Structural flexibility and the positive charges are the key factors in bacterial cell selectivity and membrane penetration of peptoid-substituted analog of Piscidin 1

Teleost Piscidins-In Silico Perspective of Natural Peptide Antibiotics from Marine Sources

Fish, like all other animals, are exposed to constant contact with microbes, both on their skin and on the surfaces of their respiratory and digestive systems. Fish have a system of non-specific immune responses that provides them with initial protection against infection and allows them to survive under normal conditions despite the presence of these potential invaders. However, fish are less protected against invading diseases than other marine vertebrates because their epidermal surface, composed primarily of living cells, lacks the keratinized skin that serves as an efficient natural barrier in other marine vertebrates. Antimicrobial peptides (AMPs) are one type of innate immune protection present in all life forms. AMPs have been shown to have a broader range of biological effects than conventional antibiotics, including antibacterial, antiviral, antiprotozoal, and antifungal effects. Although other AMPs, such as defensins and hepcidins, are found in all vertebrates and are relatively well conserved, piscidins are found exclusively in Teleost fish and are not found in any other animal. Therefore, there is less information on the expression and bioactivity of piscidins than on other AMPs. Piscidins are highly effective against Gram-positive and Gram-negative bacteria that cause disease in fish and humans and have the potential to be used as pharmacological anti-infectives in biomedicine and aquaculture. To better understand the potential benefits and limitations of using these peptides as therapeutic agents, we are conducting a comprehensive study of the Teleost piscidins included in the "reviewed" category of the UniProt database using bioinformatics tools. They all have amphipathic alpha-helical structures. The amphipathic architecture of piscidin peptides and positively charged residues influence their antibacterial activity. These alpha-helices are intriguing antimicrobial drugs due to their stability in high-salt and metal environments. New treatments for multidrug-resistant bacteria, cancer, and inflammation may be inspired by piscidin peptides.

Antibacterial activity study of a novel piscidin 5-like type 4 from Larimichthys crocea

As one of the key components of innate immune system, piscidins are likely to play pivotal role in the first defense line in fish. Piscidins own multiple resistance activity. A novel piscidin 5-like type 4 was excavated from Larimichthys crocea (termed Lc-P5L4) liver transcriptome immuned by Cryptocaryon irritans, and upregulated at 7 days post infection when secondary bacterial infection occurred. In the study, we characterized the antibacterial activity of Lc-P5L4. The liquid growth inhibition assay detected the recombinant Lc-P5L4 (rLc-P5L) had potent antibacterial activity to Photobacterium damselae. Scanning electron microscope (SEM) observed the cell surface of P. damselae collapsed to form pit, and membrane of some bacteria ruptured after co-incubation with rLc-P5L. Further, transmission electron microscope (TEM) was also employed to observe the intracellular microstructural damage, rLc-P5L4 caused cytoplasm contraction, pores formation and contents leakage. After knowing about its antibacterial effects, the preliminary antibacterial mechanism was also explored, western blot analysis showed rLc-P5L4 could bind to P. damselae through targeting to LPS. Agarose gel eletrophoresis analysis further showed rLc-P5L4 could also penetrate into cells and brought about genome DNA degradation. Therefore, rLc-P5L4 was of potential being a candidate to explore new antimicrobial drug or additive agent, especially to P. damselae.

Development of Neuropeptide Hemokinin-1 Analogues with Antimicrobial and Wound-Healing Activity

Wound healing is a complex process that can be delayed in some pathological conditions, such as infection and diabetes. Following skin injury, the neuropeptide substance P (SP) is released from peripheral neurons to promote wound healing by multiple mechanisms. Human hemokinin-1 (hHK-1) has been identified as an SP-like tachykinin peptide. Surprisingly, hHK-1 shares similar structural features with antimicrobial peptides (AMPs), but it does not display efficient antimicrobial activity. Therefore, a series of hHK-1 analogues were designed and synthesized. Among these analogues, AH-4 was found to display the greatest antimicrobial activity against a broad spectrum of bacteria. Furthermore, AH-4 rapidly killed bacteria by membrane disruption, similar to most AMPs. More importantly, AH-4 showed favorable healing activity in all tested mouse full-thickness excisional wound models. Overall, this study suggests that the neuropeptide hHK-1 can be used as a desirable template for developing promising therapeutics with multiple functions for wound healing.

S100 Proteins as Novel Therapeutic Targets in Psoriasis and Other Autoimmune Diseases

Psoriasis is one of the most common inflammatory skin diseases affecting about 1-3% of the population. One of the characteristic abnormalities in psoriasis is the excessive production of antimicrobial peptides and proteins, which play an essential role in the pathogenesis of the disease. Antimicrobial peptides and proteins can be expressed differently in normal and diseased skin, reflecting their usefulness as diagnostic biomarkers. Moreover, due to their very important functions in innate immunity, members of host defense peptides and proteins are currently considered to be promising new therapeutic targets for many inflammatory diseases. Koebnerisin (S100A15) belongs to an S100 family of antimicrobial proteins, which constitute the multigenetic group of calcium-binding proteins involved in ion-dependent cellular functions and regulation of immune mechanisms. S100A15 was first discovered to be overexpressed in 'koebnerized' psoriatic skin, indicating its involvement in the disease phenotype and the same promising potential as a new therapeutic target. This review describes the involvement of antimicrobial peptides and proteins in inflammatory diseases' development and therapy. The discussion focuses on S100 proteins, especially koebnerisin, which may be involved in the underlying mechanism of the Köebner phenomenon in psoriasis, as well as other immune-mediated inflammatory diseases described in the last decade.

