pH-dependent disruption of Escherichia coli ATCC 25922 and model membranes by the human antimicrobial peptides hepcidin 20 and 25

Bone marrow stromal cell-derived hepcidin has antimicrobial and immunomodulatory activities

Bone marrow stromal cells (BMSCs) have immunomodulatory activities in numerous species and have been used in clinical trials. BMSCs also make antibacterial agents. Since hepcidin is known to have antimicrobial effects in fish, we wondered if it might also be used as an antimicrobial agent by mammalian BMSCs. In the present study, we show hepcidin expression in both mouse (mBMSC) and human BMSCs (hBMSC). We observed a hBMSC hepcidin-dependent degradation of ferroportin in HEK-293 reporter cells in vitro. In human and mouse bone marrows (BM) we detected hepcidin-positive BMSCs in close proximity to hematopoietic progenitors. The conditioned culture medium of hBMSCs significantly reduced bacterial proliferation that was partially blocked by a hepcidin-neutralizing antibody. Similarly, medium in which hepcidin-deficient (Hamp-/-) mouse BMSCs had been grown was significantly less effective in reducing bacterial counts than the medium of wild-type cells. In a zymosan-induced peritonitis mouse model we found that mBMSC-derived hepcidin reduced the number of invading polymorphonuclear (PMN) cells in the peritoneal cavity. Our results show that BMSC-derived hepcidin has antimicrobial properties in vitro and also reduces inflammation in vivo. We conclude that hepcidin should be added to the expanding arsenal of agents available to BMSCs to fight infections and inflammation.

Antimicrobial and Cell-Penetrating Peptides: Understanding Penetration for the Design of Novel Conjugate Antibiotics

Antimicrobial peptides (AMPs) are short oligopeptides that can penetrate the bacterial inner and outer membranes. Together with cell-penetrating peptides (CPPs), they are called membrane active peptides; peptides which can translocate across biological membranes. Over the last fifty years, attempts have been made to understand the molecular features that drive the interactions of membranes with membrane active peptides. This review examines the features of a membrane these peptides exploit for translocation, as well as the physicochemical characteristics of membrane active peptides which are important for translocation. Moreover, it presents examples of how these features have been used in recent years to create conjugates consisting of a membrane active peptide, called a "vector", attached to either a current or novel antibiotic, called a "cargo" or "payload". In addition, the review discusses what properties may contribute to an ideal peptide vector able to deliver cargoes across the bacterial outer membrane as the rising issue of antimicrobial resistance demands new strategies to be employed to combat this global public health threat.

Ultra-/Small Angle X-ray Scattering (USAXS/SAXS) and Static Light Scattering (SLS) Modeling as a Tool to Determine Structural Changes and Effect on Growth in S. epidermidis

Usually, to characterize bacterial cells' susceptibility to antimicrobials, basic microbiology techniques such as serial dilutions or disk assays are used. In this work, we present an approach focused on combining static light scattering (SLS) and ultra-/small angle X-ray scattering (USAXS/SAXS). This approach was used to support microbiology techniques, with the aim of understanding the structural changes caused to bacteria when they are exposed to different stresses like pH, oxidation, and surfactants. Using USAXS/SAXS and SLS data, we developed a detailed multiscale model for a Gram-positive bacterium, S. epidermidis, and we extracted information regarding changes in the overall size and cell thickness induced by different stresses (i.e., pH and hydrogen peroxide). Increasing the concentration of hydrogen peroxide leads to a progressive reduction in cell wall thickness. Moreover, the concomitant use of pH and hydrogen peroxide provides evidence for a synergy in inhibiting the S. epidermidis growth. These promising results will be used as a starting base to further investigate more complex formulations and improve/refine the data modeling of bacteria in the small angle scattering regime.

Antimicrobial peptides and the gut microbiome in inflammatory bowel disease

Antimicrobial peptides (AMP) are highly diverse and dynamic molecules that are expressed by specific intestinal epithelial cells, Paneth cells, as well as immune cells in the gastrointestinal (GI) tract. They play critical roles in maintaining tolerance to gut microbiota and protecting against enteric infections. Given that disruptions in tolerance to commensal microbiota and loss of barrier function play major roles in the pathogenesis of inflammatory bowel disease (IBD) and converge on the function of AMP, the significance of AMP as potential biomarkers and novel therapeutic targets in IBD have been increasingly recognized in recent years. In this frontier article, we discuss the function and mechanisms of AMP in the GI tract, examine the interaction of AMP with the gut microbiome, explore the role of AMP in the pathogenesis of IBD, and review translational applications of AMP in patients with IBD.

