Comparison of NMR structures and model-membrane interactions of 15-residue antimicrobial peptides derived from bovine lactoferricin

Relationship between antimicrobial peptides-induced cell membrane damage and bactericidal activity

Most antimicrobial peptides (AMPs) act by killing bacterial cells. However, there is little information regarding the required interaction time between AMPs and bacterial cells to exert the bactericidal activity. One of the causes of the bactericidal activity is considered to be cell membrane damage, although little direct evidence is available. Here, we investigated the relationship between AMP-induced cell membrane damage in Escherichia coli and AMP-induced cell death at the single-cell level. Magainin 2, lactoferricin B, and PGLa were selected as the AMPs. First, we examined the interaction time (t) of AMPs with cells required to induce cell death using the single-cell analysis. The fraction of microcolonies containing only a single cell, Psingle (t), which indicates the fraction of dead cells, increased with time to reach ∼1 in a short time (≤5 min). Then, we examined the interaction between AMPs and single cells using confocal laser scanning microscopy in the presence of membrane-impermeable SYTOX green. Within a short time interaction, the fluorescence intensity of the cells due to SYTOX green increased, indicating that AMPs induced cell membrane damage through which the dye entered the cytoplasm. The fraction of cells in which SYTOX green entered the cytoplasm among all examined cells after the interaction time (t), Pentry (t), increased with time, reaching ∼1 in a short time (≤5 min). The values of Psingle (t) and Pentry (t) were similar at t ≥ 3 min for all AMPs. The bindings of AMPs to cells were largely reversible, whereas the AMP-induced cell membrane damages were largely irreversible because SYTOX green entered the cells after dilution of AMP concentration. Based on these results, we conclude that the rapid, substantial membrane permeabilization of cytoplasmic contents after a short interaction time with AMPs and the residual damage after dilution induce cell death.

Alanine Substitution to Determine the Effect of LR5 and YR6 Rice Peptide Structure on Antioxidant and Anti-Inflammatory Activity

The relationship between the structure of peptides LR5 (LHKFR) and YR6 (YGLYPR) and their antioxidant and anti-inflammatory activity remains unclear. Herein, leucine, tyrosine, proline, and phenylalanine at different positions in the peptides were replaced by Alanine (Ala), and two new pentapeptides (AR5 and LAR5) and four hexapeptides (AGR6, YAR6, YLR6, and YGR6) were obtained. The effect of Ala replacement on the hydrophobicity, cytotoxicity, NO inhibition rate, and active oxygen radical scavenging ability of these peptides and their antioxidant and anti-inflammatory abilities were investigated. The results indicated that the hydrophobicity of the peptides was associated with their amino acid composition and their specific sequence. However, hydrophobicity had no significant effect on cytotoxicity. Ala replacement was shown to enhance hydrophobicity and consequently increased the antioxidant and anti-inflammatory activity of the peptides. The molecular docking studies indicated that the amino acid interactions of the peptide with the Keap1 protein influenced the hydrophobicity and thus affected the antioxidant activity of the peptide.

Effect of Osmotic Pressure on Pore Formation in Lipid bilayers by the Antimicrobial Peptide Magainin 2

Osmotic pressure (Π) induces membrane tension in cells and lipid vesicles, which may affect the activity of antimicrobial peptides (AMPs) by an unknown mechanism. We recently quantitated the membrane tension...

Effect of intramolecular disulfide bond of bovine lactoferricin on its molecular structure and antibacterial activity against Trueperella pyogenes separated from cow milk with mastitis

Background

Lactoferricin (Lfcin) is an antimicrobial activity center of lactoferrin, produced by hydrolysis from the N-terminal of lactoferrin. It was hypothesized that the intramolecular disulfide bond in Lfcin could affect its antibacterial function through influencing its molecular structure. To prove this hypothesis, bovine Lfcin (bLfcin) and its two derivatives, bLfcin with an intramolecular disulfate bond (bLfcin DB) and bLfcin with a mutation C36G (bLfcin C36G), were synthesized, purified, and identified. The circular dichroism spectra of the peptides were detected in solutions with different ionic and hydrophobic strength. The antibacterial activity of the peptides against Trueperella pyogenes, separated from cow milk with mastitis, were determined.

Results

The secondary structure of bLfcin DB showed more β-turn and less random coil than the other peptides in H₂O, similar ratios of secondary structures with bLfcin and bLfcin C36G under ionic conditions, and close percentages of secondary structure with bLfcin under hydrophobic conditions. The synthetic peptides exhibited strong antimicrobial activity against T. pyogenes isolates, T. pyogenes ATCC 19,411, and E. coli ATCC 25,922. The antimicrobial activities of the three peptides were greater against T. pyogenes than against E. coli, and bLfcin DB exhibited higher antibacterial activity compared with its derivatives.

