Conformation and antimicrobial activity of linear derivatives of tachyplesin lacking disulfide bonds

Rapid Formation of Intramolecular Disulfide Bridges using Light: An Efficient Method to Control the Conformation and Function of Bioactive Peptides

Disulfide bonds are widely found in natural peptides and play a pivotal role in stabilizing their secondary structures, which are highly associated with their biological functions. Herein, we introduce a light-mediated strategy to effectively control the formation of disulfides. Our strategy is based on 2-nitroveratryl (oNv), a widely used photolabile motif, which serves both as a photocaging group and an oxidant (after photolysis). We demonstrated that irradiation of oNv-caged thiols with UV light could release free thiols that are rapidly oxidized by locally released byproduct nitrosoarene, leading to a "break-to-bond" fashion. This strategy is highlighted by the in situ restoration of the antimicrobial peptide tachyplesin I (TPI) from its external disulfide-caged analogue TPI-1. TPI-1 exhibits a distorted structure and a diminished function. However, upon irradiation, the β-hairpin structure and membrane activity of TPI were largely restored via rapid intramolecular disulfide formation. Our study proposes a powerful method to regulate the conformation and function of peptides in a spatiotemporal manner, which has significant potential for the design of disulfide-centered light-responsive systems.

Anti-Cancer Peptides: Status and Future Prospects

The dramatic rise in cancer incidence, alongside treatment deficiencies, has elevated cancer to the second-leading cause of death globally. The increasing morbidity and mortality of this disease can be traced back to a number of causes, including treatment-related side effects, drug resistance, inadequate curative treatment and tumor relapse. Recently, anti-cancer bioactive peptides (ACPs) have emerged as a potential therapeutic choice within the pharmaceutical arsenal due to their high penetration, specificity and fewer side effects. In this contribution, we present a general overview of the literature concerning the conformational structures, modes of action and membrane interaction mechanisms of ACPs, as well as provide recent examples of their successful employment as targeting ligands in cancer treatment. The use of ACPs as a diagnostic tool is summarized, and their advantages in these applications are highlighted. This review expounds on the main approaches for peptide synthesis along with their reconstruction and modification needed to enhance their therapeutic effect. Computational approaches that could predict therapeutic efficacy and suggest ACP candidates for experimental studies are discussed. Future research prospects in this rapidly expanding area are also offered.

Tachyplesin I and its derivatives: A pharmaco-chemical perspective on their antimicrobial and antitumor potential

INTRODUCTION

Increasing evidence suggests that intratumor microbiota are an intrinsic component in the tumor microenvironment across multiple cancer types, and that there is a close relationship between microbiota and tumor progression. Therefore, how to address the interaction between bacteria and malignances has become a growing concern. Tachyplesin I (TPI), a peptide with dual antimicrobial and antitumor effects, holds great promise as a therapeutic alternative for the aforementioned diseases, with the advantage of broad-spectrum activities, quick killing efficacy, and a low tendency to induce resistance.

AREAS COVERED

This review comprehensively summarizes the pharmacological mechanisms of TPI with an emphasis on its antimicrobial and antitumor potential. Furthermore, it presents advances in TPI derivatives and gives a perspective on their future development. The article is based on literature searches using PubMed and SciFinder to retrieve the most up-to-date information of TPI.

EXPERT OPINION

Bacterial infections and cancer both pose a serious threat to health due to their symbiotic interactions and drug resistance. TPI is anticipated to be a novel agent to control pathogenic bacteria and various tumors through multiple mechanisms of action. Indeed, the continuous advancements in chemical modification and innovative applications of TPI give hope for future improvements in therapeutic efficacy.

Synthetic Analogs of Antimicrobial Peptides from Limulus Inhibit the Growth of Listeria monocytogenes by Increasing Cell Membrane Permeability and Suppressing Virulence Genes

Listeria monocytogenes is a foodborne pathogen that can cause listeriosis in humans and animals. It is of significant concern to the food industry as it can grow at low temperatures. Aggravatingly, only some of the commonly used food preservatives can effectively inhibit the growth of LM. In this study, the effectiveness of synthetic analogs of antimicrobial peptides from Limulus in inhibiting the growth of LM was studied. As determined by the Kirby–Bauer disc diffusion method, the diameters of the inhibition zones produced by the synthetic antimicrobial peptides ranged from 8.24 mm to 8.86 mm, and the minimum inhibitory concentrations (MICs) of the peptides ranged from 40 μg/mL to 160 μg/mL. At their MICs, initially, the synthetic antimicrobial peptides exhibited bacteriostatic effects on LM. They permeabilized the cell membrane of the bacterium and suppressed virulence genes (the inlA, prfA, and hly genes) in the bacterium. However, the bacteriostatic effects were effective only for 1 h, after which the bacterium slowly became resistant to them. After 6 h, the bacterium resumed its growth. Although cells in treatment groups resumed their growth after 6 h, the growth of the cells was inhibited compared to the growth of cells in the control group. Further studies are necessary to reduce the resistance of LM to the antibacterial effects of the synthetic antimicrobial peptides.

In vivo immunostimulatory effect of the amoebocyte lysate and plasma of Asian horseshoe crab, Tachypleus gigas in a piscine model

Antimicrobial proteins/peptides are becoming a new generation of immunostimulants for prevention and disease control in human and animals, including aquatic animals. As the haemolymph of horseshoe crabs (Tachypleus) contains broad ranges of bioactive compounds, we have explored the in vivo immunostimulating potential of amoebocyte lysate and plasma using a fish model. Indian major carp, Labeo rohita, yearlings were injected intraperitoneally with two doses of lysate and plasma at 50 and 100 µg protein per fish. No abnormalities and/or mortalities were recorded in any group. L. rohita injected with 50 µg lysate and 100 µg plasma protein showed significant enhancement (P < 0.01) of various haematological and immunological parameters. There was a significant rise in the total protein and globulin content, myeloperoxidase and respiratory burst activity following injection with 50 µg lysate and 100 µg plasma protein. The agglutinating and haemagglutinating activities were increased albeit not significantly (P > 0.01) in any groups. On the contrary, a significantly high hemolysin titre was recorded in fish that received 100 µg plasma protein. Following challenge with Aeromonas hydrophila, both lysate and plasma protein(s) cross protected the fish after 30 days. The highest survival (50%) was recorded in group injected with 50 µg lysate protein, followed by 45% in both 100 µg lysate and plasma protein injected groups.

