well as the relevance of cell wall binding, had not been investigated. To address this, the permeabilization of Fusarium oxysporum f. sp. vasinfectum hyphae by NaD1 was investigated and compared with that by other antimicrobial peptides, including the cecropin-melittin hybrid peptide CP-29, the bovine peptide BMAP-28, and the human peptide LL-37, which are believed to act largely through membrane disruption. NaD1 appeared to permeabilize cells via a novel mechanism that required the presence of the fungal cell wall. NaD1 and Bac2A, a linear variant of the bovine peptide bactenecin, were able to enter the cytoplasm of treated hyphae, indicating that cell death is accelerated by interaction with intracellular targets.

Spotlight on Human LL-37, an Immunomodulatory Peptide with Promising Cell-Penetrating Properties

Cationic antimicrobial peptides are major components of innate immunity and help control the initial steps of the infectious process. They are expressed not only by immunocytes, but also by epithelial cells. They share an amphipathic secondary structure with a polar cationic site, which explains their tropism for prokaryote membranes and their hydrophobic site contributing to the destructuration of these membranes. LL-37 is the only cationic antimicrobial peptide derived from human cathelicidin. LL-37 can also cross the plasma membrane of eukaryotic cells, probably through special domains of this membrane called lipid rafts. This transfer could be beneficial in the context of vaccination: the activation of intracellular toll-like receptors by a complex formed between CpG oligonucleotides and LL-37 could conceivably play a major role in the building of a cellular immunity involving NK cells.

Antiparasitic activity of the antimicrobial peptide HbbetaP-1, a member of the beta-haemoglobin peptide family

A family of antimicrobial peptides (AMPs) derived from the beta-subunit of haemoglobin was recently isolated from channel catfish, Ictalurus punctatus, infected with Ichthyophthirius multifiliis (ich), an important freshwater fish parasite that causes ichthyophthiriosis. We previously discovered that one of these AMPs, HbbetaP-1, had strong cidal activity against ich as well as another ectoparasite, Tetrahymena pyriformis. HbbetaP-1 toxicity was specific, primarily affecting the trophozoite (trophont) stage of ich. Here, we show that HbbetaP-1 acts more rapidly to kill smaller (presumably less mature) trophonts of ich, taking almost twice as long to kill larger trophonts (P < 0.0001). It acts more rapidly than an unrelated AMP, piscidin 1, which is haemolytic and also lethal to ich trophonts. HbbetaP-1 is potently and selectively lethal to the trophont stage of the dinoflagellate ectoparasite, Amyloodinium ocellatum, one of the most important pathogens of warmwater marine fish. HbbetaP-1 has no effect on the fish gill cell line feeder layer (G1B cells) used to propagate Amyloodinium, further suggesting a highly selective action. These findings suggest that HbbetaP-1 or related AMPs might function in protecting marine as well as freshwater fish and that HbbetaP-1 has highly selective activity against specific life stages of important fish ectoparasites.

Using fluorine nuclear magnetic resonance to probe the interaction of membrane-active peptides with the lipid bilayer

A variety of biologically active peptides exert their function through direct interactions with the lipid membrane of the cell. These surface interactions are generally transient and highly dynamic, making them hard to study. Here we have examined the feasibility of using solution phase (19)F nuclear magnetic resonance (NMR) to study peptide-membrane interactions. Using the antimicrobial peptide MSI-78 as a model system, we demonstrate that peptide binding to either small unilamellar vesicles (SUVs) or bicelles can readily be detected by simple one-dimensional (19)F NMR experiments with peptides labeled with l-4,4,4-trifluoroethylglycine. The (19)F chemical shift associated with the peptide-membrane complex is sensitive both to the position of the trifluoromethyl reporter group (whether in the hydrophobic face or positively charged face of the amphipathic peptide) and to the curvature of the lipid bilayer (whether the peptide is bound to SUVs or bicelles). (19)F spin echo experiments using the Carr-Purcell-Meiboom-Gill pulse sequence were used to measure the transverse relaxation (T(2)) of the nucleus and thereby examine the local mobility of the MSI-78 analogues bound to bicelles. The fluorine probe positioned in the hydrophobic face of the peptide relaxes at a rate that correlates with the tumbling of the bicelle, suggesting that it is relatively immobile, whereas the probe at the positively charged face relaxes more slowly, indicating this position is much more dynamic. These results are in accord with structural models of MSI-78 bound to lipids and point to the feasibility of using fluorine-labeled peptides to monitor peptide-membrane interactions in living cells.

Peptide-induced domain formation in supported lipid bilayers: direct evidence by combined atomic force and polarized total internal reflection fluorescence microscopy

Direct visualization of the mechanism(s) by which peptides induce localized changes to the structure of membranes has high potential for enabling understanding of the structure-function relationship in antimicrobial and cell-penetrating peptides. We have applied a combined imaging strategy to track the interaction of a model antimicrobial peptide, PFWRIRIRR-amide, with bacterial membrane-mimetic supported phospholipid bilayers comprised of POPE/TOCL. Our in situ studies revealed rapid reorganization of the POPE/TOCL membrane into localized TOCL-rich domains with a concomitant change in the organization of the membranes themselves, as reflected by changes in fluorescent-membrane-probe order parameter, upon introduction of the peptide.

