Through the looking glass, mechanistic insights from enantiomeric human defensins

KCD: A prediction web server of knowledge-based circular dichroism

We present a web server that predicts the far-UV circular dichroism (CD) spectra of proteins by utilizing their three-dimensional (3D) structures from the Protein Data Bank (PDB). The main algorithm is based on the classical theory of optical activity together with a set of atomic complex polarizabilities, which are obtained from the analysis of a series of synchrotron radiation CD spectra and their related 3D structures from the PDB. The results of our knowledge-based CD method (KCD) are in good agreement with measured spectra that could include the effect of D-amino acids. Our method also delivers some of the most accurate predictions, in comparison with the calculated spectra from well-established models. Specifically, using a metric of closeness based on normalized absolute deviations between experimental and calculated spectra, the mean values for a series of 57 test proteins give the following figures for such models: 0.26 KCD, 0.27 PDBMD2CD, 0.30 SESCA, and 0.47 DichroCalc. From another point of view, it is worth mentioning the remarkable capabilities of the recent approaches based on artificial intelligence, which can precisely predict the native structure of proteins. The structure of proteins, however, is flexible and can be modified by a diversity of environmental factors such as interactions with other molecules, mechanical stresses, variations of temperature, pH, or ionic strength. Experimental CD spectra together with reliable predictions can be utilized to assess eventual secondary structural changes. A similar kind of evaluation can be done for the case of an incomplete protein structure that has been reconstructed by using different approaches. The KCD method can be freely accessed from: https://kcd.cinvestav.mx/.

PM2.5 Causes Increased Bacterial Invasion by Affecting HBD1 Expression in the Lung

Our research addresses the critical environmental issue of a fine particulate matter (PM2.5), focusing on its association with the increased infection risks. We explored the influence of PM2.5 on human beta-defensin 1 (HBD1), an essential peptide in mucosal immunity found in the airway epithelium. Using C57BL/6J mice and human bronchial epithelial cells (HBE), we examined the effects of PM2.5 exposure followed by Pseudomonas aeruginosa (P. aeruginosa) infection on HBD1 expression at both mRNA and protein levels. The study revealed that PM2.5's toxicity to epithelial cells and animals varies with time and concentration. Notably, HBE cells exposed to PM2.5 and P. aeruginosa showed increased bacterial invasion and decreased HBD1 expression compared to the cells exposed to P. aeruginosa alone. Similarly, mice studies indicated that combined exposure to PM2.5 and P. aeruginosa significantly reduced survival rates and increased bacterial invasion. These harmful effects, however, were alleviated by administering exogenous HBD1. Furthermore, our findings highlight the activation of MAPK and NF-κB pathways following PM2.5 exposure. Inhibiting these pathways effectively increased HBD1 expression and diminished bacterial invasion. In summary, our study establishes that PM2.5 exposure intensifies P. aeruginosa invasion in both HBE cells and mouse models, primarily by suppressing HBD1 expression. This effect can be counteracted with exogenous HBD1, with the downregulation mechanism involving the MAPK and NF-κB pathways. Our study endeavors to elucidate the pathogenesis of lung infections associated with PM2.5 exposure, providing a novel theoretical basis for the development of prevention and treatment strategies, with substantial clinical significance.

Defensins: A novel weapon against Mycobacterium tuberculosis?

Tuberculosis (TB) is a serious airborne communicable disease caused by organisms of the Mycobacterium tuberculosis (Mtb) complex. Although the standard treatment antimicrobials, including isoniazid, rifampicin, pyrazinamide, and ethambutol, have made great progress in the treatment of TB, problems including the rising incidence of multidrug-resistant tuberculosis (MDR-TB) and extensively drug-resistant tuberculosis (XDR-TB), the severe toxicity and side effects of antimicrobials, and the low immunity of TB patients have become the bottlenecks of the current TB treatments. Therefore, both safe and effective new strategies to prevent and treat TB have become a top priority. As a subfamily of cationic antimicrobial peptides, defensins are rich in cysteine and play a vital role in resisting the invasion of microorganisms and regulating the immune response. Inspired by studies on the roles of defensins in host defence, we describe their research history and then review their structural features and antimicrobial mechanisms, specifically for fighting Mtb in detail. Finally, we discuss the clinical relevance, therapeutic potential, and potential challenges of defensins in anti-TB therapy. We further debate the possible solutions of the current application of defensins to provide new insights for eliminating Mtb.

Inhibition of Pertussis Toxin by Human α-Defensins-1 and -5: Differential Mechanisms of Action

Whooping cough is a severe childhood disease, caused by the bacterium Bordetella pertussis, which releases pertussis toxin (PT) as a major virulence factor. Previously, we identified the human antimicrobial peptides α-defensin-1 and -5 as inhibitors of PT and demonstrated their capacity to inhibit the activity of the PT enzyme subunit PTS1. Here, the underlying mechanism of toxin inhibition was investigated in more detail, which is essential for developing the therapeutic potential of these peptides. Flow cytometry and immunocytochemistry revealed that α-defensin-5 strongly reduced PT binding to, and uptake into cells, whereas α-defensin-1 caused only a mild reduction. Conversely, α-defensin-1, but not α-defensin-5 was taken up into different cell lines and interacted with PTS1 inside cells, based on proximity ligation assay. In-silico modeling revealed specific interaction interfaces for α-defensin-1 with PTS1 and vice versa, unlike α-defensin-5. Dot blot experiments showed that α-defensin-1 binds to PTS1 and even stronger to its substrate protein Gαi in vitro. NADase activity of PTS1 in vitro was not inhibited by α-defensin-1 in the absence of Gαi. Taken together, these results suggest that α-defensin-1 inhibits PT mainly by inhibiting enzyme activity of PTS1, whereas α-defensin-5 mainly inhibits cellular uptake of PT. These findings will pave the way for optimization of α-defensins as novel therapeutics against whooping cough.

Role of Defensins in Tumor Biology

Defensins have long been considered as merely antimicrobial peptides. Throughout the years, more immune-related functions have been discovered for both the α-defensin and β-defensin subfamily. This review provides insights into the role of defensins in tumor immunity. Since defensins are present and differentially expressed in certain cancer types, researchers started to unravel their role in the tumor microenvironment. The human neutrophil peptides have been demonstrated to be directly oncolytic by permealizing the cell membrane. Further, defensins can inflict DNA damage and induce apoptosis of tumor cells. In the tumor microenvironment, defensins can act as chemoattractants for subsets of immune cells, such as T cells, immature dendritic cells, monocytes and mast cells. Additionally, by activating the targeted leukocytes, defensins generate pro-inflammatory signals. Moreover, immuno-adjuvant effects have been reported in a variety of models. Therefore, the action of defensins reaches beyond their direct antimicrobial effect, i.e., the lysis of microbes invading the mucosal surfaces. By causing an increase in pro-inflammatory signaling events, cell lysis (generating antigens) and attraction and activation of antigen presenting cells, defensins could have a relevant role in activating the adaptive immune system and generating anti-tumor immunity, and could thus contribute to the success of immune therapy.

