Investigating the effects of positive charge and hydrophobicity on the cell selectivity, mechanism of action and anti-inflammatory activity of a Trp-rich antimicrobial peptide indolicidin

Anti-inflammatory peptide therapeutics and the role of sulphur containing amino acids (cysteine and methionine) in inflammation suppression: A review

BACKGROUND

Inflammation serves as our body's immune response to combat infections, pathogens, viruses, and external stimuli. Inflammation can be classified into two types: acute inflammation and chronic inflammation. Non-steroidal anti-inflammatory medications (NSAIDs) are used to treat both acute and chronic inflammatory disorders. However, these treatments have various side effects such as reduced healing efficiency, peptic ulcers, gastrointestinal toxicities, etc. METHOD: This review assesses the potential of anti-inflammatory peptides (AIPs) derived from various natural sources, such as algae, fungi, plants, animals, and marine organisms. Focusing on peptides rich in cysteines and methionine, sulphur-containing amino acids known for their role in suppression of inflammation.

RESULT

Due to their varied biological activity, ability to penetrate cells, and low cytotoxicity, bioactive peptides have garnered interest as possible therapeutic agents. The utilisation of AIPs has shown great potential in the treatment of disorders associated with inflammation. AIPs can be obtained from diverse natural sources such as algae, fungi, plants, and animals. Cysteine and methionine are sulphur-containing amino acids that aid in the elimination of free radicals, hence assisting in the treatment of inflammatory diseases.

CONCLUSION

This review specifically examines several sources of AIPs including peptides that contain numerous cysteines and methionine. In addition, the biological characteristics of these amino acids and advancements in peptide delivery are also discussed.

The potential of glycinin basic peptide derived from soybean as a promising candidate for the natural food additive and preservative: A review

Glycinin basic peptide (GBP) is the basic polypeptide of soybean glycinin that is isolated using cheap and readily available raw materials (soybean meals). GBP can bear high-temperature processing and has good functional properties, such as emulsification and adhesion properties et al. GBP exhibits broad-spectrum antimicrobial activities against Gram-positive and Gram-negative bacteria as well as fungi. Beyond that, GBP shows enormous application potential to improve the quality and extend the shelf life of food products. This review will systematically provide information on the purification, physicochemical and functional properties of GBP. Moreover, the antimicrobial activities and multi-target antimicrobial mechanism of GBP as well as the applications of GBP in different food products are also reviewed and discussed in detail. This review aims to offer valuable insights for the applications of GBP in the food industry as a promising natural food additive and preservative.

KDEON WK-11: A short antipseudomonal peptide with promising potential

The plight of antimicrobial resistance continues to limit the availability of antibiotic treatment effective in combating resistant bacterial infections. Despite efforts made to rectify this issue and minimise its effects on both patients and the wider community, progress in this area remains minimal. Here, we de-novo designed a peptide named KDEON WK-11, building on previous work establishing effective residues and structures active in distinguished antimicrobial peptides such as lactoferrin. We assessed its antimicrobial activity against an array of bacterial strains and identified its most potent effect, against Pseudomonas aeruginosa with an MIC value of 3.12 μM, lower than its counterparts developed with similar residues and chain lengths. We then determined its anti-biofilm properties, potential mechanism of action and in vitro cytotoxicity. We identified that KDEON WK-11 had a broad range of antimicrobial activity and specific capabilities to fight Pseudomonas aeruginosa with low in vitro cytotoxicity and promising potential to express anti-lipopolysaccharide qualities, which could be exploited to expand its properties into an anti-sepsis agent.

Anti-inflammatory effects of tripeptide WLS on TNF-α-induced HT-29 cells and DSS-induced colitis in mice

Inflammatory bowel disease is a chronic disease of the intestinal tract, which is related to increased levels of various inflammatory mediators. This study aims to explore the anti-inflammatory mechanism of small molecular peptide WLS and its alleviating effect on inflammatory bowel disease (IBD). In TNF-α-induced HT-29 cells, WLS inhibited IL-8 secretion, decreased gene expression of pro-inflammatory cytokines IL-8, IL-6, IL-1β, and TNF-α, and inhibited the activation of MAPK/NF-κB signaling pathways. In the dextran sulfate sodium salt (DSS) induced colitis mouse model, WLS inhibited weight loss and disease activity index scores, increased colon length, improved colon histopathology, inhibited secretion of IL-6 and TNF-α in the colon, and down-regulated gene expression of pro-inflammatory cytokines (IL-6, TNF-α, IL-1β, IFN-γ, IL-17A). This study revealed that WLS was a novel small molecule peptide with anti-inflammatory activity and may be a potential candidate for the treatment of inflammatory bowel disease.

Production, bioactivities and bioavailability of bioactive peptides derived from walnut origin by-products: a review

Walnut-origin by-products obtained from walnut oil extraction industry are high in proteins with various physiological functions and pharmacological properties and an extensive potential for usage in producing bioactive peptides. This review presents the current research status of bioactive peptides derived from walnut by-products, including preparation, separation, purification, identification, bioactivities, and bioavailability. A plethora of walnut peptides with multiple biological activities, including antioxidative, antihypertensive, neuroprotective, antidiabetic, anticancer, and antihyperuricemia activities, were obtained from walnut-origin by-products by enzymatic hydrolysis, fermentation, and synthesis. Different bioactive peptides show various structural characteristics and amino acid composition due to their diverse mechanism of action. Furthermore, walnut protein and its hydrolysate present a high bioavailability in human gastrointestinal digestive system. Improving the bioavailability of walnut peptides is needful in the development of walnut industry. However, future research still needs to exploit energy conservation, high efficiency, environmentally friendly and low-cost production method of walnut bioactive peptide. The molecular mechanisms of different bioactive walnut peptides still need to be explored at the cell and gene levels. Additionally, the digestion, absorption, and metabolism processes of walnut peptides are also the focus of future research.

