Human beta-defensin 3 inhibits cell wall biosynthesis in Staphylococci

Multitarget Approaches against Multiresistant Superbugs

Despite efforts to develop new antibiotics, antibacterial resistance still develops too fast for drug discovery to keep pace. Often, resistance against a new drug develops even before it reaches the market. This continued resistance crisis has demonstrated that resistance to antibiotics with single protein targets develops too rapidly to be sustainable. Most successful long-established antibiotics target more than one molecule or possess targets, which are encoded by multiple genes. This realization has motivated a change in antibiotic development toward drug candidates with multiple targets. Some mechanisms of action presuppose multiple targets or at least multiple effects, such as targeting the cytoplasmic membrane or the carrier molecule bactoprenol phosphate and are therefore particularly promising. Moreover, combination therapy approaches are being developed to break antibiotic resistance or to sensitize bacteria to antibiotic action. In this Review, we provide an overview of antibacterial multitarget approaches and the mechanisms behind them.

The Antimicrobial Peptide Human Beta-Defensin 2 Inhibits Biofilm Production of Pseudomonas aeruginosa Without Compromising Metabolic Activity

Biofilm production is a key virulence factor that facilitates bacterial colonization on host surfaces and is regulated by complex pathways, including quorum sensing, that also control pigment production, among others. To limit colonization, epithelial cells, as part of the first line of defense, utilize a variety of antimicrobial peptides (AMPs) including defensins. Pore formation is the best investigated mechanism for the bactericidal activity of AMPs. Considering the induction of human beta-defensin 2 (HBD2) secretion to the epithelial surface in response to bacteria and the importance of biofilm in microbial infection, we hypothesized that HBD2 has biofilm inhibitory activity. We assessed the viability and biofilm formation of a pyorubin-producing Pseudomonas aeruginosa strain in the presence and absence of HBD2 in comparison to the highly bactericidal HBD3. At nanomolar concentrations, HBD2 - independent of its chiral state - significantly reduced biofilm formation but not metabolic activity, unlike HBD3, which reduced biofilm and metabolic activity to the same degree. A similar discrepancy between biofilm inhibition and maintenance of metabolic activity was also observed in HBD2 treated Acinetobacter baumannii, another Gram-negative bacterium. There was no evidence for HBD2 interference with the regulation of biofilm production. The expression of biofilm-related genes and the extracellular accumulation of pyorubin pigment, another quorum sensing controlled product, did not differ significantly between HBD2 treated and control bacteria, and in silico modeling did not support direct binding of HBD2 to quorum sensing molecules. However, alterations in the outer membrane protein profile accompanied by surface topology changes, documented by atomic force microscopy, was observed after HBD2 treatment. This suggests that HBD2 induces structural changes that interfere with the transport of biofilm precursors into the extracellular space. Taken together, these data support a novel mechanism of biofilm inhibition by nanomolar concentrations of HBD2 that is independent of biofilm regulatory pathways.

Influence of disulfide bonds in human beta defensin-3 on its strain specific activity against Gram-negative bacteria

Antimicrobial peptides (AMPs) play an important role in the host defense against various microbes. One of the most efficient human AMPs is the human beta defensin-3 (hBD-3) which is produced by, e.g. keratinocytes and lung epithelial cells. However, the structure-function relationship for AMPs and in particular for defensins with their typical three disulfide bonds is still poorly understood. In this study the importance of the three disulfide bonds for the activity of the AMPs is investigated with biological assays and with biophysical experiments utilizing different membrane reconstitution systems. The activities of natural hBD-3, hBD-3-c (cyclic variant with one disulfide bond), and hBD-3-l (linear variant without disulfide bonds) and fragments thereof were tested against specific Gram-negative bacteria. Furthermore, hemolytic and cytotoxic activities were analyzed as well as the potency to neutralize immune cell stimulation of lipopolysaccharide (LPS). Experiments using reconstituted lipid matrices composed of phospholipids or LPS purified from the respective Gram-negative bacteria, showed that the membrane activity of all three hBD-3 peptides is decisive for their capability to kill bacteria and to neutralize LPS. In most of the test systems the linear hBD-3-l showed the highest activity. It was also the only peptide significantly active against polymyxin B-resistant Proteus mirabilis R45. However, the stability of hBD-3 against protease activity decreases with decreasing number of disulfide bonds. This study demonstrates that the refining of AMP structures can generate more active compounds against certain strains.

Ca2+-Daptomycin targets cell wall biosynthesis by forming a tripartite complex with undecaprenyl-coupled intermediates and membrane lipids

The lipopeptide daptomycin is used as an antibiotic to treat severe infections with gram-positive pathogens, such as methicillin resistant Staphylococcus aureus (MRSA) and drug-resistant enterococci. Its precise mechanism of action is incompletely understood, and a specific molecular target has not been identified. Here we show that Ca²⁺-daptomycin specifically interacts with undecaprenyl-coupled cell envelope precursors in the presence of the anionic phospholipid phosphatidylglycerol, forming a tripartite complex. We use microbiological and biochemical assays, in combination with fluorescence and optical sectioning microscopy of intact staphylococcal cells and model membrane systems. Binding primarily occurs at the staphylococcal septum and interrupts cell wall biosynthesis. This is followed by delocalisation of components of the peptidoglycan biosynthesis machinery and massive membrane rearrangements, which may account for the pleiotropic cellular events previously reported. The identification of carrier-bound cell wall precursors as specific targets explains the specificity of daptomycin for bacterial cells. Our work reconciles apparently inconsistent previous results, and supports a concise model for the mode of action of daptomycin.

More Than a Pore: A Current Perspective on the In Vivo Mode of Action of the Lipopeptide Antibiotic Daptomycin

Daptomycin is a cyclic lipopeptide antibiotic, which was discovered in 1987 and entered the market in 2003. To date, it serves as last resort antibiotic to treat complicated skin infections, bacteremia, and right-sided endocarditis caused by Gram-positive pathogens, most prominently methicillin-resistant Staphylococcus aureus. Daptomycin was the last representative of a novel antibiotic class that was introduced to the clinic. It is also one of the few membrane-active compounds that can be applied systemically. While membrane-active antibiotics have long been limited to topical applications and were generally excluded from systemic drug development, they promise slower resistance development than many classical drugs that target single proteins. The success of daptomycin together with the emergence of more and more multi-resistant superbugs attracted renewed interest in this compound class. Studying daptomycin as a pioneering systemic membrane-active compound might help to pave the way for future membrane-targeting antibiotics. However, more than 30 years after its discovery, the exact mechanism of action of daptomycin is still debated. In particular, there is a prominent discrepancy between in vivo and in vitro studies. In this review, we discuss the current knowledge on the mechanism of daptomycin against Gram-positive bacteria and try to offer explanations for these conflicting observations.