Induction of Endogenous Antimicrobial Peptides to Prevent or Treat Oral Infection and Inflammation

Antibiotics are often used to treat oral infections. Unfortunately, excessive antibiotic use can adversely alter oral microbiomes and promote the development of antibiotic-resistant microorganisms, which can be difficult to treat. An alternate approach could be to induce the local transcription and expression of endogenous oral antimicrobial peptides (AMPs). To assess the feasibility and benefits of this approach, we conducted literature searches to identify (i) the AMPs expressed in the oral cavity; (ii) the methods used to induce endogenous AMP expression; and (iii) the roles that expressed AMPs may have in regulating oral inflammation, immunity, healing, and pain. Search results identified human neutrophil peptides (HNP), human beta defensins (HBD), and cathelicidin AMP (CAMP) gene product LL-37 as prominent AMPs expressed by oral cells and tissues. HNP, HBD, and LL-37 expression can be induced by micronutrients (trace elements, elements, and vitamins), nutrients, macronutrients (mono-, di-, and polysaccharides, amino acids, pyropeptides, proteins, and fatty acids), proinflammatory agonists, thyroid hormones, and exposure to ultraviolet (UV) irradiation, red light, or near infrared radiation (NIR). Localized AMP expression can help reduce infection, inflammation, and pain and help oral tissues heal. The use of a specific inducer depends upon the overall objective. Inducing the expression of AMPs through beneficial foods would be suitable for long-term health protection. Additionally, the specialized metabolites or concentrated extracts that are utilized as dosage forms would maintain the oral and intestinal microbiome composition and control oral and intestinal infections. Inducing AMP expression using irradiation methodologies would be applicable to a specific oral treatment area in addition to controlling local infections while regulating inflammatory and healing processes.

Mouse α-Defensins: Structural and Functional Analysis of the 17 Cryptdin Isoforms Identified from a Single Jejunal Crypt

Mouse α-defensins, better known as cryptdins, are host protective antimicrobial peptides produced in the intestinal crypt by Paneth cells. To date, more than 20 cryptdin mRNAs have been identified from mouse small intestine, of which the first six cryptdins (Crp1 to Crp6) have been isolated and characterized at the peptide level. We quantified bactericidal activities against Escherichia coli and Staphylococcus aureus of the 17 cryptdin isoforms identified by Ouellette and colleagues from a single jejunal crypt (A. J. Ouellette et al., Infect Immun 62:5040-5047, 1994), along with linearized analogs of Crp1, Crp4, and Crp14. In addition, we analyzed the most potent and weakest cryptdins in the panel with respect to their ability to self-associate in solution. Finally, we solved, for the first time, the high-resolution crystal structure of a cryptdin, Crp14, and performed molecular dynamics simulation on Crp14 and a hypothetical mutant, T14K-Crp14. Our results indicate that mutational effects are highly dependent on cryptdin sequence, residue position, and bacterial strain. Crp14 adopts a disulfide-stabilized, three-stranded β-sheet core structure and forms a noncanonical dimer stabilized by asymmetrical interactions between the two β1 strands in parallel. The killing of E. coli by cryptdins is generally independent of their tertiary and quaternary structures that are important for the killing of S. aureus, which is indicative of two distinct mechanisms of action. Importantly, sequence variations impact the bactericidal activity of cryptdins by influencing their ability to self-associate in solution. This study expands our current understanding of how cryptdins function at the molecular level.

Effect of Supplementation with Organic Selenium or Turmeric and Rosemary Mixture on Beta-Defensin Content in Goat Milk

The present study examines the effects of diet supplementation with an organic selenium or herb-spice mixture on beta-defensin1 (GBD-1) and beta-defensin2 (GBD-2) concentrations in goat milk. Herd-I, consisting of Polish White (PWI) and Fawn Improved (PFI) goats, received supplementation with organic or inorganic selenium (controls). All goats were free from parasites, mastitis, and small ruminant lentivirus infection. Herd-II, consisting of PWI goats, either received a turmeric-rosemary mixture (experimental) or not (controls). The Herd I control group demonstrated higher fat, free fatty acid, and somatic cell levels and lower protein and lactose contents than Herd II controls. The GBD-1 concentration was below the detection limit in both herds. Herd I controls demonstrated higher GBD-2 concentrations in milk than Herd II controls. In addition, lower GBD-2 concentrations were noted in PWI goat milk. Organic selenium or rosemary-turmeric mixture supplementation had no effect on the GBD-2 content in the milk of healthy goats. The higher GBD-2 concentration observed in Herd Ic than in Herd IIc may suggest that the type of basal diet affects defensin secretion.

Design of D-Amino Acids SARS-CoV-2 Main Protease Inhibitors Using the Cationic Peptide from Rattlesnake Venom as a Scaffold

The C30 endopeptidase (3C-like protease; 3CL^pro) is essential for the life cycle of SARS-CoV-2 (severe acute respiratory syndrome-coronavirus-2) since it plays a pivotal role in viral replication and transcription and, hence, is a promising drug target. Molecules isolated from animals, insects, plants, or microorganisms can serve as a scaffold for the design of novel biopharmaceutical products. Crotamine, a small cationic peptide from the venom of the rattlesnake Crotalus durissus terrificus, has been the focus of many studies since it exhibits activities such as analgesic, in vitro antibacterial, and hemolytic activities. The crotamine derivative L-peptides (L-CDP) that inhibit the 3CL protease in the low µM range were examined since they are susceptible to proteolytic degradation; we explored the utility of their D-enantiomers form. Comparative uptake inhibition analysis showed D-CDP as a promising prototype for a D-peptide-based drug. We also found that the D-peptides can impair SARS-CoV-2 replication in vivo, probably targeting the viral protease 3CL^pro.

Human defensin-inspired discovery of peptidomimetic antibiotics

We report the development of peptidomimetic antibiotics derived from a natural antimicrobial peptide, human α-defensin 5. By engaging multiple bacterial targets, the lead compound is efficacious in vitro and in vivo against bacteria with highly inducible antibiotic resistance, promising a useful therapeutic agent for the treatment of infections caused by antibiotic-resistant bacteria.