A novel hybrid peptide composed of LfcinB6 and KR-12-a4 with enhanced antimicrobial, anti-inflammatory and anti-biofilm activities

Hybridizing two known antimicrobial peptides (AMPs) is a simple and effective strategy for designing antimicrobial agents with enhanced cell selectivity against bacterial cells. Here, we generated a hybrid peptide Lf-KR in which LfcinB6 and KR-12-a4 were linked with a Pro hinge to obtain a novel AMP with potent antimicrobial, anti-inflammatory, and anti-biofilm activities. Lf-KR exerted superior cell selectivity for bacterial cells over sheep red blood cells. Lf-KR showed broad-spectrum antimicrobial activities (MIC: 4–8 μM) against tested 12 bacterial strains and retained its antimicrobial activity in the presence of salts at physiological concentrations. Membrane depolarization and dye leakage assays showed that the enhanced antimicrobial activity of Lf-KR was due to increased permeabilization and depolarization of microbial membranes. Lf-KR significantly inhibited the expression and production of pro-inflammatory cytokines (nitric oxide and tumor necrosis factor‐α) in LPS-stimulated mouse macrophage RAW264.7 cells. In addition, Lf-KR showed a powerful eradication effect on preformed multidrug-resistant Pseudomonas aeruginosa (MDRPA) biofilms. We confirmed using confocal laser scanning microscopy that a large portion of the preformed MDRPA biofilm structure was perturbed by the addition of Lf-KR. Collectively, our results suggest that Lf-KR can be an antimicrobial, anti-inflammatory, and anti-biofilm candidate as a pharmaceutical agent.

Antimicrobial-wound healing peptides: Dual-function molecules for the treatment of skin injuries

Chronic non-healing wounds caused by microbial infections extend the necessity for hospital care and constitute a public health problem and a great financial burden. Classic therapies include a wide range of approaches, from wound debridement to vascular surgery. Antimicrobial peptides (AMPs) are a preserved trait of the innate immune response among different animal species, with known effects on the immune system and microorganisms. Thus, AMPs may represent promising candidates for the treatment of chronic wounds with dual functionality in two of the main agents that lead to this condition, proliferation of microorganisms and uncontrolled inflammation. Here, our goal is to critically review AMPs with wound healing properties. We strongly believe that these dual-function peptides alone, or in combination with other wound healing strategies, constitute an underexplored field that researchers can take advantage of.

Indolicidin revisited: biological activity, potential applications and perspectives of an antimicrobial peptide not yet fully explored

The emergence of multidrug-resistant bacteria, viruses and tumors is a serious threat to public health. Among natural peptides, indolicidin, a 13-residue peptide belonging to the cathelicidin family, deserves special attention. Indolicidin has a broad spectrum of biological activity and is active against a wide range of targets, such as bacteria (Gram+  and Gram-), fungi and viruses. Here, we review the most important features of the biological activity, potential applications and perspectives of indolicidin and its analogs. Although not yet approved for commercialization, this peptide has great potential to be applied in different areas, including the medical, biomedical, food industry and other unexplored areas. To achieve this goal, a multidisciplinary team of researchers must work together to fine tune peptides that overall lead to novel analogs and formulations to combat existing and possibly future diseases.

Legumes as Functional Food for Cardiovascular Disease

Legumes are an essential food source worldwide. Their high-quality proteins, complex carbohydrates, dietary fiber, and relatively low-fat content make these an important functional food. Known to possess a multitude of health benefits, legume consumption is associated with the prevention and treatment of cardiovascular diseases (CVD). Legume crude protein isolates and purified peptides possess many cardiopreventive properties. Here, we review selected economically valued legumes, their taxonomy and distribution, biochemical composition, and their protein components and the mechanism(s) of action associated with cardiovascular health. Most of the legume protein studies had shown upregulation of low-density lipoprotein (LDL) receptor leading to increased binding and uptake, in effect significantly reducing total lipid levels in the blood serum and liver. This is followed by decreased biosynthesis of cholesterol and fatty acids. To understand the relationship of identified genes from legume studies, we performed gene network analysis, pathway, and gene ontology (GO) enrichment. Results showed that the genes were functionally interrelated while enrichment and pathway analysis revealed involvement in lipid transport, fatty acid and triglyceride metabolic processes, and regulatory processes. This review is the first attempt to collate all known mechanisms of action of legume proteins associated with cardiovascular health. This also provides a snapshot of possible targets leading to systems-level approaches to further investigate the cardiometabolic potentials of legumes.

Effects of Rationally Designed Physico-Chemical Variants of the Peptide PuroA on Biocidal Activity towards Bacterial and Mammalian Cells

Antimicrobial peptides (AMPs) often exhibit wide-spectrum activities and are considered ideal candidates for effectively controlling persistent and multidrug-resistant wound infections. PuroA, a synthetic peptide based on the tryptophan (Trp)-rich domain of the wheat protein puroindoline A, displays strong antimicrobial activities. In this work, a number of peptides were designed based on PuroA, varying in physico-chemical parameters of length, number of Trp residues, net charge, hydrophobicity or amphipathicity, D-versus L-isomers of amino acids, cyclization or dimerization, and were tested for antimicrobial potency and salt and protease tolerance. Selected peptides were assessed for effects on biofilms of methicillin-resistant Staphylococcus aureus (MRSA) and selected mammalian cells. Peptide P1, with the highest amphipathicity, six Trp and a net charge of +7, showed strong antimicrobial activity and salt stability. Peptides W7, W8 and WW (seven to eight residues) were generally more active than PuroA and all diastereomers were protease-resistant. PuroA and certain variants significantly inhibited initial biomass attachment and eradicated preformed biofilms of MRSA. Further, P1 and dimeric PuroA were cytotoxic to HeLa cells. The work has led to peptides with biocidal effects on common human pathogens and/or anticancer potential, also offering great insights into the relationship between physico-chemical parameters and bioactivities, accelerating progress towards rational design of AMPs for therapeutics.

Enhancing the antibacterial activity of antimicrobial peptide PMAP-37(F34-R) by cholesterol modification

BACKGROUND

The problem of increasing resistance against conventional antibiotics has drawn people's attention. Therefore, the development of novel antibacterial agents with effective and safe therapeutic effects is imminent. Antimicrobial peptides (AMPs) are considered a promising class of antibacterial agents due to their broad antibacterial spectrum.