Exploring Pleiotropic Functions of Canine β-Defensin 103: Nasal Cavity Expression, Antimicrobial Activity, and Melanocortin Receptor Activity

Canine β-defensin 103 (cBD103) and its common variant cBD103ΔG23 are multitasking polypeptides. As a β-defensin, cBD103 is one of many antimicrobial agents used by the innate immunity to thwart pathogenic colonization. In this study, we showed that cBD103 was expressed throughout the nasal cavity, with primary expression in the nares as well as respiratory and olfactory epithelia. In the rostral nasal concha, cBD103 was expressed in the epithelium, and to a lesser degree in the lamina propria, but was absent in goblet cells. In the main olfactory epithelium, virtually all cells in the epithelial layer and select cells associated with Bowman's glands expressed cBD103. We also showed that the ΔG23 mutation did not appreciably alter the antimicrobial activity of the peptide against several species of microorganisms tested in nutrient-rich or minimal media or minimal media with salt added. Moreover, we showed antimicrobial activity in minimal media did not necessarily predict the inhibitory action of the peptide in nutrient-rich media. Both forms of cBD103 caused ultrastructural changes (membrane blebbing, condensation of intracellular contents and cell wall lysis) in Escherichia coli and Staphylococcus aureus. As a ligand of the melanocortin receptors, we showed that cBD103ΔG23 increased ERK1/2 activation and cAMP accumulation when bound to the human or canine melanocortin-4 receptor, acting as a weak allosteric agonist.

The human milk protein-lipid complex HAMLET disrupts glycolysis and induces death in Streptococcus pneumoniae

HAMLET is a complex of human α-lactalbumin (ALA) and oleic acid and kills several Gram-positive bacteria by a mechanism that bears resemblance to apoptosis in eukaryotic cells. To identify HAMLET's bacterial targets, here we used Streptococcus pneumoniae as a model organism and employed a proteomic approach that identified several potential candidates. Two of these targets were the glycolytic enzymes fructose bisphosphate aldolase (FBPA) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Treatment of pneumococci with HAMLET immediately inhibited their ATP and lactate production, suggesting that HAMLET inhibits glycolysis. This observation was supported by experiments with recombinant bacterial enzymes, along with biochemical and bacterial viability assays, indicating that HAMLET's activity is partially inhibited by high glucose-mediated stimulation of glycolysis but enhanced in the presence of the glycolysis inhibitor 2-deoxyglucose. Both HAMLET and ALA bound directly to each glycolytic enzyme in solution and solid-phase assays and effectively inhibited their enzymatic activities. In contrast, oleic acid alone had little to no inhibitory activity. However, ALA alone also exhibited no bactericidal activity and did not block glycolysis in whole cells, suggesting a role for the lipid moiety in the internalization of HAMLET into the bacterial cells to reach its target(s). This was verified by inhibition of enzyme activity in whole cells after HAMLET but not ALA exposure. The results of this study suggest that part of HAMLET's antibacterial activity relates to its ability to target and inhibit glycolytic enzymes, providing an example of a natural antimicrobial agent that specifically targets glycolysis.

Cathelicidin Peptides Restrict Bacterial Growth via Membrane Perturbation and Induction of Reactive Oxygen Species

Antimicrobial peptides (AMPs) are an important part of the mammalian innate immune system in the battle against microbial infection. How AMPs function to control bacteria is not clear, as nearly all activity studies use nonphysiological levels of AMPs. We monitored peptide action in live bacterial cells over short time frames with single-cell resolution and found that the primary effect of cathelicidin peptides is to increase the production of oxidative molecules that cause cellular damage in Gram-positive and Gram-negative bacteria.

All metazoans produce antimicrobial peptides (AMPs) that have both broad antimicrobial and immunomodulatory activity. Cathelicidins are AMPs that preferentially kill Gram-negative bacteria in vitro, purportedly by assembling into higher-order structures that perforate the membrane. We utilized high-resolution, single-cell fluorescence microscopy to examine their mechanism of action in real time. Engineered cathelicidins rapidly bound to Gram-negative and Gram-positive cells and penetrated the cytoplasmic membrane. Rapid failure of the peptidoglycan superstructure in regions of active turnover caused leakage of cytoplasmic contents and the formation of membrane-bound blebs. A mutation anticipated to destabilize interactions between cathelicidin subunits had no effect on bactericidal activity, suggesting that cathelicidins have activities beyond perforating the membrane. Nanomolar concentrations of cathelicidins, although not bactericidal, reduced the growth rate of Gram-negative and Gram-positive bacteria. The cells exhibited expression changes in multiple essential processes, including protein synthesis, peptidoglycan biosynthesis, respiration, and the detoxification of reactive oxygen species (ROS). Time-lapse imaging revealed that ROS accumulation preceded bleb formation, and treatments that reduced cellular ROS levels overcame these bactericidal effects. We propose that that the primary effect of cathelicidins is to induce the production of ROS that damage bacterial molecules, leading to slowed growth or cell death. Given their low circulating levels in vivo, AMPs may serve to slow bacterial population expansion so that cellular immunity systems can respond to and battle the infection.

Therapeutic compounds targeting Lipid II for antibacterial purposes

Resistance against commonly used antibiotics has emerged in all bacterial pathogens. In fact, there is no antibiotic currently in clinical use against which resistance has not been reported. In particular, rapidly increasing urbanization in developing nations are sites of major concern. Additionally, the widespread practice by physicians to prescribe antibiotics in cases of viral infections puts selective pressure on antibiotics that still remain effective and it will only be a matter of time before resistance develops on a large scale. The biosynthesis pathway of the bacterial cell wall is well studied and a validated target for the development of antibacterial agents. Cell wall biosynthesis involves two major processes; 1) the biosynthesis of cell wall teichoic acids and 2) the biosynthesis of peptidoglycan. Key molecules in these pathways, including enzymes and precursor molecules are attractive targets for the development of novel antibacterial agents. In this review, we will focus on the major class of natural antibacterial compounds that target the peptidoglycan precursor molecule Lipid II; namely the glycopeptides, including the novel generation of lipoglycopeptides. We will discuss their mechanism-of-action and clinical applications. Further, we will briefly discuss additional peptides that target Lipid II such as the lantibiotic nisin and defensins. We will highlight recent developments and future perspectives.