Antibiotics with multiple mechanisms of action and broad-spectrum are urgently required to combat the growing health threat posed by resistant pathogenic microorganisms. Combining computational and medicinal chemistry tools, we used the structure of human α-defensin 5 (HD5) to design a class of peptidomimetic antibiotics with improved activity against both gram-negative and gram-positive bacteria. The most promising lead, compound 10, showed potent killing of multiple drug-resistant gram-negative bacteria isolated from patients. Compound 10 exhibited a multiplex mechanism of action through targeting membrane components—outer membrane protein A and lipopolysaccharide, as well as a potential intracellular target—70S ribosome, thus causing membrane perturbation and inhibition of protein synthesis. In vivo efficacy, stability, and safety of compound 10 were also validated. This human defensin-inspired synthetic peptidomimetic could help solve the serious problem of drug resistance to conventional antibiotics.

Real-Time Fluorescence Microscopy on Living E. coli Sheds New Light on the Antibacterial Effects of the King Penguin β-Defensin AvBD103b

(1) Antimicrobial peptides (AMPs) are a promising alternative to conventional antibiotics. Among AMPs, the disulfide-rich β-defensin AvBD103b, whose antibacterial activities are not inhibited by salts contrary to most other β-defensins, is particularly appealing. Information about the mechanisms of action is mandatory for the development and approval of new drugs. However, data for non-membrane-disruptive AMPs such as β-defensins are scarce, thus they still remain poorly understood. (2) We used single-cell fluorescence imaging to monitor the effects of a β-defensin (namely AvBD103b) in real time, on living E. coli, and at the physiological concentration of salts. (3) We obtained key parameters to dissect the mechanism of action. The cascade of events, inferred from our precise timing of membrane permeabilization effects, associated with the timing of bacterial growth arrest, differs significantly from the other antimicrobial compounds that we previously studied in the same physiological conditions. Moreover, the AvBD103b mechanism does not involve significant stereo-selective interaction with any chiral partner, at any step of the process. (4) The results are consistent with the suggestion that after penetrating the outer membrane and the cytoplasmic membrane, AvBD103b interacts non-specifically with a variety of polyanionic targets, leading indirectly to cell death.

Antimicrobial peptides properties beyond growth inhibition and bacterial killing

Antimicrobial peptides (AMPs) are versatile molecules with broad antimicrobial activity produced by representatives of the three domains of life. Also, there are derivatives of AMPs and artificial short peptides that can inhibit microbial growth. Beyond killing microbes, AMPs at grow sub-inhibitory concentrations also exhibit anti-virulence activity against critical pathogenic bacteria, including ESKAPE pathogens. Anti-virulence therapies are an alternative to antibiotics since they do not directly affect viability and growth, and they are considered less likely to generate resistance. Bacterial biofilms significantly increase antibiotic resistance and are linked to establishing chronic infections. Various AMPs can kill biofilm cells and eradicate infections in animal models. However, some can inhibit biofilm formation and promote dispersal at sub-growth inhibitory concentrations. These examples are discussed here, along with those of peptides that inhibit the expression of traits controlled by quorum sensing, such as the production of exoproteases, phenazines, surfactants, toxins, among others. In addition, specific targets that are determinants of virulence include secretion systems (type II, III, and VI) responsible for releasing effector proteins toxic to eukaryotic cells. This review summarizes the current knowledge on the anti-virulence properties of AMPs and the future directions of their research.

Knowledge-Based Atomic Polarizabilities Used to Model Circular Dichroism Spectra of Proteins

We present a model of circular dichroism for proteins, which is mainly based on both the classical theory of optical activity and a series of effective atomic polarizabilities. Such polarizabilities are extracted from the analysis of a set of synchrotron radiation circular dichroism spectra and their corresponding three-dimensional structures from the Protein Data Bank. Each modeled spectrum is obtained from the protein atomic coordinates and the identification of its secondary structure elements. The resulting spectra are in good agreement with additional experimental data and also with the predictions of some other models. Among them, only our approach is able to describe the effect of d-amino acids. Moreover, our model is also utilized to evaluate protein reconstructions as well as structural changes.

Defensins: The natural peptide antibiotic

Defensins are a family of cationic antimicrobial peptides active against a broad range of infectious microbes including bacteria, viruses and fungi, playing important roles as innate effectors and immune modulators in immunological control of microbial infection. Their antibacterial properties and unique mechanisms of action have garnered considerable interest in developing defensins into a novel class of natural antibiotic peptides to fend off pathogenic infection by bacteria, particularly those resistant to conventional antibiotics. However, serious pharmacological and technical obstacles, some of which are unique to defensins and others are common to peptide drugs in general, have hindered the development and clinical translation of defensins as anti-infective therapeutics. To overcome them, several technologies have been developed, aiming for improved functionality, prolonged circulation time, enhanced proteolytic stability and bioavailability, and efficient and controlled delivery and release of defensins to the site of infection. Additional challenges include the alleviation of potential toxicity of defensins and their cost-effective manufacturing. In this review, we briefly introduce defensin biology, focus on various transforming strategies and practical techniques developed for defensins and their derivatives as antibacterial therapeutics, and conclude with a summation of future challenges and possible solutions.

Nano- and Macroscale Imaging of Cholesterol Linoleate and Human Beta Defensin 2-Induced Changes in Pseudomonas aeruginosa Biofilms

The biofilm production of Pseudomonas aeruginosa (PA) is central to establishing chronic infection in the airways in cystic fibrosis. Epithelial cells secrete an array of innate immune factors, including antimicrobial proteins and lipids, such as human beta defensin 2 (HBD2) and cholesteryl lineolate (CL), respectively, to combat colonization by pathogens. We have recently shown that HBD2 inhibits biofilm production by PA, possibly linked to interference with the transport of biofilm precursors. Considering that both HBD2 and CL are increased in airway fluids during infection, we hypothesized that CL synergizes with HBD2 in biofilm inhibition. CL was formulated in phospholipid-based liposomes (CL-PL). As measured by atomic force microscopy of single bacteria, CL-PL alone and in combination with HBD2 significantly increased bacterial surface roughness. Additionally, extracellular structures emanated from untreated bacterial cells, but not from cells treated with CL-PL and HBD2 alone and in combination. Crystal violet staining of the biofilm revealed that CL-PL combined with HBD2 effected a significant decrease of biofilm mass and increased the number of larger biofilm particles consistent with altered cohesion of formed biofilms. These data suggest that CL and HBD2 affect PA biofilm formation at the single cell and community-wide level and that the community-wide effects of CL are enhanced by HBD2. This research may inform future novel treatments for recalcitrant infections in the airways of CF patients.