Dissecting the relationship between antimicrobial peptides and mesenchymal stem cells

Among the various biological properties presented by Mesenchymal Stem Cells (MSCs), their ability to control the immune response and fight pathogen infection through the production of antimicrobial peptides (AMPs) have been the subject of intense research in recent years. AMPs secreted by MSCs exhibit activity against a wide range of microorganisms, including bacteria, fungi, yeasts, and viruses. The main AMPs produced by these cells are hepcidin, cathelicidin LL-37, and β-defensin-2. In addition to acting against pathogens, those AMPs have also been shown to interact with MSCs to modulate MSC proliferation, migration, and regeneration, indicating that such peptides exert a more diverse biological effect than initially thought. In the present review, we discuss the production of AMPs by MSCs, revise the multiple functions of these peptides, including their influence over MSCs, and present an overview of clinical situations in which the antimicrobial properties of MSCs may be explored for therapy. Finally, we discuss possibilities of combining MSCs and AMPs to generate improved therapeutic strategies.

Role of pH in skin cleansing

BACKGROUND

The importance of maintaining the acid-mantle of human stratum corneum to maintain its healthy barrier and skin's biological functions such as desquamation and lipid biosynthesis is well recognized in the literature. An outcome of this has been an increase in the number of skincare products formulated at or near the skin pH with an implication that a product formulated at skin pH will be good for skin. Such an assumption often does not take into account the specific interactions of ingredients in the product with the stratum corneum under skin pH conditions.

OBJECTIVE

The objective of this research was to determine whether a skin cleansing product by virtue of its pH being same as "skin pH" is milder to skin.

METHODS

A well established Forearm Controlled Application Test (FCAT) protocol was used in clinical studies to compare "skin pH" cleansing systems with neutral pH cleansing systems. Specifically, certain commercially available "skin pH" cleansing bars were compared with a neutral pH syndet bar in two separate FCAT studies. Since these bars differed in their surfactant composition, in a separate FCAT study, two identical prototype bar formulations differed only in their pH were compared. Additionally, two body wash liquid prototypes, identical in composition but differing only in their pH were also compared in another FCAT study.

RESULTS

The results obtained showed that skin-cleansing systems formulated solely or predominantly with anionic surfactants under skin pH conditions can result in increased skin dryness and irritation compared to those under neutral pH conditions. The results are explained in terms of the increased electrostatic interaction of anionic surfactants with stratum corneum under low pH conditions compared to neutral pH conditions.

CONCLUSION

Skin-cleansing systems formulated solely or predominantly with anionic surfactants under skin pH conditions can result in increased skin dryness and irritation compared to those under neutral pH conditions. Any skin cleansing product by virtue of its pH being same as that of "skin pH" does not guarantee that it will be good for skin. The mildness of a cleanser will be determined by the interactions of its surfactants and other ingredients with stratum corneum under its formulated pH conditions.

The Double-Edged Sword of Beta2-Microglobulin in Antibacterial Properties and Amyloid Fibril-Mediated Cytotoxicity

Beta2-microglobulin (B2M) a key component of major histocompatibility complex class I molecules, which aid cytotoxic T-lymphocyte (CTL) immune response. However, the majority of studies of B2M have focused only on amyloid fibrils in pathogenesis to the neglect of its role of antimicrobial activity. Indeed, B2M also plays an important role in innate defense and does not only function as an adjuvant for CTL response. A previous study discovered that human aggregated B2M binds the surface protein structure in Streptococci, and a similar study revealed that sB2M-9, derived from native B2M, functions as an antibacterial chemokine that binds Staphylococcus aureus. An investigation of sB2M-9 exhibiting an early lymphocyte recruitment in the human respiratory epithelium with bacterial challenge may uncover previously unrecognized aspects of B2M in the body’s innate defense against Mycobactrium tuberculosis. B2M possesses antimicrobial activity that operates primarily under pH-dependent acidic conditions at which B2M and fragmented B2M may become a nucleus seed that triggers self-aggregation into distinct states, such as oligomers and amyloid fibrils. Modified B2M can act as an antimicrobial peptide (AMP) against a wide range of microbes. Specifically, these AMPs disrupt microbe membranes, a feature similar to that of amyloid fibril mediated cytotoxicity toward eukaryotes. This study investigated two similar but nonidentical effects of B2M: the physiological role of B2M, in which it potentially acts against microbes in innate defense and the role of B2M in amyloid fibrils, in which it disrupts the membrane of pathological cells. Moreover, we explored the pH-governing antibacterial activity of B2M and acidic pH mediated B2M amyloid fibrils underlying such cytotoxicity.