Docking on Lipid II-A Widespread Mechanism for Potent Bactericidal Activities of Antibiotic Peptides

Natural product antibiotics usually target the major biosynthetic pathways of bacterial cells and the search for new targets outside these pathways has proven very difficult. Cell wall biosynthesis maybe the most prominent antibiotic target, and ß-lactams are among the clinically most relevant antibiotics. Among cell wall biosynthesis inhibitors, glycopeptide antibiotics are a second group of important drugs, which bind to the peptidoglycan building block lipid II and prevent the incorporation of the monomeric unit into polymeric cell wall. However, lipid II acts as a docking molecule for many more naturally occurring antibiotics from diverse chemical classes and likely is the most targeted molecule in antibacterial mechanisms. We summarize current knowledge on lipid II binding antibiotics and explain, on the levels of mechanisms and resistance development, why lipid II is such a prominent target, and thus provide insights for the design of new antibiotic drugs.

Application of Antimicrobial Peptides of the Innate Immune System in Combination With Conventional Antibiotics-A Novel Way to Combat Antibiotic Resistance?

Rapidly growing resistance of pathogenic bacteria to conventional antibiotics leads to inefficiency of traditional approaches of countering infections and determines the urgent need for a search of fundamentally new anti-infective drugs. Antimicrobial peptides (AMPs) of the innate immune system are promising candidates for a role of such novel antibiotics. However, some cytotoxicity of AMPs toward host cells limits their active implementation in medicine and forces attempts to design numerous structural analogs of the peptides with optimized properties. An alternative route for the successful AMPs introduction may be their usage in combination with conventional antibiotics. Synergistic antibacterial effects have been reported for a number of such combinations, however, the molecular mechanisms of the synergy remain poorly understood and little is known whether AMPs cytotoxicy for the host cells increases upon their application with antibiotics. Our study is directed to examination of a combined action of natural AMPs with different structure and mode of action (porcine protegrin 1, caprine bactenecin ChBac3.4, human alpha- and beta-defensins (HNP-1, HNP-4, hBD-2, hBD-3), human cathelicidin LL-37), and egg white lysozyme with varied antibiotic agents (gentamicin, ofloxacin, oxacillin, rifampicin, polymyxin B, silver nanoparticles) toward selected bacteria, including drug-sensitive and drug-resistant strains, as well as toward some mammalian cells (human erythrocytes, PBMC, neutrophils, murine peritoneal macrophages and Ehrlich ascites carcinoma cells). Using “checkerboard titrations” for fractional inhibitory concentration indexes evaluation, it was found that synergy in antibacterial action mainly occurs between highly membrane-active AMPs (e.g., protegrin 1, hBD-3) and antibiotics with intracellular targets (e.g., gentamicin, rifampcin), suggesting bioavailability increase as the main model of such interaction. In some combinations modulation of dynamics of AMP-bacterial membrane interaction in presence of the antibiotic was also shown. Cytotoxic effects of the same combinations toward normal eukaryotic cells were rarely synergistic. The obtained data approve that combined application of antimicrobial peptides with antibiotics or other antimicrobials is a promising strategy for further development of new approach for combating antibiotic-resistant bacteria by usage of AMP-based therapeutics. Revealing the conventional antibiotics that increase the activity of human endogenous AMPs against particular pathogens is also important for cure strategies elaboration.

Molecular Dynamics Investigation into the Effect of Zinc(II) on the Structure and Membrane Interactions of the Antimicrobial Peptide Clavanin A

Clavanin A (ClavA) is an antimicrobial peptide (AMP) whose antimicrobial activity is enhanced in the presence of Zn(II) ions. The antimicrobial activity of ClavA has been shown to increase 16-fold in the presence of Zn(II) ions. In this study, we investigate the potential sources of this enhancement, namely, the effect of Zn(II) binding on the helical conformation of ClavA and on the ClavA interaction with a model for gram-negative bacterial membranes. In addition, we investigate the effect of Zn(II) on the membrane mechanical properties. We employed all-atom equilibrium molecular dynamics simulations initiated from both fully helical and random coil structures of ClavA. We observe that Zn(II) can stabilize an existing helical conformation in the Zn(II)-binding region, but we do not observe induction of helical conformations in systems initiated in random coil configurations. Zn(II) binding to ClavA provides more favorable electrostatics for membrane association in the C-terminal region. This is evidenced by longer and stronger C-terminal-lipid interactions. Zn(II) is also capable of modulating the membrane properties in a manner which favors ClavA insertion and the potential for enhanced translocation into the cell. This work provides insights into the role of divalent metal cations in the antimicrobial activity of ClavA. This information can be used for the development of synthetic AMPs containing motifs that can bind metals (metalloAMPs) for therapeutic and medical purposes.

G.I. pros: Antimicrobial defense in the gastrointestinal tract

The gastrointestinal tract is a complex environment in which the host immune system interacts with a diverse array of microorganisms, both symbiotic and pathogenic. As such, mobilizing a rapid and appropriate antimicrobial response depending on the nature of each stimulus is crucial for maintaining the balance between homeostasis and inflammation in the gut. Here we focus on the mechanisms by which intestinal antimicrobial peptides regulate microbial communities during dysbiosis and infection. We also discuss classes of bacterial peptides that contribute to reducing enteric pathogen outgrowth. This review aims to provide a comprehensive overview on the interplay of diverse antimicrobial responses with enteric pathogens and the gut microbiota.

Reassessing the Host Defense Peptide Landscape

Current research has demonstrated that small cationic amphipathic peptides have strong potential not only as antimicrobials, but also as antibiofilm agents, immune modulators, and anti-inflammatories. Although traditionally termed antimicrobial peptides (AMPs) these additional roles have prompted a shift in terminology to use the broader term host defense peptides (HDPs) to capture the multi-functional nature of these molecules. In this review, we critically examined the role of AMPs and HDPs in infectious diseases and inflammation. It is generally accepted that HDPs are multi-faceted mediators of a wide range of biological processes, with individual activities dependent on their polypeptide sequence. In this context, we explore the concept of chemical space as it applies to HDPs and hypothesize that the various functions and activities of this class of molecule exist on independent but overlapping activity landscapes. Finally, we outline several emerging functions and roles of HDPs and highlight how an improved understanding of these processes can potentially be leveraged to more fully realize the therapeutic promise of HDPs.