RESULTS

In this study, on the basis of our previously studied peptide PMAP-37(F34-R), a novel antimicrobial peptide Chol-37(F34-R) was developed by N-terminal cholesterol modification to increase hydrophobicity. We observed that the N-terminal cholesterol-modified Chol-37(F34-R) showed higher antimicrobial activity than PMAP-37(F34-R) in vitro. Chol-37(F34-R) also exhibited effective anti-biofilm activity and may kill bacteria by improving the permeability of their membranes. Chol-37(F34-R) exerted high stability in different pH, salt, serum, and boiling water environments. Chol-37(F34-R) also showed no hemolytic activity and substantially low toxicity. Furthermore, Chol-37(F34-R) exhibited good potency of bacteria eradication and promoted wound healing and abscess reduction in infected mice. Meanwhile, in S. aureus ATCC25923-infected peritonitis model, Chol-37(F34-R) exhibited an impressive therapeutic effect by reducing the decrease in systemic bacterial burden and alleviating organ damage.

CONCLUSIONS

Our findings suggested that the N-terminal cholesterol modification of PMAP-37(F34-R) could improve antibacterial activity. Chol-37(F34-R) displayed excellent bactericidal efficacy and impressive therapeutic effect in vivo. Thus, Chol-37(F34-R) may be a candidate for antimicrobial agents against microbial infection in the clinic.

Antibacterial activity of a scorpion-derived peptide and its derivatives in vitro and in vivo

Antimicrobial peptides have recently become extremely popular as a novel class of antimicrobial agents. AMP MK049518 (FLGLLGSVLGSVLPSIFK), identified from the crab-scorpion Didymocentrus krausi, only possesses significant antibacterial activity against Gram-positive bacteria. In this study, a derivative G2K-S3K was designed with an excellent antibacterial spectrum and significantly higher antibacterial activity compared to the natural peptide. G2K-S3K also demonstrated excellent serum- and thermal-stability and did not induce bacterial resistance. In the Staphylococcus aureus and Pseudomonas aeruginosa -induced skin infection in mice, G2K-S3K significantly decreased bacterial counts in the wound by topical application. Thus, G2K-S3K could be a potent topical anti-infective agent against the skin infection caused by S. aureus and P. aeruginosa.

Bioactive peptides from food fermentation: A comprehensive review of their sources, bioactivities, applications, and future development

Bioactive peptides (BPs) are specific protein fragments that exert various beneficial effects on human bodies and ultimately influence health, depending on their structural properties and amino acid composition and sequences. By offering promising solutions to solve diverse health issues, the production, characterization, and applications of food-derived BPs have drawn great interest in the current literature and are of particular interest to the food and pharmaceutical industries. The microbial fermentation of protein from various sources is indubitably a novel way to produce BPs with numerous beneficial health effects. Apart from its lower cost as compared to enzymes, the BPs produced from microbial fermentation can be purified without further hydrolysis. Despite these features, current literature shows dearth of information on the BPs produced from food via microbial fermentation. Hence, there is a strong necessity to explore the BPs obtained from food fermentation for the development of commercial nutraceuticals and functional foods. As such, this review focuses on the production of BPs from different food sources, including the extensively studied milk and milk products, with emphasis on microbial fermentation. The structure-activity (antihypertensive, antioxidant, antimicrobial, opiate-like, anti-inflammatory, anticancer/antiproliferative, antithrombotic, hypolipidemic, hypocholesterolemic, and mineral binding) relationship, potential applications, future development, and challenges of BPs obtained from food fermentation are also discussed.

Antimicrobial Mechanism of pBD2 against Staphylococcus aureus

Antimicrobial peptides (AMPs) show high antibacterial activity against pathogens, which makes them potential new therapeutics to prevent and cure diseases. Porcine beta defensin 2 (pBD2) is a newly discovered AMP and has shown antibacterial activity against different bacterial species including multi-resistant bacteria. In this study, the functional mechanism of pBD2 antibacterial activity against Staphylococcus aureus was investigated. After S. aureus cells were incubated with different concentrations of pBD2, the morphological changes in S. aureus and locations of pBD2 were detected by electron microscopy. The differentially expressed genes (DEGs) were also analyzed. The results showed that the bacterial membranes were broken, bulging, and perforated after treatment with pBD2; pBD2 was mainly located on the membranes, and some entered the cytoplasm. Furthermore, 31 DEGs were detected and confirmed by quantitative real-time PCR (qRT-PCR). The known functional DEGs were associated with transmembrane transport, transport of inheritable information, and other metabolic processes. Our data suggest that pBD2 might have multiple modes of action, and the main mechanism by which pBD2 kills S. aureus is the destruction of the membrane and interaction with DNA. The results imply that pBD2 is an effective bactericide for S. aureus, and deserves further study as a new therapeutic substance against S. aureus.

The Critical Role of Tryptophan in the Antimicrobial Activity and Cell Toxicity of the Duck Antimicrobial Peptide DCATH

Antimicrobial peptides (AMPs) have attracted more attention for their potential candidates for new antibiotic drugs. As a novel identified cathelicidin AMP from duck, dCATH owns broad-spectrum antimicrobial activities but with a noticeable toxicity. To explore dCATH-derived AMPs with reduced cell toxicity and improved cell selectivity, a series of truncated and tryptophan-replaced peptides of dCATH were designed. Two truncated peptides containing one of the two tryptophan (Trp) residues at the positions of 4 and 17 (W4 and W17) of dCATH, dCATH(1-16) and dCATH(5-20), showed strong antibacterial activity, but didn't show obvious hemolytic activity and cytotoxicity. The derived peptides not containing Trp didn't possess obvious antimicrobial activity, and their hemolytic and cytotoxic effect was also diminished. Also as evidence by Trp fluorescence experiment that existence of W4 and W17 was crucially important to the antimicrobial activity, hemolysis and cytotoxicity of dCATH, and one of the two Trp residues was competent and necessary to retain its antimicrobial activity. Antibacterial mechanism analysis showed that dCATH(1-16) and dCATH(5-20) killed bacterial cells by increasing permeability and causing a loss of membrane integrity. dCATH(1-16) and dCATH(5-20) possessed insignificant inhibitory activity against levels of IL-6, TNF-α, and NO in RAW 264.7 cells treated with LPS. In vivo, intraperitoneal administration of the two peptides significantly decreased mortality and provided protection against LPS-induced inflammation in mice challenged with lethal dose of LPS. The two peptides, dCATH(1-16) and dCATH(5-20), which possessed high antibacterial activity and cell selectivity, may herald development prospects as new antibacterial agents in the future.