Antimicrobial Polymer-Peptide Conjugates Based on Maximin H5 and PEG to Prevent Biofouling of E. coli and P. aeruginosa

Many pathogens, such as Pseudomonas aeruginosa and Escherichia coli bacteria can easily attach to surfaces and form stable biofilms. The formation of such biofilms in surfaces presents a problem in environmental, biomedical, and industrial processes, among many others. Aiming to provide a plausible solution to this issue, the anionic and hydrophobic peptide Maximin H5 C-terminally deaminated isoform (MH5C) has been modified with a cysteine in the C-terminal (MH5C-Cys) and coupled to polyethylene glycol (PEG) polymers of varying sizes (i.e., 2 kDa and 5 kDa) to serve as a surface protective coating. Briefly, the MH5C-Cys was bioconjugated to PEG and purified by size exclusion chromatography while the reaction was confirmed via SDS-PAGE and MALDI ToF. Moreover, the preventive antimicrobial activity of the MH5C-Cys-PEG conjugates was performed via the growth curves method, showing inhibition of bacterial growth after 24 h. The efficacy of these peptide-polymer conjugates was extensively characterized via scanning electron microscopy (SEM), minimum inhibition concentration (MIC), minimum biofilm inhibition concentration (MBIC), and minimum biofilm eradication concentration (MBEC) assays to evaluate their ability to eradicate and prevent the biofilms. Interestingly, this work demonstrated a critical PEG polymer weight of 5 kDa as ideal when coupled to the peptide to achieve inhibition and eradication of the biofilm formation in both bacteria strains. According to the MICs (40 μM) and MBICs (300 μM), we can conclude that this conjugate (MH5C-Cys-5 kDa) has an action that prevents/inhibits the formation of biofilms and the eradication of biofilms (MBEC 500 μM). In contrast, the MH5C-Cys peptide with PEG polymer of 2 kDa did not show inhibition or eradication of the biofilms.

Trichoderma/pathogen/plant interaction in pre-harvest food security

Large losses before crop harvesting are caused by plant pathogens, such as viruses, bacteria, oomycetes, fungi, and nematodes. Among these, fungi are the major cause of losses in agriculture worldwide. Plant pathogens are still controlled through application of agrochemicals, causing human disease and impacting environmental and food security. Biological control provides a safe alternative for the control of fungal plant pathogens, because of the ability of biocontrol agents to establish in the ecosystem. Some Trichoderma spp. are considered potential agents in the control of fungal plant diseases. They can interact directly with roots, increasing plant growth, resistance to diseases, and tolerance to abiotic stress. Furthermore, Trichoderma can directly kill fungal plant pathogens by antibiosis, as well as via mycoparasitism strategies. In this review, we will discuss the interactions between Trichoderma/fungal pathogens/plants during the pre-harvest of crops. In addition, we will highlight how these interactions can influence crop production and food security. Finally, we will describe the future of crop production using antimicrobial peptides, plants carrying pathogen-derived resistance, and plantibodies.

ACPred: A Computational Tool for the Prediction and Analysis of Anticancer Peptides

Anticancer peptides (ACPs) have emerged as a new class of therapeutic agent for cancer treatment due to their lower toxicity as well as greater efficacy, selectivity and specificity when compared to conventional small molecule drugs. However, the experimental identification of ACPs still remains a time-consuming and expensive endeavor. Therefore, it is desirable to develop and improve upon existing computational models for predicting and characterizing ACPs. In this study, we present a bioinformatics tool called the ACPred, which is an interpretable tool for the prediction and characterization of the anticancer activities of peptides. ACPred was developed by utilizing powerful machine learning models (support vector machine and random forest) and various classes of peptide features. It was observed by a jackknife cross-validation test that ACPred can achieve an overall accuracy of 95.61% in identifying ACPs. In addition, analysis revealed the following distinguishing characteristics that ACPs possess: (i) hydrophobic residue enhances the cationic properties of α-helical ACPs resulting in better cell penetration; (ii) the amphipathic nature of the α-helical structure plays a crucial role in its mechanism of cytotoxicity; and (iii) the formation of disulfide bridges on β-sheets is vital for structural maintenance which correlates with its ability to kill cancer cells. Finally, for the convenience of experimental scientists, the ACPred web server was established and made freely available online.

Influences of disulfide connectivity on structure and antimicrobial activity of tachyplesin I

Tachyplesin I is a potent antimicrobial peptide with broad spectrum of antimicrobial activity. It has 2 disulfide bonds and can form 3 disulfide bond isomers. In this study, the structure and antimicrobial activity of 3 tachyplesin I isomers (tachyplesin I, 3C12C, 3C7C) were investigated using molecular dynamic simulations, circular dichroism structural study, as well as antimicrobial activity and hemolysis assay. Our results suggest that in comparison to the native peptide, the 2 isomers (3C12C, 3C7C) have substantial structural and activity variations. The native peptide is in the ribbon conformation, while 3C12C and 3C7C possess remarkably different secondary structures, which are referred as "globular" and "beads" isomers, respectively. The substantially decreased hemolysis effects for these 2 isomers is accompanied by significantly decreased anti-gram-positive bacterial activity.

Identification of a novel antimicrobial peptide from the sea star Patiria pectinifera

Antimicrobial peptides (AMPs) are components of innate immunity found in many forms of life. However, there have been no reports of AMPs in sea star (Phylum Echinodermata). Here we report the isolation and characterization of a novel antimicrobial peptide from the coelomic epithelium extract of the sea star Patiria pectinifera. The isolated peptide comprises 38 amino acid residues, is cationic (pI 9.2), has four cysteine residues that form two disulfide bonds (C1-C3 and C2-C4), is amidated at the C-terminus, and is designated P. pectinifera cysteine-rich antimicrobial peptide (PpCrAMP). Synthetic PpCrAMP identical to the native peptide exhibited the most potent antimicrobial activity compared to analogs with different disulfide bond configurations. Expression analysis of PpCrAMP precursor transcripts revealed constitutive expression in the coelomic epithelium and tube feet of P. pectinifera. Analysis of genomic DNA and cDNA encoding the PpCrAMP precursor protein revealed that an intron splits the coding region of the mature peptide into a positively charged N-terminal domain and a C-terminal domain harboring four cysteine residues and a glycine for C-terminal amidation. No significant homology with other known AMPs was observed, while orthologs of PpCrAMP were found in other echinoderm species. These findings indicate that PpCrAMP is the prototype of a family a novel cysteine-rich AMPs that participate in mechanisms of innate immunity in echinoderms. Furthermore, the discovery of PpCrAMP may lead to the identification of related AMPs in vertebrates and protostome invertebrates.

