Functional interaction of human neutrophil peptide-1 with the cell wall precursor lipid II

Human Defensins: Structure, Function, and Potential as Therapeutic Antimicrobial Agents with Highlights Against SARS CoV-2

The human defensins are a group of cationic antimicrobial peptides that range in size from 2 to 5 kDa and share a common structural motif of six disulphide-linked cysteines. Several naturally occurring human α- and β-defensins have been identified over the past two decades. They have a wide variety of antimicrobial effects, and their potential to avoid the development of resistance to antimicrobial treatment makes them attractive as therapeutic agents. Human defensins have recently been the focus of medical and molecular biology studies due to their promising application in medicine and the pharmaceutical industry. This work aims to provide a comprehensive summary of the current developments of human defensins, including their identification, categorization, molecular features, expression, modes of action, and potential application in medical settings. Current obstacles and future opportunities for using human defensins are also covered. Furthermore, we shed light on the potential of this class as an antiviral agent, particularly against SARS CoV-2, by providing an in silico-based investigation of their plausible mechanisms of action.

Deep Sequencing of Crohn's Disease Lamina Propria Phagocytes Identifies Pathobionts and Correlates With Pro-Inflammatory Gene Expression

BACKGROUND

Crohn's disease (CD) is characterized by an inflammatory response to gut microbiota. Macrophages and dendritic cells play an active role in CD inflammation. Specific microbiota have been implicated in the pathogenesis of ileal CD. We investigated the phagocyte-associated microbiome using an unbiased sequencing approach to identify potential pathobionts and elucidate the host response to these microbes.

METHODS

We collected ileal and colonic mucosal biopsies from CD patients and controls without inflammatory bowel disease (IBD), isolated lamina propria phagocytes (CD11b+ cells), and performed deep RNA sequencing (n = 37). Reads were mapped to the human genome for host gene expression analysis and a prokaryotic database for microbiome taxonomic and metatranscriptomic profiling. Results were confirmed in a second IBD cohort (n = 17). Lysed lamina propria cells were plated for bacterial culturing; isolated colonies underwent whole genome sequencing (n = 11).

RESULTS

Crohn's disease ileal phagocytes contained higher relative abundances of Escherichia coli, Ruminococcus gnavus, and Enterocloster spp. than those from controls. CD phagocyte-associated microbes had increased expression of lipopolysaccharide (LPS) biosynthesis pathways. Phagocytes with a higher pathobiont burden showed increased expression of pro-inflammatory and antimicrobial genes, including PI3 (antimicrobial peptide) and BPIFB1 (LPS-binding molecule). E. coli isolated from the CD lamina propria had more flagellar motility and antibiotic resistance genes than control-derived strains.

CONCLUSIONS

Lamina propria resident phagocytes harbor bacterial strains that may act as pathobionts in CD. Our findings shed light on the role of pathobionts and the immune response in CD pathogenesis and suggest new targets for therapies.

Natural Human Antimicrobial Peptides and Female Reproductive Tract Infections

Female reproductive tract infections (RTIs) are a major health challenge worldwide and are the leading cause of infertility and adverse pregnancy outcomes. The rising incidence of RTIs highlights their status as a major public health issue. Microbial dysbiosis, particularly bacterial, fungal, and viral infections, constitutes the primary etiological factor disrupting female reproductive health. Antimicrobial peptides (AMPs) are evolutionarily conserved host defense molecules that exhibit broad-spectrum antimicrobial activity against pathogens, as well as anti-inflammatory and immunomodulatory properties. This review systematically summarizes the structural diversity, biological sources, and mechanistic pathways of human-derived AMPs in combating RTIs, with a particular emphasis on their therapeutic potential in fertility preservation. Emerging evidence suggests AMPs as promising alternatives to conventional antibiotics in the post-antibiotic era.

Novel antimicrobial defensin peptides from different coleopteran insects (Coleoptera: Insecta): identification, characterisation and antimicrobial properties

Antimicrobial peptides are crucial components of the immune systems of both vertebrates and invertebrates. Here, defensins, the most studied class of antimicrobial molecules in arthropods were investigated in four coleopteran insect species: Harpalus rufipes (DeGeer, 1774), Mylabris quadripunctata (Linnaeus, 1767), Sphaeridium marginatum (Linnaeus, 1758), and Ocypus mus (Brullé, 1832). The peptides synthesized with over 95% purity and their antimicrobial activities were evaluated by MIC test method. As a result, it was determined that Mylabris quadripunctata defensin (MqDef) showed high antimicrobial activity against Staphylococcus aureus and MRSA, whereas Sphaeridium marginatum (SmDef) and Harpalus rufipes (HrDef) defensins against Candida tropicalis.