Inhibitory Effects of Walnut (Juglans regia) Peptides on Neuroinflammation and Oxidative Stress in Lipopolysaccharide-Induced Cognitive Impairment Mice

Increasing level of inflammation and oxidative stress could lead to memory impairment. The purpose of this study was to determine the neuroprotective effects of walnut peptides against memory deficits induced by lipopolysaccharide (LPS) in mice and further to explore the underlying anti-inflammatory mechanisms against LPS-elicited inflammation in BV-2 cells. Results showed that walnut protein hydrolysate (WPH) and its low-molecular-weight fraction (WPHL) could ameliorate the memory deficits induced by LPS via normalizing the inflammatory response and oxidative stress in brain, especially WPHL. Furthermore, 18 peptides with anti-inflammatory activities on LPS-activated BV-2 cells were identified from WPHL and it was found that Trp, Gly, and Leu residues in peptides might contribute to the anti-inflammation. Meanwhile, the strong anti-inflammatory effects of LPF, GVYY, and APTLW might be related to their hydrophobic and aromatic amino acid residues as well. LPF, GVYY, and APTLW could reduce the content of proinflammatory mediators and cytokines by downregulating related enzyme expressions and mRNA expressions. Additionally, ROS and mitochondria homeostasis might also contribute to their anti-inflammatory effects.

Little Antimicrobial Peptides with Big Therapeutic Roles

Antimicrobial Peptides (AMPs) are short amphipathic biological molecules generally with less than 100 amino acids. AMPs not only present high bioactivities against bacteria, fungi or protists-induced infections, but also play important roles in anticancer activity, immune response and inflammation regulation. AMPs are classified as ribosomally synthesized, non-ribosomally synthesized and post-translationally modified, non-ribosomally synthesized ones and several synthetic or semisynthetic peptides according to their synthesis with or without the involvement of ribosomes. The molecular characterization and bioactivity action mechanisms are summarized for several ribosomally synthesized AMPs and main non-ribosomally synthesized members (cyclopeptides, lipopeptides, glycopeptides, lipoglycopeptides). We also analyze challenges and new strategies to overcome drug resistance and application limitations for AMP discovery. In conclusion, the growing novel small molecular AMPs have huge therapeutic potentials of antibacterial, antiviral, anticancer and immunoregulatory bioactivities through new techniquesdriven drug discovery strategy including bioinformatics prediction, de novo rational design and biosynthesis.

Enhancing the antibacterial activity of PMAP-37 by increasing its hydrophobicity

With increasing resistance against conventional antibiotics, there is an urgent need to discover novel substances to replace antibiotics. This need provides an opportunity for the development of antimicrobial peptides (AMPs). To develop new AMPs with effective and safe therapeutic effects, two PMAP-37 analogs called PMAP-37(R13-I) and PMAP-37(K20/27-I) were designed to increase hydrophobicity. Antimicrobial susceptibility testing and animal infection models were used to assess their antibacterial activity. The results showed that the minimal inhibitory concentrations of PMAP-37(R13-I) were lower than those of PMAP-37 for two gram-negative strains. Compared with PMAP-37, PMAP-37(K20/27-I) not only inhibited the growth of most bacterial strains, but also exhibited antibacterial activity against Shigella flexneri CICC21534. In addition, PMAP-37(K20/27-I) exhibited pH and thermal stability. PMAP-37(R13-I) had a therapeutic effect only in mice infected with Salmonella typhimurium SL1344. However, PMAP-37(K20/27-I) exhibited the therapeutic effects, whether in the clinical symptoms, the tissue lesions, or the tissue bacterial loads and the survival rates in mice infected with Staphylococcus aureus ATCC25923 or S. typhimurium SL1344. Therefore, PMAP-37(K20/27-I) can be used as a substitute for antibiotics against infection with bacterial strains.

Design of therapeutically improved analogue of the antimicrobial peptide, indolicidin, using a glycosylation strategy

Indolicidin is a member of cathelicidin family which displays broad spectrum antimicrobial activity. Severe toxicity and aggregation propensity associated with indolicidin pose a huge limitation to its probable therapeutic application. We are reporting the use of glycosylation strategy to design an analogue of indolicidin and subsequently explore structural and functional effects of sugar on it. Our study led to the design of a potent antibacterial glycosylated peptide, [βGlc-T9,K7]indolicidin, which showed decreased toxicity against erythrocytes and macrophage cells and thus a higher therapeutic selectivity. The incorporation of sugar also increased the solubility of the peptide. The mode of bacterial killing, functional stability, LPS binding, and cytokine inhibitory potential of the peptide, however, seemed unaffected upon glycosylation. Absence of significant changes in structure upon glycosylation accounts for the possibly retained functions and mode of action of the peptide. Our report thus presents the designing of an indolicidin analogue with improved therapeutic potential by substituting aromatic amino acid with glycosylated amino acid as a promising strategy for the first time.

Design, expression, and characterization of a novel cecropin A-derived peptide with high antibacterial activity

In recent years, antimicrobial peptides have received increased interest and are potential substitutes for antibiotics. However, natural antimicrobial peptides are always toxic to mammalian cells and usually exhibit weak antibacterial activity, which restrict their wide application. In this study, a novel antibacterial peptide named PEW300 was designed with three mutations to the parental peptide cecropin A. As predicted by bioinformatic programs, the positive charge of PEW300 increased from + 6 to + 9 compared with cecropin A, and the grand average of hydropathicity increased from - 0.084 to - 0.008. Expression of PEW300 resulted in serious inhibition of Escherichia coli BL21(DE3) cells, indicating designed PEW300 may have stronger antibacterial activity. A simple, fast, and low-cost approach without tedious protein purification steps was selected for the efficient production of PEW300 by fusion with ELK16 and about 7.38 μg/mg wet cell weight PEW300 was eventually obtained. Purified PEW300 exhibited strong antibacterial activity against various Gram-positive and Gram-negative bacteria which was enhanced four- to sevenfold compared with the parental peptide cecropin A. Besides, PEW300 had no hemolytic activity toward mammalian cells even at high concentration (224 ng/μl). PEW300 showed good stability in neutral and alkaline solutions. Moreover, PEW300 was thermally stable even at up to 100 °C and resistant to proteinase K, pepsin, snailase, and trypsin. The incubation with human serum had no effect on the antibacterial activity of PEW300. All these results demonstrated that PEW300 designed in this work may have good potential as a candidate pharmaceutical agent.