Mg(OH)2 nanoparticles enhance the antibacterial activities of macrophages by activating the reactive oxygen species

Infection often causes disastrous consequences in all fields of clinical medicine, especially orthopedics. Hence, critical efforts are being made to engineer novel nanomaterials for the treatment of orthopedic infections due to the high biocompatibility and antibacterial properties they possess. The purpose of this study was to investigate the antibacterial effects of magnesium hydroxide (Mg(OH)₂ ) nanoparticles (NPs) in vitro and determine their possible mechanisms of action. In this study, Escherichia coli was selected as the pathogenic bacteria and it was found that Mg(OH)₂ NPs significantly inhibited the growth of E. coli by promoting nucleic acid leakage, inhibiting protein synthesis, and suppressing the metabolic activity. The minimum inhibitory concentration for these bacteria was determined to be 4.4 μg/ml. In vitro flow cytometry and immunofluorescence tests indicated that Mg(OH)₂ NPs induced the macrophages to generate reactive oxygen species to kill the bacteria. To understand the mechanisms involved in this process, western blotting was performed and it was found that Mg(OH)₂ NPs activated the phosphatidylinositol-3-kinase/serine-threonine kinase (PI3K/Akt) signaling pathway of macrophages to enhance their phagocytosis with no obvious cytotoxicity. Thus, Mg(OH)₂ NPs are a suitable choice to develop promising agents or coating materials for the treatment of clinically widespread infections in view of their safety, biocompatibility, and powerful antibacterial properties.

Human Antimicrobial Peptide Hepcidin 25-Induced Apoptosis in Candida albicans

Hepcidin 25 (hep 25) is a cysteine-rich 25-amino acid antimicrobial peptide containing the amino-terminal Cu(II)/Ni(II)-binding (ATCUN) motif. Upon metal binding, the ATCUN motif is known to be involved in the generation of reactive oxygen species (ROS), especially hydrogen peroxide and hydroxyl radicals, which act against different bacterial species. However, the antifungal activity and its correlation to the Cu(II)-ATCUN complex of Hep 25 are still poorly understood. Here, we found that ROS accumulation plays an important role in the fungicidal activity of hep 25 against Candida albicans. In addition, Annexin V-FITC staining and TUNEL assay results provide clues about the apoptosis induced by hep 25. Moreover, hep 25 also increases the generation of ROS, possibly because of copper binding to the ATCUN motif, which is relevant to its activity against C. albicans. Finally, the C. albicans killing action of hep 25 is an energy- and temperature-dependent process that does not involve targeting the membrane. Taken together, our results provide new insights into the mechanisms of hep 25 against C. albicans cells and the potential use of hep 25 and its derivatives as novel antifungal agents.

Opossum Cathelicidins Exhibit Antimicrobial Activity Against a Broad Spectrum of Pathogens Including West Nile Virus

This study aimed to characterize cathelicidins from the gray short-tailed opossum in silico and experimentally validate their antimicrobial effects against various pathogenic bacteria and West Nile virus (WNV). Genome-wide in silico analysis against the current genome assembly of the gray short-tailed opossum yielded 56 classical antimicrobial peptides (AMPs) from eight different families, among which 19 cathelicidins, namely ModoCath1 – 19, were analyzed in silico to predict their antimicrobial domains and three of which, ModoCath1, -5, and -6, were further experimentally evaluated for their antimicrobial activity, and were found to exhibit a wide spectrum of antimicroial effects against a panel of gram-positive and gram-negative bacterial strains. In addition, these peptides displayed low-to-moderate cytotoxicity in mammalian cells as well as stability in serum and various salt and pH conditions. Circular dichroism analysis of the spectra resulting from interactions between ModoCaths and lipopolysaccharides (LPS) showed formation of a helical structure, while a dual-dye membrane disruption assay and scanning electron microscopy analysis revealed that ModoCaths exerted bactericidal effects by causing membrane damage. Furthermore, ModoCath5 displayed potent antiviral activity against WNV by inhibiting viral replication, suggesting that opossum cathelicidins may serve as potentially novel antimicrobial endogenous substances of mammalian origin, considering their large number. Moreover, analysis of publicly available RNA-seq data revealed the expression of eight ModoCaths from five different tissues, suggesting that gray short-tailed opossums may be an interesting source of cathelicidins with diverse characteristics.

Hepcidin cDNA evolution in vertebrates

Hepcidin, belonging to the β-defensin family, was isolated for the first time from plasma and human urine. It is a cationic peptide, rich in cysteine bound with four disulfide bridges, which plays a major role in innate immunity and iron homeostasis. Some vertebrate species have multiple hepcidin homolog genes and each contains only one copy that functions as an iron regulator except hepcidin sequences in the pigeon (Columba livia). The aim of this chapter is to investigate the molecular evolution of several hepcidin gene from searches of the literature and public genomic databases from 17 different species, all among the vertebrates.

Exposure of Escherichia coli to human hepcidin results in differential expression of genes associated with iron homeostasis and oxidative stress

Hepcidin belongs to the antimicrobial peptide family but has weak activity with regards to bacterial killing. The regulatory function of hepcidin in humans serves to maintain an iron-restricted environment that limits the growth of pathogens; this study explored whether hepcidin affected bacterial iron homeostasis and oxidative stress using the model organism Escherichia coli. Using the Miller assay it was determined that under low iron availability exposure to sub-inhibitory doses of hepcidin (4-12μM) led to 2-fold and 4-fold increases in the expression of ftnA and bfd, respectively (P < 0.05), in both a wild type (WT) and Δfur (ferric uptake regulator) background. Quantitative real-time PCR analysis of oxyR and sodA, treated with 4 or 8 μM of hepcidin showed that expression of these genes was significantly (P < 0.05) increased, whereas expression of lexA was unchanged, indicating that hepcidin likely mediated oxidative stress but did not induce DNA damage.