Acyclic peptides incorporating the d-Phe-2-Abz turn motif: Investigations on antimicrobial activity and propensity to adopt β-hairpin conformations

Three linear peptides incorporating d-Phe-2-Abz as the turn motif are reported. Peptide 1, a hydrophobic β-hairpin, served as a proof of principle for the design strategy with both NMR and CD spectra strongly suggesting a β-hairpin conformation. Peptides 2 and 3, designed as amphipathic antimicrobials, exhibited broad spectrum antimicrobial activity, with potency in the nanomolar range against Staphylococcus aureus. Both compounds possess a high degree of selectivity, proving non-haemolytic at concentrations 500 to 800 times higher than their respective minimal inhibitory concentrations (MICs) against S. aureus. Peptide 2 induced cell membrane and cell wall disintegration in both S. aureus and Pseudomonas aeruginosa as observed by transmission electron microscopy. Peptide 2 also demonstrated moderate antifungal activity against Candida albicans with an MIC of 50 μM. Synergism was observed with sub-MIC levels of amphotericin B (AmB), leading to nanomolar MICs against C. albicans for peptide 2. Based on circular dichroism spectra, both peptides 2 and 3 appear to exist as a mixture of conformers with the β-hairpin as a minor conformer in aqueous solution, and a slight increase in hairpin population in 50% trifluoroethanol, which was more pronounced for peptide 3. NMR spectra of peptide 2 in a 1:1 CD₃ CN/H₂ O mixture and 30 mM deuterated sodium dodecyl sulfate showed evidence of an extended backbone conformation of the β-strand residues. However, inter-strand rotating frame Overhauser effects (ROE) could not be detected and a loosely defined divergent hairpin structure resulted from ROE structure calculation in CD₃ CN/H₂ O. The loosely defined hairpin conformation is most likely a result of the electrostatic repulsions between cationic strand residues which also probably contribute towards maintaining low haemolytic activity.

Excavating differentially expressed antimicrobial peptides from transcriptome of Larimichthys crocea liver in response to Cryptocaryon irritans

Larimichthys crocea, the special marine economy fish, owns the largest annual yield for a single species in China. One of the most significant factors affecting large yellow croaker culture is the diseases, especially the threat of marine white spot disease which caused by a protozoan Cryptocaryon irritans. Antimicrobial peptides (AMPs) have been demonstrated to be active against bacterium, fungi and parasites, showing their potential usefulness in aquaculture as substitutes for antibiotics. Many researches have been carried out about the AMPs concentrating on the activity resist on C. irritans, and piscidin-like of L. crocea owning widely antibacterial spectrum and strong activity against C. irritans was screened in our team. In the paper, taking advantage of the large yellow croaker hepatic comparison transcriptome in response to C. irritans at 3d post infection, seven kinds of AMPs have been excavated from the differently expressed genes, including LEAP2 like, LEAP-2A, hepcidin, hepcidin-like, piscidin-5-like, piscidin-5-like type 4 and bactericidal permeability increasing protein (BPI). Hepcidin, hepcidin-like, piscidin-5-like, piscidin-5-like type4 and BPI were up-regulated to protect large yellow croaker from being damaged by C. irritans infection; while LEAP2 like and LEAP-2A were down-regulated, they might be as a negative-feedback regulation factor or some other regulatory mechanisms to adjust the immune response in the process of C. irritans infection. The differential expression changes were verified with quantitative real-time PCR (qRT-PCR) to illustrate the reliability of the sequenced data. Hearteningly, piscidin-5-like type 4 was a novel type which was high similar to other piscidin-5-like types. Interestingly, the infection may well cause alternative splicing of LEAP-2A mRNA, which was a surprised phenomenon and finding after C. irritans infection, but more further study was needed to be conducted. Therefore, the data showed that these AMPs were involved in the immune response to the C. irritans infection. In all, these results implied that the immune response of AMPs to C. irritans infection was a complex and sophisticated regulatory process.

Structure-Activity and -Toxicity Relationships of the Antimicrobial Peptide Tachyplesin-1

Tachyplesin-1 (TP1; 1) is a cationic β-hairpin antimicrobial peptide with a membranolytic mechanism of action. While it possesses broad-spectrum, potent antimicrobial activity, 1 is highly hemolytic against mammalian erythrocytes, which precludes it from further development. In this study, we report a template-based approach to investigate the structure-function and structure-toxicity relationships of each amino acid of 1. We modulated charge and hydrophobicity by residue modification and truncation of the peptide. Antimicrobial activity was then assessed against six key bacterial pathogens and two fungi, with toxicity profiled against mammalian cells. The internal disulfide bridge Cys7-Cys12 of 1 was shown to play an important role in broad-spectrum antimicrobial activity against all pathogenic strains tested. Novel peptides based on the progenitor were then designed, including 5 (TP1[F4A]), 12 (TP1[I11A]), and 19 (TP1[C3A,C16A]). These had 26- to 64-fold improved activity/toxicity indices and show promise for further development. Structural studies of 5 (TP1[F4A]) and 12 (TP1[I11A]) identified a conserved β-hairpin secondary structure motif correlating with their very high stablility in mouse and human plasma. Membrane binding affinity determined by surface plasmon resonance confirmed their selectivity toward bacterial membranes, but the degree of membrane binding did not correlate with the degree of hemolysis, suggesting that other factors may drive toxicity.

Single channel planar lipid bilayer recordings of the melittin variant MelP5

MelP5 is a 26 amino acid peptide derived from melittin, the main active constituent of bee venom, with five amino acid replacements. The pore-forming activity of MelP5 in lipid membranes is attracting attention because MelP5 forms larger pores and induces dye leakage through liposome membranes at a lower concentration than melittin. Studies of MelP5 have so far focused on ensemble measurements of membrane leakage and impedance; here we extend this characterization with an electrophysiological comparison between MelP5 and melittin using planar lipid bilayer recordings. These experiments reveal that MelP5 pores in lipid membranes composed of 3:1 phosphatidylcholine:cholesterol consist of an average of 10 to 12 monomers compared to an average of 3 to 9 monomers for melittin. Both peptides form transient pores with dynamically varying conductance values similar to previous findings for melittin, but MelP5 occasionally also forms stable, well-defined pores with single channel conductance values that vary greatly and range from 50 to 3000pS in an electrolyte solution containing 100mM KCl.

HemoPred: a web server for predicting the hemolytic activity of peptides

AIM

Toxicity arising from hemolytic activity of peptides hinders its further progress as drug candidates.

MATERIALS & METHODS

This study describes a sequence-based predictor based on a random forest classifier using amino acid composition, dipeptide composition and physicochemical descriptors (named HemoPred).

RESULTS

This approach could outperform previously reported method and typical classification methods (e.g., support vector machine and decision tree) verified by fivefold cross-validation and external validation with accuracy and Matthews correlation coefficient in excess of 95% and 0.91, respectively. Results revealed the importance of hydrophobic and Cys residues on α-helix and β-sheet, respectively, on the hemolytic activity.

CONCLUSION

A sequence-based predictor which is publicly available as the web service of HemoPred, is proposed to predict and analyze the hemolytic activity of peptides.