Conclusions

The intramolecular disulfide bond could change the molecular structure of bLfcin under alternative ionic strengths and hydrophobic effects, and the formation of the disulfide bond is beneficial to executing the antibacterial function of bLfcin.

The role of membrane tension in the action of antimicrobial peptides and cell-penetrating peptides in biomembranes

For antimicrobial peptides (AMPs) with antimicrobial and bactericidal activities and cell-penetrating peptides (CPPs) with activity to permeate through plasma membrane, their interactions with lipid bilayer region in plasma membrane play important roles in these functions. However, the elementary processes and mechanisms of their functions have not been clear. The single giant unilamellar vesicle (GUV) method has revealed the details of elementary processes of interaction of some AMPs and CPPs with lipid vesicles. In this review, we summarize the mode of action of AMPs such as magainin 2 (Mag) and CPPs such as transportan 10 (TP10), revealed by the single GUV methods, and especially we focus on the role of membrane tension in actions of Mag and TP10 and the mechanisms of their actions. First, we explain the characteristics of the single GUV method briefly. Next, we summarize the recent view on the effect of tension on physical properties of lipid bilayers and describe the role of tension in actions of Mag and TP10. Some experimental results indicate that Mag-induced pore is a stretch-activated pore. The effect of packing of transbilayer asymmetric lipid on Mag-induced pore formation is described. On the other hand, entry of fluorescent dye, carboxyfluorescein (CF)-labeled TP10 (i.e., CF-TP10), into single GUVs without pore formation is affected by tension and high concentration of cholesterol. Pre-pore model for translocation of CF-TP10 across lipid bilayer is described. The experimental methods and their analysis described here are useful for investigation of functions of the other types of AMPs, CPPs, and proteins.

Theoretical insights into the competitive metal bioaffinity of lactoferrin as a metal ion carrier: a DFT study

Metal–protein complexes, specifically lactoferrin (Lf), an iron-binding glycoprotein found naturally in milk and several other body fluids play a pivotal role in all living organisms.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Characterization of bactericidal efficiency, cell selectivity, and mechanism of short interspecific hybrid peptides

Facing rising global antibiotics resistance, physical membrane-damaging antimicrobial peptides (AMPs) represent promising antimicrobial agents. Various strategies to design effective hybrid peptides offer many advantages in overcoming the adverse effects of natural AMPs. In this study, hybrid peptides from different species were investigated, and three hybrid antimicrobial peptides, LI, LN, and LC, were designed by combining the typical fragment of human cathelicidin-derived LL37 with either indolicidin, pig nematode cecropin P1 (CP-1) or rat neutrophil peptide-1 (NP-1). In an aqueous solution, all hybrid peptides had an unordered conformation. In simulated membrane conditions, the hybrid peptide LI displayed more β-turn and β-hairpin structures, whereas LN and LC folded into α-helix structures. The three interspecific hybrid peptides LI, LN, and LC exhibited different levels of antimicrobial activity against Gram-positive and Gram-negative bacteria. LI demonstrated the highest antimicrobial activity and cell selectivity. The results of the swimming motility indicated that LI repressed bacterial motility in a concentration-dependent method. Endotoxin binding assay demonstrated that hybrid peptide LI conserved the binding ability to LPS (polyanionic lipopolysaccharides) of its parental peptides. Fluorescence assays, flow cytometry, and SEM further revealed that hybrid peptide LI acted through different bacteriostatic mechanisms than LL37 and indolicidin and that LI killed bacterial cells via membrane damage. In summary, this study demonstrated that hybrid peptide LI produced by interspecific hybrid synthesis possessed strong cell selectivity and is a promising therapeutic candidate for drug-resistant bacteria infection.

