Ultrashort cationic lipopeptides and lipopeptoids selectively induce cytokine production in macrophages

Structure-activity study of oncocin: On-resin guanidinylation and incorporation of homoarginine, 4-hydroxyproline or 4,4-difluoroproline residues

Antimicrobial peptides (AMPs) often display guanidinium functionalities, and hence robust synthetic procedures are needed to facilitate access to analogues with unnatural homologues of arginine (Arg = R). Initially, a resin-bound Arg/Pro-rich fluoren-9-yl-methyloxycarbonyl-protected fragment (Fmoc-RPRPPR) of the AMP oncocin (i.e., VDKPPYLPRPRPPRRIYNR-NH2) was employed in a comparative on-resin assessment of commercial guanidinylation reagents head-to-head with the recently studied bis-Boc-protected triazole-based reagent, 1H-triazole-1-[N,N'-bis(tert-butoxycarbonyl)]-carboxamidine, which was synthesized by a chromatography-free procedure. This reagent was found to enable quantitative conversion in solid-phase peptide synthesis (SPPS) of peptides displaying homoarginine (Har) residues and/or an N-terminal guanidinium group. SPPS was used to obtain analogues of the 18-mer oncocin with single as well as multiple Arg → Har modifications. In addition, the effect of replacement of proline (Pro) residues in oncocin was explored by incorporating single or multiple trans-4-hydroxy-l-proline (Hyp) or 4,4-difluoro-l-proline (Dfp) residues, which both affected hydrophobicity. The resulting peptide library was tested against both Gram-negative and Gram-positive bacteria. Analysis of the minimal inhibitory concentrations (MICs) showed that analogues, displaying modifications at positions 4, 5 and 12 (originally Pro residues), had retained or slightly improved antimicrobial activity. Next, an oncocin analogue with two stabilizing l-Arg → d-Arg replacements in the C-terminal part was further modified by triple-replacement of Pro by either Dfp or Hyp in positions 4, 5, and 12. The resulting analogue displaying three Pro → Dfp modifications proved to possess the best activity profile: MICs of 1-2 µg/mL against E. coli and Klebsiella pneumoniae, less than 1% hemolysis at 800 µg/mL, and an IC50 above 1280 µg/mL in HepG2 cells. Thus, incorporation of bis-fluorinated Pro residues appears to constitute a novel tool in structure-activity studies aimed at optimization of Pro-rich AMPs.

Surface-Active Compounds Produced by Microorganisms: Promising Molecules for the Development of Antimicrobial, Anti-Inflammatory, and Healing Agents

Surface-active compounds (SACs), biomolecules produced by bacteria, yeasts, and filamentous fungi, have interesting properties, such as the ability to interact with surfaces as well as hydrophobic or hydrophilic interfaces. Because of their advantages over other compounds, such as biodegradability, low toxicity, antimicrobial, and healing properties, SACs are attractive targets for research in various applications in medicine. As a result, a growing number of properties related to SAC production have been the subject of scientific research during the past decade, searching for potential future applications in biomedical, pharmaceutical, and therapeutic fields. This review aims to provide a comprehensive understanding of the potential of biosurfactants and emulsifiers as antimicrobials, modulators of virulence factors, anticancer agents, and wound healing agents in the field of biotechnology and biomedicine, to meet the increasing demand for safer medical and pharmacological therapies.

Effect of Disulfide Cyclization of Ultrashort Cationic Lipopeptides on Antimicrobial Activity and Cytotoxicity

Ultrashort cationic lipopeptides (USCLs) are considered to be a promising class of antimicrobials with high activity against a broad-spectrum of microorganisms. However, the majority of these compounds are characterized by significant toxicity toward human cells, which hinders their potential application. To overcome those limitations, several approaches have been advanced. One of these is disulfide cyclization that has been shown to improve drug-like characteristics of peptides. In this article the effect of disulfide cyclization of the polar head of N-palmitoylated USCLs on in vitro biological activity has been studied. Lipopeptides used in this study consisted of three or four basic amino acids (lysine and arginine) and cystine in a cyclic peptide. In general, disulfide cyclization of the lipopeptides resulted in peptides with reduced cytotoxicity. Disulfide-cyclized USCLs exhibited improved selectivity between Candida sp., Gram-positive strains and normal cells in contrast to their linear counterparts. Interactions between selected USCLs and membranes were studied by molecular dynamics simulations using a coarse-grained force field. Moreover, membrane permeabilization properties and kinetics were examined. Fluorescence and transmission electron microscopy revealed damage to Candida cell membrane and organelles. Concluding, USCLs are strong membrane disruptors and disulfide cyclization of polar head can have a beneficial effect on its in vitro selectivity between Candida sp. and normal human cells.