Quantitative Proteomic Profiling of Tachyplesin I Targets in U251 Gliomaspheres

Tachyplesin I is a cationic peptide isolated from hemocytes of the horseshoe crab and its anti-tumor activity has been demonstrated in several tumor cells. However, there is limited information providing the global effects and mechanisms of tachyplesin I on glioblastoma multiforme (GBM). Here, by using two complementary proteomic strategies (2D-DIGE and dimethyl isotope labeling-based shotgun proteomics), we explored the effect of tachyplesin I on the proteome of gliomaspheres, a three-dimensional growth model formed by a GBM cell line U251. In total, the expression levels of 192 proteins were found to be significantly altered by tachyplesin I treatment. Gene ontology (GO) analysis revealed that many of them were cytoskeleton proteins and lysosomal acid hydrolases, and the mostly altered biological process was related to cellular metabolism, especially glycolysis. Moreover, we built protein-protein interaction network of these proteins and suggested the important role of DNA topoisomerase 2-alpha (TOP2A) in the signal-transduction cascade of tachyplesin I. In conclusion, we propose that tachyplesin I might down-regulate cathepsins in lysosomes and up-regulate TOP2A to inhibit migration and promote apoptosis in glioma, thus contribute to its anti-tumor function. Our results suggest tachyplesin I is a potential candidate for treatment of glioma.

Hairpin structure stability plays a role in the activity of two antimicrobial peptides

Many naturally occurring antimicrobial peptides (AMPs) are amphipathic with a β-hairpin conformation stabilized by cross-strand disulfides across the associated β-strands. Here, we show that the disulfides are not essential. Other structuring means such as better β-turns and noncovalent cross-strand interactions can, with proper design, replace the disulfides with no loss in antimicrobial activity. Our results also demonstrate that the hairpin turn region may play a role in membrane recognition for at least one member of this class, since a homodimeric turnless β-sheet analog showed no antimicrobial activity. We also examined the effects of N-terminal fatty acid adducts on AMPs. Surprisingly, the large hydrophobic carboxylic moieties examined completely eliminated the antimicrobial activity of previously active β-hairpin peptides.

Antiviral function of Tachyplesin I against iridovirus and nodavirus

Antimicrobial peptides (AMPs) are ubiquitously found in living organisms and are an important component in innate immune response. Tachyplesin I is a potent antimicrobial peptide isolated from the hemocytes of the horseshoe crab, Tachypleus tridentatus. Previous studies have shown that the 17-residue peptide exhibits a wide spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria, fungi, protozoa, and viruses. However, the efficiencies and defense mechanisms of the Tachyplesin I against fish viruses are still unknown. In this study, Tachyplesin I showed a key role in inhibiting the infection and replication of two kinds of newly emerging marine fish viruses, an enveloped DNA virus of Singapore grouper iridovirus (SGIV), and a non-enveloped RNA virus of viral nervous necrosis virus (RGNNV). Synthetic peptides of Tachyplesin I incubated with virus or cells before infection reduced the viral infectivity. Synthetic peptides of Tachyplesin I drastically decreased SGIV and RGNNV titers and viral gene expression. Grouper spleen (GS) and brain (GB) cells over-expressing Tachyplesin I (GS/pcDNA3.1-flag-Tac I and GB/pcDNA3.1-flag-Tac I) support the inhibition of viral infection. Tachyplesin I activated type I IFN and Interferon-sensitive response element (ISRE) in vitro. The promoter activity of IFN-β and ISRE were significantly up-regulated in cells transfected with pcDNA3.1-flag-Tac I after infection with SGIV and VNNV. These results suggest that Tachyplesin I is importantly involved in host immune responses to invasion of viral pathogens.

Armadillidin H, a Glycine-Rich Peptide from the Terrestrial Crustacean Armadillidium vulgare, Displays an Unexpected Wide Antimicrobial Spectrum with Membranolytic Activity

Antimicrobial peptides (AMPs) are key components of innate immunity and are widespread in nature, from bacteria to vertebrate animals. In crustaceans, there are currently 15 distinct AMP families published so far in the literature, mainly isolated from members of the Decapoda order. Up to now, armadillidin is the sole non-decapod AMP isolated from the haemocytes of Armadillidium vulgare, a crustacean isopod. Its first description demonstrated that armadillidin is a linear glycine-rich (47%) cationic peptide with an antimicrobial activity directed toward Bacillus megaterium. In the present work, we report identification of armadillidin Q, a variant of armadillidin H (earlier known as armadillidin), from crude haemocyte extracts of A. vulgare using LC-MS approach. We demonstrated that both armadillidins displayed broad spectrum antimicrobial activity against several Gram-positive and Gram-negative bacteria, fungi, but were totally inactive against yeasts. Membrane permeabilization assays, only performed with armadillidin H, showed that the peptide is membrane active against bacterial and fungal strains leading to deep changes in cell morphology. This damaging activity visualized by electronic microscopy correlates with a rapid decrease of cell viability leading to highly blebbed cells. In contrast, armadillidin H does not reveal cytotoxicity toward human erythrocytes. Furthermore, no secondary structure could be defined in this study [by circular dichroism (CD) and nuclear magnetic resonance (NMR)] even in a membrane mimicking environment. Therefore, armadillidins represent interesting candidates to gain insight into the biology of glycine-rich AMPs.

Contribution of Amphipathicity and Hydrophobicity to the Antimicrobial Activity and Cytotoxicity of β-Hairpin Peptides

Bacteria have acquired extensive resistance mechanisms to protect themselves against antibiotic action. Today the bacterial membrane has become one of the “final frontiers” in the search for new compounds acting on novel targets to address the threat of multi-drug resistant (MDR) and XDR bacterial pathogens. β-Hairpin antimicrobial peptides are amphipathic, membrane-binding antibiotics that exhibit a broad range of activities against Gram-positive, Gram-negative, and fungal pathogens. However, most members of the class also possess adverse cytotoxicity and hemolytic activity that preclude their development as candidate antimicrobials. We examined peptide hydrophobicity, amphipathicity, and structure to better dissect and understand the correlation between antimicrobial activity and toxicity, membrane binding, and membrane permeability. The hydrophobicity, pI, net charge at physiological pH, and amphipathic moment for the β-hairpin antimicrobial peptides tachyplesin-1, polyphemusin-1, protegrin-1, gomesin, arenicin-3, and thanatin were determined and correlated with key antimicrobial activity and toxicity data. These included antimicrobial activity against five key bacterial pathogens and two fungi, cytotoxicity against human cell lines, and hemolytic activity in human erythrocytes. Observed antimicrobial activity trends correlated with compound amphipathicity and, to a lesser extent, with overall hydrophobicity. Antimicrobial activity increased with amphipathicity, but unfortunately so did toxicity. Of note, tachyplesin-1 was found to be 8-fold more amphipathic than gomesin. These analyses identify tachyplesin-1 as a promising scaffold for rational design and synthetic optimization toward an antibiotic candidate.