Amphibian-Derived Antimicrobial Peptides: Essential Components of Innate Immunity and Potential Leads for New Antibiotic Development

Like other vertebrates, amphibians possess innate and adaptive immune systems. At the center of the adaptive immune system is the Major Histocompatibility Complex. The important molecules of innate immunity are antimicrobial peptides (AMPs). These peptides are secreted by granular glands in the skin and protect the animal against microorganisms entering its body through the skin. AMPs offer an effective and rapid defense against pathogenic microorganisms and have cationic and amphiphilic structures. These peptides are small gene-encoded molecules of 8-50 amino acid residues synthesized by ribosomes. These small molecules typically exhibit activity against bacteria, viruses, fungi, and even cancer cells. It is known that today's amphibian AMPs originated from a common precursor gene 150 million years ago and that the origin of these peptides is preprodermaseptins. Today, antibiotic resistance has occurred due to the incorrect use of antibiotics. Traditional antibiotics are becoming increasingly inadequate. AMPs are considered promising candidates for the development of new-generation antibiotics. Therefore, new antibiotic discoveries are needed. AMPs are suitable molecules for new-generation antibiotics that are both fast and have different killing mechanisms. One of the biggest problems in the clinical applications of AMPs is their poor stability. AMPs generally have limited tropical applications because they are sensitive to protease degradation. Coating these peptides with nanomaterials to make them more stable can solve this problem.

Novel structural determinants and bacterial death-related regulatory effects of the scorpion defensin BmKDfsin4 against gram-positive bacteria

Numerous defensins constitute a family of cationic antimicrobial peptides with high degrees of sequence variability, and in-depth characterization of their structural basis and antibacterial mechanisms remains limited. Here, a representative scorpion defensin, BmKDfsin4, with two distinct hydrophobic and basic residue clusters, was extensively investigated. The hydrophobic residue cluster, formed by Phe2, Pro5, Phe6, Phe28 and Leu29 residues, strongly influences the antibacterial activity of BmKDfsin4 against Gram-positive bacteria. Compared with the three scattered Lys13, Lys30 and Arg32 residues, the basic residue cluster, consisting of the Arg19, Arg20, Arg21 and Arg37 residues, played a less important role. The synergistic interaction between the basic residue cluster and Arg32 significantly affected BmKDfsin4 function. The bacterial growth inhibition by BmKDfsin4 was associated with regulating expression levels of cell division-related genes, cell wall synthesis-related genes and bacterial autolysis-related genes rather than destroying the bacterial cell membrane. The coincubation of BmKDfsin4 with the bacterial strains induced gradual changes in the bacterial surface from a rough and thin surface to a noticeably wrinkled surface together with abundant white spots and even complete cavities within the bacteria. These findings revealed novel structural determinants and bacterial death-related regulatory effects of the defensin BmKDfsin4 and highlighted diverse antibacterial mechanisms of defensins.

Lung antimicrobial proteins and peptides: from host defense to therapeutic strategies

Representing severe morbidity and mortality globally, respiratory infections associated with chronic respiratory diseases, including complicated pneumonia, asthma, interstitial lung disease, and chronic obstructive pulmonary disease, are a major public health concern. Lung health and the prevention of pulmonary disease rely on the mechanisms of airway surface fluid secretion, mucociliary clearance, and adequate immune response to eradicate inhaled pathogens and particulate matter from the environment. The antimicrobial proteins and peptides contribute to maintaining an antimicrobial milieu in human lungs to eliminate pathogens and prevent them from causing pulmonary diseases. The predominant antimicrobial molecules of the lung environment include human α- and β-defensins and cathelicidins, among numerous other host defense molecules with antimicrobial and antibiofilm activity such as PLUNC (palate, lung, and nasal epithelium clone) family proteins, elafin, collectins, lactoferrin, lysozymes, mucins, secretory leukocyte proteinase inhibitor, surfactant proteins SP-A and SP-D, and RNases. It has been demonstrated that changes in antimicrobial molecule expression levels are associated with regulating inflammation, potentiating exacerbations, pathological changes, and modifications in chronic lung disease severity. Antimicrobial molecules also display roles in both anticancer and tumorigenic effects. Lung antimicrobial proteins and peptides are promising alternative therapeutics for treating and preventing multidrug-resistant bacterial infections and anticancer therapies.

The Antimicrobial Activity of Human Defensins at Physiological Non-Permeabilizing Concentrations Is Caused by the Inhibition of the Plasma Membrane H+-ATPases

Human defensins are cysteine-rich peptides (Cys-rich peptides) of the innate immune system. Defensins contain an ancestral structural motif (i.e., γ-core motif) associated with the antimicrobial activity of natural Cys-rich peptides. In this study, low concentrations of human α- and β-defensins showed microbicidal activity that was not associated with cell membrane permeabilization. The cell death pathway was similar to that previously described for human lactoferrin, also an immunoprotein containing a γ-core motif. The common features were (1) cell death not related to plasma membrane (PM) disruption, (2) the inhibition of microbicidal activity via extracellular potassium, (3) the influence of cellular respiration on microbicidal activity, and (4) the influence of intracellular pH on bactericidal activity. In addition, in yeast, we also observed (1) partial K+-efflux mediated via Tok1p K+-channels, (2) the essential role of mitochondrial ATP synthase in cell death, (3) the increment of intracellular ATP, (4) plasma membrane depolarization, and (5) the inhibition of external acidification mediated via PM Pma1p H+-ATPase. Similar features were also observed with BM2, an antifungal peptide that inhibits Pma1p H+-ATPase, showing that the above coincident characteristics were a consequence of PM H+-ATPase inhibition. These findings suggest, for the first time, that human defensins inhibit PM H+-ATPases at physiological concentrations, and that the subsequent cytosolic acidification is responsible for the in vitro microbicidal activity. This mechanism of action is shared with human lactoferrin and probably other antimicrobial peptides containing γ-core motifs.