Antimicrobial α-defensins as multi-target inhibitors against amyloid formation and microbial infection

Amyloid aggregation and microbial infection are considered as pathological risk factors for developing amyloid diseases, including Alzheimer's disease (AD), type II diabetes (T2D), Parkinson's disease (PD), and medullary thyroid carcinoma (MTC). Due to the multifactorial nature of amyloid diseases, single-target drugs and treatments have mostly failed to inhibit amyloid aggregation and microbial infection simultaneously, thus leading to marginal benefits for amyloid inhibition and medical treatments. Herein, we proposed and demonstrated a new "anti-amyloid and antimicrobial hypothesis" to discover two host-defense antimicrobial peptides of α-defensins containing β-rich structures (human neutrophil peptide of HNP-1 and rabbit neutrophil peptide of NP-3A), which have demonstrated multi-target, sequence-independent functions to (i) prevent the aggregation and misfolding of different amyloid proteins of amyloid-β (Aβ, associated with AD), human islet amyloid polypeptide (hIAPP, associated with T2D), and human calcitonin (hCT, associated with MTC) at sub-stoichiometric concentrations, (ii) reduce amyloid-induced cell toxicity, and (iii) retain their original antimicrobial activity upon the formation of complexes with amyloid peptides. Further structural analysis showed that the sequence-independent amyloid inhibition function of α-defensins mainly stems from their cross-interactions with amyloid proteins via β-structure interactions. The discovery of antimicrobial peptides containing β-structures to inhibit both microbial infection and amyloid aggregation greatly expands the new therapeutic potential of antimicrobial peptides as multi-target amyloid inhibitors for better understanding pathological causes and treatments of amyloid diseases.

Insight into the Mechanism of Action and Peptide-Membrane Interactions of Aib-Rich Peptides: Multitechnique Experimental and Theoretical Analysis

The increase in resistant bacterial strains necessitates the identification of new antimicrobial molecules. Antimicrobial peptides (AMPs) are an attractive option because of evidence that bacteria cannot easily develop resistance to AMPs. The peptaibols, a class of naturally occurring AMPs, have shown particular promise as antimicrobial drugs, but their development has been hindered by their mechanism of action not being clearly understood. To explore how peptaibols might interact with membranes, circular dichroism, vibrational circular dichroism, linear dichroism, Raman spectroscopy, Raman optical activity, neutron reflectivity and molecular dynamics simulations have been used to study a small library of peptaibol mimics, the Aib-rich peptides. All the peptides studied quickly partitioned and oriented in membranes, and we found evidence of chiral interactions between the phospholipids and membrane-embedded peptides. The protocols presented in this paper open new ground by showing how chiro-optical spectroscopies can throw light on the mechanism of action of AMPs.

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.

Defensins: A Double-Edged Sword in Host Immunity

Defensins are a major family of host defense peptides expressed predominantly in neutrophils and epithelial cells. Their broad antimicrobial activities and multifaceted immunomodulatory functions have been extensively studied, cementing their role in innate immunity as a core host-protective component against bacterial, viral and fungal infections. More recent studies, however, paint defensins in a bad light such that they are "alleged" to promote viral and bacterial infections in certain biological settings. This mini review summarizes the latest findings on the potential pathogenic properties of defensins against the backdrop of their protective roles in antiviral and antibacterial immunity. Further, a succinct description of both tumor-proliferative and -suppressive activities of defensins is also given to highlight their functional and mechanistic complexity in antitumor immunity. We posit that given an enabling environment defensins, widely heralded as the "Swiss army knife," can function as a "double-edged sword" in host immunity.

Is the Mirror Image a True Reflection? Intrinsic Membrane Chirality Modulates Peptide Binding

Peptides with pharmaceutical activities are attractive drug leads, and knowledge of their mode-of-action is essential for translation into the clinic. Comparison of native and enantiomeric peptides has long been used as a powerful approach to discriminate membrane- or receptor-mediated modes-of-action on the basis of the assumption that interactions with cell membranes are independent of peptide chirality. Here, we revisit this paradigm with the cyclotide kalata B1, a drug scaffold with intrinsic membrane-binding activity whose enantiomer is less potent than native peptide. To investigate this chirality dependence, we compared peptide-lipid binding using mirror image model membranes. We synthesized phospholipids with non-natural chirality and demonstrate that native kalata B1 binds with higher affinity to phospholipids with chirality found in eukaryotic membranes. This study shows for the first time that the chiral environment of lipid bilayers can modulate the function of membrane-active peptides and challenges the view that peptide-lipid interactions are achiral.

Chirality-Dependent Adsorption between Amphipathic Peptide and POPC Membrane

The interactions between chiral molecules and cell membranes have attracted more and more attention in recent decades, due to their importance in molecular science and medical applications. It is observed that some peptides composed of different chiral amino acids may have distinct interactions with a membrane. How does the membrane exhibit a selective behavior related to the chirality of the peptides? Microscopically, the interactions between the peptides and the membrane are poorly understood. In this work, we study the interactions between an amphipathic peptide (C6) and POPC membrane with simulations. The kinetics and thermodynamics of peptide enantiomers during the adsorption to the membrane are characterized with direct simulations and umbrella sampling. It is observed that there are slow kinetics for the peptide composed of D-type amino acids. Along the observed pathways, the free energy landscapes are determined with umbrella sampling techniques. A free-energy barrier for the peptide composed of D-amino acids is observed, which is consistent with the kinetic observations. The results indicate the concurrent adsorption and rotation of the peptide helix. The local interactions between the peptides and the membrane are examined in detail, including the contact interactions between the peptides and the membrane, and the distributions of the lipids around the peptide. There are observable differences of the local interactions for the cases related to different peptide enantiomers. These results further demonstrate the importance of the rotation of peptide helix during the adsorption. More interestingly, all these kinetic differences between peptide enantiomers can be explained based on the conformations of the residue Trp and interactions between Trp and lipid molecules. These results give us a molecular understanding of the mechanism of the chirality-dependent peptide-membrane interactions, and may provide clues to designing systems which are sensitive to the chirality of membranes.

A review on anti-tuberculosis peptides: Impact of peptide structure on anti-tuberculosis activity

Antibiotic resistance is a major public health problem globally. Particularly concerning amongst drug-resistant human pathogens is Mycobacterium tuberculosis that causes the deadly infectious tuberculosis (TB) disease. Significant issues associated with current treatment options for drug-resistant TB and the high rate of mortality from the disease makes the development of novel treatment options against this pathogen an urgent need. Antimicrobial peptides are part of innate immunity in all forms of life and could provide a potential solution against drug-resistant TB. This review is a critical analysis of antimicrobial peptides that are reported to be active against the M tuberculosis complex exclusively. However, activity on non-TB strains such as Mycobacterium avium and Mycobacterium intracellulare, whenever available, have been included at appropriate sections for these anti-TB peptides. Natural and synthetic antimicrobial peptides of diverse sequences, along with their chemical structures, are presented, discussed, and correlated to their observed antimycobacterial activities. Critical analyses of the structure allied to the anti-mycobacterial activity have allowed us to draw important conclusions and ideas for research and development on these promising molecules to realise their full potential. Even though the review is focussed on peptides, we have briefly summarised the structures and potency of the various small molecule drugs that are available and under development, for TB treatment.

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.