Improved strategy for recombinant production and purification of antimicrobial peptide tachyplesin I and its analogs with high cell selectivity

Here, we report an efficient procedure for recombinant production and purification of tachyplesin I (THI) with a final yield of 17 mg/L of the culture medium. The peptide was expressed in Escherichia coli as a part of the thioredoxin fusion protein. With the use of soluble expression followed by immobilized metal-ion affinity chromatography, the recombinant protein cleavage and reversed-phase high-performance liquid chromatography, a yield of THI did not exceed 6.5 mg/L of the culture medium. Further optimization studies were carried out to improve the protein expression level and simplify purification procedure of the target peptide. To achieve better yield of the peptide, we used high-cell-density bacterial expression. The formed inclusion bodies were highly enriched with the fusion protein, which allowed us to perform direct chemical cleavage of the inclusion bodies solubilized in 6 M guanidine-HCl with subsequent selective precipitation of proteins with trifluoroacetic acid. This enabled us to avoid an extra step of purification by immobilized metal-ion affinity chromatography. The developed procedure has made it possible to obtain biologically active THI and was used for screening a number of its mutant analogs. As a result, several selective and nonhemolytic analogs were developed. Significant reduction in hemolytic activity without losing antimicrobial activity was achieved by substitution of tyrosine or isoleucine residue in the β-turn region of the molecule with hydrophilic serine. The present study affords further insight into molecular mechanism of antimicrobial action of tachyplesin and gains a better understanding of structure-activity relationships in its analogs. This is aimed at searching for novel antibiotics on the basis of antimicrobial peptides with reduced cytotoxicity.

Characterization, evolution and functional analysis of the liver-expressed antimicrobial peptide 2 (LEAP-2) gene in miiuy croaker

As an evolutionarily older defense strategy, the innate immune is the dominant immune system and provides a first line of antimicrobial host defense in teleost. Liver-expressed antimicrobial peptide-2 (LEAP-2) is a critical molecule of the innate immune system and plays a very important role in resistance against bacterial infections. We reported comprehensive analysis and characterization of LEAP-2 gene from miiuy croaker (Miichthys miiuy) in here. The complete cDNA of miiuy croaker LEAP-2 consists 2360 bp, including a 5' terminal untranslated region (UTR) of 170 bp, an open reading frame (ORF) of 312 bp, and a 3'-UTR of 1878 bp. Interestingly, two polyadenylation signals (AATTAAA) which may involve the stability, translation efficiency, or localization of an mRNA in a tissue were found in 3'-UTR. Genomic DNA of miiuy croaker LEAP-2 includes three exons and two introns, which is similar to LEAP-2 genes in other mammals and fish. The deduced 103 amino acids consist of signal peptide, prodomain and mature peptide. Four highly conserved cysteine residues involved two disulfide bridges in mature peptide. Real-time PCR results showed that LEAP-2 was ubiquitously expressed in all tissues and the expression level was highest in liver. Significantly, the expression levels were increased after infection with Vibrio anguillarum in liver and spleen. The antimicrobial activity analysis result of LEAP-2 in vitro indicated that LEAP-2 of miiuy croaker was effective in controlling Aeromonas hydrophila. In addition, we performed evolutionary analysis in order to estimate the selective constraints on the LEAP-2 gene. The result indicated that no positive selection exists in LEAP-2 gene sequences, which may be on account of irreplaceable function constrains. Meanwhile, we compared the structure of LEAP-2 with that of another Liver-expressed antimicrobial peptide (LEAP-1, also named HAMP), and found the LEAP-2 from miiuy croaker comprises of α-helix, β-sheet, and β-turn while the LEAP-1 of miiuy croaker only contains β-sheet.

Chemical synthesis, structure-activity relationship, and properties of shepherin I: a fungicidal peptide enriched in glycine-glycine-histidine motifs

Although glycine-rich antimicrobial peptides (AMPs) are found in animals and plants, very little has been reported on their chemistry, structure activity-relationship, and properties. We investigated those topics for Shepherin I (Shep I), a glycine-rich AMP with the unique amino acid sequence G(1)YGGHGGHGGHGGHGGHGGHGHGGGGHG(28). Shep I and analogues were synthesized by the solid-phase method at 60 °C using conventional heating. Purification followed by chemical characterization confirmed the products' identities and high purity. Amino acid analysis provided their peptide contents. All peptides were active against the clinically important Candida species, but ineffective against bacteria and mycelia fungi. Truncation of the N- or C-terminal portion reduced Shep I antifungal activity, the latter being more pronounced. Carboxyamidation of Shep I did not affect the activity against C. albicans or C. tropicalis, but increased activity against S. cerevisiae. Carboxyamidated analogues Shep I (3-28)a and Shep I (6-28)a were equipotent to Shep I and Shep Ia against Candida species. As with most cationic AMPs, all peptides had their activity significantly reduced in high-salt concentrations, a disadvantage that is defeated if 10 µM ZnCl2 is present. At 100 µM, the peptides were practically not hemolytic. Shep Ia also killed C. albicans MDM8 and ATCC 90028 cells. Fluo-Shep Ia, an analogue labeled with 5(6)-carboxyfluorescein, was rapidly internalized by C. albicans MDM8 cells, a salt-sensitive process dependent on metabolic energy and temperature. Altogether, such results shed light on the chemistry, structural requirements for activity, and other properties of candidacidal glycine-rich peptides. Furthermore, they show that Shep Ia may have strong potential for use in topical application.

Interaction between tachyplesin I, an antimicrobial peptide derived from horseshoe crab, and lipopolysaccharide

Lipopolysaccharide (LPS) is a major constituent of the outer membrane of Gram-negative bacteria and is the very first site of interactions with antimicrobial peptides (AMPs). In order to gain better insight into the interaction between LPS and AMPs, we determined the structure of tachyplesin I (TP I), an antimicrobial peptide derived from horseshoe crab, in its bound state with LPS and proposed the complex structure of TP I and LPS using a docking program. CD and NMR measurements revealed that binding to LPS slightly extends the two β-strands of TP I and stabilizes the whole structure of TP I. The fluorescence wavelength of an intrinsic tryptophan of TP I and fluorescence quenching in the presence or absence of LPS indicated that a tryptophan residue is incorporated into the hydrophobic environment of LPS. Finally, we succeeded in proposing a structural model for the complex of TP I and LPS by using a docking program. The calculated model structure suggested that the cationic residues of TP I interact with phosphate groups and saccharides of LPS, whereas hydrophobic residues interact with the acyl chains of LPS.

Linear bactenecin analogs with cell selectivity and anti-endotoxic activity

Bactenecin (Bac) is a 12-residue disulfide-linked antimicrobial peptide isolated from the granules of bovine neutrophils. In this study, to develop novel linear Bac analogs with cell selectivity and anti-endotoxic activity, we designed and synthesized a series of linear Bac analogs with amino acid substitution in Cys3,11 and/or Val6,7 of Bac. Among Bac analogs, some analogs (Bac-W, Bac-KW, Bac-L, Bac-KL, Bac-LW, and Bac-KLW) with higher hydrophobicity showed the amalgamated property of cell selectivity and anti-endotoxic activity. Furthermore, Bac-W, Bac-KW, Bac-LW, and Bac-KLW showed serum stability comparable with that of disulfide-bonded Bac. Therefore, these Bac analogs (Bac-W, Bac-KW, Bac-LW, and Bac-KLW) can serve as promising antibiotics for the development of therapeutic agents for treatment against endotoxic shock and bacterial infection. In addition, our results suggest that a little increase in hydrophobicity may be responsible for the decreased cell selectivity of the multiple Arg-containing peptides (Bac-W, Bac-L, and Bac-LW) over the multiple Lys-containing peptides (Bac-KW, Bac-KL, and Bac-KLW).