Entry of a Six-Residue Antimicrobial Peptide Derived from Lactoferricin B into Single Vesicles and Escherichia coli Cells without Damaging their Membranes

Lactoferricin B (LfcinB) and shorter versions of this peptide have antimicrobial activity. However, the elementary processes of interactions of these peptides with lipid membranes and bacteria are still not well understood. To elucidate the mechanism of their antimicrobial activity, we investigated the interactions of LfcinB (4-9) (its sequence of RRWQWR) with Escherichia coli cells and giant unilamellar vesicles (GUVs). LfcinB (4-9) and lissamine rhodamine B red-labeled LfcinB (4-9) (Rh-LfcinB (4-9)) did not induce an influx of a membrane-impermeant fluorescent probe, SYTOX green, from the outside of E. coli cells into their cytoplasm, indicating that no damage occurred in their plasma membrane. To examine the activity of LfcinB (4-9) to enter E. coli cytoplasm, we investigated the interaction of Rh-LfcinB (4-9) with single cells of E. coli containing calcein using confocal microscopy. We found that Rh-LfcinB (4-9) entered the cytoplasm without leakage of calcein. Next, we investigated the interactions of Rh-LfcinB (4-9) with single GUVs of dioleoylphosphatidylglycerol (DOPG) and dioleoylphosphatidylcholine (DOPC) mixtures containing a fluorescent probe, Alexa Fluor 647 hydrazide (AF647), using the single GUV method. The results indicate that Rh-LfcinB (4-9) outside the GUV translocated through the GUV membrane and entered its lumen without leakage of AF647. Interaction of Rh-LfcinB (4-9) with DNA increased its fluorescence intensity greatly. Therefore, we can conclude that Rh-LfcinB (4-9) can translocate across lipid membrane regions of the plasma membrane of E. coli cells to enter their cytoplasm without leakage of calcein and its antimicrobial activity is not due to damage of their plasma membranes.

Membrane solid-state NMR in Canada: A historical perspective

This manuscript presents an overview of more than 40years of membrane solid-state nuclear magnetic resonance (NMR) research in Canada. This technique is a method of choice for the study of the structure and dynamics of lipid bilayers; bilayer interactions with a variety of molecules such as membrane peptides, membrane proteins and drugs; and to investigate membrane peptide and protein structure, dynamics, and topology. Canada has a long tradition in this field of research, starting with pioneering work on natural and model membranes in the 1970s in a context of emergence of biophysics in the country. The 1980s and 1990s saw an emphasis on studying lipid structures and dynamics, and peptide-lipid and protein-lipid interactions. The study of bicelles began in the 1990s, and in the 2000s there was a rise in the study of membrane protein structures. Novel perspectives include using dynamic nuclear polarization (DNP) for membrane studies and using NMR in live cells. This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Role of phenylalanine and valine10 residues in the antimicrobial activity and cytotoxicity of piscidin-1

Piscidin-1 (Pis-1) is a linear antibacterial peptide derived from mast cells of aquacultured hybrid striped bass that comprises 22 amino acids with a phenylalanine-rich amino-terminus. Pis-1 exhibits potent antibacterial activity against pathogens but is not selective for distinguishing between bacterial and mammalian cells. To determine the key residues for its antibacterial activity and those for its cytotoxicity, we investigated the role of each Phe residue near the N-terminus as well as the Val¹⁰ residue located near the boundary of the hydrophobic and hydrophilic sectors of the helical wheel diagram. Fluorescence dye leakage and tryptophan fluorescence experiments were used to study peptide-lipid interactions, showing comparable depths of insertion of substituted peptides in different membranes. Phe² was found to be the most deeply inserted phenylalanine in both bacterial- and mammalian-mimic membranes. Each Phe was substituted with Ala or Lys to investigate its functional role. Phe² plays key roles in the cytotoxicity as well as the antibacterial activities of Pis-1, and Phe⁶ is essential for the antibacterial activities of Pis-1. We also designed and synthesized a piscidin analog, Pis-V10K, in which Lys was substituted for Val¹⁰, resulting in an elevated amphipathic α-helical structure. Pis-V10K showed similar antibacterial activity (average minimum inhibitory concentration (MIC)  = 1.6 µM) to Pis-1 (average MIC  = 1.5 µM). However, it exhibited much lower cytotoxicity than Pis-1. Lys¹⁰-substituted analogs, Pis-F1K/V10K, Pis-F2K/V10K, and Pis-F6K/V10K in which Lys was substituted for Phe retained antibacterial activity toward standard and drug-resistant bacterial strains with novel bacterial cell selectivity. They exert anti-inflammatory activities via inhibition of nitric oxide production, TNF-α secretion, and MIP-1 and MIP-2 production. They may disrupt the binding of LPS to toll-like receptors, eventually suppressing MAPKs-mediated signaling pathways. These peptides may be good candidates for the development of peptide antibiotics with potent antibacterial activity but without cytotoxicity.