Determination of the Relationships between the Chemical Structure and Antimicrobial Activity of a GAPDH-Related Fish Antimicrobial Peptide and Analogs Thereof

The structure–activity relationships and mode of action of synthesized glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-related antimicrobial peptides were investigated. Including the native skipjack tuna GAPDH-related peptide (SJGAP) of 32 amino acid residues (model for the study), 8 different peptide analogs were designed and synthesized to study the impact of net charge, hydrophobicity, amphipathicity, and secondary structure on both antibacterial and antifungal activities. A net positive charge increase, by the substitution of anionic residues or C-terminal amidation, improved the antimicrobial activity of the SJGAP analogs (minimal inhibitory concentrations of 16–64 μg/mL), whereas the alpha helix content, as determined by circular dichroism, did not have a very definite impact. The hydrophobicity of the peptides was also found to be important, especially for the improvement of antifungal activity. Membrane permeabilization assays showed that the active peptides induced significant cytoplasmic membrane permeabilization in the bacteria and yeast tested, but that this permeabilization did not cause leakage of 260 nm-absorbing intracellular material. This points to a mixed mode of action involving both membrane pore formation and targeting of intracellular components. This study is the first to highlight the links between the physicochemical properties, secondary structure, antimicrobial activity, and mechanism of action of antimicrobial peptides from scombrids or homologous to GAPDH.

Chemical modifications to increase the therapeutic potential of antimicrobial peptides

The continued use of antibiotics has been accompanied by the rapid emergence and spread of antibiotic-resistant strains of bacteria. Antimicrobial peptides (AMPs), also known as host defense peptides, show multiple features as an ideal antimicrobial agent, including potent, rapid, and broad-spectrum antimicrobial activity, low promotion of antimicrobial resistance, potent anti-biofilm activity, and lethality against metabolically inactive microorganisms. However, several crucial drawbacks constrain the use of AMPs as clinical drugs, e.g., liability in vivo, toxicity when used systemically, and high production costs. Based on recent findings and our own experiences, here we summarize some chemical modifications and key design strategies to increase the therapeutic potential of AMPs, including 1) enhancing antimicrobial activities, 2) improving in vivo effectiveness, and 3) reduction in toxicity, which may facilitate the design and optimization of AMPs for the development of drug candidates. We also discuss the present challenges in the optimization of AMPs and future concerns about the resistance and cross-resistance to AMPs in the development of AMPs as therapeutic drugs.

A review emphasizing on utility of heptad repeat sequence as a tool to design pharmacologically safe peptide-based antibiotics

Extensive usage of antibiotics has created an unprecedented scenario of the rapid emergence of many drug-resistant bacteria, which has become an alarming public health concern around the globe. Search for better alternatives that are as efficacious as antibiotics led to the discovery of antimicrobial peptides (AMPs). These small cationic amphiphilic peptides have emerged as a promising option as antimicrobial agents, owing to their multifaceted implications against varied pathogens. Recent years have witnessed tremendous growth in research on AMPs resulting in them being tested in clinical trials of which six got approved for topical application. The relatively less successful outcome has been attributed to the poor cell selectivity shown by most of the naturally occurring AMPs. This drawback needs to be circumvented by identifying strategies to design safe and effective peptides. In the present review, we have emphasized the importance of heptad repeat sequence (leucine and/or phenylalanine zipper motif) as a tool that has shown great promise in remodeling the toxic AMPs to safe antimicrobial agents.

Synthesis, characterisation and cytotoxicity of gold microwires for ultra-sensitive biosensor development

With the long-term goal of developing an ultra-sensitive microcantilever-based biosensor for versatile biomarker detection, new controlled bioreceptor-analytes systems are being explored to overcome the disadvantages of conventional ones. Gold (Au) microwires have been used as a probe to overcome the tolerance problem that occurs in response to changes in environmental conditions. However, the cytotoxicity of Au microwires is still unclear. Here, we examined the cytotoxicity of Au microwires systems using both commercial and as-synthesised Au microwires. In vitro experiments show that commercial Au microwires with an average quoted length of 5.6 µm are highly toxic against Gram-negative Escherichia coli (E. coli) at 50 µg/mL. However, this toxicity is due to the presence of CTAB surfactant not by the microwires. Conversely, the as-synthesised Au microwires show non-cytotoxicity even at the maximum viable concentration (330 µg/mL). These findings may lead to the development of potentially life-saving cytotoxicity-free biosensors for an early diagnostic of potential diseases.

Cyclic peptides bearing the d-Phe-2-Abz turn motif: Structural characterization and antimicrobial potential

The effect on secondary structure and antimicrobial activity of introducing different cyclic constraints in linear β-hairpin antimicrobial peptides has been investigated with the intention of generating cyclic β sheets as promising antimicrobials with improved therapeutic potential. The linear peptides were cyclized head to tail either directly or after the addition of either a second turn motif or a disulfide bridge. The propensity of these peptides to adopt a cyclic β-sheet structure has been correlated to their antibacterial activity. All cyclic peptides showed enhanced activity, compared with their linear counterparts against methicillin-resistant Staphylococcus aureus. Scanning electron microscopy and transmission electron microscopy studies showed that this family kills bacteria through membrane lysis. The peptide that showed the best efficacy against all strains (exhibiting nanomolar activity), while retaining low haemolysis, bears two symmetrical, homochiral d-phe-2-Abz-d-ala turns and adopted a flexible structure. Its twin peptide that bears heterochiral turns (one with d-ala and one with L-Ala) showed reduced antibacterial activity and higher percentage of haemolysis. Circular dichroism and nuclear magnetic resonance spectroscopy indicate that heterochirality in the two turns leads to oligomerization of the peptide at higher concentrations, stabilizing the β-sheet secondary structure. More rigid secondary structure is associated with lower activity against bacteria and loss of selectivity.