β-Defensins: Farming the Microbiome for Homeostasis and Health

Diverse commensal populations are now regarded as key to physiological homeostasis and protection against disease. Although bacteria are the most abundant component of microbiomes, and the most intensively studied, the microbiome also consists of viral, fungal, archael, and protozoan communities, about which comparatively little is known. Host-defense peptides (HDPs), originally described as antimicrobial, now have renewed significance as curators of the pervasive microbial loads required to maintain homeostasis and manage microbiome diversity. Harnessing HDP biology to transition away from non-selective, antibiotic-mediated treatments for clearance of microbes is a new paradigm, particularly in veterinary medicine. One family of evolutionarily conserved HDPs, β-defensins which are produced in diverse combinations by epithelial and immune cell populations, are multifunctional cationic peptides which manage the cross-talk between host and microbes and maintain a healthy yet dynamic equilibrium across mucosal systems. They are therefore key gatekeepers to the oral, respiratory, reproductive and enteric tissues, preventing pathogen-associated inflammation and disease and maintaining physiological normality. Expansions in the number of genes encoding these natural antibiotics have been described in the genomes of some species, the functional significance of which has only recently being appreciated. β-defensin expression has been documented pre-birth and disruptions in their regulation may play a role in maladaptive neonatal immune programming, thereby contributing to subsequent disease susceptibility. Here we review recent evidence supporting a critical role for β-defensins as farmers of the pervasive and complex prokaryotic ecosystems that occupy all body surfaces and cavities. We also share some new perspectives on the role of β-defensins as sensors of homeostasis and the immune vanguard particularly at sites of immunological privilege where inflammation is attenuated.

Systematic mutational analysis of human neutrophil α-defensin HNP4

Defensins are a family of cationic antimicrobial peptides of innate immunity with immunomodulatory properties. The prototypic human α-defensins, also known as human neutrophil peptides 1-3 or HNP1-3, are extensively studied for their structure, function and mechanisms of action, yet little is known about HNP4 - the much less abundant "distant cousin" of HNP1-3. Here we report a systematic mutational analysis of HNP4 with respect to its antibacterial activity against E. coli and S. aureus, inhibitory activity against anthrax lethal factor (LF), and binding activity for LF and HIV-1 gp120. Except for nine conserved and structurally important residues (6xCys, 1xArg, 1xGlu and 1xGly), the remaining 24 residues of HNP4 were each individually mutated to Ala. The crystal structures of G23A-HNP4 and T27A-HNP4 were determined, both exhibiting a disulfide-stabilized canonical α-defensin dimer identical to wild-type HNP4. Unlike HNP1-3, HNP4 preferentially killed the Gram-negative bacterium, a property largely attributable to three clustered cationic residues Arg10, Arg11 and Arg15. The cationic cluster was also important for HNP4 killing of S. aureus, inhibition of LF and binding to LF and gp120. However, F26A, while functionally inconsequential for E. coli killing, was far more deleterious than any other mutations. Similarly, N-methylation of Leu20 to destabilize the HNP4 dimer had little effect on E. coli killing, but significantly reduced the ability of HNP4 to kill S. aureus, inhibit LF, and bind to LF and gp120. Our findings unveil the molecular determinants of HNP4 function, completing the atlas of structure and function relationships for all human neutrophil α-defensins.

Bacteria Hunt Bacteria through an Intriguing Cyclic Peptide

In the last few decades, peptides have been victorious over small molecules as therapeutics due to their broad range of applications, high biological activity, and high specificity. However, the main challenges to overcome if peptides are to become effective drugs is their low oral bioavailability and instability under physiological conditions. Cyclic peptides play a vital role in this context because they show higher stability under physiological conditions, higher membrane permeability, and greater oral bioavailability than that of their corresponding linear analogues. In this regard, cyclic antimicrobial peptides (AMPs) have gained considerable attention in the field of novel antibiotic development. Bacterial strains produce cyclic AMPs through two pathways: ribosomal and nonribosomal. This review provides an overview of the chemical classification of cyclic AMPs isolated from bacteria, and provides a description of their biological activity and mode of action.

Pleiotropic Anti-Infective Effects of Defensin-Derived Antimicrobial Compounds

We identified low-molecular weight compounds derived from the antimicrobial peptide human neutrophil peptide-1 that bind to Lipid II, an essential precursor of bacterial cell wall biosynthesis. These compounds act as antibacterials on multiple biosynthesis pathways with specificity against gram-positive organisms. Here, we have tested a small subset of our most promising leads against the bacterium Clostridium perfringens and sporozoites of Eimeria tenella, an intracellular protozoan parasite that causes intestinal disease in poultry. We found one compound, 1611-0203 (2-{2,3,5,6-tetrafluoro-4-[2,3,5,6-tetrafluoro-4-(2-hydroxyphenoxy)phenyl]phenoxy}phenol), specifically to inhibit growth of both agents out of all compounds tested. Additionally, compound 1611-0203 inhibits Staphylococcus aureus and Enterococcus spp. Mechanism-of-action studies further reveal that 1611-0203 affects cell wall biosynthesis and inhibits additional biosynthetic pathways. Combined, our results indicate that compounds such as 1611-0203 have therapeutic potential to act as anti-infectives against various organisms simultaneously.

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.

Natural molecules induce and synergize to boost expression of the human antimicrobial peptide β-defensin-3

Antimicrobial peptides (AMPs) are mucosal defense effectors of the human innate immune response. In the intestine, AMPs are produced and secreted by epithelial cells to protect the host against pathogens and to support homeostasis with commensals. The inducible nature of AMPs suggests that potent inducers could be used to increase their endogenous expression for the prevention or treatment of diseases. Here we aimed at identifying molecules from the natural pharmacopoeia that induce expression of human β-defensin-3 (HBD3), one of the most efficient AMPs, without modifying the production of proinflammatory cytokines. By screening, we identified three molecules isolated from medicinal plants, andrographolide, oridonin, and isoliquiritigenin, which induced HBD3 production in human colonic epithelial cells. This effect was observed without activation of the NF-κB pathway or the expression of associated proinflammatory cytokines. We identified the EGF receptor as the target of these compounds and characterized the downstream-activated MAPK pathways. At the chromatin level, molecules increased phosphorylation of histone H3 on serine S10 and recruitment of the c-Fos, c-Jun, and Elk1 or c-Myc transcription factors at the HBD3 promoter. Interestingly, stimulating cells with a combination of andrographolide and isoliquiritigenin synergistically enhanced HBD3 induction 10-fold more than observed with each molecule alone. Finally, we investigated the molecular basis governing the synergistic effect, confirmed our findings in human colonic primary cells, and demonstrated that synergism increased cellular antimicrobial activity. This work shows the capability of small molecules to achieve induction of epithelial antimicrobial defenses while simultaneously avoiding the deleterious risks of an inflammatory response.