Position-Dependent Influence of the Three Trp Residues on the Membrane Activity of the Antimicrobial Peptide, Tritrpticin

Antimicrobial peptides (AMPs) constitute promising candidates for the development of new antibiotics. Among the ever-expanding family of AMPs, tritrpticin has strong antimicrobial activity against a broad range of pathogens. This 13-residue peptide has an unusual amino acid sequence that is almost symmetrical and features three central Trp residues with two Arg residues near each end of the peptide. In this work, the role of the three sequential Trp residues in tritrpticin was studied in a systematic fashion by making a series of synthetic peptides with single-, double- and triple-Trp substitutions to Tyr or Ala. ¹H NMR and fluorescence spectroscopy demonstrated the ability of all of the tritrpticin-analog peptides to interact with negatively-charged membranes. Consequently, most tritrpticin analogs exhibited the ability to permeabilize synthetic ePC:ePG (egg-yolk phosphatidylcholine (ePC), egg-yolk phosphatidylglycerol (ePG)) vesicles and live Escherichia coli bacteria. The membrane perturbation characteristics were highly dependent on the location of the Trp residue substitution, with Trp6 being the most important residue and Trp8 the least. The membrane permeabilization activity of the peptides in synthetic and biological membranes was directly correlated with the antimicrobial potency of the peptides against E. coli. These results contribute to the understanding of the role of each of the three Trp residues to the antimicrobial activity of tritrpticin.

High-quality 3D structures shine light on antibacterial, anti-biofilm and antiviral activities of human cathelicidin LL-37 and its fragments

Host defense antimicrobial peptides are key components of human innate immunity that plays an indispensible role in human health. While there are multiple copies of cathelicidin genes in horses, cattle, pigs, and sheep, only one cathelicidin gene is found in humans. Interestingly, this single cathelicidin gene can be processed into different forms of antimicrobial peptides. LL-37, the most commonly studied form, is not only antimicrobial but also possesses other functional roles such as chemotaxis, apoptosis, wound healing, immune modulation, and cancer metastasis. This article reviews recent advances made in structural and biophysical studies of human LL-37 and its fragments, which serve as a basis to understand their antibacterial, anti-biofilm and antiviral activities. High-quality structures were made possible by using improved 2D NMR methods for peptide fragments and 3D NMR spectroscopy for intact LL-37. The two hydrophobic domains in the long amphipathic helix (residues 2-31) of LL-37 separated by a hydrophilic residue serine 9 explain its cooperative binding to bacterial lipopolysaccharides (LPS). Both aromatic rings (F5, F6, F17, and F27) and interfacial basic amino acids of LL-37 directly interact with anionic phosphatidylglycerols (PG). Although the peptide sequences reported in the literature vary slightly, there is a consensus that the central helix of LL-37 is essential for disrupting superbugs (e.g., MRSA), bacterial biofilms, and viruses such as human immunodeficiency virus 1 (HIV-1) and respiratory syncytial virus (RSV). In the central helix, the central arginine R23 is of particular importance in binding to bacterial membranes or DNA. Mapping the functional roles of the cationic amino acids of the major antimicrobial region of LL-37 provides a basis for designing antimicrobial peptides with desired properties. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Characterization of Antimicrobial Peptides toward the Development of Novel Antibiotics

Antimicrobial agents have eradicated many infectious diseases and significantly improved our living environment. However, abuse of antimicrobial agents has accelerated the emergence of multidrug-resistant microorganisms, and there is an urgent need for novel antibiotics. Antimicrobial peptides (AMPs) have attracted attention as a novel class of antimicrobial agents because AMPs efficiently kill a wide range of species, including bacteria, fungi, and viruses, via a novel mechanism of action. In addition, they are effective against pathogens that are resistant to almost all conventional antibiotics. AMPs have promising properties; they directly disrupt the functions of cellular membranes and nucleic acids, and the rate of appearance of AMP-resistant strains is very low. However, as pharmaceuticals, AMPs exhibit unfavorable properties, such as instability, hemolytic activity, high cost of production, salt sensitivity, and a broad spectrum of activity. Therefore, it is vital to improve these properties to develop novel AMP treatments. Here, we have reviewed the basic biochemical properties of AMPs and the recent strategies used to modulate these properties of AMPs to enhance their safety.