A highly thermostable antimicrobial peptide from Aspergillus clavatus ES1: biochemical and molecular characterization

Antimicrobial peptides (AMPs) are extremely attractive candidates as therapeutic agents due to their wide spectrum of antimicrobial activity and mechanism of action, which differs from that of small-molecule antibiotics. In this study, a 6.0-kDa antimicrobial peptide from Aspergillus clavatus ES1, designated as AcAMP, was isolated by a one-step heat treatment. AcAMP was sensitive to proteolytic enzymes, stable between pH 5.0 and 10.0, and heat resistant (15 min at 100 degrees C). The acamp gene encoding AcAMP peptide was isolated by reverse-transcriptase polymerase chain reaction (RT-PCR) and cloned in pCRII-TOPO vector. Sequence analysis of the complementary DNA (cDNA) acamp gene revealed an open reading frame of 282 bp encoding a peptide of 94 amino acid residues consisting of a 21-aa signal peptide, a 22-aa pro-peptide, and a 51-aa mature peptide. The deduced amino acid sequence showed high identity with other ascomycete antifungal peptides. AcAMP belongs to the group of small, cysteine-rich, basic proteins with antimicrobial activity. In addition to its antifungal activity, AcAMP is the first fungal peptide exhibiting antibacterial activity against several Gram-positive and Gram-negative bacteria. Based on all these features, AcAMP can be considered as a promising new member of the restraint family of ascomycete antimicrobial peptides that might be used in biological control of plant diseases and also for potential applications in food preservation.

Functional interaction of human neutrophil peptide-1 with the cell wall precursor lipid II

Defensins constitute a major class of cationic antimicrobial peptides in mammals and vertebrates, acting as effectors of innate immunity against infectious microorganisms. It is generally accepted that defensins are bactericidal by disrupting the anionic microbial membrane. Here, we provide evidence that membrane activity of human alpha-defensins does not correlate with antibacterial killing. We further show that the alpha-defensin human neutrophil peptide-1 (HNP1) binds to the cell wall precursor lipid II and that reduction of lipid II levels in the bacterial membrane significantly reduces bacterial killing. The interaction between defensins and lipid II suggests the inhibition of cell wall synthesis as a novel antibacterial mechanism of this important class of host defense peptides.

Lipopolysaccharide interaction is decisive for the activity of the antimicrobial peptide NK-2 against Escherichia coli and Proteus mirabilis

Phosphatidylglycerol is a widely used mimetic to study the effects of AMPs (antimicrobial peptides) on the bacterial cytoplasmic membrane. However, the antibacterial activities of novel NK-2-derived AMPs could not be sufficiently explained by using this simple model system. Since the LPS (lipopolysaccharide)-containing outer membrane is the first barrier of Gram-negative bacteria, in the present study we investigated interactions of NK-2 and a shortened variant with viable Escherichia coli WBB01 and Proteus mirabilis R45, and with model membranes composed of LPS isolated from these two strains. Differences in net charge and charge distribution of the two LPS have been proposed to be responsible for the differential sensitivity of the respective bacteria to other AMPs. As imaged by TEM (transmission electron microscopy) and AFM (atomic force microscopy), NK-2-mediated killing of these bacteria was corroborated by structural alterations of the outer and inner membranes, the release of E. coli cytoplasma, and the formation of unique fibrous structures inside P. mirabilis, suggesting distinct and novel intracellular targets. NK-2 bound to and intercalated into LPS bilayers, and eventually induced the formation of transient heterogeneous lesions in planar lipid bilayers. However, the discriminative activity of NK-2 against the two bacterial strains was independent of membrane intercalation and lesion formation, which both were indistinguishable for the two LPS. Instead, differences in activity originated from the LPS-binding step, which could be demonstrated by NK-2 attachment to intact bacteria, and to solid-supported LPS bilayers on a surface acoustic wave biosensor.

Spectroscopic and thermodynamic evidence for antimicrobial peptide membrane selectivity

In our laboratory we developed a series of antimicrobial peptides that exhibit selectivity and potency for prokaryotic over eukaryotic cells (Hicks et al., 2007). Circular dichroism (CD), isothermal calorimetry (ITC) and calcein leakage assays were conducted to determine the mechanism of lipid binding of a representative peptide 1 (Ac-GF-Tic-Oic-GK-Tic-Oic-GF-Tic-Oic-GK-Tic-KKKK-CONH(2)) to model membranes. POPC liposomes were used as a simple model for eukaryotic membranes and 4:1 POPC:POPG liposomes were used as a simple model for prokaryotic membranes. CD, ITC and calcein leakage data clearly indicate that compound 1 interacts via very different mechanisms with the two different liposome membranes. Compound 1 exhibits weaker binding and induces less calcein leakage in POPC liposomes than POPC:POPG (4:1 mole ratio) liposomes. The predominant binding mechanism to POPC appears to be limited to surface interactions while the mechanism of binding to 4:1 POPC:POPG most likely involves some type of pore formation.