DEFA1A3 DNA gene-dosage regulates the kidney innate immune response during upper urinary tract infection

Antimicrobial peptides (AMPs) are host defense effectors with potent neutralizing and immunomodulatory functions against invasive pathogens. The AMPs α-Defensin 1-3/DEFA1A3 participate in innate immune responses and influence patient outcomes in various diseases. DNA copy-number variations in DEFA1A3 have been associated with severity and outcomes in infectious diseases including urinary tract infections (UTIs). Specifically, children with lower DNA copy numbers were more susceptible to UTIs. The mechanism of action by which α-Defensin 1-3/DEFA1A3 copy-number variations lead to UTI susceptibility remains to be explored. In this study, we use a previously characterized transgenic knock-in of the human DEFA1A3 gene mouse to dissect α-Defensin 1-3 gene dose-dependent antimicrobial and immunomodulatory roles during uropathogenic Escherichia coli (UPEC) UTI. We elucidate the relationship between kidney neutrophil- and collecting duct intercalated cell-derived α-Defensin 1-3/DEFA1A3 expression and UTI. We further describe cooperative effects between α-Defensin 1-3 and other AMPs that potentiate the neutralizing activity against UPEC. Cumulatively, we demonstrate that DEFA1A3 directly protects against UPEC meanwhile impacting pro-inflammatory innate immune responses in a gene dosage-dependent manner.

Development strategies and application of antimicrobial peptides as future alternatives to in-feed antibiotics

The use of in-feed antibiotics has been widely restricted due to the significant environmental pollution and food safety concerns they have caused. Antimicrobial peptides (AMPs) have attracted widespread attention as potential future alternatives to in-feed antibiotics owing to their demonstrated antimicrobial activity and environment friendly characteristics. However, the challenges of weak bioactivity, immature stability, and low production yields of natural AMPs impede practical application in the feed industry. To address these problems, efforts have been made to develop strategies for approaching the AMPs with enhanced properties. Herein, we summarize approaches to improving the properties of AMPs as potential alternatives to in-feed antibiotics, mainly including optimization of structural parameters, sequence modification, selection of microbial hosts, fusion expression, and industrially fermentation control. Additionally, the potential for application of AMPs in animal husbandry is discussed. This comprehensive review lays a strong theoretical foundation for the development of in-feed AMPs to achieve the public health globally.

Antimicrobial peptides as promising antibiotic adjuvants to combat drug-resistant pathogens

The widespread antimicrobial resistance (AMR) calls for the development of new antimicrobial strategies. Antibiotic adjuvant rescues antibiotic activity and increases the life span of the antibiotics, representing a more productive, timely, and cost-effective strategy in fighting drug-resistant pathogens. Antimicrobial peptides (AMPs) from synthetic and natural sources are considered new-generation antibacterial agents. Besides their direct antimicrobial activity, growing evidence shows that some AMPs effectively enhance the activity of conventional antibiotics. The combinations of AMPs and antibiotics display an improved therapeutic effect on antibiotic-resistant bacterial infections and minimize the emergence of resistance. In this review, we discuss the value of AMPs in the age of resistance, including modes of action, limiting evolutionary resistance, and their designing strategies. We summarise the recent advances in combining AMPs and antibiotics against antibiotic-resistant pathogens, as well as their synergistic mechanisms. Lastly, we highlight the challenges and opportunities associated with the use of AMPs as potential antibiotic adjuvants. This will shed new light on the deployment of synergistic combinations to address the AMR crisis.

Multifaceted Materials for Enhanced Osteogenesis and Antimicrobial Properties on Bioplastic Polyetheretherketone Surfaces: A Review

Implant-associated infections and the increasing number of bone implants loosening and falling off after implantation have become urgent global challenges, hence the need for intelligent alternative solutions to combat implant loosening and falling off. The application of polyetheretherketone (PEEK) in biomedical and medical therapy has aroused great interest, especially because its elastic modulus close to bone provides an effective alternative to titanium implants, thereby preventing the possibility of bone implants loosening and falling off due to the mismatch of elastic modulus. In this Review, we provide a comprehensive overview of recent advances in surface modifications to prevent bone binding deficiency and bacterial infection after implantation of bone implants, starting with inorganics for surface modification, followed by organics that can effectively promote bone integration and antimicrobial action. In addition, surface modifications derived from cells and related products of biological activity have been proposed, and there is increasing evidence of clinical potential. Finally, the advantages and future challenges of surface strategies against medical associated poor osseointegration and infection are discussed, with promising prospects for developing novel osseointegration and antimicrobial PEEK materials.

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.