Antimicrobial peptides: Promising alternatives in the post feeding antibiotic era

Antimicrobial peptides (AMPs), critical components of the innate immune system, are widely distributed throughout the animal and plant kingdoms. They can protect against a broad array of infection-causing agents, such as bacteria, fungi, parasites, viruses, and tumor cells, and also exhibit immunomodulatory activity. AMPs exert antimicrobial activities primarily through mechanisms involving membrane disruption, so they have a lower likelihood of inducing drug resistance. Extensive studies on the structure-activity relationship have revealed that net charge, hydrophobicity, and amphipathicity are the most important physicochemical and structural determinants endowing AMPs with antimicrobial potency and cell selectivity. This review summarizes the recent advances in AMPs development with respect to characteristics, structure-activity relationships, functions, antimicrobial mechanisms, expression regulation, and applications in food, medicine, and animals.

Ultrashort cationic lipopeptides and lipopeptoids: Evaluation and mechanistic insights against epithelial cancer cells

Peptides present an attractive scaffold for the development of new anticancer lead agents due to their accessibility and ease of modification. Synthetic ultrashort cationic lipopeptides, with four amino acids or less conjugated to a fatty acid, were developed to retain the biological activity of longer peptides in a smaller molecular size. Herein, we report the activity of amphiphilic lipotripeptides, lipotripeptoids and lipotetrapeptides against breast (MDA-MB-231, JIMT-1), prostate (DU145) and pancreas (MiaPaCa2) epithelial cancer cell lines. The lipotripeptide C16-KKK-NH2 and lipotetrapeptide C16-PCatPHexPHexPCat-NH2 were identified to possess anticancer activity. The latter lipotetrapeptide possess a short polyproline scaffold consisting of only two L-4R-aminoproline (PCat) and two L-4R-hexyloxyproline (PHex). However, all the prepared lipotripeptoids lack anticancer activity. The amphiphilic C16-PCatPHexPHexPCat-NH2 exhibited similar anticancer potency to the surfactant benzethonium chloride while superior activity was observed in comparison to myristylamine. Mechanistic studies revealed that the peptides do not lyse ovine erythrocytes nor epithelial cancer cells, thus ruling out necrosis as the mechanism of cell death. Surprisingly, the two lipopeptides exhibit different mechanisms of action that result in cancer cell death. The lipotripeptide C16-KKK-NH2 was found to induce caspase-mediated apoptosis while C16-PCatPHexPHexPCat-NH2 kills tumor cells independent of caspases.

Exploring architectures at the nanoscale: the interplay between hydrophobic twin lipid chains and head groups of designer peptide amphiphiles in the self-assembly process and application

The self-assembly of peptide amphiphiles (PAs) is found to be governed by the hydrophobic interactions induced by the hydrophobic groups/number of alkyl chains and the hydrophilic head groups. In this study, an assessment of the nanostructures formed by the self-assembly of simple twin chained PAs was carried out and compared to their single chain/short analogues. The spectroscopic and microscopic analysis revealed the fact that the twin chained amphiphiles had a high inclination to form β-sheet nanofibers and further towards hydrogelation. The mixture of twin chained PAs also exhibited cooperative self-assembly with improved aggregation behavior, although not much augmentation in β-type structuring was found. In contrast, the single chain/short analogue containing PAs showed very less of β-sheet type structures to a lesser extent and no hydrogelating behavior but resulted in mostly random conformations. The increase in the number or alteration of polar head groups in double chained PAs induced higher extent of β-type conformation and better gelling capability due to the combined hydrophobic effect of the twin chains. The overall results delineated the dominance of hydrophobic interactions. Finally, calcium phosphate bio-mineralization was done in the hydrogels of twin chained PAs with the aim of developing future biomaterials.

Surviving sepsis in the era of antibiotic resistance: are there any alternative approaches to antibiotic therapy?