Analysis of the antimicrobial mechanism of porcine beta defensin 2 against E. coli by electron microscopy and differentially expressed genes

Porcine beta defensin 2 (pBD2) is a cationic antimicrobial peptide with broad spectrum antibacterial activity, which makes it a potential alternative to antibiotics to prevent and cure diseases of pigs. However, development of pBD2 as an effective antibiotic agent requires molecular understanding of its functional mechanism against pathogens. In this study, we investigated the functional mechanism of pBD2 antibacterial activity. Escherichia coli was incubated with different pBD2 concentrations for different times. Electron microscopy was used to analyze the locations of pBD2 and its induced morphological changes in E. coli. Gene expression analysis was also performed to further understand the molecular changes of E. coli in response to pBD2 incubation. The results demonstrated that E. coli membranes were broken, holed, and wrinkled after treatment with pBD2, and pBD2 was located on the cell membranes and manly in the cytoplasm. Furthermore, 38 differentially expressed genes (DEGs) were detected, successfully sequenced and confirmed by quantitative real-time PCR (qRT-PCR). Most of the known functional DEGs were associated with DNA transcription and translation and located in the cytoplasm. Collectively, the results suggest that pBD2 could have multiple modes of action and the main mechanism for killing E. coli might be influence on DNA transcription and translation by targeting intracellular molecules after membrane damage, although transport and metabolism proteins were also affected.

Structural-features of food-derived bioactive peptides with anti-inflammatory activity: A brief review

Chronic inflammatory conditions such as obesity, type-2 diabetes, and cardiovascular diseases are the leading causes of mortality worldwide. Hence, much research interest in bioactive peptides has been stimulated due to lack of potent pharmacological interventions. Although many such peptides have been identified from food proteins, insufficient information is available on their structure-function relationship. Presence of hydrophobic and positively charged amino acids is a common occurrence for the peptides with anti-inflammatory properties. However, inconsistent findings have also been reported. Most of the food-derived peptides exhibited their anti-inflammatory activities primarily by inhibiting signaling components of either NF-κB or MAPK pathway, which are the two major pathways involved in chronic inflammation following uncontrolled signal activation. This review highlighted the structural requirements of the peptides to exhibit anti-inflammatory activity based on the current knowledge about food-derived anti-inflammatory peptides and their underlying molecular mechanisms of action. PRACTICAL APPLICATIONS: While research in the food-derived bioactive peptide is gaining momentum, but the ability to translate these new findings into the commercial product such as nutraceuticals and functional foods, remains delayed. The most prominent reasons for this delay are the lack of detailed research on, (i) the structure-function relationship of the peptide and the underlying molecular mechanisms of these bioactive peptides, and (ii) the interaction of these peptides with different cellular elements in the disease pathophysiology. This review gives an insight into the structure-activity relationship of bioactive peptides involved in anti-inflammatory responses. The information provided here would be highly beneficial to describe the possible anti-inflammatory activity of any newly identified peptides from different food sources.

Cospreading of Anionic Phospholipids with Peptides of the Structure (KX)4K at the Air-Water Interface: Influence of Lipid Headgroup Structure and Hydrophobicity of the Peptide on Monolayer Behavior

Mixtures of anionic phospholipids (PG, PA, PS, and CL) with cationic peptides were cospread from a common organic solvent at the air-water interface. The compression of the mixed film was combined with epifluorescence microscopy or infrared reflection adsorption spectroscopy (IRRAS) to gain information on the interactions of the peptide with the different lipids. To evaluate the influence of the amino acid X of peptides with the sequence (KX)₄K on the binding, 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) was mixed with different peptides with increasing hydrophobicity of the uncharged amino acid X. The monolayer isotherms of DPPG/(KX)₄K mixtures show an increased area for the lift-off due to incorporation of the peptide into the liquid-expanded (LE) state of the lipid. The surface pressure for the transition from LE to the liquid-condensed (LC) state is slightly increased for peptides with amino acids X with moderate hydrophobicity. For the most hydrophobic peptide (KL)₄K two plateaus are seen at a charge ratio PG to K of 5:1, and a strongly increased transition pressure is observed for a charge ratio of 1:1. Epifluorescence microscopy images and infrared spectroscopy show that the lower plateau corresponds to the LE-LC phase transition of the lipid. The upper plateau is connected with a squeeze-out of the peptide into the subphase. To test the influence of the lipid headgroup structure on peptide binding (KL)₄K was cospread with different anionic phospholipids. The shift of the isotherm to larger areas for lift-off and to higher surface pressure for the LE-LC phase transition was observed for all tested anionic lipids. Epifluorescence microscopy reveals the formation of LC domains with extended filaments indicating a decrease in line tension due to accumulation of the peptides at the LC-domain boundaries. This effect depends on the size of the headgroup of the anionic phospholipid.

Differential Ability of Bovine Antimicrobial Cathelicidins to Mediate Nucleic Acid Sensing by Epithelial Cells

Cathelicidins encompass a family of cationic peptides characterized by antimicrobial activity and other functions, such as the ability to enhance the sensing of nucleic acids by the innate immune system. The present study aimed to investigate the ability of the bovine cathelicidins indolicidin, bactenecin (Bac)1, Bac5, bovine myeloid antimicrobial peptide (BMAP)-27, BMAP-28, and BMAP-34 to inhibit the growth of bacteria and to enhance the sensing of nucleic acid by the host’s immune system. BMAP-27 was the most effective at killing Staphylococcus aureus, Streptococcus uberis, and Escherichia coli, and this was dependent on its amphipathic structure and cationic charge. Although most cathelicidins possessed DNA complexing activity, only the alpha-helical BMAP cathelicidins and the cysteine-rich disulfide-bridged Bac1 were able to enhance the sensing of nucleic acids by primary epithelial cells. We also compared these responses with those mediated by neutrophils. Activation of neutrophils with phorbol myristate acetate resulted in degranulation and release of cathelicidins as well as bactericidal activity in the supernatants. However, only supernatants from unstimulated neutrophils were able to promote nucleic acid sensing in epithelial cells. Collectively, the present data support a role for certain bovine cathelicidins in helping the innate immune system to sense nucleic acids. The latter effect is observed at concentrations clearly below those required for direct antimicrobial functions. These findings are relevant in development of future strategies to promote protection at mucosal surfaces against pathogen invasion.