Bioinspired synthetic peptide-based biomaterials regenerate bone through biomimicking of extracellular matrix

There have been remarkable advancements in regenerative medicine for bone regeneration, tackling the worldwide health concern of tissue loss. Tissue engineering uses the body's natural capabilities and applies biomaterials and bioactive molecules to replace damaged or lost tissues and restore their functionality. While synthetic ceramics have overcome some challenges associated with allografts and xenografts, they still need essential growth factors and biomolecules. Combining ceramics and bioactive molecules, such as peptides derived from biological motifs of vital proteins, is the most effective approach to achieve optimal bone regeneration. These bioactive peptides induce various cellular processes and modify scaffold properties by mimicking the function of natural osteogenic, angiogenic and antibacterial biomolecules. The present review aims to consolidate the latest and most pertinent information on the advancements in bioactive peptides, including angiogenic, osteogenic, antimicrobial, and self-assembling peptide nanofibers for bone tissue regeneration, elucidating their biological effects and potential clinical implications.

Resistance is futile: targeting multidrug-resistant bacteria with de novo Cys-rich cyclic polypeptides

The search for novel antimicrobial agents to combat microbial pathogens is intensifying in response to rapid drug resistance development to current antibiotic therapeutics. The use of disulfide-rich head-to-tail cyclized polypeptides as molecular frameworks for designing a new type of peptide antibiotics is gaining increasing attention among the scientific community and the pharmaceutical industry. The use of macrocyclic peptides, further constrained by the presence of several disulfide bonds, makes these peptide frameworks remarkably more stable to thermal, biological, and chemical degradation showing better activities when compared to their linear analogs. Many of these novel peptide scaffolds have been shown to have a high tolerance to sequence variability in those residues not involved in disulfide bonds, able to cross biological membranes, and efficiently target complex biomolecular interactions. Hence, these unique properties make the use of these scaffolds ideal for many biotechnological applications, including the design of novel peptide antibiotics. This article provides an overview of the new developments in the use of several disulfide-rich cyclic polypeptides, including cyclotides, θ-defensins, and sunflower trypsin inhibitor peptides, among others, in the development of novel antimicrobial peptides against multidrug-resistant bacteria.

Mechanisms and regulation of defensins in host defense

As a family of cationic host defense peptides, defensins are mainly synthesized by Paneth cells, neutrophils, and epithelial cells, contributing to host defense. Their biological functions in innate immunity, as well as their structure and activity relationships, along with their mechanisms of action and therapeutic potential, have been of great interest in recent years. To highlight the key research into the role of defensins in human and animal health, we first describe their research history, structural features, evolution, and antimicrobial mechanisms. Next, we cover the role of defensins in immune homeostasis, chemotaxis, mucosal barrier function, gut microbiota regulation, intestinal development and regulation of cell death. Further, we discuss their clinical relevance and therapeutic potential in various diseases, including infectious disease, inflammatory bowel disease, diabetes and obesity, chronic inflammatory lung disease, periodontitis and cancer. Finally, we summarize the current knowledge regarding the nutrient-dependent regulation of defensins, including fatty acids, amino acids, microelements, plant extracts, and probiotics, while considering the clinical application of such regulation. Together, the review summarizes the various biological functions, mechanism of actions and potential clinical significance of defensins, along with the challenges in developing defensins-based therapy, thus providing crucial insights into their biology and potential clinical utility.

Antimicrobial Peptides and Small Molecules Targeting the Cell Membrane of Staphylococcus aureus

Clinical management of Staphylococcus aureus infections presents a challenge due to the high incidence, considerable virulence, and emergence of drug resistance mechanisms. The treatment of drug-resistant strains, such as methicillin-resistant S. aureus (MRSA), is further complicated by the development of tolerance and persistence to antimicrobial agents in clinical use. To address these challenges, membrane disruptors, that are not generally considered during drug discovery for agents against S. aureus, should be explored. The cell membrane protects S. aureus from external stresses and antimicrobial agents, but membrane-targeting antimicrobial agents are probably less likely to promote bacterial resistance. Nontypical linear cationic antimicrobial peptides (AMPs), highly modified AMPs such as daptomycin (lipopeptide), bacitracin (cyclic peptide), and gramicidin S (cyclic peptide), are currently in clinical use. Recent studies have demonstrated that AMPs and small molecules can penetrate the cell membrane of S. aureus, inhibit phospholipid biosynthesis, or block the passage of solutes between the periplasm and the exterior of the cell. In addition to their primary mechanism of action (MOA) that targets the bacterial membrane, AMPs and small molecules may also impact bacteria through secondary mechanisms such as targeting the biofilm, and downregulating virulence genes of S. aureus. In this review, we discuss the current state of research into cell membrane-targeting AMPs and small molecules and their potential mechanisms of action against drug-resistant physiological forms of S. aureus, including persister cells and biofilms.

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.