Differential Interactions of Piscidins with Phospholipids and Lipopolysaccharides at Membrane Interfaces

Piscidins 1 and 3 (P1 and P3) are potent antimicrobial peptides isolated from striped bass. Their mechanism of action involves formation of amphipathic α-helices on contact with phospholipids and destabilization of the microbial cytoplasmic membrane. The peptides are active against both Gram-positive and Gram-negative bacteria, suggesting easy passage across the outer membrane. Here, we performed a comparative study of these two piscidins at the air-water interface on lipopolysaccharide (LPS) monolayers modeling the outer bacterial surface of Gram-negative organisms and on phospholipid monolayers, which mimic the inner membrane. The results show that P1 and P3 are highly surface active (log K_AW ∼ 6.8) and have similar affinities to phospholipid monolayers (log K_lip ≈ 7.7). P1, which is more potent against Gram negatives, exhibits a much stronger partitioning into LPS monolayers (log K_LPS = 8.3). Pressure-area isotherms indicate that under increasing lateral pressures, inserted P1 repartitions from phospholipid monolayers back to the subphase or to a more shallow position with in-plane areas of ∼170 Ų per peptide, corresponding to fully folded amphipathic α-helices. In contrast, peptide expulsion from LPS occurs with areas of ∼35 Ų, suggesting that the peptides may not form the similarly oriented, rigid secondary structures when they avidly intercalate between LPS molecules. Patch-clamp experiments on Escherichia coli spheroplasts show that when P1 and P3 reach the outer surface of the bacterial cytoplasmic membrane, they produce fluctuating conductive structures at voltages above 80 mV. The data suggests that the strong activity of these piscidins against Gram-negative bacteria begins with the preferential accumulation of peptides in the outer LPS layer followed by penetration into the periplasm, where they form stable amphipathic α-helices upon contact with phospholipids and attack the energized inner membrane.

Peptide/Peptoid Hybrid Oligomers: The Influence of Hydrophobicity and Relative Side-Chain Length on Antibacterial Activity and Cell Selectivity

Previous optimisation studies of peptide/peptoid hybrids typically comprise comparison of structurally related analogues displaying different oligomer length and diverse side chains. The present work concerns a systematically constructed series of 16 closely related 12-mer oligomers with an alternating cationic/hydrophobic design, representing a wide range of hydrophobicity and differences in relative side-chain lengths. The aim was to explore and rationalise the structure-activity relationships within a subclass of oligomers displaying variation of three structural features: (i) cationic side-chain length, (ii) hydrophobic side-chain length, and (iii) type of residue that is of a flexible peptoid nature. Increased side-chain length of cationic residues led to reduced hydrophobicity till the side chains became more extended than the aromatic/hydrophobic side chains, at which point hydrophobicity increased slightly. Evaluation of antibacterial activity revealed that analogues with lowest hydrophobicity exhibited reduced activity against E. coli, while oligomers with the shortest cationic side chains were most potent against P. aeruginosa. Thus, membrane-disruptive interaction with P. aeruginosa appears to be promoted by a hydrophobic surface of the oligomers (comprised of the aromatic groups shielding the cationic side chains). Peptidomimetics with short cationic side chains exhibit increased hemolytic properties as well as give rise to decreased HepG2 (hepatoblastoma G2 cell line) cell viability. An optimal hydrophobicity window could be defined by a threshold of minimal hydrophobicity conferring activity toward E. coli and a threshold for maximal hydrophobicity, beyond which cell selectivity was lost.

An insight into piscidins: The discovery, modulation and bioactivity of greater amberjack, Seriola dumerili, piscidin

Antimicrobial peptides (AMPs) play an important role in the innate immune response of vertebrates by creating a hostile environment for any invading pathogens. Piscidins are potent teleost specific AMPs, which have a broad spectrum activity. We have identified a novel piscidin active peptide, in the greater amberjack, Seriola dumerili, that consists of 25 aa, which forms an amphipathic helix with distinct hydrophobic and positively charged regions. Following homology and phylogenetic analysis the greater amberjack piscidin was deemed to belong to the group 3 family of piscidins. Piscidin was expressed constitutively at immune sites, with transcript level highest in the spleen and gut, at an intermediate level in the gills and lowest in the head kidney. Following in vivo stimulation with PAMPs (poly I:C, LPS and flagellin) piscidin transcript level increased in gills in response to flagellin, in gut and spleen in response to poly I:C, and in head kidney in response to poly I:C, LPS and flagellin. Head kidney and spleen cells were then isolated from greater amberjack and incubated with each of the PAMPs for 4, 12 and 24 h. Piscidin expression was unchanged at 4 and 12 h post PAMP stimulation in head kidney cells but at 24 h transcript level was markedly upregulated compared to control (unstimulated) cells, especially with the bacterial PAMPs. In contrast, spleen cells upregulated piscidin expression by 4 h post stimulation with poly I:C and flagellin, and remained upregulated to 24 h with flagellin exposure, but had returned to baseline levels by 12 h using poly I:C. To determine if piscidin expression could be modulated by diet, greater amberjack were fed diets supplemented with MOS and cMOS for 30 days when transcript level was determined. It was found that MOS supplemented diets increased expression in the spleen, cMOS supplemented diets upregulated transcript levels in the gills and head kidney, whilst a diet containing both MOS and cMOS upregulated transcript in the gut, when compared to fish fed the control diet. Finally, a synthetic greater amberjack piscidin was produced and showed bacteriostatic activity against a number of bacterial strains, including both Gram positive and Gram negative fish pathogens.