From Antimicrobial Peptides to Antimicrobial Poly(α-amino acid)s

Conventional small-molecule antibiotics are facing a significant challenge of the rapidly developed drug resistance of pathogens. In contrast, antimicrobial peptides (AMPs), an important component for innate host defenses, are now under intensive investigation as a promising antimicrobial agent for combating drug resistant pathogens. Most AMPs can effectively kill a broad spectrum of pathogens via physical disruption of microbial cellular membranes, which is identified to be difficult to develop resistance. However, the clinical applications of AMPs are still greatly limited by several inherent impediments, such as high cost of production, potential hemolysis or toxicity, and liability to proteinase degradation. Recently, cationic poly(α-amino acid)s with structures mimicking the AMPs are found to have excellent antimicrobial activity. These polymers, termed "antimicrobial poly(α-amino acid)s (APAAs)," have some advantages over AMPs, such as easy production and modification, prolonged antimicrobial activity, low cytotoxicity, and enhanced stability to protease degradation. Here, a brief introduction of mechanisms and affecting factors of microbial killing by AMPs is first presented, followed by a systematic illustration of recent advances in design and preparation of biomimetic APAAs and a perspective in this field.

Cationic Amphipathic Antimicrobial Peptides Perturb the Inner Membrane of Germinated Spores Thus Inhibiting Their Outgrowth

The mode of action of four cationic amphipathic antimicrobial peptides (AMPs) was evaluated against the non-pathogenic, Gram-positive, spore-forming bacterium, Bacillus subtilis. The AMPs were TC19, TC84, BP2, and the lantibiotic Nisin A. TC19 and TC84 were derived from the human thrombocidin-1. Bactericidal peptide 2 (BP2) was derived from the human bactericidal permeability increasing protein (BPI). We employed structured illumination microscopy (SIM), fluorescence microscopy, Alexa 488-labeled TC84, B. subtilis mutants producing proteins fused to the green fluorescent protein (GFP) and single-cell live imaging to determine the effects of the peptides against spores. TC19, TC84, BP2, and Nisin A showed to be bactericidal against germinated spores by perturbing the inner membrane, thus preventing outgrowth to vegetative cells. Single cell live imaging showed that the AMPs do not affect the germination process, but the burst time and subsequent generation time of vegetative cells. Alexa 488-labeled TC84 suggested that the TC84 might be binding to the dormant spore-coat. Therefore, dormant spores were also pre-coated with the AMPs and cultured on AMP-free culture medium during single-cell live imaging. Pre-coating of the spores with TC19, TC84, and BP2 had no effect on the germination process, and variably affected the burst time and generation time. However, the percentage of spores that burst and grew out into vegetative cells was drastically lower when pre-coated with Nisin A, suggesting a novel application potential of this lantibiotic peptide against spores. Our findings contribute to the understanding of AMPs and show the potential of AMPs as eventual therapeutic agents against spore-forming bacteria.

Benzo(a)pyrene suppresses tracheal antimicrobial peptide gene expression in bovine tracheal epithelial cells

Respiratory disease is an important cause of morbidity and mortality in cetaceans, which are also threatened by environmental degradation caused by crude oil spills. Following oil spills, cetaceans at the water surface may inhale droplets of oil containing toxic polycyclic aromatic hydrocarbons (PAHs), which could potentially alter respiratory immunity via activation of the aryl hydrocarbon receptor (AHR) and its subsequent interaction with nuclear factor kappa B (NF-κB). β-defensins are antimicrobial peptides secreted by airway epithelial cells and their expression is known to be dependent on NF-κB. We hypothesized that PAHs may suppress the expression of β-defensins, and thereby contribute to the pathogenesis of pneumonia. This hypothesis was modeled by measuring the in vitro effects of benzo(a)pyrene (BAP), phenanthrene, and naphthalene on tracheal antimicrobial peptide (TAP) gene expression in bovine tracheal epithelial cells. Stimulation with lipopolysaccharide (LPS) induced 20 ± 17-fold (mean ± SD) increased TAP gene expression. Exposure of tracheal epithelial cells to 5 μM BAP for 4 or 8 h, followed by incubation with a combination of LPS and 5 μM BAP for another 16 h, significantly (P = 0.002) suppressed LPS-induced TAP gene expression by 40.6 ± 21.8% (mean ± SD) in tracheal epithelial cells from 9 calves tested. BAP-induced suppression of TAP gene expression coincided with induction of cytochrome P450 1A1 gene expression. In contrast, phenanthrene and naphthalene had no consistent effect, and exposure to PAHs did not significantly affect constitutive TAP gene expression (i.e. without LPS). These findings characterize the suppressive effects of BAP-a toxic pollutant found in crude oil-on this respiratory innate immune response.

Expression of recombinant HBD3 protein that reduces Mycobacterial infection capacity

Bovine tuberculosis is a disease caused by Mycobacterium bovis (M. bovis) that leads to great economic losses in cattle production. The discovery of a reasonable bioagent to reduce M. bovis infection risk and environment contamination becomes significant and urgent. Previous study reported that human β-defensin-3 (HBD3) participated in Mycobacterial immunity and was recognized as a suitable candidate reagent. However, its minimal inhibitory concentration to M. bovis is not yet reported. In this study, we first purified HBD3 protein by recombinant-DNA technology and prokaryotic expression system. Subsequently, anti-bacterial tests were used to evaluate the basic bioactivity of the protein. Results revealed that recombinant HBD3 (rHBD3) protein inhibits Staphylococcus multiplication but not the host Escherichia coli. The growth curve of M. bovis showed that rHBD3 protein controls the proliferation of M. bovis in 20 μg/ml concentration. In addition, rHBD3 protein-incubated M. bovis exhibited reduced infectivity to alveolar epithelial cells and macrophages. In conclusion, the expression of rHBD3 protein is a potential ideal bio-regent for reducing M. bovis infection.

Enteric α-defensins on the verge of intestinal immune tolerance and inflammation

The gut is the biggest immune organ in the body that encloses commensal microbiota which aids in food digestion. Paneth cells, positioned at the frontline of host-microbiota interphase, can modulate the composition of microbiota. Paneth cells achieve this via the delivery of microbicidal substances, among which enteric α-defensins play the primary role. If microbiota is dysregulated, it can impact the function of the local mucosal immune system. Importantly, this system is also exposed to an enormous number of antigens which are derived from the gut-resident microbiota and processed food, and may potentially trigger undesirable local inflammatory responses. To understand the intricate regulations and liaisons between Paneth cells, microbiota and the immune system in this intestinal-specific setting, one must consider their mode of interaction in a wider context of regulatory processes which impose immune tolerance not only to self, but also to microbiota and food-derived antigens. These include, but are not limited to, tolerogenic mechanisms of central tolerance in the thymus and peripheral tolerance in the secondary lymphoid organs, and the intestine itself. Defects in these processes can compromise homeostasis in the intestinal mucosal immunity. In this review, which is focused on tolerance to intestinal antigens and its relevance for the pathogenesis of gut immune diseases, we provide an outline of such multilayered immune control mechanisms and highlight functional links that underpin their cooperative nature.