Dynamics of human defensin 5 (HD5) self-assembly in solution: Molecular simulations/insights

Human α -defensin 5 (HD5) is a 32-residue cysteine-rich host-defense peptide that exhibits broad-spectrum antimicrobial activity and plays an essential role in innate immunity in the human gut and other organ systems. Although its antimicrobial mechanism of action remains unclear, the high salt concentration seems to attenuate the antimicrobial function of HD5 via an unknown mechanism. In this work, we employ Molecular Dynamics (MD) simulations to analyse the oligomerization behaviour of HD5 when exposed to different salt concentration. We demonstrate that the presence of salt, such as sodium chloride (NaCl), promotes HD5 to form higher-order oligomers (up to heptamers) in our simulations. In addition, we also analyse the electrostatic interactions between the two Glu residues (E14 and E21) and their neighbouring residues. Our data confirm that the E14 residue is essential for the structural integrity, whereas the E21 residue contributes to the dimerization of HD5, suggesting that these Glu residues are important for the antimicrobial function of this peptide.

Critical determinants of human neutrophil peptide 1 for enhancing host epithelial adhesion of Shigella flexneri

Human neutrophil peptides (HNPs), also known as human myeloid α-defensins degranulated by infiltrating neutrophils at bacterial infection loci, exhibit broad antomicrobial activities against bacteria, fungi, and viruses. We have made a surprising recent finding that Shigella, a highly contagious, yet poorly adhesive enteric pathogen, exploits human α-defensins including HNP1 to enhance its adhesion to and invasion of host epithelial cells. However, the critical molecular determinants responsible for HNP1-enhanced Shigella adhesion and invasion have yet to be investigated. Using cultured epithelial cells and polarised Caco2 cells as an in vitro infection model, we demonstrated that HNP1 promoted Shigella infection in a structure- and sequence-dependent manner, with two bulky hydrophobic residues, Trp26 and Phe28 important for HNP1 self-assembly, being most critical. The functional importance of hydrophobicity for HNP1-enhanced Shigella infection was further verified by substitutions for Trp26 of a series of unnatural amino acids with straight aliphatic side chains of different lengths. Dissection of the Shigella infection process revealed that bacteria-rather than host cells-bound HNP1 contributed most to the enhancement. Further, mutagenesis analysis of bacterial surface components, while precluding the involvement of lipopolysaccharides (LPS) in the interaction with HNP1, identified outer membrane proteins and the Type 3 secretion apparatus as putative binding targets of HNP1 involved in enhanced Shigella adhesion and invasion. Our findings provide molecular and mechanistic insights into the mode of action of HNP1 in promoting Shigella infection, thus showcasing another example of how innate immune factors may serve as a double-edged sword in health and disease.

Newly Identified HNP-F from Human Neutrophil Peptide-1 Promotes Hemostasis

Active hemostatic agents can play a crucial role in saving patients' lives during surgery. Active hemostats have several advantages including utilization of natural blood coagulation and biocompatibility. Among them, although human neutrophil peptide-1 (HNP-1) has been previously reported with the hemostatic mechanism, which part of HNP-1 facilitates the hemostatic activity is not known. Here, a partial peptide (HNP-F) promoting hemostasis, originating from HNP-1, has been newly identified by the blood coagulation ability test. HNP-F shows the best hemostatic effect between the anterior half and posterior half of peptides. Moreover, microscopic images show platelet aggregation and an increase in the concentration of platelet factor 4, and the scanning electron microscope image of platelets support platelet activation by HNP-F. Thromboelastography indicates decreased clotting time and increased physical properties of blood clotting. Mouse liver experiments demonstrate improved hemostatic effect by treatment of peptide solution. Cell viability and hemolysis assays confirm the HNP-F's biosafety. It is hypothesized that the surface charge and structure of HNP-F could be favorable to interact with fibrinogen or thrombospondin-1. Collectively, because HNP-F as an active peptide hemostat has many advantages, it could be expected to become a potent hemostatic biomaterial, additive or pharmaceutical candidate for various hemostatic applications.

Review of antimicrobial peptides with anti-Helicobacter pylori activity

BACKGROUND

The emergence of antibiotic-resistant Helicobacter pylori strains in recent years has increased the need for finding an alternative in the post-antibiotic era. One of the fields being considered for this purpose is antimicrobial peptides. The aim of this review was to provide an obvious scheme from the studied anti-H. pylori peptides and to investigate their common features.

METHOD

First, all of the antimicrobial peptides with their anti-H. pylori effects have been proved up to September 2018 were selected and their information including structure, mechanism of action, and function was reviewed. To achieve this, three databases of PubMed, Scopus, and Web of science were used.

RESULTS

A total of 9 groups containing 22 antimicrobial peptides were found with demonstrated anti-H. pylori effects. The nine groups included pexiganan, tilapia piscidins, epinecidin-1, cathelicidins, defensins, bicarinalin, odorranain-HP, PGLa-AM1, and bacteriocins. Most of the antimicrobial peptides, not all, had common features such as the ability to kill antibiotic-resistant strains, having α-helical structure, being cationic, with high positive charge and isoelectric point.

CONCLUSION

Antimicrobial peptides with anti-H. pylori effects have the potential to replace the antibiotics, especially in the post-antibiotic era, if a rapid and low-cost production method would be found.

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.

The adsorption of human defensin 5 on bacterial membranes: simulation studies

Human α-defensin 5 (HD5) is one of the important antimicrobial peptides (AMPs) used against a broad-spectrum of pathogens, especially Gram-negative bacteria. HD5 kills by disrupting and making a pore in the bacterial membrane. The presence of lipopolysaccharide (LPS), located on a membrane surface, is found to have an impact on HD5's activity, where such binding mechanism in microscopic detail remains unclear. In this work, we therefore employed molecular dynamics (MD) simulations to investigate the binding mechanisms of HD5 on LPS in comparison to a bare DMPC lipid membrane. Two oligomers, dimer and tetramer, are studied here. Apparently, the membrane structure influences the protein binding affinity. HD5 binds tighter to a lipid membrane than LPS. Both dimeric and tetrameric HD5 can penetrate deeply into a phosphate layer in a lipid membrane, whereas only facial contacts are observed for LPS systems. The proteins appear to stay in the polar area instead of diving into a hydrophobic region. Furthermore, it happens in all cases that residues in the active region (A1, T2, R6, R13, R32) contribute to the membrane adsorption. The breakdown of tetramer into two dimers is also found. This implies that the dimer is more favorable for membrane binding. Moreover, both dimeric and tetrameric HD5 can significantly disrupt a LPS layer, whilst no serious distortion of lipid membrane is obtained. This emphasizes the importance of LPS on HD5 activity.