Chimerization of lactoferricin and lactoferrampin peptides strongly potentiates the killing activity against Candida albicans

Bovine lactoferrin harbors 2 antimicrobial sequences (LFcin and LFampin), situated in close proximity in the N1-domain. To mimic their semi parallel configuration we have synthesized a chimeric peptide (LFchimera) in which these sequences are linked in a head-to-head fashion to the α- and ε-amino group, respectively, of a single lysine. In line with previously described bactericidal effects, this peptide was also a stronger candidacidal agent than the antimicrobial peptides LFcin17-30 and LFampin265-284, or a combination of these 2. Conditions that strongly reduced the candidacidal activities of LFcin17-30 and LFampin265-284, such as high ionic strength and energy depletion, had little influence on the activity of LFchimera. Freeze-fracture electron microscopy showed that LFchimera severely affected the membrane morphology, resulting in disintegration of the membrane bilayer and in an efflux of small and high molecular weight molecules such as ATP and proteins. The differential effects displayed by the chimeric peptide and a mixture of its constituent peptides clearly demonstrate the synergistic effect of linking these peptides in a fashion that allows a similar spatial arrangement as in the parent protein, suggesting that in bovine lactoferrrin the corresponding fragments act in concert in its candidacidal activity.

The role of lactoferrin binding protein B in mediating protection against human lactoferricin

Bacteria that inhabit the mucosal surfaces of the respiratory and genitourinary tracts of mammals encounter an iron-deficient environment because of iron sequestration by the host iron-binding proteins transferrin and lactoferrin. Lactoferrin is also present in high concentrations at sites of inflammation where the cationic, antimicrobial peptide lactoferricin is produced by proteolysis of lactoferrin. Several Gram-negative pathogens express a lactoferrin receptor that enables the bacteria to use lactoferrin as an iron source. The receptor is composed of an integral membrane protein, lactoferrin binding protein A (LbpA), and a membrane-bound lipoprotein, lactoferrin binding protein B (LbpB). LbpA is essential for growth with lactoferrin as the sole iron source, whereas the role of LbpB in iron acquisition is not yet known. In this study, we demonstrate that LbpB from 2 different species is capable of providing protection against the killing activity of a human lactoferrin-derived peptide. We investigated the prevalence of lactoferrin receptors in bacteria and examined their sequence diversity. We propose that the protection against the cationic antimicrobial human lactoferrin-derived peptide is associated with clusters of negatively charged amino acids in the C-terminal lobe of LbpB that is a common feature of this protein.

Structural and biophysical characterization of an antimicrobial peptide chimera comprised of lactoferricin and lactoferrampin

Lactoferricin and lactoferrampin are two antimicrobial peptides found in the N-terminal lobe of bovine lactoferrin with broad spectrum antimicrobial activity against a range of Gram-positive and Gram-negative bacteria as well as Candida albicans. A heterodimer comprised of lactoferrampin joined to a fragment of lactoferricin was recently reported in which these two peptides were joined at their C-termini through the two amino groups of a single Lys residue (Bolscher et al., 2009, Biochimie 91(1):123-132). This hybrid peptide, termed LFchimera, has significantly higher antimicrobial activity compared to the individual peptides or an equimolar mixture of the two. In this work, the underlying mechanism behind the increased antibacterial activity of LFchimera was investigated. Differential scanning calorimetry studies demonstrated that all the peptides influenced the thermotropic phase behaviour of anionic phospholipid suspensions. Calcein leakage and vesicle fusion experiments with anionic liposomes revealed that LFchimera had enhanced membrane perturbing properties compared to the individual peptides. Peptide structures were evaluated using circular dichroism and NMR spectroscopy to gain insight into the structural features of LFchimera that contribute to the increased antimicrobial activity. The NMR solution structure, determined in a miscible co-solvent mixture of chloroform, methanol and water, revealed that the Lys linkage increased the helical content in LFchimera compared to the individual peptides, but it did not fix the relative orientations of lactoferricin and lactoferrampin with respect to each other. The structure of LFchimera provides insight into the conformation of this peptide in a membranous environment and improves our understanding of its antimicrobial mechanism of action.

Designed low amphipathic peptides with alpha-helical propensity exhibiting antimicrobial activity via a lipid domain formation mechanism

Although several low amphipathic peptides have been known to exhibit antimicrobial activity, their mode of action has not been completely elucidated. In this study, using designed low amphipathic peptides that retain different alpha-helical content and hydrophobicity, we attempted to investigate the mechanism of these properties. Calorimetric and thermodynamic analyses demonstrated that the peptides induce formation of two lipid domains in an anionic liposome at a high peptide-to-lipid ratio. On the other hand, even at a low peptide-to-lipid ratio, they caused minimal membrane damage, such as flip-flop of membrane lipids or leakage of calcein molecules from liposomes, and never translocated across membranes. Interaction energies between the peptides and anionic liposomes showed good correlation with antimicrobial activity for both Escherichia coli and Bacillus subtilis. We thus propose that the domain formation mechanism in which antimicrobial peptides exhibit activity solely by forming lipid domains without membrane damage is a major determinant of the antimicrobial activity of low amphipathic peptides. These peptides appear to stiffen the membrane such that it is deprived of the fluidity necessary for biological functions. We also showed that to construct the lipid domains, peptides need not form stable and cooperative structures. Rather, it is essential for peptides to only interact tightly with the membrane interface via strong electrostatic interactions, and slight differences in binding strength are invoked by differences in hydrophobicity. The peptides thus designed might pave the way for "clean" antimicrobial reagents that never cause release of membrane elements and efflux of their inner components.