Perspectives for clinical use of engineered human host defense antimicrobial peptides

Infectious diseases caused by bacteria, viruses or fungi are among the leading causes of death worldwide. The emergence of drug-resistance mechanisms, especially among bacteria, threatens the efficacy of all current antimicrobial agents, some of them already ineffective. As a result, there is an urgent need for new antimicrobial drugs. Host defense antimicrobial peptides (HDPs) are natural occurring and well-conserved peptides of innate immunity, broadly active against Gram-negative and Gram-positive bacteria, viruses and fungi. They also are able to exert immunomodulatory and adjuvant functions by acting as chemotactic for immune cells, and inducing cytokines and chemokines secretion. Moreover, they show low propensity to elicit microbial adaptation, probably because of their non-specific mechanism of action, and are able to neutralize exotoxins and endotoxins. HDPs have the potential to be a great source of novel antimicrobial agents. The goal of this review is to provide an overview of the advances made in the development of human defensins as well as the cathelicidin LL-37 and their derivatives as antimicrobial agents against bacteria, viruses and fungi for clinical use.

Recombinant β-defensin 126 promotes bull sperm binding to bovine oviductal epithelia

Primate β-defensin 126 regulates the ability of spermatozoa to bind to oviductal epithelial cells in vitro. Bovine β-defensin 126 (BBD126) exhibits preferential expression in the cauda epididymis of the bull, but there have been few studies on its functional role in cattle. The aim of the present study was to examine the role of BBD126 in bull sperm binding to bovine oviductal epithelial cell (BOEC) explants. BBD126 has been shown to be highly resistant to the standard methods of dissociation used in other species and, as a result, corpus epididymal spermatozoa, which have not been exposed to the protein, were used to study the functional role of BBD126. Corpus epididymal spermatozoa were incubated with recombinant (r) BBD126 in the absence or presence of anti-BBD126 antibody. Addition of rBBD126 significantly enhanced the ability of epididymal spermatozoa to bind to BOEC explants (P < 0.05). Anti-BBD126 antibody blocked the BBD126-mediated increase in sperm binding capacity. Ejaculated spermatozoa, which are coated with native BBD126 protein but also a large number of seminal plasma proteins in vivo, were incubated with rBBD126 in the absence or presence of the anti-BBD126 antibody. Addition of rBBD126 significantly enhanced the ability of ejaculated spermatozoa to bind to BOEC explants (P < 0.05), whereas rBBD126 also reduced corpus sperm agglutination (P < 0.05). These results suggest that, similar to the role of its analogue in the macaque, spermatozoa with more BBD126 in their acrosome may represent spermatozoa with more oviduct binding capacity.

Differential daptomycin resistance development in Staphylococcus aureus strains with active and mutated gra regulatory systems

The first-in-class lipopeptide antibiotic daptomycin (DAP) is highly active against Gram-positive pathogens including ß-lactam and glycopeptide resistant strains. Its molecular mode of action remains enigmatic, since a defined target has not been identified so far and multiple effects, primarily on the cell envelope have been observed. Reduced DAP susceptibility has been described in S. aureus and enterococci after prolonged treatment courses. In line with its pleiotropic antibiotic activities, a unique, defined molecular mechanism of resistance has not emerged, instead non-susceptibility appears often accompanied by alterations in membrane composition and changes in cell wall homeostasis. We compared S. aureus strains HG001 and SG511, which differ primarily in the functionality of the histidine kinase GraS, to evaluate the impact of the GraRS regulatory system on the development of DAP non-susceptibility. After extensive serial passing, both DAP^R variants reached a minimal inhibitory concentration of 31 μg/ml and shared some phenotypic characteristics (e.g. thicker cell wall, reduced autolysis). However, based on comprehensive analysis of the underlying genetic, transcriptomic and proteomic changes, we found that both strains took different routes to achieve DAP resistance. Our study highlights the impressive genetic and physiological capacity of S. aureus to counteract pleiotropic activities of cell wall- and membrane-active compounds even when a major cell wall regulatory system is dysfunctional.

The potential role of fecal microbiota transplantation in the treatment of inflammatory Bowel disease

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of an unknown etiology. Its pathogenesis involves an interplay of infectious, genetic, environmental, and immunological factors. The current therapeutic options have various limitations in terms of cost, side effect profile, and the development of drug resistance and dependence. Therefore, there is a need to develop future therapeutic options which are safe and effective to control the inflammatory process. This review focuses in a method for the administration of fecal matters (which contains a mixture of various commensals) from a healthy donor to the inflamed colon called fecal microbiota transplantation (FMT) aiming to correct the underlying dysbiosis in the gut as one of the major driving force for the inflammatory process. IBD patients have reduced number of protective (e.g., clostridia and bacteroids) and increased number of pathogenic (e.g., adhesive invasive E. coli and mycobacterium avium paratuberculosis) commensals, and this method is aimed to shift these changes in the gut. Recent studies from animal models and clinical trials suggest promising effects of this method in treating patients with IBD, but more studies are urgently needed to confirm its efficacy and safety, since the etiology of this chronic inflammatory disease is not fully understood and caution should be taken when transplanting fecal matters between individuals which might transfer other infectious organisms and diseases.

The synthetic human beta-defensin-3 C15 peptide exhibits antimicrobial activity against Streptococcus mutans, both alone and in combination with dental disinfectants

Streptococcus mutans is a major etiologic agent of human dental caries that forms biofilms on hard tissues in the human oral cavity, such as tooth and dentinal surfaces. Human β-defensin-3 (HBD3) is a 45-amino-acid natural antimicrobial peptide that has broad spectrum antimicrobial activity against bacteria and fungi. A synthetic peptide consisting of the C-terminal 15 amino acids of HBD3 (HBD3-C15) was recently shown to be sufficient for its antimicrobial activity. Thus, clinical applications of this peptide have garnered attention. In this study, we investigated whether HBD3-C15 inhibits the growth of the representative cariogenic pathogen Streptococcus mutans and its biofilm formation. HBD3-C15 inhibited bacterial growth, exhibited bactericidal activity, and attenuated bacterial biofilm formation in a dose-dependent manner. HBD3-C15 potentiated the bactericidal and anti-biofilm activity of calcium hydroxide (CH) and chlorhexidine digluconate (CHX), which are representative disinfectants used in dental clinics, against S. mutans. Moreover, HBD3-C15 showed antimicrobial activity by inhibiting biofilm formation by S. mutans and other dentinophilic bacteria such as Enterococcus faecalis and Streptococcus gordonii, which are associated with dental caries and endodontic infection, on human dentin slices. These effects were observed for HBD3-C15 alone and for HBD3-C15 in combination with CH or CHX. Therefore, we suggest that HBD3-C15 is a potential alternative or additive disinfectant that can be used for the treatment of oral infectious diseases, including dental caries and endodontic infections.