Substitution of lysine for isoleucine at the center of the nonpolar face of the antimicrobial peptide, piscidin-1, leads to an increase in the rapidity of bactericidal activity and a reduction in toxicity

Purpose: Piscidin-1 is an effective antimicrobial peptide (AMP) against a variety of microbes. However, its toxicity has been reported as a limitation for its potential therapeutic applications. The toxicity of piscidin-1 may be related to the long nonpolar face of this AMP. Here, we investigated different piscidin-1 analogs to reach a peptide with the reduced toxicity.

Material and methods: In vitro and in vivo antibacterial activity and toxicity of piscidin-1 analogs generated by replacement of isoleucine at the border (I9) or the center (I16) of the nonpolar face of piscidin-1 by alanine or lysine were investigated.

Results: The results indicated that among all peptides, piscidin-1 with the highest HPLC retention time (RT) and I16K-piscidin-1 with the lowest RT had the highest and lowest cytotoxicity, respectively. Although I16K-piscidin-1 possessed the same MIC value as the parent peptide (piscidin-1) and other analogs, I16K-piscidin-1 exhibited a higher rapidity of bactericidal action at 5×MIC. The β-galactosidase leakage and propidium iodide staining assays indicated a higher pore-forming capacity of I16K-piscidin-1 relative to the parent peptide (piscidin-1). Taken together, RT is suggested to have a direct association with the toxicity and an inverse association with the rapidity of bactericidal action and pore-forming capacity. After infection of mice with clinical colistin-resistant Acinetobacter baumannii or clinical methicillin-resistant Staphylococcus aureus strains, treatment with I16K-piscidin-1, but not piscidin-1 and other analogs, resulted in a significantly stronger bactericidal potency. Furthermore, I16K-piscidin-1 exhibited the lowest in vivo toxicity. 

Conclusion: Overall, in vitro and in vivo comparison of piscidin-1 and its analogs together documented that replacement of isoleucine at the center of the nonpolar face of piscidin-1(I16) by lysine leads to not only a decrease in toxicity potential but also an increase in bactericidal potential.

Studies on acid stability and solid-phase block synthesis of peptide-peptoid hybrids: ligands for formyl peptide receptors

α-Peptoids as well as peptide/α-peptoid hybrids and peptide/β-peptoid hybrids constitute major classes of proteolytically stable peptidomimetics that have been extensively investigated as mimetics of biologically active peptides. Representatives of lipidated peptide/β-peptoid hybrids have been identified as promising immunomodulatory lead compounds, and hence access to these via protocols suitable for gram-scale synthesis is warranted to enable animal in vivo studies. Recent observations indicated that several byproducts appear in crude mixtures of relatively short benzyl-based peptide/β-peptoid oligomers, and that these were most predominant when the β-peptoid units displayed an α-chiral benzyl side chain. This prompted an investigation of their stability under acidic conditions. Simultaneous deprotection and cleavage of peptidomimetics containing either α-chiral α- or β-peptoid residues required treatment with strong acid only for a short time to minimize the formation of partially debenzylated byproducts. The initial work on peptide/β-peptoid oligomers with an alternating design established that it was beneficial to form the amide bond between the carboxyl group of the α-amino acid and the congested amino functionality of the β-peptoid residue in solution. To further simplify oligomer assembly on solid phase, we now present a protocol for purification-free solid-phase synthesis of tetrameric building blocks. Next, syntheses of peptidomimetic ligands via manual solid-phase methodologies involving tetrameric building blocks were found to give more readily purified products as compared to those obtained with dimeric building blocks. Moreover, the tetrameric building blocks could be utilized in automated synthesis with microwave-assisted heating, albeit the purity of the crude products was not increased.

Flexibility vs rigidity of amphipathic peptide conjugates when interacting with lipid bilayers

For the first time, the photoisomerization of a diarylethene moiety (DAET) in peptide conjugates was used to probe the effects of molecular rigidity/flexibility on the structure and behavior of model peptides bound to lipid membranes. The DAET unit was incorporated into the backbones of linear peptide-based constructs, connecting two amphipathic sequences (derived from the β-stranded peptide (KIGAKI)₃ and/or the α-helical peptide BP100). A β-strand-DAET-α-helix and an α-helix-DAET-α-helix models were synthesized and studied in phospholipid membranes. Light-induced photoisomerization of the linker allowed the generation of two forms of each conjugate, which differed in the conformational mobility of the junction between the α-helical and/or the β-stranded part of these peptidomimetic molecules. A detailed study of their structural, orientational and conformational behavior, both in isotropic solution and in phospholipid model membranes, was carried out using circular dichroism and solid-state ¹⁹F-NMR spectroscopy. The study showed that the rigid and flexible forms of the two conjugates had appreciably different structures only when embedded in an anisotropic lipid environment and only in the gel phase. The influence of the rigidity/flexibility of the studied conjugates on the lipid thermotropic phase transition was also investigated by differential scanning calorimetry. Both models were found to destabilize the lamellar gel phases.