Membrane-Active Cyclic Amphiphilic Peptides: Broad-Spectrum Antibacterial Activity Alone and in Combination with Antibiotics

We designed a library of 24 cyclic peptides containing arginine (R) and tryptophan (W) residues in a sequential manner [RnWn] (n = 2-7) to study the impact of the hydrophilic/hydrophobic ratio, charge, and ring size on the antibacterial activity against Gram-positive and Gram-negative strains. Among peptides, 5a and 6a demonstrated the highest antimicrobial activity. In combination with 11 commercially available antibiotics, 5a and 6a showed remarkable synergism against a large panel of resistant pathogens. Hemolysis (HC50 = 340 μg/mL) and cell viability against mammalian cells demonstrated the selective lethal action of 5a against bacteria over mammalian cells. Calcein dye leakage and scanning electron microscopy studies revealed the membranolytic effect of 5a. Moreover, the stability in human plasma (t1/2 = 3 h) and the negligible ability of pathogens to develop resistance further reflect the potential of 5a for further development as a peptide-based antibiotic.

Antimicrobial peptides for bone tissue engineering: Diversity, effects and applications

Bone tissue engineering has been becoming a promising strategy for surgical bone repair, but the risk of infection during trauma repair remains a problematic health concern worldwide, especially for fracture and infection-caused bone defects. Conventional antibiotics fail to effectively prevent or treat bone infections during bone defect repair because of drug-resistance and recurrence, so novel antibacterial agents with limited resistance are highly needed for bone tissue engineering. Antimicrobial peptides (AMPs) characterized by cationic, hydrophobic and amphipathic properties show great promise to be used as next-generation antibiotics which rarely induce resistance and show potent antibacterial efficacy. In this review, four common structures of AMPs (helix-based, sheet-based, coil-based and composite) and related modifications are presented to identify AMPs and design novel analogs. Then, potential effects of AMPs for bone infection during bone repair are explored, including bactericidal activity, anti-biofilm, immunomodulation and regenerative properties. Moreover, we present distinctive applications of AMPs for topical bone repair, which can be either used by delivery system (surface immobilization, nanoparticles and hydrogels) or used in gene therapy. Finally, future prospects and ongoing challenges are discussed.

Defensins: defenders of human reproductive health

BACKGROUND

Reproductive tract infection is an important factor leading to male and female infertility. Among female infertility factors, microbial and viral infections are the main factors affecting female reproductive health and causing tubal infertility, ectopic tubal pregnancy and premature delivery. Among male infertility factors, 13-15% of male infertility is related to infection. Defensins are cationic antibacterial and antiviral peptides, classified into α-defensins, β-defensins and θ-defensins. Humans only have α-defensins and β-defensins. Apart from their direct antimicrobial functions, defensins have an immunomodulatory function and are involved in many physiological processes. Studies have shown that defensins are widely distributed in the female reproductive tract (FRT) and male reproductive tract (MRT), playing a dual role of host defence and fertility protection. However, to our knowledge, the distribution, regulation and function of defensins in the reproductive tract and their relation to reproduction have not been reviewed.

OBJECTIVE AND RATIONALE

This review summarizes the expression, distribution and regulation of defensins in the reproductive tracts to reveal the updated research on the dual role of defensins in host defence and the protection of fertility.

SEARCH METHODS

A systematic search was conducted in PubMed using the related keywords through April 2022. Related data from original researches and reviews were integrated to comprehensively review the current findings and understanding of defensins in the human reproductive system. Meanwhile, female and male transcriptome data in the GEO database were screened to analyze defensins in the human reproductive tracts.

OUTCOMES

Two transcriptome databases from the GEO database (GSE7307 and GSE150852) combined with existing researches reveal the expression levels and role of the defensins in the reproductive tracts. In the FRT, a high expression level of α-defensin is found, and the expression levels of defensins in the vulva and vagina are higher than those in other organs. The expression of defensins in the endometrium varies with menstrual cycle stages and with microbial invasion. Defensins also participate in the local immune response to regulate the risk of spontaneous preterm birth. In the MRT, a high expression level of β-defensins is also found. It is mainly highly expressed in the epididymal caput and corpus, indicating that defensins play an important role in sperm maturation. The expression of defensins in the MRT varies with androgen levels, age and the status of microbial invasion. They protect the male reproductive system from bacterial infections by neutralizing lipopolysaccharide and downregulating pro-inflammatory cytokines. In addition, animal and clinical studies have shown that defensins play an important role in sperm maturation, motility and fertilization.

WIDER IMPLICATIONS

As a broad-spectrum antimicrobial peptide without drug resistance, defensin has great potential for developing new natural antimicrobial treatments for reproductive tract infections. However, increasing evidence has shown that defensins can not only inhibit microbial invasion but can also promote the invasion and adhesion of some microorganisms in certain biological environments, such as human immunodeficiency virus. Therefore, the safety of defensins as reproductive tract anti-infective drugs needs more in-depth research. In addition, the modulatory role of defensins in fertility requires more in-depth research since the current conclusions are based on small-size samples. At present, scientists have made many attempts at the clinical transformation of defensins. However, defensins have problems such as poor stability, low bioavailability and difficulties in their synthesis. Therefore, the production of safe, effective and low-cost drugs remains a challenge.