Structural Insights into the Mode of Action of the Peptide Antibiotic Copsin

Defensins make up a class of cysteine-rich antimicrobial peptides, expressed by virtually all eukaryotes as part of their innate immune response. Because of their unique mode of action and rapid killing of pathogenic microbes, defensins are considered promising alternatives to clinically applied antibiotics. Copsin is a defensin-like peptide, previously identified in the mushroom Coprinopsis cinerea. It exerts its activity against a range of Gram-positive bacteria by binding to the peptidoglycan precursor lipid II and prevention of proper cell wall formation. In this study, we present a new workflow for the generation, production, and activity-driven selection of copsin derivatives, based on their expression in Pichia pastoris. One hundred fifty-two single-amino acid mutants and combinations thereof allowed the identification of k-copsin, a peptide variant exhibiting significantly enhanced activity against Bacillus subtilis and Staphylococcus aureus. Furthermore, we performed in silico characterizations of membrane interactions of copsin and k-copsin, in the presence and absence of lipid II. The molecular dynamics data highlighted a high variability in lipid II binding, with a preference for the MurNAc moiety with 47 and 35% of the total contacts for copsin and k-copsin, respectively. Mutated amino acids were located in loop regions of k-copsin and shown to be crucial in the perturbation of the bacterial membrane. These structural studies provide a better understanding of how defensins can be developed toward antibacterial therapies less prone to resistance issues.

Targeting a cell wall biosynthesis hot spot

Covering: up to 2017History points to the bacterial cell wall biosynthetic network as a very effective target for antibiotic intervention, and numerous natural product inhibitors have been discovered. In addition to the inhibition of enzymes involved in the multistep synthesis of the macromolecular layer, in particular, interference with membrane-bound substrates and intermediates essential for the biosynthetic reactions has proven a valuable antibacterial strategy. A prominent target within the peptidoglycan biosynthetic pathway is lipid II, which represents a particular "Achilles' heel" for antibiotic attack, as it is readily accessible on the outside of the cytoplasmic membrane. Lipid II is a unique non-protein target that is one of the structurally most conserved molecules in bacterial cells. Notably, lipid II is more than just a target molecule, since sequestration of the cell wall precursor may be combined with additional antibiotic activities, such as the disruption of membrane integrity or disintegration of membrane-bound multi-enzyme machineries. Within the membrane bilayer lipid II is likely organized in specific anionic phospholipid patches that form a particular "landing platform" for antibiotics. Nature has invented a variety of different "lipid II binders" of at least 5 chemical classes, and their antibiotic activities can vary substantially depending on the compounds' physicochemical properties, such as amphiphilicity and charge, and thus trigger diverse cellular effects that are decisive for antibiotic activity.

Human β-defensin 3 increases the TLR9-dependent response to bacterial DNA

Human β-defensin 3 (hBD3) is a cationic antimicrobial peptide with potent bactericidal activity in vitro. HBD3 is produced in response to pathogen challenge and can modulate immune responses. The amplified recognition of self-DNA by human plasmacytoid dendritic cells has been previously reported, but we show here that hBD3 preferentially enhances the response to bacterial DNA in mouse Flt-3 induced dendritic cells (FLDCs) and in human peripheral blood mononuclear cells. We show the effect is mediated through TLR9 and although hBD3 significantly increases the cellular uptake of both E. coli and self-DNA in mouse FLDCs, only the response to bacterial DNA is enhanced. Liposome transfection also increases uptake of bacterial DNA and amplifies the TLR9-dependent response. In contrast to hBD3, lipofection of self-DNA enhances inflammatory signaling, but the response is predominantly TLR9-independent. Together, these data show that hBD3 has a role in the innate immune-mediated response to pathogen DNA, increasing inflammatory signaling and promoting activation of the adaptive immune system via antigen presenting cells including dendritic cells. Therefore, our data identify an additional immunomodulatory role for this copy-number variable defensin, of relevance to host defence against infection and indicate a potential for the inclusion of HBD3 in pathogen DNA-based vaccines.

Intracellular Targeting Mechanisms by Antimicrobial Peptides

Antimicrobial peptides (AMPs) are expressed in various living organisms as first-line host defenses against potential harmful encounters in their surroundings. AMPs are short polycationic peptides exhibiting various antimicrobial activities. The principal antibacterial activity is attributed to the membrane-lytic mechanism which directly interferes with the integrity of the bacterial cell membrane and cell wall. In addition, a number of AMPs form a transmembrane channel in the membrane by self-aggregation or polymerization, leading to cytoplasm leakage and cell death. However, an increasing body of evidence has demonstrated that AMPs are able to exert intracellular inhibitory activities as the primary or supportive mechanisms to achieve efficient killing. In this review, we focus on the major intracellular targeting activities reported in AMPs, which include nucleic acids and protein biosynthesis and protein-folding, protease, cell division, cell wall biosynthesis, and lipopolysaccharide inhibition. These multifunctional AMPs could serve as the potential lead peptides for the future development of novel antibacterial agents with improved therapeutic profiles.

Ubiquitously expressed Human Beta Defensin 1 (hBD1) forms bacteria-entrapping nets in a redox dependent mode of action

Ever since the discovery of endogenous host defense antimicrobial peptides it has been discussed how these evolutionary conserved molecules avoid to induce resistance and to remain effective. Human ß-defensin 1 (hBD1) is an ubiquitously expressed endogenous antimicrobial peptide that exhibits qualitatively distinct activities between its oxidized and reduced forms. Here, we explore these antimicrobial mechanisms. Surprisingly, using electron microscopy we detected a so far unknown net-like structure surrounding bacteria, which were treated with the reduced but not the oxidized form of hBD1. A transmigration assay demonstrated that hBD1-derived nets capture bacteria and inhibit bacterial transmigration independent of bacterial killing. The presence of nets could completely prevent migration of hBD1 resistant pathogens and are stable in the presence of human duodenal secretion with a high amount of proteases. In contrast to HD6, cysteins are necessary for net formation. This redox-dependent function serves as an additional mechanism of action for hBD1 and differs from net formation by other defensins such as Paneth cell-derived human α-defensin 6 (HD6). While hBD1red and hBD1ox have distinct antimicrobial profiles and functions, only the reduced form provides additional host protection by entrapping bacteria in extracellular net structures preventing bacterial invasion. Better understanding of the modes of action of endogenous host peptides will help to find new antimicrobial strategies.