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DAET building blocks can be used to study rigidity/flexibility effects in supramolecular model systems.

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Photoswitchable DAET linkers perturb only up to 3–4 adjacent amino acid residues.

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Membrane-bound amphiphilic secondary structure elements exert a negligible influence on each other when linked by DAET.

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The rigidity of peptide conjugates affected their structural behavior only in the lipid gel phase.

Advances in Development of Antimicrobial Peptidomimetics as Potential Drugs

The rapid emergence of multidrug-resistant pathogens has evolved into a global health problem as current treatment options are failing for infections caused by pan-resistant bacteria. Hence, novel antibiotics are in high demand, and for this reason antimicrobial peptides (AMPs) have attracted considerable interest, since they often show broad-spectrum activity, fast killing and high cell selectivity. However, the therapeutic potential of natural AMPs is limited by their short plasma half-life. Antimicrobial peptidomimetics mimic the structure and biological activity of AMPs, but display extended stability in the presence of biological matrices. In the present review, focus is on the developments reported in the last decade with respect to their design, synthesis, antimicrobial activity, cytotoxic side effects as well as their potential applications as anti-infective agents. Specifically, only peptidomimetics with a modular structure of residues connected via amide linkages will be discussed. These comprise the classes of α-peptoids (N-alkylated glycine oligomers), β-peptoids (N-alkylated β-alanine oligomers), β³-peptides, α/β³-peptides, α-peptide/β-peptoid hybrids, α/γ N-acylated N-aminoethylpeptides (AApeptides), and oligoacyllysines (OAKs). Such peptidomimetics are of particular interest due to their potent antimicrobial activity, versatile design, and convenient optimization via assembly by standard solid-phase procedures.

Ovalbumin with Glycated Carboxyl Groups Shows Membrane-Damaging Activity

The aim of the present study was to investigate whether glycated ovalbumin (OVA) showed novel activity at the lipid-water interface. Mannosylated OVA (Man-OVA) was prepared by modification of the carboxyl groups with p-aminophenyl α-dextro (d)-mannopyranoside. An increase in the number of modified carboxyl groups increased the membrane-damaging activity of Man-OVA on cell membrane-mimicking vesicles, whereas OVA did not induce membrane permeability in the tested phospholipid vesicles. The glycation of carboxyl groups caused a notable change in the gross conformation of OVA. Moreover, owing to their spatial positions, the Trp residues in Man-OVA were more exposed, unlike those in OVA. Fluorescence quenching studies suggested that the Trp residues in Man-OVA were located on the interface binds with the lipid vesicles, and their microenvironment was abundant in positively charged residues. Although OVA and Man-OVA showed a similar binding affinity for lipid vesicles, the lipid-interacting feature of Man-OVA was distinct from that of OVA. Chemical modification studies revealed that Lys and Arg residues, but not Trp residues, played a crucial role in the membrane-damaging activity of Man-OVA. Taken together, our data suggest that glycation of carboxyl groups causes changes in the structural properties and membrane-interacting features of OVA, generating OVA with membrane-perturbing activities at the lipid-water interface.

LL-37-derived membrane-active FK-13 analogs possessing cell selectivity, anti-biofilm activity and synergy with chloramphenicol and anti-inflammatory activity

Although the human-derived antimicrobial peptide (AMP) LL-37 has potent antimicrobial and anti-inflammatory activities, its therapeutic application is limited by its low cell selectivity and high production cost due to its large size. To overcome these problems, we tried to develop novel LL-37-derived short α-helical AMPs with improved cell selectivity and without a significant loss of anti-inflammatory activity relative to that of parental LL-37. Using amino acid substitution, we designed and synthesized a series of FK13 analogs based on the sequence of the 13-meric short FK13 peptide (residues 17-29 of LL-37) that has been identified as the region responsible for the antimicrobial activity of LL-37. Among the designed FK13 analogs, FK-13-a1 and FK-13-a7 showed high cell selectivity and retained the anti-inflammatory activity. The therapeutic index (a measure of cell selectivity) of FK-13-a1 and FK-13-a7 was 6.3- and 2.3-fold that of parental LL-37, respectively. Furthermore, FK-13-a1 and FK-13-a7 displayed more potent antimicrobial activity against antibiotic-resistant bacteria including MRSA, MDRPA, and VREF, than did LL-37. In addition, FK-13-a1 and FK-13-a7 exhibited greater synergistic effects with chloramphenicol against MRSA and MDRPA and were more effective anti-biofilm agents against MDRPA than LL-37 was. Moreover, FK-13-a1 and FK-13-a7 maintained their activities in the presence of physiological salts and human serum. SYTOX green uptake, membrane depolarization and killing kinetics revealed that FK13-a1 and FK13-a7 kills microbial cells by permeabilizing the cell membrane and damaging membrane integrity. Taken together, our results suggest that FK13-a1 and FK13-a7 can be developed as novel antimicrobial/anti-inflammatory agents.