Anticancer β-hairpin peptides: membrane-induced folding triggers activity

Several cationic antimicrobial peptides (AMPs) have recently been shown to display anticancer activity via a mechanism that usually entails the disruption of cancer cell membranes. In this work, we designed an 18-residue anticancer peptide, SVS-1, whose mechanism of action is designed to take advantage of the aberrant lipid composition presented on the outer leaflet of cancer cell membranes, which makes the surface of these cells electronegative relative to the surface of noncancerous cells. SVS-1 is designed to remain unfolded and inactive in aqueous solution but to preferentially fold at the surface of cancer cells, adopting an amphiphilic β-hairpin structure capable of membrane disruption. Membrane-induced folding is driven by electrostatic interaction between the peptide and the negatively charged membrane surface of cancer cells. SVS-1 is active against a variety of cancer cell lines such as A549 (lung carcinoma), KB (epidermal carcinoma), MCF-7 (breast carcinoma), and MDA-MB-436 (breast carcinoma). However, the cytotoxicity toward noncancerous cells having typical membrane compositions, such as HUVEC and erythrocytes, is low. CD spectroscopy, appropriately designed peptide controls, cell-based studies, liposome leakage assays, and electron microscopy support the intended mechanism of action, which leads to preferential killing of cancerous cells.

Antimicrobial peptides: key components of the innate immune system

Life-threatening infectious diseases are on their way to cause a worldwide crisis, as treating them effectively is becoming increasingly difficult due to the emergence of antibiotic resistant strains. Antimicrobial peptides (AMPs) form an ancient type of innate immunity found universally in all living organisms, providing a principal first-line of defense against the invading pathogens. The unique diverse function and architecture of AMPs has attracted considerable attention by scientists, both in terms of understanding the basic biology of the innate immune system, and as a tool in the design of molecular templates for new anti-infective drugs. AMPs are gene-encoded short (<100 amino acids), amphipathic molecules with hydrophobic and cationic amino acids arranged spatially, which exhibit broad spectrum antimicrobial activity. AMPs have been the subject of natural evolution, as have the microbes, for hundreds of millions of years. Despite this long history of co-evolution, AMPs have not lost their ability to kill or inhibit the microbes totally, nor have the microbes learnt to avoid the lethal punch of AMPs. AMPs therefore have potential to provide an important breakthrough and form the basis for a new class of antibiotics. In this review, we would like to give an overview of cationic antimicrobial peptides, origin, structure, functions, and mode of action of AMPs, which are highly expressed and found in humans, as well as a brief discussion about widely abundant, well characterized AMPs in mammals, in addition to pharmaceutical aspects and the additional functions of AMPs.

Structure of chemokine-derived antimicrobial Peptide interleukin-8alpha and interaction with detergent micelles and oriented lipid bilayers

Interleukin-8alpha (IL-8alpha) is an antimicrobial peptide derived from the chemokine IL-8. Solution NMR was used to determine the atomic-resolution structure of IL-8alpha in SDS micelles. Solid-state NMR and tryptophan fluorescence were used to probe the interaction of IL-8alpha with model membranes. The peptide interacted differently with anionic versus purely zwitterionic micelles or bilayers. Tryptophan fluorescence demonstrated a deeper position of Trp4 in SDS micelles and POPC/POPG bilayers compared to pure POPC bilayers, consistent with (2)H order parameters, which also indicated a deeper position of the peptide in POPC/POPG bilayers compared to POPC bilayers. Paramagnetic probe data showed that IL-8alpha was situated roughly parallel to the SDS micelle surface, with a slight tilt that positioned the N-terminus more deeply in the micelle compared to the C-terminus. (15)N solid-state NMR spectra indicated a similar, nearly parallel position for the peptide in POPC/POPG bilayers. (31)P and (2)H solid-state NMR demonstrated that the peptide did not induce the formation of any nonlamellar phases and did not significantly disrupt bilayer orientation in aligned model membranes composed of POPC or POPC and POPG.

In vitro activity of Tachyplesin III alone and in combination with terbinafine against clinical isolates of dermatophytes

Aim of our study was to investigate the in vitro effects of Tachyplesin III (TP), a potent disulfide-linked peptide, in dermatophytes infections, with respect to or in combination with terbinafine (TERB), against 20 clinical isolates of dermatophytes belonging to four species. A broth microdilution method following the CLSI recommendations (M38-A) was used for testing drugs alone and in combination. TERB MICs were significantly lower than those observed for TP (p<0.001). Testing for antifungal agents in combination was performed for TERB with TP for all the 20 isolates. TERB activity in combination with TP showed indifferent activity for 14 of the 20 isolates (70%); synergic activity for 6 of the 20 isolates (30%); no antagonistic activity was observed. Further experiments were conducted with Microsporum canis 133, Trichophyton rubrum 62 and Trichophyton mentagrophytes 91 for fungal biomass. TP and TERB did not show a significant growth reduction compared to the control against T. mentagrophytes and T. rubrum. A significant difference of growth reduction both for TP and TERB compared to controls was observed for M. canis (p<0.01). In conclusion our study demonstrated that Tachyplesin III has potential activity against dermatophytes. In addition, we observed that the in vitro activity of Tachyplesin III can be enhanced upon combination with terbinafine. Synergy could permit lower doses of the individual antifungal agents to be used more effectively and/or safely.

The chitin-binding capability of Cy-AMP1 from cycad is essential to antifungal activity

Antimicrobial peptides are important components of the host innate immune responses by exerting broad-spectrum microbicidal activity against pathogenic microbes. Cy-AMP1 found in the cycad (Cycas revoluta) seeds has chitin-binding ability, and the chitin-binding domain was conserved in knottin-type and hevein-type antimicrobial peptides. The recombinant Cy-AMP1 was expressed in Escherichia coli and purified to study the role of chitin-binding domain. The mutants of Cy-AMP1 lost chitin-binding ability completely, and its antifungal activity was markedly decreased in comparison with native Cy-AMP1. However, the antimicrobial activities of the mutant peptides are nearly identical to that of native one. It was suggested that the chitin-binding domain plays an essential role in antifungal, but not antimicrobial, activity of Cy-AMP1.