Neutralization of Pseudomonas auruginosa Exotoxin A by human neutrophil peptide 1

Pseudomonas aeruginosa produces a large number of virulence factors, including the extracellular protein, Exotoxin A (ETA). Human Neutrophil Peptide 1 (HNP1) neutralizes the Exotoxin A. HNP1 belongs to the family of α-defensins, small effector peptides of the innate immune system that combat against microbial infections. Neutralization of bacterial toxins such as ETA by HNP1 is a novel biological function in addition to direct killing of bacteria. In this study, we report on the interaction between HNP-1 and Exotoxin A at the molecular level to allow for the design and development of potent antibacterial peptides as alternatives to classical antibiotics.

Important Late-Stage Symbiotic Role of the Sinorhizobium meliloti Exopolysaccharide Succinoglycan

Sinorhizobium meliloti enters into beneficial symbiotic interactions with Medicago species of legumes. Bacterial exopolysaccharides play critical signaling roles in infection thread initiation and growth during the early stages of root nodule formation. After endocytosis of S. meliloti by plant cells in the developing nodule, plant-derived nodule-specific cysteine-rich (NCR) peptides mediate terminal differentiation of the bacteria into nitrogen-fixing bacteroids. Previous transcriptional studies showed that the intensively studied cationic peptide NCR247 induces expression of the exo genes that encode the proteins required for succinoglycan biosynthesis. In addition, genetic studies have shown that some exo mutants exhibit increased sensitivity to the antimicrobial action of NCR247. Therefore, we investigated whether the symbiotically active S. meliloti exopolysaccharide succinoglycan can protect S. meliloti against the antimicrobial activity of NCR247. We discovered that high-molecular-weight forms of succinoglycan have the ability to protect S. meliloti from the antimicrobial action of the NCR247 peptide but low-molecular-weight forms of wild-type succinoglycan do not. The protective function of high-molecular-weight succinoglycan occurs via direct molecular interactions between anionic succinoglycan and the cationic NCR247 peptide, but this interaction is not chiral. Taken together, our observations suggest that S. meliloti exopolysaccharides not only may be critical during early stages of nodule invasion but also are upregulated at a late stage of symbiosis to protect bacteria against the bactericidal action of cationic NCR peptides. Our findings represent an important step forward in fully understanding the complete set of exopolysaccharide functions during legume symbiosis.IMPORTANCE Symbiotic interactions between rhizobia and legumes are economically important for global food production. The legume symbiosis also is a major part of the global nitrogen cycle and is an ideal model system to study host-microbe interactions. Signaling between legumes and rhizobia is essential to establish symbiosis, and understanding these signals is a major goal in the field. Exopolysaccharides are important in the symbiotic context because they are essential signaling molecules during early-stage symbiosis. In this study, we provide evidence suggesting that the Sinorhizobium meliloti exopolysaccharide succinoglycan also protects the bacteria against the antimicrobial action of essential late-stage symbiosis plant peptides.

Mining the tree of life: Host defense peptides as antiviral therapeutics

Discovering new therapeutics for human viral diseases is important for combatting emerging infectious viruses and omnipresent circulating viruses as well as those that can become resistant to the drugs we currently have available. The innate host defense peptide (HDP) repertoire present in animals is a wealth of potential antimicrobial agents that could be mined to meet these needs. While much of the body of research regarding HDPs is in the context of bacteria, there is increasing evidence that they can be an effective source for antivirals. Peptides can be identified in a number of ways, including eco-conservation-minded approaches. Those shown to have antiviral properties can be modified to exhibit desired properties as the relationship between structure and function is elucidated and then developed into therapeutics for human use. This review looks at the discovery and therapeutic potential of HDPs for human viral infections.

A Simplified Derivative of Human Defensin 5 with Potent and Efficient Activity against Multidrug-Resistant Acinetobacter baumannii

The increasing incidence of multidrug-resistant Acinetobacter baumannii (MDRAb) infections worldwide has necessitated the development of novel antibiotics. Human defensin 5 (HD5) is an endogenous peptide with a complex architecture and antibacterial activity against MDRAb In the present study, we attempted to simplify the structure of HD5 by removing disulfide bonds. We found that the Cys2-4 bond was most indispensable for HD5 to inactivate MDRAb, although the antibacterial activity of the derivative was significantly attenuated. We then replaced the noncationic and nonhydrophobic residues with electropositive Arg to increase the antibacterial activity of HD5 derivative that contains a Cys2-4 bond, obtaining another derivative termed HD5d5. The in vitro antibacterial assay and irradiation-wound-infection animal experiment both showed that HD5d5 was much more effective than HD5 at eliminating MDRAb Further investigations revealed that HD5d5 efficiently bound to outer membrane lipid A and penetrated membranes, leading to bacterial collapse and peptide translocation. Compared to HD5, more HD5d5 molecules were located in the cytoplasm of MDRAb, and HD5d5 was more efficient at reducing the activities of superoxide dismutase and catalase, causing the accumulation of reactive oxygen species that are detrimental to microbes. In addition, HD5 failed to suppress the pathogenic outer membrane protein A of Acinetobacter baumannii (AbOmpA) at concentrations up to 50 μg/ml, whereas HD5d5 strongly bound to AbOmpA and exhibited a dramatic toxin-neutralizing ability, thus expanding the repertoire of drugs that is available to treat MDRAb infections.

Targeting and inactivation of bacterial toxins by human defensins

Defensins, as a prominent family of antimicrobial peptides (AMP), are major effectors of the innate immunity with a broad range of immune modulatory and antimicrobial activities. In particular, defensins are the only recognized fast-response molecules that can neutralize a broad range of bacterial toxins, many of which are among the deadliest compounds on the planet. For a decade, the mystery of how a small and structurally conserved group of peptides can neutralize a heterogeneous group of toxins with little to no sequential and structural similarity remained unresolved. Recently, it was found that defensins recognize and target structural plasticity/thermodynamic instability, fundamental physicochemical properties that unite many bacterial toxins and distinguish them from the majority of host proteins. Binding of human defensins promotes local unfolding of the affected toxins, destabilizes their secondary and tertiary structures, increases susceptibility to proteolysis, and leads to their precipitation. While the details of toxin destabilization by defensins remain obscure, here we briefly review properties and activities of bacterial toxins known to be affected by or resilient to defensins, and discuss how recognized features of defensins correlate with the observed inactivation.