Recent progress on the application of 2H solid-state NMR to probe the interaction of antimicrobial peptides with intact bacteria

Discoveries relating to innate immunity and antimicrobial peptides (AMPs) granted Bruce Beutler and Jules Hoffmann a Nobel prize in medicine in 2011, and opened up new avenues for the development of therapies against infections, and even cancers. The mechanisms by which AMPs interact with, and ultimately disrupt, bacterial cell membranes is still, to a large extent, incompletely understood. Up until recently, this mechanism was studied using model lipid membranes that failed to reproduce the complexity of molecular interactions present in real cells comprising lipids but also membrane proteins, a cell wall containing peptidoglycan or lipopolysaccharides, and other molecules. In this review, we focus on recent attempts to study, at the molecular level, the interaction between cationic AMPs and intact bacteria, by ²H solid-state NMR. Specifically-labeled lipids allow us to focus on the interaction of AMPs with the heart of the bacterial membrane, and measure the lipid order and its variation upon interaction with various peptides. We will review the important parameters to consider in such a study, and summarize the results obtained in the past 5years on various peptides, in particular aurein 1.2, caerin 1.1, MSI-78 and CA(1-8)M(1-10). This article is part of a Special Issue entitled: Biophysics in Canada, edited by Lewis Kay, John Baenziger, Albert Berghuis and Peter Tieleman.

Crabrolin, a natural antimicrobial peptide: structural properties

A joint application of experimental and computational approaches has revealed the exceptionally high attitude of crabrolin, a 13-residue peptide with sequence FLPLILRKIVTAL-NH₂ , to adopt alpha-helix conformation not only in membrane-mimicking solvents but also in the presence of a not negligible amount of water. Our study shows that this propensity essentially resides in the intrinsic thermodynamic stability of alpha-helix conformation whose kinetic stability is drastically reduced in water solvent. Our analysis suggests that this is due to two effects enhanced by water: a more local effect consisting of the demolition of intra-peptide H-bonds, essential for the alpha-helix formation, and a bulk - electrostatic - effect favoring conformational states more polar than alpha-helix. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Mapping membrane activity in undiscovered peptide sequence space using machine learning

There are some ∼1,100 known antimicrobial peptides (AMPs), which permeabilize microbial membranes but have diverse sequences. Here, we develop a support vector machine (SVM)-based classifier to investigate ⍺-helical AMPs and the interrelated nature of their functional commonality and sequence homology. SVM is used to search the undiscovered peptide sequence space and identify Pareto-optimal candidates that simultaneously maximize the distance σ from the SVM hyperplane (thus maximize its "antimicrobialness") and its ⍺-helicity, but minimize mutational distance to known AMPs. By calibrating SVM machine learning results with killing assays and small-angle X-ray scattering (SAXS), we find that the SVM metric σ correlates not with a peptide's minimum inhibitory concentration (MIC), but rather its ability to generate negative Gaussian membrane curvature. This surprising result provides a topological basis for membrane activity common to AMPs. Moreover, we highlight an important distinction between the maximal recognizability of a sequence to a trained AMP classifier (its ability to generate membrane curvature) and its maximal antimicrobial efficacy. As mutational distances are increased from known AMPs, we find AMP-like sequences that are increasingly difficult for nature to discover via simple mutation. Using the sequence map as a discovery tool, we find a unexpectedly diverse taxonomy of sequences that are just as membrane-active as known AMPs, but with a broad range of primary functions distinct from AMP functions, including endogenous neuropeptides, viral fusion proteins, topogenic peptides, and amyloids. The SVM classifier is useful as a general detector of membrane activity in peptide sequences.

pH Dependent Antimicrobial Peptides and Proteins, Their Mechanisms of Action and Potential as Therapeutic Agents