Inducible Expression of the De-Novo Designed Antimicrobial Peptide SP1-1 in Tomato Confers Resistance to Xanthomonas campestris pv. vesicatoria

Antimicrobial peptides (AMPs) are small peptides with less than 50 amino acids and are part of the innate immune response in almost all organisms, including bacteria, vertebrates, invertebrates and plants. AMPs are active against a broad-spectrum of pathogens. The inducible expression of AMPs in plants is a promising approach to combat plant pathogens with minimal negative side effects, such as phytotoxicity or infertility. In this study, inducible expression of the de-novo designed AMP SP1-1 in Micro Tom tomato protected tomato fruits against bacterial spot disease caused by Xanthomonas campestris pv. vesicatoria. The peptide SP1-1 was targeted to the apoplast which is the primary infection site for plant pathogens, by fusing SP1-1 peptide to the signal peptide RsAFP1 of radish (Raphanus sativus). The pathogen inducibility of the expression was enabled by using an optimized inducible 4XW2/4XS promoter. As a result, the tomato fruits of independently generated SP1-1 transgenic lines were significantly more resistant to X. campestris pv. vesicatoria than WT tomato fruits. In transgenic lines, bacterial infection was reduced up to 65% in comparison to the infection of WT plants. Our study demonstrates that the combination of the 4XW2/4XS cis-element from parsley with the synthetic antimicrobial peptide SP1-1 is a good alternative to protect tomato fruits against infections with X. campestris pv. vesicatoria.

Influence of Cysteine and Tryptophan Substitution on DNA-Binding Activity on Maize α-Hairpinin Antimicrobial Peptide

For almost four decades, antimicrobial peptides have been studied, and new classes are being discovered. However, for therapeutic use of these molecules, issues related to the mechanism of action must be answered. In this work, the antimicrobial activity of the hairpinin MBP-1 was studied by the synthesis of two variants, one replacing cysteines and one tryptophan with alanine. Antibacterial activity was abolished in both variants. No membrane disturbance, even in concentrations higher than those required to inhibit the bacteria, was observed in SEM microscopy. The gel retardation assay showed that MBP-1 possesses a higher DNA-binding ability than variants. Finally, molecular modelling showed that the lack of cysteines resulted in structure destabilization and lack of tryptophan resulted in a less flexible peptide, with less solvent assessable surface area, both characteristics that could contribute to absence of activity. In summary, the data here reported add more information about the multiple mechanisms of action of α-hairpinins.

The Disulfide Bond of the Peptide Thanatin Is Dispensible for Its Antimicrobial Activity In Vivo and In Vitro

Thanatin (THA) displays potent antibiotic activity, especially against extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli both in vitro and in vivo, with minimal hemolytic toxicity and satisfactory stability in plasma. However, the high cost of thanatin significantly limits its development and clinical application. To reduce the cost of peptide synthesis, a formulation of cyclic thanatin (C-thanatin) called linear thanatin (L-thanatin) was synthesized and its activity was evaluated in vivo and in vitro Results showed that C-thanatin and L-thanatin MICs did not differ against eight Gram-negative and two Gram-positive bacterial strains. Furthermore, the survival rates of ESBL-producing-E. coli-infected mice were consistent after C-thanatin or L-thanatin treatment at 5 or 10 mg/kg of body weight. Neither C-thanatin nor L-thanatin showed toxicity for human red blood cells (hRBCs) and human umbilical vein endothelial cells (HUVECs) at a concentration as high as 256 μg/ml. Results of circular dichroism spectroscopy indicated that the secondary structure of L-thanatin is extremely similar to that of C-thanatin. Membrane permeabilization and depolarization assays showed that C-thanatin and L-thanatin have similar abilities to permeabilize the outer and inner membranes and to induce membrane depolarization in ESBL-producing E. coli However, neither of them caused significant HUVEC membrane permeability. These findings indicate that the two peptides have similar effects on bacterial cell membranes and that the disulfide bond in thanatin is not essential for its antimicrobial activities in vivo and in vitro L-thanatin is thus a promising low-cost peptide candidate for treating ESBL-producing E. coli infections.

Antibiotic development challenges: the various mechanisms of action of antimicrobial peptides and of bacterial resistance

Antimicrobial peptides (AMPs) are natural antibiotics produced by various organisms such as mammals, arthropods, plants, and bacteria. In addition to antimicrobial activity, AMPs can induce chemokine production, accelerate angiogenesis, and wound healing and modulate apoptosis in multicellular organisms. Originally, their antimicrobial mechanism of action was thought to consist solely of an increase in pathogen cell membrane permeability, but it has already been shown that several AMPs do not modulate membrane permeability in the minimal lethal concentration. Instead, they exert their effects by inhibiting processes such as protein and cell wall synthesis, as well as enzyme activity, among others. Although resistance to these molecules is uncommon several pathogens developed different strategies to overcome AMPs killing such as surface modification, expression of efflux pumps, and secretion of proteases among others. This review describes the various mechanisms of action of AMPs and how pathogens evolve resistance to them.

The antimicrobial domains of wheat puroindolines are cell-penetrating peptides with possible intracellular mechanisms of action

The puroindoline proteins (PINA and PINB) of wheat display lipid-binding properties which affect the grain texture, a critical parameter for wheat quality. Interestingly, the same proteins also display antibacterial and antifungal properties, attributed mainly to their Tryptophan-rich domain (TRD). Synthetic peptides based on this domain also display selectivity towards bacterial and fungal cells and do not cause haemolysis of mammalian cells. However, the mechanisms of these activities are unclear, thus limiting our understanding of the in vivo roles of PINs and development of novel applications. This study investigated the mechanisms of antimicrobial activities of synthetic peptides based on the TRD of the PINA and PINB proteins. Calcein dye leakage tests and transmission electron microscopy showed that the peptides PuroA, Pina-M and Pina-W→F selectively permeabilised the large unilamellar vesicles (LUVs) made with negatively charged phospholipids mimicking bacterial membranes, but were ineffective against LUVs made with zwitterionic phospholipids mimicking eukaryotic membranes. Propidium iodide fluorescence tests of yeast (Saccharomyces cerevisiae) cells showed the peptides were able to cause loss of membrane integrity, PuroA and Pina-M being more efficient. Scanning electron micrographs of PINA-based peptide treated yeast cells showed the formation of pits or pores in cell membranes and release of cellular contents. Gel retardation assays indicated the peptides were able to bind to DNA in vitro, and the induction of filamental growth of E. coli cells indicated in vivo inhibition of DNA synthesis. Together, the results strongly suggest that the PIN-based peptides exert their antimicrobial effects by pore formation in the cell membrane, likely by a carpet-like mechanism, followed by intracellular mechanisms of activity.