Building a Better Quaternary Ammonium Compound (QAC): Branched Tetracationic Antiseptic Amphiphiles

Bacteria contaminate surfaces in a wide variety of environments, causing severe problems across a number of industries. In a continuation of our campaign to develop novel antibacterial quaternary ammonium compounds (QACs) as useful antiseptics, we have identified a starting material bearing four tertiary amines, enabling the rapid synthesis of several tris- and tetracationic QACs. Herein we report the synthesis and biological activity of a series of 24 multiQACs deemed the "superT" family, and an investigation of the role of cationic charge in antimicrobial and anti-biofilm activity, as well as toxicity. This class represents the most potent series of QACs reported to date against methicillin-resistant Staphylococcus aureus (MRSA), with minimum inhibitory concentrations (MICs) and minimum biofilm eradication concentrations (MBECs) as low as 0.25 and 25 μm, respectively. Based on the significant cell-surface-charge differences between bacterial and eukaryotic cells, in certain cases we observed excellent efficacy-to-toxicity profiles, exceeding a 100-fold differential. This work further elucidates the chemical underpinnings of disinfectant efficacy versus toxicity based on cationic charge.

Single Amino Acid Substitutions at Specific Positions of the Heptad Repeat Sequence of Piscidin-1 Yielded Novel Analogs That Show Low Cytotoxicity and In Vitro and In Vivo Antiendotoxin Activity

Piscidin-1 possesses significant antimicrobial and cytotoxic activities. To recognize the primary amino acid sequence(s) in piscidin-1 that could be important for its biological activity, a long heptad repeat sequence located in the region from amino acids 2 to 19 was identified. To comprehend the possible role of this motif, six analogs of piscidin-1 were designed by selectively replacing a single isoleucine residue at a d (5th) position or at an a (9th or 16th) position with either an alanine or a valine residue. Two more analogs, namely, I5F,F6A-piscidin-1 and V12I-piscidin-1, were designed for investigating the effect of interchanging an alanine residue at a d position with an adjacent phenylalanine residue and replacing a valine residue with an isoleucine residue at another d position of the heptad repeat of piscidin-1, respectively. Single alanine-substituted analogs exhibited significantly reduced cytotoxicity against mammalian cells compared with that of piscidin-1 but appreciably retained the antibacterial and antiendotoxin activities of piscidin-1. All the single valine-substituted piscidin-1 analogs and I5F,F6A-piscidin-1 showed cytotoxicity greater than that of the corresponding alanine-substituted analogs, antibacterial activity marginally greater than or similar to that of the corresponding alanine-substituted analogs, and also antiendotoxin activity superior to that of the corresponding alanine-substituted analogs. Interestingly, among these peptides, V12I-piscidin-1 showed the highest cytotoxicity and antibacterial and antiendotoxin activities. Lipopolysaccharide (12 mg/kg of body weight)-treated mice, further treated with I16A-piscidin-1, the piscidin-1 analog with the highest therapeutic index, at a single dose of 1 or 2 mg/kg of body weight, showed 80 and 100% survival, respectively. Structural and functional characterization of these peptides revealed the basis of their biological activity and demonstrated that nontoxic piscidin-1 analogs with significant antimicrobial and antiendotoxin activities can be designed by incorporating single alanine substitutions in the piscidin-1 heptad repeat.

Peptides and Peptidomimetics for Antimicrobial Drug Design

The purpose of this paper is to introduce and highlight a few classes of traditional antimicrobial peptides with a focus on structure-activity relationship studies. After first dissecting the important physiochemical properties that influence the antimicrobial and toxic properties of antimicrobial peptides, the contributions of individual amino acids with respect to the peptides antibacterial properties are presented. A brief discussion of the mechanisms of action of different antimicrobials as well as the development of bacterial resistance towards antimicrobial peptides follows. Finally, current efforts on novel design strategies and peptidomimetics are introduced to illustrate the importance of antimicrobial peptide research in the development of future antibiotics.

Functional Roles of Aromatic Residues and Helices of Papiliocin in its Antimicrobial and Anti-inflammatory Activities

A cecropin-like peptide, papiliocin, isolated from the swallowtail butterfly Papilio xuthus, possesses high selectivity against gram-negative bacteria. Since Trp(2) and Phe(5) are highly conserved residues in cecropin-like peptides, we investigated the role of Trp(2) and Phe(5) in antibacterial activity. Substitution of Trp(2) and Phe(5) in papiliocin with Ala (papiliocin-2A and papiliocin-5A) revealed that Trp(2) is a key residue in its antibacterial activities. In order to understand the structural requirements for papiliocin function and to design shorter, but more potent, peptide antibiotics, we designed papiliocin constructs, PapN (residues Arg(1)-Ala(22) from the N-terminal amphipathic helix). PapN exhibited significant broad-spectrum antibacterial activities without cytotoxicity. Bactericidal kinetics of peptides against E.coli showed that papiliocin completely and rapidly killed E.coli in less than 10 minutes at 2× MIC concentration, while papiliocin-2A and papiliocin-5A killed four times more slowly than papiliocin. The PapN series peptides permeabilized bacterial membranes less effectively than papiliocin, showing no antibacterial activities in an hour. The results imply that the Trp(2) and Phe(5) in the amphipathic N-terminal helix are important in the rapid permeabilization of the gram-negative bacterial membrane. The hydrophobic C-terminal residues permeabilize the hydrophobic bacterial cell membrane synergistically with these aromatic residues, providing selectivity against gram-negative bacteria.