Study of the interaction of lactoferricin B with phospholipid monolayers and bilayers

Bovine lactoferricin (LfcinB) is an antimicrobial peptide obtained from the pepsin cleavage of lactoferrin. The activity of LfcinB has been extensively studied on diverse pathogens, but its mechanism of action still has to be elucidated. Because of its nonspecificity, its mode of action is assumed to be related to interactions with membranes. In this study, the interaction of LfcinB with a negatively charged monolayer of dipalmitoylphosphatidylglycerol has been investigated as a function of the surface pressure of the lipid film using in situ Brewster angle and polarization modulation infrared reflection absorption spectroscopy and on transferred monolayers by atomic force microscopy and polarized attenuated total reflection infrared spectroscopy. The data show clearly that LfcinB forms stable films at the air-water interface. They also reveal that the interaction of LfcinB with the lipid monolayer is modulated by the surface pressure. At low surface pressure, LfcinB inserts within the lipid film with its long molecular axis oriented mainly parallel to the acyl chains, while at high surface pressure, LfcinB is adsorbed under the lipid film, the hairpin being preferentially aligned parallel to the plane of the interface. The threshold for which the behavior changes is 20 mN/m. At this critical surface pressure, LfcinB interacts with the monolayer to form discoidal lipid-peptide assemblies. This structure may actually represent the mechanism of action of this peptide. The results obtained on monolayers are correlated by fluorescent probe release measurements of dye-containing vesicles made of lipids in different phases and support the important role of the lipid fluidity and packing on the activity of LfcinB.

Altered activity and physicochemical properties of short cationic antimicrobial peptides by incorporation of arginine analogues

The incorporation of nongenetically encoded amino acids is a well established strategy to alter the behavior of several types of promising cationic antimicrobial peptides. Generally, these elements have been improved mimics of the hydrophobic amino acids yielding peptides with increased stability and potency. In this initial study, the effect of systematic replacement of Arg in a well-defined moderately antimicrobial tripeptide library is described. It is shown that the arginine analogues need to display a strong basicity to produce active peptides. It is further revealed that the hydrophobic units needed for activity in these peptides can be effectively incorporated in the direct vicinity of the cationic charge to produce compounds with improved antibacterial properties. A well-defined facial amphiphilic structure, which remains intact upon introduction of hydrophobic elements in the cationic side chains, is seen for the majority of the tested peptides. Microcalorimetric studies revealed a peptide binding to large anionic unilamellar vesicles (LUVs) mimicking the Gram-positive bacterial membrane as well as a potentially competitive binding to human serum albumin in the low- to mid-micromolar range. No considerable alterations in binding to either albumin or the LUVs were seen for the analogue containing peptides. A neutral LUV mimicking the eukaryotic cell membrane showed no significant binding to any of the peptides. The oral absorption of this class of short lactoferricin based peptides was investigated for the first time and revealed that incorporation of weaker bases than Arg produced peptides with much improved permeability in a recently developed permeation model, the phospholipid vesicle based barrier assay. Collectively, the results presented here show that there is ample room to toggle the activity and physical properties of short cationic antimicrobial peptides by incorporation of arginine analogues.

Expression, purification and structural studies of a short antimicrobial peptide

We have produced a small antimicrobial peptide PFWRIRIRR in bacteria utilizing production in the form of insoluble fusion protein with ketosteroid isomerase. The recombinant peptide was rapidly and efficiently isolated by acidic cleavage of the fusion protein based on the acid labile Asp-Pro bond at the N-terminus of the peptide. The peptide has antibacterial activity and neutralizes macrophage activation by LPS. The selectivity of the peptide against bacteria correlates with preferential binding to acidic phospholipid vesicles. Solution structure of the peptide in SDS and DPC micelles was determined by NMR. The peptide adopts a well-defined structure, comprising a short helical segment. Cationic and hydrophobic clusters are segregated along the molecular axis of the short helix, which is positioned perpendicular to the membrane plane. The position of the helix is shifted in two micellar types and more nonpolar surface is exposed in anionic micelles. Overall structure explains the advantageous role of the N-terminal proline residue, which forms an integral part of the hydrophobic cluster.