Antimicrobial peptides (AMPs) are potent antibiotics of the innate immune system that have been extensively investigated as a potential solution to the global problem of infectious diseases caused by pathogenic microbes. A group of AMPs that are increasingly being reported are those that utilise pH dependent antimicrobial mechanisms, and here we review research into this area. This review shows that these antimicrobial molecules are produced by a diverse spectrum of creatures, including vertebrates and invertebrates, and are primarily cationic, although a number of anionic examples are known. Some of these molecules exhibit high pH optima for their antimicrobial activity but in most cases, these AMPs show activity against microbes that present low pH optima, which reflects the acidic pH generally found at their sites of action, particularly the skin. The modes of action used by these molecules are based on a number of major structure/function relationships, which include metal ion binding, changes to net charge and conformational plasticity, and primarily involve the protonation of histidine, aspartic acid and glutamic acid residues at low pH. The pH dependent activity of pore forming antimicrobial proteins involves mechanisms that generally differ fundamentally to those used by pH dependent AMPs, which can be described by the carpet, toroidal pore and barrel-stave pore models of membrane interaction. A number of pH dependent AMPs and antimicrobial proteins have been developed for medical purposes and have successfully completed clinical trials, including kappacins, LL-37, histatins and lactoferrin, along with a number of their derivatives. Major examples of the therapeutic application of these antimicrobial molecules include wound healing as well as the treatment of multiple cancers and infections due to viruses, bacteria and fungi. In general, these applications involve topical administration, such as the use of mouth washes, cream formulations and hydrogel delivery systems. Nonetheless, many pH dependent AMPs and antimicrobial proteins have yet to be fully characterized and these molecules, as a whole, represent an untapped source of novel biologically active agents that could aid fulfillment of the urgent need for alternatives to conventional antibiotics, helping to avert a return to the pre-antibiotic era.

Recombinant overexpression of camel hepcidin cDNA in Pichia pastoris: purification and characterization of the polyHis-tagged peptide HepcD-His

Hepcidin, a liver-expressed antimicrobial peptide, has been demonstrated to act as an iron regulatory hormone as well as to exert a wide spectrum of antimicrobial activity. The aim of this work was the expression, as secreted peptide, purification, and characterization of a new recombinant polyHis-tagged camel hepcidin (HepcD-His) in yeast Pichia pastoris. The use of this eukaryotic expression system, for the production of HepcD-His, having 6 histidine residues at its C terminus, was simpler and more efficient compared with the use of the prokaryotic system Escherichia coli. Indeed, a single purification step was required to isolate the soluble hepcidin with purity estimated more that 94% and a yield of 2.8 against 0.2 mg/L for the E coli system. Matrix-assisted laser desorption/ionization time-of-flight (TOF)/TOF mass spectrometry of the purified HepcD-His showed 2 major peaks at m/z 4524.64 and 4634.56 corresponding to camel hepcidin with 39 and 40 amino acids. Evaluation of disulfide bond connectivity with the Ellman method showed an absence of free thiol groups, testifying that the 8 cysteine residues in the peptide are displayed, forming 4 disulfide bridges. Circular dichroism spectroscopy showed that camel hepcidin structure was significantly modified at high temperature of 90°C and returns to its original structure when incubation temperature drops back to 20°C. Interestingly, this peptide showed also a greater bactericidal activity, at low concentration of 9.5μM, against E coli, than the synthetic analog DH3. Thus, the production, at a large scale, of the recombinant camel hepcidin, HepcD-His, may be helpful for future therapeutic applications including bacterial infection diseases.

Low pH Enhances the Action of Maximin H5 against Staphylococcus aureus and Helps Mediate Lysylated Phosphatidylglycerol-Induced Resistance

Maximin H5 (MH5) is an amphibian antimicrobial peptide specifically targeting Staphylococcus aureus. At pH 6, the peptide showed an improved ability to penetrate (ΔΠ = 6.2 mN m(-1)) and lyse (lysis = 48%) Staphylococcus aureus membrane mimics, which incorporated physiological levels of lysylated phosphatidylglycerol (Lys-PG, 60%), compared to that at pH 7 (ΔΠ = 5.6 mN m(-1) and lysis = 40% at pH 7) where levels of Lys-PG are lower (40%). The peptide therefore appears to have optimal function at pH levels known to be optimal for the organism's growth. MH5 killed S. aureus (minimum inhibitory concentration of 90 μM) via membranolytic mechanisms that involved the stabilization of α-helical structure (approximately 45-50%) and showed similarities to the "Carpet" mechanism based on its ability to increase the rigidity (Cs(-1) = 109.94 mN m(-1)) and thermodynamic stability (ΔGmix = -3.0) of physiologically relevant S. aureus membrane mimics at pH 6. On the basis of theoretical analysis, this mechanism might involve the use of a tilted peptide structure, and efficacy was noted to vary inversely with the Lys-PG content of S. aureus membrane mimics for each pH studied (R(2) ∼ 0.97), which led to the suggestion that under biologically relevant conditions, low pH helps mediate Lys-PG-induced resistance in S. aureus to MH5 antibacterial action. The peptide showed a lack of hemolytic activity (<2% hemolysis) and merits further investigation as a potential template for development as an antistaphylococcal agent in medically and biotechnically relevant areas.