Synthesis of side chain N,N'-diaminoalkylated derivatives of basic amino acids for application in solid-phase peptide synthesis

Despite the enormous therapeutic potential, the clinical use of peptides has been limited by their poor bioavailability and low stability under physiological conditions. Hence, efforts have been undertaken to alter peptide structure in ways to improve their pharmacological properties. Inspired by the importance of basic amino acids in biological systems and the remarkable versatility displayed by lysine during the synthesis of complex peptide scaffolds, this chapter describes a simple procedure that enables rapid access to protected N,N'-diaminoalkylated basic amino acid building blocks suitable for standard solid-phase peptide synthesis. This procedure allows preparation of symmetrical, as well as unsymmetrical, dialkylated amino acid derivatives that can be further modified, enhancing their synthetic utility. The suitability of the synthesized branched basic amino acid building blocks for use in standard solid-phase peptide synthesis has been demonstrated by synthesis of an indolicidin analog in which the lysine residue was substituted with its synthetic polyamino derivate. The substitution provided indolicidin analog with increase net positive charge, more ordered secondary structure in biological membranes mimicking conditions, and enhanced antibacterial activity without altering hemolytic activity. Taking into consideration the increasing interest for peptides with unusual structural features due to their improved biological properties, the described synthesis of polyfunctional amino acid building blocks is of particular practical value.

Structure-activity relationship of synthetic variants of the milk-derived antimicrobial peptide αs2-casein f(183-207)

Template-based studies on antimicrobial peptide (AMP) derivatives obtained through manipulation of the amino acid sequence are helpful to identify properties or residues that are important for biological activity. The present study sheds light on the importance of specific amino acids of the milk-derived αs2-casein f(183-207) peptide to its antibacterial activity against the food-borne pathogens Listeria monocytogenes and Cronobacter sakazakii. Trimming of the peptide revealed that residues at the C-terminal end of the peptide are important for activity. Removal of the last 5 amino acids at the C-terminal end and replacement of the Arg at position 23 of the peptide sequence by an Ala residue significantly decreased activity. These findings suggest that Arg23 is very important for optimal activity of the peptide. Substitution of the also positively charged Lys residues at positions 15 and 17 of the αs2-casein f(183-207) peptide also caused a significant reduction of the effectiveness against C. sakazakii, which points toward the importance of the positive charge of the peptide for its biological activity. Indeed, simultaneous replacement of various positively charged amino acids was linked to a loss of bactericidal activity. On the other hand, replacement of Pro residues at positions 14 and 20 resulted in a significantly increased antibacterial potency, and hydrophobic end tagging of αs2-casein f(193-203) and αs2-casein f(197-207) peptides with multiple Trp or Phe residues significantly increased their potency against L. monocytogenes. Finally, the effect of pH (4.5 to 7.4), temperature (4°C to 37°C), and addition of sodium and calcium salts (1% to 3%) on the activity of the 15-amino-acid αs2-casein f(193-207) peptide was also determined, and its biological activity was shown to be completely abolished in high-saline environments.

Direct access to side chain N,N'-diaminoalkylated derivatives of basic amino acids suitable for solid-phase peptide synthesis

A simple and efficient one-pot procedure that enables rapid access to orthogonally protected N,N'-diaminoalkylated basic amino acid building blocks fully compatible with standard Boc and Fmoc solid-phase peptide synthesis is reported. Described synthetic approach includes double reductive alkylation of N (α)-protected diamino acids with N-protected amino aldehydes in the presence of sodium cyanoborohydride. This approach allows preparation of symmetrical, as well as unsymmetrical, basic amino acid derivatives with branched side-chains that can be further modified, enhancing their synthetic utility. The suitability of the synthesized branched basic amino acid building blocks for use in standard solid-phase peptide synthesis has been demonstrated by synthesis of an indolicidin analogue in which the lysine residue was substituted with the synthetic derivative N (α)-(9H-fluorenyl-9-methoxycarbonyl)-N (β),N (β) '-bis[2-(tert-butoxycarbonylamino)ethyl]-L-2,3-diaminopropionic acid. This substitution resulted in an analogue with more ordered secondary structure in 2,2,2-trifluoroethanol and enhanced antibacterial activity without altering hemolytic activity.

Extensive manipulation of caseicins A and B highlights the tolerance of these antimicrobial peptides to change

Caseicins A and B are low-molecular-weight antimicrobial peptides which are released by proteolytic digestion of sodium caseinate. Caseicin A (IKHQGLPQE) is a nine-amino-acid cationic peptide, and caseicin B (VLNENLLR) is a neutral eight-amino-acid peptide; both have previously been shown to exhibit antibacterial activity against a number of pathogens, including Cronobacter sakazakii. Previously, four variants of each caseicin which differed subtly from their natural counterparts were generated by peptide synthesis. Antimicrobial activity assays revealed that the importance of a number of the residues within the peptides was dependent on the strain being targeted. In this study, this engineering-based approach was expanded through the creation of a larger collection of 26 peptides which are altered in a variety of ways. The investigation highlights the generally greater tolerance of caseicin B to change, the fact that changes have a more detrimental impact on anti-Gram-negative activity, and the surprising number of variants which exhibit enhanced activity against Staphylococcus aureus.

Molecular characterization of hepcidin AS-hepc2 and AS-hepc6 in black porgy (Acanthopagrus schlegelii): expression pattern responded to bacterial challenge and in vitro antimicrobial activity

There are more diversified isoforms of the hepcidin gene that exist in fishes than in mammals, and elucidating the differences between these isoforms should provide insight into the functioning of hepcidin in fishes. In our study, AS-hepc2 and AS-hepc6 hepcidin isoforms from black porgy were characterized for their in vivo expression patterns following bacterial challenge, and their in vitro antimicrobial activities against Gram-positive and Gram-negative bacteria as well as fungi. As a result, two isoforms were observed to be widely distributed in all the tissues tested. AS-hepc2 was a liver-expressed hepcidin peptide which was always highly more expressed in the liver than in the other tissues tested no matter whether this was before or after bacterial challenge. AS-hepc6 was detected mainly in the head kidney and trunk kidney of normal fish, but, in the challenged fish, its expression involved more tissues than just the kidneys. The mature peptides of AS-hepc2 and AS-hepc6 were modeled for 3D structure and then synthesized for antimicrobial assay. AS-hepc6 had a wider antimicrobial spectrum than AS-hepc2 and, in particular, had more potent antifungal activity. Our study indicated that the two hepcidin isoforms had different characteristics in terms of their expression patterns and antimicrobial activity, and they were assumed to play an overlapping role in the innate immune system of black porgy against invading pathogens.