Structure-Based Design of Hepatitis C Virus Vaccines That Elicit Neutralizing Antibody Responses to a Conserved Epitope

Despite recent advances in therapeutic options, hepatitis C virus (HCV) remains a severe global disease burden, and a vaccine can substantially reduce its incidence. Due to its extremely high sequence variability, HCV can readily escape the immune response; thus, an effective vaccine must target conserved, functionally important epitopes. Using the structure of a broadly neutralizing antibody in complex with a conserved linear epitope from the HCV E2 envelope glycoprotein (residues 412 to 423; epitope I), we performed structure-based design of immunogens to induce antibody responses to this epitope. This resulted in epitope-based immunogens based on a cyclic defensin protein, as well as a bivalent immunogen with two copies of the epitope on the E2 surface. We solved the X-ray structure of a cyclic immunogen in complex with the HCV1 antibody and confirmed preservation of the epitope conformation and the HCV1 interface. Mice vaccinated with our designed immunogens produced robust antibody responses to epitope I, and their serum could neutralize HCV. Notably, the cyclic designs induced greater epitope-specific responses and neutralization than the native peptide epitope. Beyond successfully designing several novel HCV immunogens, this study demonstrates the principle that neutralizing anti-HCV antibodies can be induced by epitope-based, engineered vaccines and provides the basis for further efforts in structure-based design of HCV vaccines.IMPORTANCE Hepatitis C virus is a leading cause of liver disease and liver cancer, with approximately 3% of the world's population infected. To combat this virus, an effective vaccine would have distinct advantages over current therapeutic options, yet experimental vaccines have not been successful to date, due in part to the virus's high sequence variability leading to immune escape. In this study, we rationally designed several vaccine immunogens based on the structure of a conserved epitope that is the target of broadly neutralizing antibodies. In vivo results in mice indicated that these antigens elicited epitope-specific neutralizing antibodies, with various degrees of potency and breadth. These promising results suggest that a rational design approach can be used to generate an effective vaccine for this virus.

Defensins, lectins, mucins, and secretory immunoglobulin A: microbe-binding biomolecules that contribute to mucosal immunity in the human gut

In the intestine, the mucosal immune system plays essential roles in maintaining homeostasis between the host and microorganisms, and protecting the host from pathogenic invaders. Epithelial cells produce and release a variety of biomolecules into the mucosa and lumen that contribute to immunity. In this review, we focus on a subset of these remarkable host-defense factors - enteric α-defensins, select lectins, mucins, and secretory immunoglobulin A - that have the capacity to bind microbes and thereby contribute to barrier function in the human gut. We provide an overview of the intestinal epithelium, describe specialized secretory cells named Paneth cells, and summarize our current understanding of the biophysical and functional properties of these select microbe-binding biomolecules. We intend for this compilation to complement prior reviews on intestinal host-defense factors, highlight recent advances in the field, and motivate investigations that further illuminate molecular mechanisms as well as the interplay between these molecules and microbes.

Defensins, lectins, mucins, and secretory immunoglobulin A: microbe-binding biomolecules that contribute to mucosal immunity in the human gut

In the intestine, the mucosal immune system plays essential roles in maintaining homeostasis between the host and microorganisms, and protecting the host from pathogenic invaders. Epithelial cells produce and release a variety of biomolecules into the mucosa and lumen that contribute to immunity. In this review, we focus on a subset of these remarkable host-defense factors - enteric α-defensins, select lectins, mucins, and secretory immunoglobulin A - that have the capacity to bind microbes and thereby contribute to barrier function in the human gut. We provide an overview of the intestinal epithelium, describe specialized secretory cells named Paneth cells, and summarize our current understanding of the biophysical and functional properties of these select microbe-binding biomolecules. We intend for this compilation to complement prior reviews on intestinal host-defense factors, highlight recent advances in the field, and motivate investigations that further illuminate molecular mechanisms as well as the interplay between these molecules and microbes.

Mirror Images of Antimicrobial Peptides Provide Reflections on Their Functions and Amyloidogenic Properties

Enantiomeric forms of BTD-2, PG-1, and PM-1 were synthesized to delineate the structure and function of these β-sheet antimicrobial peptides. Activity and lipid-binding assays confirm that these peptides act via a receptor-independent mechanism involving membrane interaction. The racemic crystal structure of BTD-2 solved at 1.45 Å revealed a novel oligomeric form of β-sheet antimicrobial peptides within the unit cell: an antiparallel trimer, which we suggest might be related to its membrane-active form. The BTD-2 oligomer extends into a larger supramolecular state that spans the crystal lattice, featuring a steric-zipper motif that is common in structures of amyloid-forming peptides. The supramolecular structure of BTD-2 thus represents a new mode of fibril-like assembly not previously observed for antimicrobial peptides, providing structural evidence linking antimicrobial and amyloid peptides.

Reduction Impairs the Antibacterial Activity but Benefits the LPS Neutralization Ability of Human Enteric Defensin 5

Oxidized human defensin 5 (HD5_OX), a Paneth cell-secreted antibacterial peptide with three characteristic disulfide bonds, protects the host from invasion by morbigenous microbes in the small intestine. HD5_OX can be reduced by thioredoxin (Trx) in vitro, while the biochemical properties of the reduced linear peptide, HD5_RED, remain unclear. Here, we first confirm that HD5_RED does exist in vivo. Furthermore, we reveal that the recruitment of HD5_RED to the outer membrane of Gram-negative bacteria and to the anionic lipid A is lower than that of HD5_OX, and HD5_RED is less efficient in penetrating bacterial outer and inner membranes and inducing membrane depolarization, which confers an attenuated antibacterial activity to HD5_RED. However, due to its higher structural flexibility, the binding of HD5_RED to bacterial lipopolysaccharide (LPS) is markedly stronger than that of HD5_OX. Consequently, HD5_RED is more effective in suppressing the production of the pro-inflammatory cytokine TNF-α in LPS-stimulated macrophages by blocking the interaction between LPS and LPS-binding protein, thus suggesting that HD5_RED might act as a scavenger to neutralize LPS in the gut. This study provides insights into the antibacterial and immunoregulatory effects of HD5_RED and expands the known repertoire of the enteric defensins.

Contribution of microbiota to the intestinal physicochemical barrier

The large number of intestinal microorganisms, which exceeds the total number of human cells by ten folds, alludes to a significant contribution to human health. This is vivid in enteric and some systemic diseases emanating from disruption of the microbiota. As life style keeps shifting towards disruption of the microbiota in most societies worldwide, interest in the contribution of the microbiota to gut health has grown enormously. Many studies have been conducted to elucidate the exact contribution of the microbiota to human health. The knowledge gained from these studies indicates that the microbiota interacts with the intestinal milieu to maintain gut health. In this review, the crosstalk of microbiota with the intestinal physicochemical barrier pivotal to the gut innate immunity is highlighted. In particular, the review focuses on the role of the microbiota on competitive exclusion of pathogens, intestinal pH, epithelial mechanical barrier integrity, apical actin cytoskeleton, antimicrobial peptides, and the mucus layer. Understanding this microbe-host relationship will provide useful insight into overcoming some diseases related to the disruption of the host microbiota.

Loss of Innate Host Defense Following Unprotected Vaginal Sex

BACKGROUND

Multiple host defense mechanisms protect the female genital tract from pathogens, but the impact of sexual intercourse on defense is unknown.