Thermodynamics of the interactions of tryptophan-rich cathelicidin antimicrobial peptides with model and natural membranes

Tritrpticin and indolicidin are short 13-residue tryptophan-rich antimicrobial peptides that hold potential as future alternatives for antibiotics. Isothermal titration calorimetry (ITC) has been applied as the main tool in this study to investigate the thermodynamics of the interaction of these two cathelicidin peptides as well as five tritrpticin analogs with large unilamellar vesicles (LUVs), representing model and natural anionic membranes. The anionic LUVs were composed of (a) 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPE/POPG) (7:3) and (b) natural E. coli polar lipid extract. 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was used to make model zwitterionic membranes. Binding isotherms were obtained to characterize the antimicrobial peptide binding to the LUVs, which then allowed for calculation of the thermodynamic parameters of the interaction. All peptides exhibited substantially stronger binding to anionic POPE/POPG and E. coli membrane systems than to the zwitterionic POPC system due to strong electrostatic attractions between the highly positively charged peptides and the negatively charged membrane surface, and results with tritrpticin derivatives further revealed the effects of various amino acid substitutions on membrane binding. No significant improvement was observed upon increasing the Tritrp peptide charge from +4 to +5. Replacement of Arg residues with Lys did not substantially change peptide binding to anionic vesicles but moderately decreased the binding to zwitterionic LUVs. Pro to Ala substitutions in tritrpticin, allowing the peptide to adopt an alpha-helical structure, resulted in a significant increase of the binding to both anionic and zwitterionic vesicles and therefore reduced the selectivity for bacterial and mammalian membranes. In contrast, substitution of Trp with other aromatic amino acids significantly decreased the peptide's ability to bind to anionic LUVs and essentially eliminated binding to zwitterionic LUVs. The ITC results were consistent with the outcome of fluorescence spectroscopy membrane binding and perturbation studies. Overall, our work showed that a natural E. coli polar lipid extract as a bacterial membrane model was advantageous compared to the simpler and more widely used POPE/POPG lipid system.

Unifying themes in host defence effector polypeptides

It is said that nature is the greatest innovator, yet molecular conservation can be equally powerful. One key requirement for the survival of any host is its ability to defend against infection, predation and competition. Recent discoveries, including the presence of a multidimensional structural signature, have revealed a previously unforeseen structural and functional congruence among host defence effector molecules spanning all kingdoms of life. Antimicrobial peptides, kinocidins, polypeptide venoms and other molecules that were once thought to be distinct in form and function now appear to be members of an ancient family of host defence effectors. These molecules probably descended from archetype predecessors that emerged during the beginning of life on earth. Understanding how nature has sustained these host defence molecules with a potent efficacy in the face of dynamic microbial evolution should provide new opportunities to prevent or treat life-threatening infections.

Interactions of tryptophan-rich cathelicidin antimicrobial peptides with model membranes studied by differential scanning calorimetry

The 13-residue cathelicidins indolicidin and tritrpticin are part of a group of relatively short tryptophan-rich antimicrobial peptides that hold potential as future substitutes for antibiotics. Differential scanning calorimetry (DSC) has been applied here to study the effect of indolicidin and tritrpticin as well as five tritrpticin analogs on the phase transition behaviour of model membranes made up of zwitterionic dimyristoylphosphatidylcholine (DMPC, DMPC/cholesterol) and anionic dimyristoylphosphatidyl glycerol (DMPG) phospholipids. Most of the peptides studied significantly modified the phase transition profile, suggesting the importance of hydrophobic forces for the peptide interactions with the lipid bilayers and their insertion into the bilayer. Indolicidin and tritrpticin are both known to be flexible in aqueous solution, but they adopt turn-turn structures when they bind to and insert in a membrane surface. Pro-to-Ala substitutions in tritrpticin, which result in the formation of a stable alpha-helix in this peptide, lead to a substantial increase in the peptide interactions with both zwitterionic and anionic phospholipid vesicles. In contrast, the substitution of the three Trp residues by Tyr or Phe resulted in a significant decrease of the peptide's interaction with anionic vesicles and virtually eliminated binding of these peptides to the zwitterionic vesicles. An increase of the cationic charge of the peptide induced much smaller changes to the peptide interaction with all lipid systems than substitution of particular amino acids or modification of the peptide conformation. The presence of multiple lipid domains with a non-uniform peptide distribution was noticed. Slow equilibration of the lipid-peptide systems due to peptide redistribution was observed in some cases. Generally good agreement between the present DSC data and peptide antimicrobial activity data was obtained.