A Rapid and Quantitative Flow Cytometry Method for the Analysis of Membrane Disruptive Antimicrobial Activity

We describe a microbial flow cytometry method that quantifies within 3 hours antimicrobial peptide (AMP) activity, termed Minimum Membrane Disruptive Concentration (MDC). Increasing peptide concentration positively correlates with the extent of bacterial membrane disruption and the calculated MDC is equivalent to its MBC. The activity of AMPs representing three different membranolytic modes of action could be determined for a range of Gram positive and negative bacteria, including the ESKAPE pathogens, E. coli and MRSA. By using the MDC50 concentration of the parent AMP, the method provides high-throughput, quantitative screening of AMP analogues. A unique feature of the MDC assay is that it directly measures peptide/bacteria interactions and lysed cell numbers rather than bacteria survival as with MIC and MBC assays. With the threat of multi-drug resistant bacteria, this high-throughput MDC assay has the potential to aid in the development of novel antimicrobials that target bacteria with improved efficacy.

Differential scanning calorimetry of whole Escherichia coli treated with the antimicrobial peptide MSI-78 indicate a multi-hit mechanism with ribosomes as a novel target

Differential Scanning Calorimetry (DSC) of intact Escherichia coli (E. coli) was used to identify non-lipidic targets of the antimicrobial peptide (AMP) MSI-78. The DSC thermograms revealed that, in addition to its known lytic properties, MSI-78 also has a striking effect on ribosomes. MSI-78’s effect on DSC scans of bacteria was similar to that of kanamycin, an antibiotic drug known to target the 30S small ribosomal subunit. An in vitro transcription/translation assay helped confirm MSI-78’s targeting of ribosomes. The scrambled version of MSI-78 also affected the ribosome peak of the DSC scans, but required greater amounts of peptide to cause a similar effect to the unscrambled peptide. Furthermore, the effect of the scrambled peptide was not specific to the ribosomes; other regions of the DSC thermogram were also affected. These results suggest that MSI-78’s effects on E. coli are at least somewhat dependent on its particular structural features, rather than a sole function of its overall charge and hydrophobicity. When considered along with earlier work detailing MSI-78’s membrane lytic properties, it appears that MSI-78 operates via a multi-hit mechanism with multiple targets.

Antimicrobial peptides and their interaction with biofilms of medically relevant bacteria

Biofilm-associated infections represent one of the major threats of modern medicine. Biofilm-forming bacteria are encased in a complex mixture of extracellular polymeric substances (EPS) and acquire properties that render them highly tolerant to conventional antibiotics and host immune response. Therefore, there is a pressing demand of new drugs active against microbial biofilms. In this regard, antimicrobial peptides (AMPs) represent an option taken increasingly in consideration. After dissecting the peculiar biofilm features that may greatly affect the development of new antibiofilm drugs, the present article provides a general overview of the rationale behind the use of AMPs against biofilms of medically relevant bacteria and on the possible mechanisms of AMP-antibiofilm activity. An analysis of the interactions of AMPs with biofilm components, especially those constituting the EPS, and the obstacles and/or opportunities that may arise from such interactions in the development of new AMP-based antibiofilm strategies is also presented and discussed. This article is part of a Special Issue entitled: Antimicrobial Peptides edited by Karl Lohner and Kai Hilpert.

Insights into the antimicrobial properties of hepcidins: advantages and drawbacks as potential therapeutic agents

The increasing frequency of multi-drug resistant microorganisms has driven research into alternative therapeutic strategies. In this respect, natural antimicrobial peptides (AMPs) hold much promise as candidates for the development of novel antibiotics. However, AMPs have some intrinsic drawbacks, such as partial degradation by host proteases or inhibition by host body fluid composition, potential toxicity, and high production costs. This review focuses on the hepcidins, which are peptides produced by the human liver with a known role in iron homeostasis, as well by numerous other organisms (including fish, reptiles, other mammals), and their potential as antibacterial and antifungal agents. Interestingly, the antimicrobial properties of human hepcidins are enhanced at acidic pH, rendering these peptides appealing for the design of new drugs targeting infections that occur in body areas with acidic physiological pH. This review not only considers current research on the direct killing activity of these peptides, but evaluates the potential application of these molecules as coating agents preventing biofilm formation and critically assesses technical obstacles preventing their therapeutic application.

Hepcidin Mitigates Renal Ischemia-Reperfusion Injury by Modulating Systemic Iron Homeostasis