Peptoid-Substituted Hybrid Antimicrobial Peptide Derived from Papiliocin and Magainin 2 with Enhanced Bacterial Selectivity and Anti-inflammatory Activity

Antimicrobial peptides (AMPs) are important components of the host innate immune system. Papiliocin is a 37-residue AMP purified from larvae of the swallowtail butterfly Papilio xuthus. Magainin 2 is a 23-residue AMP purified from the skin of the African clawed frog Xenopus laevis. We designed an 18-residue hybrid peptide (PapMA) incorporating N-terminal residues 1-8 of papiliocin and N-terminal residues 4-12 of magainin 2, joined by a proline (Pro) hinge. PapMA showed high antimicrobial activity but was cytotoxic to mammalian cells. To decrease PapMA cytotoxicity, we designed a lysine (Lys) peptoid analogue, PapMA-k, which retained high antimicrobial activity but displayed cytotoxicity lower than that of PapMA. Fluorescent dye leakage experiments and confocal microscopy showed that PapMA targeted bacterial cell membranes whereas PapMA-k penetrated bacterial cell membranes. Nuclear magnetic resonance experiments revealed that PapMA contained an N-terminal α-helix from Lys(3) to Lys(7) and a C-terminal α-helix from Lys(10) to Lys(17), with a Pro(9) hinge between them. PapMA-k also had two α-helical structures in the same region connected with a flexible hinge residue at Nlys(9), which existed in a dynamic equilibrium of cis and trans conformers. Using lipopolysaccharide-stimulated RAW264.7 macrophages, the anti-inflammatory activity of PapMA and PapMA-k was confirmed by inhibition of nitric oxide and inflammatory cytokine production. In addition, treatment with PapMA and PapMA-k decreased the level of ultraviolet irradiation-induced expression of genes encoding matrix metalloproteinase-1 (MMP-1), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in human keratinocyte HaCaT cells. Thus, PapMA and PapMA-k are potent peptide antibiotics with antimicrobial and anti-inflammatory activity, with PapMA-k displaying enhanced bacterial selectivity.

Membrane lipid therapy: Modulation of the cell membrane composition and structure as a molecular base for drug discovery and new disease treatment

Nowadays we understand cell membranes not as a simple double lipid layer but as a collection of complex and dynamic protein-lipid structures and microdomains that serve as functional platforms for interacting signaling lipids and proteins. Membrane lipids and lipid structures participate directly as messengers or regulators of signal transduction. In addition, protein-lipid interactions participate in the localization of signaling protein partners to specific membrane microdomains. Thus, lipid alterations change cell signaling that are associated with a variety of diseases including cancer, obesity, neurodegenerative disorders, cardiovascular pathologies, etc. This article reviews the newly emerging field of membrane lipid therapy which involves the pharmacological regulation of membrane lipid composition and structure for the treatment of diseases. Membrane lipid therapy proposes the use of new molecules specifically designed to modify membrane lipid structures and microdomains as pharmaceutical disease-modifying agents by reversing the malfunction or altering the expression of disease-specific protein or lipid signal cascades. Here, we provide an in-depth analysis of this emerging field, especially its molecular bases and its relevance to the development of innovative therapeutic approaches.

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.

Antimicrobial peptides from fish

Antimicrobial peptides (AMPs) are found widely distributed through Nature, and participate in the innate host defense of each species. Fish are a great source of these peptides, as they express all of the major classes of AMPs, including defensins, cathelicidins, hepcidins, histone-derived peptides, and a fish-specific class of the cecropin family, called piscidins. As with other species, the fish peptides exhibit broad-spectrum antimicrobial activity, killing both fish and human pathogens. They are also immunomodulatory, and their genes are highly responsive to microbes and innate immuno-stimulatory molecules. Recent research has demonstrated that some of the unique properties of fish peptides, including their ability to act even in very high salt concentrations, make them good potential targets for development as therapeutic antimicrobials. Further, the stimulation of their gene expression by exogenous factors could be useful in preventing pathogenic microbes in aquaculture.

The effect of an adding histidine on biological activity and stability of Pc-pis from Pseudosciaena crocea

Pc-pis is a novel piscidin-like antimicrobial polypeptide that was identified in Pseudosciaena crocea. Although active against most bacteria tested, Pc-pis was inactive against Aeromonas hydrophila and Pseudomonas aeruginosa. The Pc-pis analogue Pc-pis-His was designed by adding a histidine residue at the carboxyl terminal. Pc-pis-His demonstrated a more broad-spectrum and stronger antimicrobial activity against a representative set of microorganisms and more potent antiparasitic activity against Cryptocaryon irritans trophonts than Pc-pis. The stability assay revealed that Pc-pis-His was active against Staphylococcus aureus not only in acidic (pH 5.5-7.3) and relatively low concentration monovalent cation (0-160 mM NaCl) environments but also in alkaline (pH 7.5-9.5), divalent cation (1.25-160 mM MgCl2 and 1.25-40 mM CaCl2) and high concentration monovalent cation (320-2560 mM NaCl) environments, which indicates that the added histidine residue conferred better salt-, acid- and alkali-tolerance to Pc-pis-His. Pc-pis-His also possessed the desired heat-tolerance, which was reflected by the antimicrobial activity of the peptide after being boiled for 10-60 minutes. Hemolytic activity analysis revealed that Pc-pis-His at concentrations up to 6 µM exhibited no hemolysis against human erythrocytes, with 6 µM being a concentration that is highly active against most of the microorganisms tested, although the hemolytic activity of Pc-pis-His was enhanced compared to Pc-pis. These results provide a unique, reasonable basis for designing novel piscidins with potent, broad-spectrum and stable antimicrobial activity and new insight into the future development of piscidins as potential therapeutic agents against microbial and external protozoan parasite infections.