Solution structures and model membrane interactions of lactoferrampin, an antimicrobial peptide derived from bovine lactoferrin

Bovine lactoferrampin (LFampinB) has been identified as a novel antimicrobial peptide, which is derived from the N-terminal lobe of bovine lactoferrin. In this study, the solution structure of LFampinB bound to negatively charged sodium dodecyl sulphate micelles and zwitterionic dodecyl phosphocholine micelles was determined using 2-dimensional nuclear magnetic resonance (NMR) spectroscopy. The interaction between LFampinB and multilamellar phospholipid vesicles, containing choline and glycerol head groups, was examined using differential scanning calorimetry (DSC). In addition, the interaction between the N-terminal tryptophan residue and model membranes of varying composition was analyzed by fluorescence spectroscopy. LFampinB adopts an amphipathic alpha-helical conformation across the first 11 residues of the peptide but remains relatively unstructured at the C-terminus. The hydrophobic surface of the amphipathic helix is bordered by the side chains of Trp1 and Phe11, and is seen in both micelle-bound structures. The fluorescence results suggest that Trp1 inserts into the membrane at the lipid/water interface. The phenyl side chain of Phe11 is oriented in the same direction as the indole ring of Trp1, allowing these two residues to serve as anchors for the lipid bilayer. The DSC results also indicate that LFampinB interacts with glycerol head groups in multilamellar vesicles but has little effect on acyl chain packing. Our results support a two step model of antimicrobial activity where the initial attraction of LFampinB is mediated by the cluster of positive charges on the C-terminus followed by the formation of the N-terminal helix which binds to the surface of the bacterial lipid bilayer.

Structure-function relationships and conformational properties of alpha-MSH(6-13) analogues with candidacidal activity

Alpha-melanocyte-stimulating hormone (alpha-MSH) is an endogenous linear tridecapeptide with potent anti-inflammatory effects. We firstly demonstrated that alpha-MSH and its C-terminal sequence Lys-Pro-Val [alpha-MSH(11-13)] have antimicrobial effects against two major and representative pathogens: Staphylococcus aureus and Candida albicans. Successively, in an attempt to improve the candidacidal activity of alpha-MSH and to better understand the peptide structure-antifungal activity relations, we have recently designed and synthesized novel peptide analogues. We focused on the sequence alpha-MSH(6-13), which contains the invariant melanocortin core sequence His-Phe-Arg-Trp (6-9) and also contains the sequence Lys-Pro-Val (11-13) important for antimicrobial activity. In that structure-activity study, we discovered several compounds that have greater candidacidal activity than alpha-MSH, among which the peptide [d-Nal-7,Phe-12]-alpha-MSH(6-13) was the most potent. Here, we report a detailed conformational analysis by spectroscopic and computational methods of three peptides, alpha-MSH(6-13) (1), [d-Nal-7,Phe-12]-alpha-MSH(6-13) (2) and [d-Nal-7,Asp-12]-alpha-MSH(6-13) (3). Peptides were chosen on the basis of their candidacidal activities and were studied in membrane mimetic environment (SDS micelles). Different turn structures were observed for the three peptides and a conformation-activity model was developed based on these results. This study offers a structural basis for the design of novel peptide and non-peptide analogues to be used as new antimicrobial agents.

The anti-papillomavirus activity of human and bovine lactoferricin

Human papillomavirus (HPV) cause common warts, laryngeal papilloma and genital condylomata and is necessary for the development of cervical cancer. We have previously found that lactoferrin has antiviral activity against HPV-16 and others have demonstrated that lactoferricin, an N-terminal fragment of lactoferrin, has inhibitory activities against several viruses. Two cell lines and two virus types, HPV-5 and HPV-16, were used to study if lactoferrin and lactoferricin could inhibit HPV pseudovirus (PsV) infection. We demonstrated that bovine lactoferrin (bLf) and human lactoferrin (hLf) were both potent inhibitors of HPV-5 and -16 PsV infections. Among the four lactoferricin derivatives we analyzed, a 15 amino acid peptide from bovine lactoferricin (bLfcin) 17-31 was the most potent inhibitor of both HPV-5 and HPV-16 PsV infection. Among the other derivatives, the human lactoferricin (hLfcin) 1-49 showed some antiviral activity against HPV PsV infection while bLfcin 17-42 inhibited only HPV-5 PsV infection in one of the cell lines. When we studied initial attachment of HPV-16, only bLfcin 17-42 and hLfcin 1-49 had an antiviral effect. This is the first time that lactoferricin was demonstrated to have an inhibitory effect on HPV infection and the antiviral activity differed depending on size, charge and structures of the lactoferricin.