Iron-mediated oxidative stress is implicated in the pathogenesis of renal ischemia-reperfusion injury. Hepcidin is an endogenous acute phase hepatic hormone that prevents iron export from cells by inducing degradation of the only known iron export protein, ferroportin. In this study, we used a mouse model to investigate the effect of renal ischemia-reperfusion injury on systemic iron homeostasis and determine if dynamic modulation of iron homeostasis with hepcidin has therapeutic benefit in the treatment of AKI. Renal ischemia-reperfusion injury induced hepatosplenic iron export through increased ferroportin expression, which resulted in hepatosplenic iron depletion and an increase in serum and kidney nonheme iron levels. Exogenous hepcidin treatment prevented renal ischemia-reperfusion-induced changes in iron homeostasis. Hepcidin also decreased kidney ferroportin expression and increased the expression of cytoprotective H-ferritin. Hepcidin-induced restoration of iron homeostasis was accompanied by a significant reduction in ischemia-reperfusion-induced tubular injury, apoptosis, renal oxidative stress, and inflammatory cell infiltration. Hepcidin -: deficient mice demonstrated increased susceptibility to ischemia-reperfusion injury compared with wild-type mice. Reconstituting hepcidin-deficient mice with exogenous hepcidin induced hepatic iron sequestration, attenuated the reduction in renal H-ferritin and reduced renal oxidative stress, apoptosis, inflammation, and tubular injury. Hepcidin-mediated protection was associated with reduced serum IL-6 levels. In summary, renal ischemia-reperfusion injury results in profound alterations in systemic iron homeostasis. Hepcidin treatment restores iron homeostasis and reduces inflammation to mediate protection in renal ischemia-reperfusion injury, suggesting that hepcidin-ferroportin pathway holds promise as a novel therapeutic target in the treatment of AKI.

Lipopolysaccharide induces amyloid formation of antimicrobial peptide HAL-2

Lipopolysaccharide (LPS), the important component of the outer membrane of Gram-negative bacteria, contributes to the integrity of the outer membrane and protects the cell against bactericidal agents, including antimicrobial peptides. However, the mechanisms of interaction between antimicrobial peptides and LPS are not clearly understood. Halictines-2 (HAL-2), one of the novel antimicrobial peptides, was isolated from the venom of the eusocial bee Halictus sexcinctus. HAL-2 has exhibited potent antimicrobial activity against Gram-positive and Gram-negative bacteria and even against cancer cells. Here, we studied the interactions between HAL-2 and LPS to elucidate the antibacterial mechanism of HAL-2 in vitro. Our results show that HAL-2 adopts a significant degree of β-strand structure in the presence of LPS. LPS is capable of inducing HAL-2 amyloid formation, which may play a vital role in its antimicrobial activity.

Molecular characterization of a novel hepcidin (HepcD) from Camelus dromedarius. Synthetic peptide forms exhibit antibacterial activity

Hepcidin is a cysteine-rich peptide widely characterized in immunological processes and antimicrobial activity in several vertebrate species. Obviously, this hormone plays a central role in the regulation of systemic iron homeostasis. However, its role in camelids' immune response and whether it is involved in antibacterial immunity have not yet been proven. In this study, we characterized the Arabian camel hepcidin nucleotide sequence with an open reading frame of 252 bp encoding an 83-amino acid preprohepcidin peptide. Eight cysteine key residues conserved in all mammalian hepcidin sequences were identified. The model structure analysis of hepcidin-25 peptide showed a high homology structure and sequence identity to the human hepcidin. Two different hepcidin-25 analogs manually synthesized by SPPS shared significant cytotoxic capacity toward the Gram-negative bacterium Escherichia coli American Type Culture Collection (ATCC) 8739 as well as the Gram-positive bacteria Bacillus subtilis ATCC 11779 and Staphylococcus aureus ATCC 6538 in vitro. The three disulfide bridges hepcidin analog demonstrated bactericidal activity, against B. subtilis ATCC 11779 and S. aureus ATCC 6538 strains, at the concentration of 15 μM (50 µg/ml) or above at pH 6.2. This result correlates with the revealed structural features suggesting that camel hepcidin is proposed to be involved in antibacterial process of innate immune response.

Human antimicrobial peptides and proteins

As the key components of innate immunity, human host defense antimicrobial peptides and proteins (AMPs) play a critical role in warding off invading microbial pathogens. In addition, AMPs can possess other biological functions such as apoptosis, wound healing, and immune modulation. This article provides an overview on the identification, activity, 3D structure, and mechanism of action of human AMPs selected from the antimicrobial peptide database. Over 100 such peptides have been identified from a variety of tissues and epithelial surfaces, including skin, eyes, ears, mouths, gut, immune, nervous and urinary systems. These peptides vary from 10 to 150 amino acids with a net charge between -3 and +20 and a hydrophobic content below 60%. The sequence diversity enables human AMPs to adopt various 3D structures and to attack pathogens by different mechanisms. While α-defensin HD-6 can self-assemble on the bacterial surface into nanonets to entangle bacteria, both HNP-1 and β-defensin hBD-3 are able to block cell wall biosynthesis by binding to lipid II. Lysozyme is well-characterized to cleave bacterial cell wall polysaccharides but can also kill bacteria by a non-catalytic mechanism. The two hydrophobic domains in the long amphipathic α-helix of human cathelicidin LL-37 lays the basis for binding and disrupting the curved anionic bacterial membrane surfaces by forming pores or via the carpet model. Furthermore, dermcidin may serve as ion channel by forming a long helix-bundle structure. In addition, the C-type lectin RegIIIα can initially recognize bacterial peptidoglycans followed by pore formation in the membrane. Finally, histatin 5 and GAPDH(2-32) can enter microbial cells to exert their effects. It appears that granulysin enters cells and kills intracellular pathogens with the aid of pore-forming perforin. This arsenal of human defense proteins not only keeps us healthy but also inspires the development of a new generation of personalized medicine to combat drug-resistant superbugs, fungi, viruses, parasites, or cancer. Alternatively, multiple factors (e.g., albumin, arginine, butyrate, calcium, cyclic AMP, isoleucine, short-chain fatty acids, UV B light, vitamin D, and zinc) are able to induce the expression of antimicrobial peptides, opening new avenues to the development of anti-infectious drugs.