Chicken cathelicidin-2-derived peptides with enhanced immunomodulatory and antibacterial activities against biological warfare agents

Host defence peptides (HDPs) are considered to be excellent candidates for the development of novel therapeutic agents. Recently, it was demonstrated that the peptide C1-15, an N-terminal segment of chicken HDP cathelicidin-2, exhibits potent antibacterial activity while lacking cytotoxicity towards eukaryotic cells. In the present study, we report that C1-15 is active against bacteria such as Bacillus anthracis and Yersinia pestis that may potentially be used by bioterrorists. Substitution of single and multiple phenylalanine (Phe) residues to tryptophan (Trp) in C1-15 resulted in variants with improved antibacterial activity against B. anthracis and Y. pestis as well as decreased salt sensitivity. In addition, these peptides exhibited enhanced neutralisation of lipopolysaccharide (LPS)-induced release of pro-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs). The antibacterial and LPS-neutralising activities of these C1-15-derived peptides are exerted at concentrations far below the concentrations that are toxic to human PBMCs. Taken together, we show that Phe-->Trp substitutions in C1-15 variants enhances the antibacterial and LPS-neutralising activities against pathogenic bacteria, including those that may potentially be used as biological warfare agents.

Control of alpha-alumina surface charge with carboxylic acids

In this work, we studied the surface charge of alpha-alumina treated with carboxylic acids with different carbon chain length. The results show the possibility of controlling surface charges of alumina by using different concentrations of carboxylic acids or changing the size of the carbon chain of the acids. We also report that part of the acid found on the surface is strongly bound, therefore making it possible to obtain pH-resistant samples of alpha-alumina with an isoelectric point (IEP) of 5.5. It is found, that IEP values obtained for modified samples have a linear correlation with the number of carbon atoms of dicarboxylic acids for up to five carbon atoms. From a practical perspective, the method presented in this work has many advantages. First, it maintains the same hydrophilicity of the alumina surface. Second, the modification of the surface is stable in a long-range of pH. Finally, the presented method is easy-to-use and cheap, as the modification consists of only two simple steps carried out at low temperatures with inexpensive and nontoxic reagents.

Multifunctional host defense peptides: intracellular-targeting antimicrobial peptides

There is widespread acceptance that cationic antimicrobial peptides, apart from their membrane-permeabilizing/disrupting properties, also operate through interactions with intracellular targets, or disruption of key cellular processes. Examples of intracellular activity include inhibition of DNA and protein synthesis, inhibition of chaperone-assisted protein folding and enzymatic activity, and inhibition of cytoplasmic membrane septum formation and cell wall synthesis. The purpose of this minireview is to question some widely held views about intracellular-targeting antimicrobial peptides. In particular, I focus on the relative contributions of intracellular targeting and membrane disruption to the overall killing strategy of antimicrobial peptides, as well as on mechanisms whereby some peptides are able to translocate spontaneously across the plasma membrane. Currently, there are no more than three peptides that have been convincingly demonstrated to enter microbial cells without the involvement of stereospecific interactions with a receptor/docking molecule and, once in the cell, to interfere with cellular functions. From the limited data currently available, it seems unlikely that this property, which is isolated in particular peptide families, is also shared by the hundreds of naturally occurring antimicrobial peptides that differ in length, amino acid composition, sequence, hydrophobicity, amphipathicity, and membrane-bound conformation. Microbial cell entry and/or membrane damage associated with membrane phase/transient pore or long-lived transitions could be a feature common to intracellular-targeting antimicrobial peptides and mammalian cell-penetrating peptides that have an overrepresentation of one or two amino acids, i.e. Trp and Pro, His, or Arg. Differences in membrane lipid composition, as well as differential lipid recruitment by peptides, may provide a basis for microbial cell killing on one hand, and mammalian cell passage on the other.

In vitro antibacterial activity of acyl-lysyl oligomers against Helicobacter pylori

The gastric pathogen Helicobacter pylori has developed resistance to virtually all current antibiotics; thus, there is a pressing need to develop new anti-H. pylori therapies. The goal of this work was to evaluate the antibacterial effect of oligo-acyl-lysyl (OAK) antimicrobial peptidomimetics to determine if they might represent alternatives to conventional antibiotic treatment of H. pylori infection. A total of five OAK sequences were screened for growth-inhibitory and/or bactericidal effects against H. pylori strain G27; four of these sequences had growth-inhibitory and bactericidal effects. The peptide with the highest efficacy against strain G27, C12K-2beta12, was selected for further characterization against five additional H. pylori strains (26695, J99, 7.13, SS1, and HPAG1). C12K-2beta12 displayed MIC and minimum bactericidal concentration (MBC) ranges of 6.5 to 26 microM and 14.5 to 90 microM, respectively, across the six strains after 24 h of exposure. G27 was the most sensitive H. pylori strain (MIC = 6.5 to 7 microM; MBC = 15 to 20 microM), whereas 26695 was the least susceptible strain (MIC = 25 to 26 microM; MBC = 70 to 90 microM). H. pylori was completely killed after 6 to 8 h of incubation in liquid cultures containing two times the MBC of C12K-2beta12. The OAK demonstrated strong in vitro stability, since efficacy was maintained after incubation at extreme temperatures (4 degrees C, 37 degrees C, 42 degrees C, 50 degrees C, 55 degrees C, 60 degrees C, and 95 degrees C) and at low pH, although reduced killing kinetics were observed at pH 4.5. Additionally, upon transient exposure to the bacteria, C12K-2beta12 showed irreversible and significant antibacterial effects and was also nonhemolytic. Our results show a significant in vitro effect of C12K-2beta12 against H. pylori and suggest that OAKs may be a valuable resource for the treatment of H. pylori infection.