Identification of a two-component regulatory system involved in antimicrobial peptide resistance in Streptococcus pneumoniae

Two-component regulatory systems (TCS) are among the most widespread mechanisms that bacteria use to sense and respond to environmental changes. In the human pathogen Streptococcus pneumoniae, a total of 13 TCS have been identified and many of them have been linked to pathogenicity. Notably, TCS01 strongly contributes to pneumococcal virulence in several infection models. However, it remains one of the least studied TCS in pneumococci and its functional role is still unclear. In this study, we demonstrate that TCS01 cooperates with a BceAB-type ABC transporter to sense and induce resistance to structurally-unrelated antimicrobial peptides of bacterial origin that all target undecaprenyl-pyrophosphate or lipid II, which are essential precursors of cell wall biosynthesis. Even though tcs01 and bceAB genes do not locate in the same gene cluster, disruption of either of them equally sensitized the bacterium to the same set of antimicrobial peptides. We show that the key function of TCS01 is to upregulate the expression of the transporter, while the latter appears the main actor in resistance. Electrophoretic mobility shift assays further demonstrated that the response regulator of TCS01 binds to the promoter region of the bceAB genes, implying a direct control of these genes. The BceAB transporter was overexpressed and purified from E. coli. After reconstitution in liposomes, it displayed substantial ATPase and GTPase activities that were stimulated by antimicrobial peptides to which it confers resistance to, revealing new functional features of a BceAB-type transporter. Altogether, this inducible defense mechanism likely contributes to the survival of the opportunistic microorganism in the human host, in which competition among commensal microorganisms is a key determinant for effective host colonization and invasive path.

Inhibition of SARS-CoV-2 Infection by Human Defensin HNP1 and Retrocyclin RC-101

Severe acute respiratory syndrome coronavirus (SARS-CoV)-2 is an enveloped virus responsible for the COVID-19 pandemic. The emergence of new potentially more transmissible and vaccine-resistant variants of SARS-CoV-2 is an ever-present threat. Thus, it remains essential to better understand innate immune mechanisms that can inhibit the virus. One component of the innate immune system with broad antipathogen, including antiviral, activity is a group of cationic immune peptides termed defensins. The ability of defensins to neutralize enveloped and non-enveloped viruses and to inactivate numerous bacterial toxins correlate with their ability to promote the unfolding of proteins with high conformational plasticity. We found that human neutrophil α-defensin HNP1 binds to SARS-CoV-2 Spike protein with submicromolar affinity that is more than 20 fold stronger than its binding to serum albumin. As such, HNP1, as well as a θ-defensin retrocyclin RC-101, both interfere with Spike-mediated membrane fusion, Spike-pseudotyped lentivirus infection, and authentic SARS-CoV-2 infection in cell culture. These effects correlate with the abilities of the defensins to destabilize and precipitate Spike protein and inhibit the interaction of Spike with the ACE2 receptor. Serum reduces the anti-SARS-CoV-2 activity of HNP1, though at high concentrations, HNP1 was able to inactivate the virus even in the presence of serum. Overall, our results suggest that defensins can negatively affect the native conformation of SARS-CoV-2 Spike, and that α- and θ-defensins may be valuable tools in developing SARS-CoV-2 infection prevention strategies.

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.

Defensin-lipid interactions in membrane targeting: mechanisms of action and opportunities for the development of antimicrobial and anticancer therapeutics

Defensins are a class of host defence peptides (HDPs) that often harbour antimicrobial and anticancer activities, making them attractive candidates as novel therapeutics. In comparison with current antimicrobial and cancer treatments, defensins uniquely target specific membrane lipids via mechanisms distinct from other HDPs. Therefore, defensins could be potentially developed as therapeutics with increased selectivity and reduced susceptibility to the resistance mechanisms of tumour cells and infectious pathogens. In this review, we highlight recent advances in defensin research with a particular focus on membrane lipid-targeting in cancer and infection settings. In doing so, we discuss strategies to harness lipid-binding defensins for anticancer and anti-infective therapies.

Immunomodulatory and Allergenic Properties of Antimicrobial Peptides

With the growing problem of the emergence of antibiotic-resistant bacteria, the search for alternative ways to combat bacterial infections is extremely urgent. While analyzing the effect of antimicrobial peptides (AMPs) on immunocompetent cells, their effect on all parts of the immune system, and on humoral and cellular immunity, is revealed. AMPs have direct effects on neutrophils, monocytes, dendritic cells, T-lymphocytes, and mast cells, participating in innate immunity. They act on B-lymphocytes indirectly, enhancing the induction of antigen-specific immunity, which ultimately leads to the activation of adaptive immunity. The adjuvant activity of AMPs in relation to bacterial and viral antigens was the reason for their inclusion in vaccines and made it possible to formulate the concept of a “defensin vaccine” as an innovative basis for constructing vaccines. The immunomodulatory function of AMPs involves their influence on cells in the nearest microenvironment, recruitment and activation of other cells, supporting the response to pathogenic microorganisms and completing the inflammatory process, thus exhibiting a systemic effect. For the successful use of AMPs in medical practice, it is necessary to study their immunomodulatory activity in detail, taking into account their pleiotropy. The degree of maturity of the immune system and microenvironment can contribute to the prevention of complications and increase the effectiveness of therapy, since AMPs can suppress inflammation in some circumstances, but aggravate the response and damage of organism in others. It should also be taken into account that the real functions of one or another AMP depend on the types of total regulatory effects on the target cell, and not only on properties of an individual peptide. A wide spectrum of biological activity, including direct effects on pathogens, inactivation of bacterial toxins and influence on immunocompetent cells, has attracted the attention of researchers, however, the cytostatic activity of AMPs against normal cells, as well as their allergenic properties and low stability to host proteases, are serious limitations for the medical use of AMPs. In this connection, the tasks of searching for compounds that selectively affect the target and development of an appropriate method of application become critically important. The scope of this review is to summarize the current concepts and newest advances in research of the immunomodulatory activity of natural and synthetic AMPs, and to examine the prospects and limitations of their medical use.