Inhibitory effect of the human liver-derived antimicrobial peptide hepcidin 20 on biofilms of polysaccharide intercellular adhesin (PIA)-positive and PIA-negative strains of Staphylococcus epidermidis

Staphylococcus epidermidis plays a major role in biofilm-related medical device infections. Herein the anti-biofilm activity of the human liver-derived antimicrobial peptide hepcidin 20 (hep20) was evaluated against polysaccharide intercellular adhesin (PIA)-positive and PIA-negative clinical isolates of S. epidermidis. Hep20 markedly inhibited biofilm formation and bacterial cell metabolism of PIA-positive and PIA-negative strains, but the decrease in biofilm biomass only partially correlated with a decrease in viable bacteria. Confocal microscope images revealed that, in the presence of hep20, both PIA-positive and PIA-negative strains formed biofilms with altered architectures and reduced amounts of extracellular matrix. Co-incubation of hep20 with vancomycin produced no synergistic effect, evaluated as number of viable cells, both in preventing biofilm formation and in treating preformed biofilms. In contrast, biofilms obtained in the presence of hep20, and then exposed to vancomycin, displayed an increased susceptibility to vancomycin. These results suggest that hep20 may inhibit the production/accumulation of biofilm extracellular matrix.

Neisseria gonorrhoeae modulates iron-limiting innate immune defenses in macrophages

Neisseria gonorrhoeae is a strict human pathogen that causes the sexually transmitted infection termed gonorrhea. The gonococcus can survive extracellularly and intracellularly, but in both environments the bacteria must acquire iron from host proteins for survival. However, upon infection the host uses a defensive response by limiting the bioavailability of iron by a number of mechanisms including the enhanced expression of hepcidin, the master iron-regulating hormone, which reduces iron uptake from the gut and retains iron in macrophages. The host also secretes the antibacterial protein NGAL, which sequesters bacterial siderophores and therefore inhibits bacterial growth. To learn whether intracellular gonococci can subvert this defensive response, we examined expression of host genes that encode proteins involved in modulating levels of intracellular iron. We found that N. gonorrhoeae can survive in association (tightly adherent and intracellular) with monocytes and macrophages and upregulates a panel of its iron-responsive genes in this environment. We also found that gonococcal infection of human monocytes or murine macrophages resulted in the upregulation of hepcidin, NGAL, and NRAMP1 as well as downregulation of the expression of the gene encoding the short chain 3-hydroxybutyrate dehydrogenase (BDH2); BDH2 catalyzes the production of the mammalian siderophore 2,5-DHBA involved in chelating and detoxifying iron. Based on these findings, we propose that N. gonorrhoeae can subvert the iron-limiting innate immune defenses to facilitate iron acquisition and intracellular survival.

Antifungal Activity of the Noncytotoxic Human Peptide Hepcidin 20 against Fluconazole-Resistant Candida glabrata in Human Vaginal Fluid

Vaginal infections caused by Candida glabrata are difficult to eradicate due to this species' scarce susceptibility to azoles. Previous studies have shown that the human cationic peptide hepcidin 20 (Hep-20) exerts fungicidal activity in sodium phosphate buffer against a panel of C. glabrata clinical isolates with different levels of susceptibility to fluconazole. In addition, the activity of the peptide was potentiated under acidic conditions, suggesting an application in the topical treatment of vaginal infections. To investigate whether the peptide activity could be maintained in biological fluids, in this study the antifungal activity of Hep-20 was evaluated by a killing assay in (i) a vaginal fluid simulant (VFS) and in (ii) human vaginal fluid (HVF) collected from three healthy donors. The results obtained indicated that the activity of the peptide was maintained in VFS and HVF supplemented with EDTA. Interestingly, the fungicidal activity of Hep-20 was enhanced in HVF compared to that observed in VFS, with a minimal fungicidal concentration of 25 μM for all donors. No cytotoxic effect on human cells was exerted by Hep-20 at concentrations ranging from 6.25 to 100 μM, as shown by 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide tetrazolium salt (XTT) reduction assay and propidium iodide staining. A piece of indirect evidence of Hep-20 stability was also obtained from coincubation experiments of the peptide with HVF at 37°C for 90 min and for 24 h. Collectively, these results indicate that this peptide should be further studied as a novel therapeutic agent for the topical treatment of vaginal C. glabrata infections.