Antimicrobial peptide cWFW kills by combining lipid phase separation with autolysis

The synthetic cyclic hexapeptide cWFW (cyclo(RRRWFW)) has a rapid bactericidal activity against both Gram-positive and Gram-negative bacteria. Its detailed mode of action has, however, remained elusive. In contrast to most antimicrobial peptides, cWFW neither permeabilizes the membrane nor translocates to the cytoplasm. Using a combination of proteome analysis, fluorescence microscopy, and membrane analysis we show that cWFW instead triggers a rapid reduction of membrane fluidity both in live Bacillus subtilis cells and in model membranes. This immediate activity is accompanied by formation of distinct membrane domains which differ in local membrane fluidity, and which severely disrupts membrane protein organisation by segregating peripheral and integral proteins into domains of different rigidity. These major membrane disturbances cause specific inhibition of cell wall synthesis, and trigger autolysis. This novel antibacterial mode of action holds a low risk to induce bacterial resistance, and provides valuable information for the design of new synthetic antimicrobial peptides.

β-Defensins in the Fight against Helicobacter pylori

Antimicrobial peptides (AMPs) play a pivotal role in the innate immune responses to Helicobacter pylori (Hp) in humans. β-Defensins, a class of cationic arginine-rich AMPs, are small peptides secreted by immune cells and epithelial cells that exert antimicrobial activity against a broad spectrum of microorganisms, including Gram-positive and Gram-negative bacteria and fungi. During Hp infections, AMP expression is able to eradicate the bacteria, thereby preventing Hp infections in gastrointestinal tract. It is likely that gastric β-defensins expression is increased during Hp infection. The aim of this review is to focus on increased knowledge of the role of β-defensins in response to Hp infection. We also briefly discuss the potential use of AMPs, either alone or in combination with conventional antibiotics, for the treatment of Hp infection.

Prediction of the impact of coding missense and nonsense single nucleotide polymorphisms on HD5 and HBD1 antibacterial activity against Escherichia coli

Defensins confer host defense against microorganisms and are important for human health. Single nucleotide polymorphisms (SNPs) in defensin gene-coding regions could lead to less active variants. Using SNP data available at the dbSNP database and frequency information from the 1000 Genomes Project, two DEFA5 (L26I and R13H) and eight DEFB1 (C35S, K31T, K33R, R29G, V06I, C12Y, Y28* and C05*) missense and nonsense SNPs that are located within mature regions of the coded defensins were retrieved. Such SNPs are rare and population restricted. In order to assess their antibacterial activity against Escherichia coli, two linear regression models were used from a previous work, which models the antibacterial activity as a function of solvation potential energy, using molecular dynamics data. Regarding only the antibacterial predictions, for HD5, no biological differences between wild-type and its variants were observed; while for HBD1, the results suggest that the R29G, K31T, Y28* and C05* variants could be less active than the wild-type one. The data here reported could lead to a substantial improvement in knowledge about the impact of missense SNPs in human defensins and their world distribution. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 633-644, 2016.

Convergent evolution of defensin sequence, structure and function

Defensins are a well-characterised group of small, disulphide-rich, cationic peptides that are produced by essentially all eukaryotes and are highly diverse in their sequences and structures. Most display broad range antimicrobial activity at low micromolar concentrations, whereas others have other diverse roles, including cell signalling (e.g. immune cell recruitment, self/non-self-recognition), ion channel perturbation, toxic functions, and enzyme inhibition. The defensins consist of two superfamilies, each derived from an independent evolutionary origin, which have subsequently undergone extensive divergent evolution in their sequence, structure and function. Referred to as the cis- and trans-defensin superfamilies, they are classified based on their secondary structure orientation, cysteine motifs and disulphide bond connectivities, tertiary structure similarities and precursor gene sequence. The utility of displaying loops on a stable, compact, disulphide-rich core has been exploited by evolution on multiple occasions. The defensin superfamilies represent a case where the ensuing convergent evolution of sequence, structure and function has been particularly extreme. Here, we discuss the extent, causes and significance of these convergent features, drawing examples from across the eukaryotes.

The Interplay between Defensins and Microbiota in Crohn's Disease

Crohn's disease (CD) is a chronic inflammation of the intestinal mucosa, characterized by periods of acute recurrence and remission. Depending on the specific region affected, CD is classified as ileal CD or colonic CD. It is largely accepted that the intestinal microbiota is involved in the onset of the pathology. Indeed, a reduced immune tolerance to components of the intestinal commensal microbiota and inflammation of the intestinal barrier typifies patients with CD. Several studies have shown defective expression of intestinal antimicrobial peptides (AMPs) in patients with CD compared to controls, particularly defensins. A reduction in α-defensins is observed in ileal CD, while β-defensins are increased in colonic CD. In addition to an immunological basis, the disease is frequently associated with genetic alterations including mutations of NOD2 gene. Several therapeutic strategies to circumvent the dysfunction observed in CD are currently under investigation. These include the use of delivery systems to administer endogenous AMPs and the engineering of peptidomimetics that could ameliorate the severity of CD. In this review, the role defensins play in CD and the strategies aimed at overcoming bacterial resistance will be discussed.

Comparative Analysis of the Bacterial Membrane Disruption Effect of Two Natural Plant Antimicrobial Peptides

In the Medicago truncatula genome about 700 genes code for nodule-specific cysteine-rich (NCR) small peptides that are expressed in the symbiotic organ, the root nodule, where they control terminal differentiation of the endosymbiotic rhizobium bacteria to nitrogen-fixing bacteroids. Cationic NCR peptides were predicted to have antimicrobial activities. Here antibacterial activities of NCR247, NCR335, polymyxin B (PMB), and streptomycin were investigated and compared on two foodborne pathogens Salmonella enterica and Listeria monocytogenes as representatives of Gram-negative and Gram-positive bacteria. The integrity of the bacterial membrane was seriously compromised by these NCR peptides. Different localization was observed for NCR247 and NCR335 in the treated bacteria, the peptides mostly accumulated in the cytosol in S. enterica while they remained in the bacterial membrane in L. monocytogenes. Scanning electron microscopy revealed distinct membrane morphology of the peptide-treated bacteria. Complete cell disruption was induced by PMB and NCR335 in S. enterica while NCR247 treatment resulted in extensive budding observed on the cell surface of Salmonella. PMB had no effect on L. monocytogenes while NCR335 and NCR247 provoked morphological changes on this bacterium, the whole Listeria cell content was released in response to NCR335 treatment.