Self-assembling peptides: sequence, secondary structure in solution and film formation

Peptides of alternating charge and hydrophobic amino acids have a tendency to adopt unusually stable beta-sheet structures that can form insoluble macroscopic aggregates under physiological conditions. In this study, analogues of a well-known self-assembling peptide, characterized by the same polar/nonpolar periodicity but with different residues, were designed to study the relationship between sequence, conformation in solution and film-forming capacity in saline solution. Peptide conformation, evaluated by circular dichroism, correlated with film forming capacity observed by inverted optical microscopy after addition of saline solution and subsequent drying. We found that polar/nonpolar periodicity of several analogues is not criterion enough to induce beta-sheet and thus film formation and that conformations different from beta-sheet also allow self-assemblage. Furthermore, addition of the short adhesive sequence RGD to a known self-assembling sequence was shown to not prevent the self-assembling process. This finding might prove useful for the design of biomimetic scaffolds.

Synthetic antimicrobial peptides as agricultural pesticides for plant-disease control

There is a need of antimicrobial compounds in agriculture for plant-disease control, with low toxicity and reduced negative environmental impact. Antimicrobial peptides are produced by living organisms and offer strong possibilities in agriculture because new compounds can be developed based on natural structures with improved properties of activity, specificity, biodegradability, and toxicity. Design of new molecules has been achieved using combinatorial-chemistry procedures coupled to high-throughput screening systems and data processing with design-of-experiments (DOE) methodology to obtain QSAR equation models and optimized compounds. Upon selection of best candidates with low cytotoxicity and moderate stability to protease digestion, anti-infective activity has been evaluated in plant-pathogen model systems. Suitable compounds have been submitted to acute toxicity testing in higher organisms and exhibited a low toxicity profile in a mouse model. Large-scale production can be achieved by solution organic or chemoenzymatic procedures in the case of very small peptides, but, in many cases, production can be performed by biotechnological methods using genetically modified microorganisms (fermentation) or transgenic crops (plant biofactories).

Linear analogues of human beta-defensin 3: concepts for design of antimicrobial peptides with reduced cytotoxicity to mammalian cells

A series of engineered linear analogues [coded as F6, W6, Y6, A6, S6 and C(Acm)6] were modeled, designed, synthesized and structurally characterized by mass spectra, circular dichroism, hydrophobicity analysis and molecular modeling. We have screened antimicrobial activity, hemolysis to rabbit erythrocytes, and cytotoxicity to human conjunctival epithelial cells. No significant hemolytic effect was observed for hBD3 or from five of the six analogues [F6, Y6, A6, S6 and C(Acm)6] over the range of 3-100 microg mL(-1). The six linear analogues have reduced cytotoxicity to human conjunctival epithelial cells over the range of 6-100 microg mL(-1) compared to hBD3. By tuning the overall hydrophobicity of linear hBD3 analogues, reduced cytotoxicity and hemolysis were obtained while preserving the antimicrobial properties. The decreased cytotoxicity of the linear analogues is suggested to be structurally related to the removal of disulfide bridges, and the flexible structure of the linear forms, which seem to be associated with loss of secondary structure. These results suggest a new approach for guiding the design of new linear analogues of defensin peptides with strong antibiotic properties and reduced cytotoxicity to mammalian cells.

NMR based structure-activity relationship analysis of an antimicrobial peptide, thanatin, engineered by site-specific chemical modification: Activity improvement and spectrum alteration

Activity improvement of an antimicrobial peptide, thanatin, has been achieved up to 4-fold higher than natural original one by site-specific chemical modifications with tert-butyl group at two cysteine residues which form an intramoleular disulfide bridge. The chemically modified thanatin (C11tBu/C18tBu) exhibited improved antimicrobial activity toward Gram-positive bacteria, Micrococcus luteus, whereas lowered activity toward Gram-negative bacteria, Escherichia coli. This finding suggests that disulfide-bridge formation is not only indispensable for exhibition of antimicrobial activity of thanatin but also closely related to the activity specificity towards bacteria. NMR analysis indicates that thanatin acts against E.coli stereospecifically by taking advantage of its C-terminal beta-hairpin structure, while the activity against M. luteus does not relate to structures and correlates very well to side-chain hydrophobicity.

Studies on anticancer activities of antimicrobial peptides

In spite of great advances in cancer therapy, there is considerable current interest in developing anticancer agents with a new mode of action because of the development of resistance by cancer cells towards current anticancer drugs. A growing number of studies have shown that some of the cationic antimicrobial peptides (AMPs), which are toxic to bacteria but not to normal mammalian cells, exhibit a broad spectrum of cytotoxic activity against cancer cells. Such studies have considerably enhanced the significance of AMPs, both synthetic and from natural sources, which have been of importance both for an increased understanding of the immune system and for their potential as clinical antibiotics. The electrostatic attraction between the negatively charged components of bacterial and cancer cells and the positively charged AMPs is believed to play a major role in the strong binding and selective disruption of bacterial and cancer cell membranes, respectively. However, it is unclear why some host defense peptides are able to kill cancer cells when others do not. In addition, it is not clear whether the molecular mechanism(s) underlying the antibacterial and anticancer activities of AMPs are the same or different. In this article, we review various studies on different AMPs that exhibit cytotoxic activity against cancer cells. The suitability of cancer cell-targeting AMPs as cancer therapeutics is also discussed.

Conformation, dynamics, and insertion of a noncysteine-containing protegrin-1 analogue in lipid membranes from solid-state NMR spectroscopy

Disulfide-bonded beta-hairpin structures are common among antimicrobial peptides. Disulfide bonds are known to be important for antimicrobial activity, but the underlying structural reason is not well understood. We have investigated the membrane-bound structure of a disulfide-deleted analogue of the antimicrobial peptide protegrin-1, in which the four Cys residues were replaced by Ala. The secondary structure, dynamics, and topology of this Ala-PG1 peptide in the membrane were determined by using magic-angle-spinning NMR spectroscopy. Conformation-dependent (13)C isotropic chemical shifts of multiple (13)C-labeled residues were obtained from 1D cross-polarization and direct-polarization spectra, and from 2D J-coupling-mediated (13)C-(13)C correlation spectra. Most labeled residues exhibited two conformations: a random coil and a beta-sheet structure. The dual-conformation property was present in both anionic lipid bilayers, which mimic the bacterial membrane, and zwitterionic cholesterol-containing bilayers, which mimic the eukaryotic cell membrane. The mobility of the peptide was measured by using a 2D C-H dipolar-shift correlation experiment. The random-coil fraction was highly mobile whereas the beta-sheet component was rigid. (1)H spin diffusion from the lipid chains to the peptide indicates that the beta-sheet component was well inserted into the anionic membrane, but surface bound in the cholesterol-containing neutral membrane. Thus, the removal of disulfide bonds changed some PG-1 molecules to highly mobile random coils that were poorly associated with the lipid membrane, but other molecules retained a beta-sheet conformation and had a similar membrane-binding topology to the parent peptide. Thus, the reduced antimicrobial activity of Ala-PG1 was largely due to the reduced number of insertion-competent beta-sheet molecules, rather than uniformly weakened activity of identically structured peptides.