METHODS

As part of a hypothesis-generating study, 17 women provided cervicovaginal lavage (CVL) specimens at baseline (all had abstained from sexual intercourse, masturbation, and vaginal product use for 72 hours prior to screening), 2-6 hours and 10-14 hours after vaginal intercourse with a male condom, and 2-6 hours and 10-14 hours after vaginal intercourse without a male condom (5 visits total, including the baseline visit). Vaginal pH, concentrations of immune molecules, and antimicrobial activity at postcoital visits were compared to baseline values.

RESULTS

Vaginal pH and the transforming growth factor β1 level increased, but human beta-defensin 2 (HBD-2), HBD-3, and interleukin 8 levels decreased after unprotected sex. Median Escherichia coli inhibitory activity in CVL specimens decreased significantly from baseline at the visit 2-6 hours after unprotected sex (63% [range, -34% to 99%] vs 5% [range, -51% to 100%]; P = .02) and remained low at the visit 10-14 hours after unprotected sex (6% [range, -19% to 92%]; P = .02). Pooled human seminal plasma enhanced E. coli growth in vitro in a dose-dependent manner and, when added to CVL samples with high anti-E. coli activity, reversed the inhibition.

CONCLUSIONS

Unprotected vaginal sex results in a reduction in endogenous anti-E. coli activity, which may reflect, in part, enhancement of bacterial growth by seminal plasma. This finding may contribute to the risk of E. coli vaginal colonization following sexual intercourse.

Activity of Genital Tract Secretions and Synthetic Antimicrobial Peptides against Group B Streptococcus

PROBLEM

Genital tract secretions inhibit Escherichia coli (E. coli) through antimicrobial peptides (AMP) secreted by the host and vaginal microbiota. However, there are limited data against group B Streptococcus (GBS).

METHOD OF STUDY

Group B Streptococcus were incubated with cervico-vaginal lavage (CVL) samples from healthy non-pregnant women (n = 12) or synthetic AMP and monitored for bacterial growth using a turbidimetric approach. E. coli inhibitory activity was determined by a colony-forming unit assay.

RESULTS

None of the CVL samples inhibited GBS. The human neutrophil peptide-1 and human defensin 5 inhibited GBS growth by ≥80% at concentrations ≥20 μg/mL and ≥50 μg/mL, respectively, while human beta-defensin 2 and LL-37 did not inhibit at highest concentration tested (100 μg/mL). In contrast, all AMP inhibited E. coli.

CONCLUSIONS

Antimicrobial peptides may protect against E. coli colonization but have more limited activity against GBS. Future studies will focus on augmenting host defense with specific AMP to prevent genitourinary infection with these pathogenic organisms.

Antimicrobial activity of human α-defensin 5 and its linear analogs: N-terminal fatty acylation results in enhanced antimicrobial activity of the linear analogs

Human α-defensin 5 (HD5) exhibits broad spectrum antimicrobial activity and plays an important role in mucosal immunity of the small intestine. Although there have been several studies, the structural requirements for activity and mechanism of bacterial killing is yet to be established unequivocally. In this study, we have investigated the antimicrobial activity of HD5 and linear analogs. Cysteine deletions attenuated the antibacterial activity considerably. Candidacidal activity was affected to a lesser extent. Fatty acid conjugated linear analogs showed antimicrobial activity comparable activity to HD5. Effective surface charge neutralization of bacteria was observed for HD5 as compared to the non-fatty acylated linear analogs. Our results show that HD5 and non-fatty acylated linear analogs enter the bacterial cytoplasm without causing damage to the bacterial inner membrane. Although fatty acylated peptides exhibited antimicrobial activity comparable to HD5, their mechanism of action involved permeabilization of the Escherichia coli inner membrane. HD5 and analogs had the ability to bind plasmid DNA. HD5 had greater binding affinity to plasmid DNA as compared to the analogs. The three dimensional structure of HD5 favors greater interaction with the bacterial cell surface and also with DNA. Antibacterial activity of HD5 involves entry into bacterial cytoplasm and binding to DNA which would result in shut down of the bacterial metabolism leading to cell death. We show how a moderately active linear peptide derived from the α-defensin HD5 can be engineered to enhance antimicrobial activity almost comparable to the native peptide.

Peptides and Peptidomimetics for Antimicrobial Drug Design

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

What's one phosphate between friends (and foe)?

Body surfaces are colonized by resident microbes that are remarkably resilient to recurrent immune responses. In the latest issue of Science, Cullen et al. (2015) report that, contrary to prevailing assumptions, bacteria of the colonizing microbiota are resistant to antimicrobial peptides, and identify a common mechanism of resistance.

Visualizing attack of Escherichia coli by the antimicrobial peptide human defensin 5

Human α-defensin 5 (HD5) is a 32-residue cysteine-rich host-defense peptide that exhibits broad-spectrum antimicrobial activity and contributes to innate immunity in the human gut and other organ systems. Despite many years of investigation, its antimicrobial mechanism of action remains unclear. In this work, we report that HD5ox, the oxidized form of this peptide that exhibits three regiospecific disulfide bonds, causes distinct morphological changes to Escherichia coli and other Gram-negative microbes. These morphologies include bleb formation, cellular elongation, and clumping. The blebs are up to ∼1 μm wide and typically form at the site of cell division or cell poles. Studies with E. coli expressing cytoplasmic GFP reveal that HD5ox treatment causes GFP emission to localize in the bleb. To probe the cellular uptake of HD5ox and subsequent localization, we describe the design and characterization of a fluorophore-HD5 conjugate family. By employing these peptides, we demonstrate that fluorophore-HD5ox conjugates harboring the rhodamine and coumarin fluorophores enter the E. coli cytoplasm. On the basis of the fluorescence profiles, each of these fluorophore-HD5ox conjugates localizes to the site of cell division and cell poles. These studies support the notion that HD5ox, at least in part, exerts its antibacterial activity against E. coli and other Gram-negative microbes in the cytoplasm.

Quantification of polysaccharides fixed to Gram stained slides using lactophenol cotton blue and digital image processing

Dark blue rings and circles emerged when the non-specific polysaccharide stain lactophenol cotton blue was added to Gram stained slides. The dark blue staining is attributable to the presence of capsular polysaccharides and bacterial slime associated with clumps of Gram-negative bacteria.  Since all bacterial cells are glycosylated and concentrate polysaccharides from the media, the majority of cells stain light blue. The contrast between dark and light staining is sufficient to enable a digital image processing thresholding technique to be quantitative with little background noise. Prior to the addition of lactophenol cotton blue, the Gram-stained slides appeared unremarkable, lacking ubiquitous clumps or stained polysaccharides.  Adding lactophenol cotton blue to Gram stained slides is a quick and inexpensive way to screen cell cultures for bacterial slime, clumps and biofilms that are invisible using the Gram stain alone.