Potent bactericidal activity of reduced cryptdin-4 derived from its hydrophobicity and mediated by bacterial membrane disruption

Defensin is a cysteine-rich antimicrobial peptide with three disulphide bonds under normal oxidative conditions. Cryptdin-4 (Crp4) is a defensin secreted by Paneth cells in the small intestine of mice, and only reduced Crp4 (Crp4_red) shows activity against enteric commensal bacteria, although both oxidised Crp4 (Crp4_ox) and Crp4_red can kill non-commensal bacteria. To investigate the molecular factors that affect the potent antimicrobial activity of Crp4_red, the bactericidal activities of Crp4_ox and Crp4_red, Crp4 with all Cys residues substituted with Ser peptide (6C/S-Crp4), and Crp4 with all thiol groups modified by N-ethylmaleimide (NEM-Crp4) were assessed. All peptides showed bactericidal activity against non-commensal bacteria, whereas Crp4_red and NEM-Crp4 showed bactericidal activity against commensal bacteria. These potent peptides exhibited high hydrophobicity, which was strongly correlated with membrane insertion. Intriguingly, Crp4_ox formed electrostatic interactions with the membrane surface of bacteria, even without exerting bactericidal activity. Moreover, the bactericidal activity of both oxidised and reduced forms of Crp4 was abolished by inhibition of electrostatic interactions; this finding suggests that Crp4_red targets bacterial membranes. Finally, a liposome leakage assay against lipids extracted from commensal bacteria demonstrated a correlation with bactericidal activity. These results suggest that the potent bactericidal activity of Crp4_red is derived from its hydrophobicity, and the bactericidal mechanism involves disruption of the bacterial membrane. Findings from this study provide a better understanding of the bactericidal mechanism of both Crp4_ox and Crp4_red.

Antimicrobial peptides, conventional antibiotics, and their synergistic utility for the treatment of drug-resistant infections

Antimicrobial peptides (AMPs), also known as host defense peptides (HDPs), are important effector immune defense molecules in multicellular organisms. AMPs exert their antimicrobial activities through several mechanisms; thus far, induction of drug resistance through AMPs has been regarded as unlikely. Therefore, they have great potential as new generation antimicrobial agents. To date, more than 30 AMP-related drugs are in the clinical trial phase. In recent years, studies show that some AMPs and conventional antibiotics have synergistic effects. The combined use of AMPs and antibiotics can kill drug-resistant pathogens, prevent drug resistance, and significantly improve the therapeutic effects of antibiotics. In this review, we discuss the progress in synergistic studies on AMPs and conventional antibiotics. An overview of the current understanding of the functional scope of AMPs, ongoing clinical trials, and challenges in the development processes are also presented.

The evolution of the antimicrobial peptide database over 18 years: Milestones and new features

The antimicrobial peptide database (APD) has served the antimicrobial peptide field for 18 years. Because it is widely used in research and education, this article documents database milestones and key events that have transformed it into the current form. A comparison is made for the APD peptide statistics between 2010 and 2020, validating the major database findings to date. We also describe new additions ranging from peptide entries to search functions. Of note, the APD also contains antimicrobial peptides from host microbiota, which are important in shaping immune systems and could be linked to a variety of human diseases. Finally, the database has been re-programmed to the web branding and latest security compliance of the University of Nebraska Medical Center. The reprogrammed APD can be accessed at https://aps.unmc.edu.