Short KR-12 analogs designed from human cathelicidin LL-37 possessing both antimicrobial and antiendotoxic activities without mammalian cell toxicity

KR-12 (residues 18-29 of LL-37) was known to be the smallest peptide of human cathelicidin LL-37 possessing antimicrobial activity. In order to optimize α-helical short antimicrobial peptides having both antimicrobial and antiendotoxic activities without mammalian cell toxicity, we designed and synthesized a series of KR-12 analogs. Highest hydrophobic analogs KR-12-a5 and KR-12-a6 displayed greater inhibition of lipopolysaccharide (LPS)-stimulated tumor necrosis factor-α production and higher LPS-binding activity. We have observed that antimicrobial activity is independent of charge, but LPS neutralization requires a balance of hydrophobicity and net positive charge. Among KR-12 analogs, KR-12-a2, KR-12-a3 and KR-12-a4 showed much higher cell specificity for bacteria over erythrocytes and retained antiendotoxic activity, relative to parental LL-37. KR-12-a5 displayed the strongest antiendotoxic activity but almost similar cell specificity as compared with LL-37. Also, these KR-12 analogs (KR-12-a2, KR-12-a3, KR-12-a4 and KR-12-a5) exhibited potent antimicrobial activity (minimal inhibitory concentration: 4 μM) against methicillin-resistant Staphylococcus aureus. Taken together, these KR-12 analogs have the potential for future development as a novel class of antimicrobial and anti-inflammatory therapeutic agents.

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

AIMS

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

METHODS AND RESULTS

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

CONCLUSIONS

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

SIGNIFICANCE AND IMPACT OF THE STUDY

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

Conformational flexibility determines selectivity and antibacterial, antiplasmodial, and anticancer potency of cationic α-helical peptides

We used a combination of fluorescence, circular dichroism (CD), and NMR spectroscopies in conjunction with size exclusion chromatography to help rationalize the relative antibacterial, antiplasmodial, and cytotoxic activities of a series of proline-free and proline-containing model antimicrobial peptides (AMPs) in terms of their structural properties. When compared with proline-free analogs, proline-containing peptides had greater activity against Gram-negative bacteria, two mammalian cancer cell lines, and intraerythrocytic Plasmodium falciparum, which they were capable of killing without causing hemolysis. In contrast, incorporation of proline did not have a consistent effect on peptide activity against Mycobacterium tuberculosis. In membrane-mimicking environments, structures with high α-helix content were adopted by both proline-free and proline-containing peptides. In solution, AMPs generally adopted disordered structures unless their sequences comprised more hydrophobic amino acids or until coordinating phosphate ions were added. Proline-containing peptides resisted ordering induced by either method. The roles of the angle subtended by positively charged amino acids and the positioning of the proline residues were also investigated. Careful positioning of proline residues in AMP sequences is required to enable the peptide to resist ordering and maintain optimal antibacterial activity, whereas varying the angle subtended by positively charged amino acids can attenuate hemolytic potential albeit with a modest reduction in potency. Maintaining conformational flexibility improves AMP potency and selectivity toward bacterial, plasmodial, and cancerous cells while enabling the targeting of intracellular pathogens.

Prokaryotic selectivity and LPS-neutralizing activity of short antimicrobial peptides designed from the human antimicrobial peptide LL-37

To develop novel antimicrobial peptides (AMPs) with shorter lengths, improved prokaryotic selectivity and retained lipolysaccharide (LPS)-neutralizing activity compared to human cathelicidin AMP, LL-37, a series of amino acid-substituted analogs based on IG-19 (residues 13-31 of LL-37) were synthesized. Among the IG-19 analogs, the analog a4 showed the highest prokaryotic selectivity, but much lower LPS-neutralizing activity compared to parental LL-37. The analogs, a5, a6, a7 and a8 with higher hydrophobicity displayed LPS-neutralizing activity comparable to that of LL-37, but much lesser prokaryotic selectivity. These results indicate that the proper hydrophobicity of the peptides is crucial to exert the amalgamated property of LPS-neutralizing activity and prokaryotic selectivity. Furthermore, to increase LPS-neutralizing activity of the analog a4 without a remarkable decrease in prokaryotic selectivity, we synthesized Trp-substituted analogs (a4-W1 and a4-W2), in which Phe(5) or Phe(15) of a4 is replaced by Trp. Despite their same prokaryotic selectivity, a4-W2 displayed much higher LPS-neutralizing activity compared to a4-W1. When compared with parental LL-37, a4-W2 showed retained LPS-neutralizing activity and 2.8-fold enhanced prokaryotic selectivity. These results suggest that the effective site for Trp-substitution when designing novel AMPs with higher LPS-neutralizing activity, without a remarkable reduction in prokaryotic selectivity, is the amphipathic interface between the end of the hydrophilic side and the start of the hydrophobic side rather than the central position of the hydrophobic side in their α-helical wheel projection. Taken together, the analog a4-W2 can serve as a promising template for the development of therapeutic agents for the treatment of endotoxic shock and bacterial infection.