Sepsis, a serious cause of morbidity in humans, has no proper single medication dedicated to it. In this review, we look at the current treatment modalities, the different approaches attempted towards treating it and alternative approaches that could be implemented to counter this neglected disease condition. The use of antibiotics towards treatment of sepsis, use of combinations and strategies derived from natural antimicrobial peptides have been dealt in detail. The social and technical difficulties associated with treating sepsis and the possible ways of combating them have also been discussed.

Lipidated α-peptide/β-peptoid hybrids with potent anti-inflammatory activity

In this study, we investigated, optimized, and characterized a novel subclass of host defense peptide (HDP) mimics based on α-peptide/β-peptoid hybrid oligomers with an alternating cationic/hydrophobic design with respect to their ability to modulate the pro-inflammatory response by human primary leukocytes upon exposure to bacterial components. Structure-activity studies revealed that certain lipidated α-peptide/β-peptoid hybrid oligomers possess anti-inflammatory activities in the submicromolar range with low cytotoxicity, and that the anti-inflammatory activity of the HDP mimics is dependent on the length and position of the lipid element(s). The resulting lead compound, Pam-(Lys-βNSpe)6-NH2, blocks LPS-induced cytokine secretion with a potency comparable to that of polymyxin B. The mode of action of this HDP mimic appears not to involve direct LPS interaction since it, in contrast to polymyxin B, displayed only minor activity in the Limulus amebocyte lysate assay. Flow cytometry data showed specific interaction of a fluorophore-labeled lipidated α-peptide/β-peptoid hybrid with monocytes and granulocytes indicating a cellular target expressed by these leukocyte subsets.

Structure-activity relationships in ultrashort cationic lipopeptides: the effects of amino acid ring constraint on antibacterial activity

Taking a minimalistic approach in efforts to lower the cost for the development of new synthetic antimicrobial peptides, ultrashort cationic lipopeptides were designed to mimic the amphiphilic nature crucial for their activity but with only a very short peptide sequence ligated to a lipidic acid. Nine ultrashort cationic lipopeptides were prepared to study the effects of ring constraint in the amino acid side chain of the peptide component. USCL-PCat1, consisting of only four L-4R-aminoproline residues and acylated with palmitic acid at the N-terminus, was found to populate a polyproline II helical secondary conformation that is stable to different pHs and temperatures using circular dichroism. The synthesized lipopeptides were found to have a micellar structure in water using negative staining transmission electron microscopy. We found that constraining the side chain of the amino acid component is not beneficial to the antimicrobial activity. USCL-Dab1, USCL-Dab3 and USCL-K1 showed promising activity against a panel of laboratory reference and clinically isolated Gram-positive and Gram-negative bacterial strains, some of which are multidrug resistant. No appreciable cytotoxicity against human monocytic THP-1 cells was observed up to concentrations of 20-40 µM for all synthesized compounds. Moreover, all USCLs did not induce the production of either pro-inflammatory cytokines or chemokines up to 40 µM.

The self-assembly of a cyclic lipopeptides mixture secreted by a B. megaterium strain and its implications on activity against a sensitive Bacillus species

Cyclic lipopeptides are produced by a soil Bacillus megaterium strain and several other Bacillus species. In this work, they are detected both in the Bacillus intact cells and the cells culture medium by MALDI-TOF mass spectrometry. The cyclic lipopeptides self-assemble in water media producing negatively charged and large aggregates (300–800 nm of mean hydrodynamic radius) as evaluated by dynamic light scattering and zeta-potential analysis. The aggregate size depends on pH and ionic strength. However, it is not affected by changes in the osmolarity of the outer medium suggesting the absence of an internal aqueous compartment despite the occurrence of low molecular weight phospholipids in their composition as determined from inorganic phosphorus analysis. The activity against a sensitive Bacillus cereus strain was evaluated from inhibition halos and B. cereus lysis. Essential features determining the antibiotic activity on susceptible Bacillus cereus cells are the preserved cyclic moiety conferring cyclic lipopeptides resistance to proteases and the medium pH. The aggregates are inactive per se at the pH of the culture medium which is around 6 or below. The knock out of the sensitive cells only takes place when the aggregates are disassembled due to a high negative charge at pH above 6.