Eukaryotic expression and antimicrobial spectrum determination of the peptide tachyplesin II

Ta0-a, the gene encoding the mature antimicrobial peptide tachyplesin II, was engineered to optimize the coding sequence according to codon usage bias in yeast. Ta0-a was efficiently expressed in the methylotrophic yeast Pichia pastoris strain SMD1168. The recombinant peptide Ta0 reached 150mg/L after methanol induction for 6 d. Ta0 was rapidly purified to homogeneity by a single step of size-exclusion chromatography. The minimal lethal concentrations of Ta0 to the Escherichia coli strain K12 was 30 microg/mL. Ta0 exhibited a wide range of antimicrobial activity: the growth of 26 bacterial and fungal strains, including some typical food/feed spoilage microorganisms, was all substantially inhibited. This result indicates the potential practical application of the recombinant peptide in various industrial products.

Intrinsic flexibility and structural adaptability of Plasticins membrane-damaging peptides as a strategy for functional versatility

The Plasticins are a family of antimicrobial, 23-29-residue Gly-Leu-rich ortholog peptides from the frog skin that have very similar amino acid sequences, hydrophobicities, and amphipathicities but differ markedly in their conformational plasticity and spectrum of activity. The intrinsic flexibility and structural malleability of Plasticins modulate their ability to bind to and disrupt the bilayer membranes of prokaryotic and eukaryotic cells, and/or to reach intracellular targets, therefore, triggering functional versatility. The discussion is opened herein on several examples of other membrane-active peptides, like viral fusion peptides, cell-penetrating peptides, that are able to display antimicrobial activity. Hence, Plasticins could be regarded as models of multipotent membrane-active peptides guided by structural plasticity.

Antimicrobial peptides and plant disease control

Several diseases caused by viruses, bacteria and fungi affect plant crops, resulting in losses and decreasing the quality and safety of agricultural products. Plant disease control relies mainly on chemical pesticides that are currently subject to strong restrictions and regulatory requirements. Antimicrobial peptides are interesting compounds in plant health because there is a need for new products in plant protection that fit into the new regulations. Living organisms secrete a wide range of antimicrobial peptides produced through ribosomal (defensins and small bacteriocins) or non-ribosomal synthesis (peptaibols, cyclopeptides and pseudopeptides). Several antimicrobial peptides are the basis for the design of new synthetic analogues, have been expressed in transgenic plants to confer disease protection or are secreted by microorganisms that are active ingredients of commercial biopesticides.

Action mechanism of tachyplesin I and effects of PEGylation

PEGylation of protein and peptide drugs is frequently used to improve in vivo efficacy. We investigated the action mechanism of tachyplesin I, a membrane-acting cyclic antimicrobial peptide from Tachypleus tridentatus and the effects of PEGylation on the mechanism. The PEGylated peptide induced the leakage of calcein from egg yolk L-alpha-phosphatidylglycerol/egg yolk L-alpha-phosphatidylcholine large unilamellar vesicles similarly to the parent peptide. Both peptides induced lipid flip-flop coupled to leakage and was translocated into the inner leaflet of the bilayer, indicating that tachyplesin I forms a toroidal pore and that PEGylation did not alter the basic mechanism of membrane permeabilization of the parent peptide. Despite their similar activities against model membranes, the peptides showed very different biological activities. The cytotoxicity of tachyplesin I was greatly reduced by PEGylation, although the antimicrobial activity was significantly weakened. We investigated the enhancement of the permeability of inner membranes induced by the peptides. Our results suggested that outer membranes and peptidoglycan layers play an inhibitory role in the permeation of the PEG moiety. Furthermore, a reduction in DNA binding by PEGylation may also contribute to the weak activity of the PEGylated peptide.

Solution structures and biological functions of the antimicrobial peptide, arenicin-1, and its linear derivative

Arenicin-1 (AR-1) is a novel antimicrobial peptide that was isolated from coelomocytes of the marine polychaeta lugworm Arenicola marina and shown to contain a single disulfide bond between Cys3 and Cys20, forming an 18-residue ring [Ovchinnikova, T. V. et al., FEBS Lett 2004, 577, 209-214]. To determine the role of this disulfide bond, we synthesized AR-1 (RWCVYAYVRVRGVLVRYRRCW) and its linear derivative, arenicin-1-S (AR-1-S: RWSVYAYVRVRGVLVRYRRSW). Activity assays revealed that AR-1-S is somewhat less active against bacterial cells than AR-1. Both peptides were very hydrophobic, and displayed cytotoxicity against human red blood cells. Analysis of the tertiary structures of AR-1 and AR-1-S by NMR spectroscopy disclosed that AR-1 has two-stranded antiparallel beta-sheet structures with amphipathicity, while AR-1-S displays a random coil structure in DMSO. Our biological data for AR-1 and AR-1-S indicate that the hydrophobic-hydrophilic balance, disulfide bridge, and the amphipathic beta-sheet structure of the peptides play important roles in their biological activities. Elucidation of the structure of AR-1 and its derivative should facilitate the design of novel non-cytotoxic peptide antibiotics with potent antibacterial activities.

Structural malleability of plasticins: Preorganized conformations in solution and relevance for antimicrobial activity

Plasticins (23 long-residue glycine-leucine-rich dermaseptin-related peptides produced by the skin of South American hylids) have very similar amino acid sequences, hydrophobicities, and amphipathicities, but differ in their membrane-damaging properties and structurations (i.e. destabilized helix states, beta-hairpin, beta-sheet, and disordered states) at anionic and zwitterionic membrane interfaces. Structural malleability of plasticins in aqueous solutions together with parameters that may govern their ability to fold within beta-hairpin like structures were analyzed through circular dichroism and FTIR spectroscopic studies completed by molecular dynamics simulations in polar mimetic media. The goal of this study was to probe to which extent pre-existent peptide conformations, i.e. intrinsic "conformational landscape", may be responsible for variability in bioactive conformation and antimicrobial/hemolytic mechanisms of action of these peptides in relation with their various membrane disturbing properties. All plasticins present a turn region that does not always result in folding into a beta-hairpin shaped conformation. Residue at position 8 plays a major role in initiating the folding, while position 12 is not critical. Conformational stability has no major impact on antimicrobial efficacy. However, preformed beta-hairpin in solution may act as a conformational lock that prevents switch to alpha-helical structure. This lock lowers the antimicrobial efficiency and explains subtle differences in potencies of the most active antimicrobial plasticins.

Antibacterial activity of linear peptides spanning the carboxy-terminal beta-sheet domain of arthropod defensins

The antibacterial activity of peptides without disulfide bridges, spanning the carboxy-terminal segment of arthropod defensins, has been investigated. Although all the peptides have net positive charges, they exhibited varying antibacterial potencies and spectra. Atomic force and fluorescence microscopic analyses indicate that the peptides exert their activity by permeabilizing the outer and inner membranes of Gram-negative bacteria such as Escherichia coli. It appears that the plasticity observed in the activity of mammalian defensins with respect to sequence, number of disulfide bridges or net positive charge, is also observed in insect defensins.