Rational Designed Hybrid Peptides Show up to a 6-Fold Increase in Antimicrobial Activity and Demonstrate Different Ultrastructural Changes as the Parental Peptides Measured by BioSAXS

Antimicrobial peptides (AMPs) are a promising class of compounds being developed against multi-drug resistant bacteria. Hybridization has been reported to increase antimicrobial activity. Here, two proline-rich peptides (consP1: VRKPPYLPRPRPRPL-CONH₂ and Bac5-v291: RWRRPIRRRPIRPPFWR-CONH₂) were combined with two arginine-isoleucine-rich peptides (optP1: KIILRIRWR-CONH₂ and optP7: KRRVRWIIW-CONH₂). Proline-rich antimicrobial peptides (PrAMPs) are known to inhibit the bacterial ribosome, shown also for Bac5-v291, whereas it is hypothesized a “dirty drug” model for the arginine-isoleucine-rich peptides. That hypothesis was underpinned by transmission electron microscopy and biological small-angle X-ray scattering (BioSAXS). The strength of BioSAXS is the power to detect ultrastructural changes in millions of cells in a short time (seconds) in a high-throughput manner. This information can be used to classify antimicrobial compounds into groups according to the ultrastructural changes they inflict on bacteria and how the bacteria react towards that assault. Based on previous studies, this correlates very well with different modes of action. Due to the novelty of this approach direct identification of the target of the antimicrobial compound is not yet fully established, more research is needed. More research is needed to address this limitation. The hybrid peptides showed a stronger antimicrobial activity compared to the proline-rich peptides, except when compared to Bac5-v291 against E. coli. The increase in activity compared to the arginine-isoleucine-rich peptides was up to 6-fold, however, it was not a general increase but was dependent on the combination of peptides and bacteria. BioSAXS experiments revealed that proline-rich peptides and arginine-isoleucine-rich peptides induce very different ultrastructural changes in E. coli, whereas a hybrid peptide (hyP7B5GK) shows changes, different to both parental peptides and the untreated control. These different ultrastructural changes indicated that the mode of action of the parental peptides might be different from each other as well as from the hybrid peptide hyP7B5GK. All peptides showed very low haemolytic activity, some of them showed a 100-fold or larger therapeutic window, demonstrating the potential for further drug development.

Multidrug resistance crisis during COVID-19 pandemic: Role of anti-microbial peptides as next-generation therapeutics

The decreasing effectiveness of conventional drugs due to multidrug-resistance is a major challenge for the scientific community, necessitating development of novel antimicrobial agents. In the present era of coronavirus 2 (COVID-19) pandemic, patients are being widely exposed to antimicrobial drugs and hence the problem of multidrug-resistance shall be aggravated in the days to come. Consequently, revisiting the phenomena of multidrug resistance leading to formulation of effective antimicrobial agents is the need of the hour. As a result, this review sheds light on the looming crisis of multidrug resistance in wake of the COVID-19 pandemic. It highlights the problem, significance and approaches for tackling microbial resistance with special emphasis on anti-microbial peptides as next-generation therapeutics against multidrug resistance associated diseases. Antimicrobial peptides exhibit exceptional mechanism of action enabling rapid killing of microbes at low concentration, antibiofilm activity, immunomodulatory properties along with a low tendency for resistance development providing them an edge over conventional antibiotics. The review is unique as it discusses the mode of action, pharmacodynamic properties and application of antimicrobial peptides in areas ranging from therapeutics to agriculture.

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.

Antimicrobial Peptides: An Update on Classifications and Databases

Antimicrobial peptides (AMPs) are distributed across all kingdoms of life and are an indispensable component of host defenses. They consist of predominantly short cationic peptides with a wide variety of structures and targets. Given the ever-emerging resistance of various pathogens to existing antimicrobial therapies, AMPs have recently attracted extensive interest as potential therapeutic agents. As the discovery of new AMPs has increased, many databases specializing in AMPs have been developed to collect both fundamental and pharmacological information. In this review, we summarize the sources, structures, modes of action, and classifications of AMPs. Additionally, we examine current AMP databases, compare valuable computational tools used to predict antimicrobial activity and mechanisms of action, and highlight new machine learning approaches that can be employed to improve AMP activity to combat global antimicrobial resistance.

Antimicrobial peptides: mechanism of action, activity and clinical potential

The management of bacterial infections is becoming a major clinical challenge due to the rapid evolution of antibiotic resistant bacteria. As an excellent candidate to overcome antibiotic resistance, antimicrobial peptides (AMPs) that are produced from the synthetic and natural sources demonstrate a broad-spectrum antimicrobial activity with the high specificity and low toxicity. These peptides possess distinctive structures and functions by employing sophisticated mechanisms of action. This comprehensive review provides a broad overview of AMPs from the origin, structural characteristics, mechanisms of action, biological activities to clinical applications. We finally discuss the strategies to optimize and develop AMP-based treatment as the potential antimicrobial and anticancer therapeutics.

Natural antimicrobial peptides as a source of new antiviral agents

Current antiviral drugs are limited because of their adverse side effects and increased rate of resistance. In recent decades, much scientific effort has been invested in the discovery of new synthetic and natural compounds with promising antiviral properties. Among this new generation of compounds, antimicrobial peptides with antiviral activity have been described and are attracting attention due to their mechanism of action and biological properties. To understand the potential of antiviral peptides (AVPs), we analyse the antiviral activity of well-known AVP families isolated from different natural sources, discuss their physical-chemical properties, and demonstrate how AVP databases can guide us to design synthetic AVPs with better therapeutic properties. All considerations in this sphere of antiviral therapy clearly demonstrate the remarkable contribution that AVPs may make in conquering old as well as newly emerging viruses that plague humanity.

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.

Identification of a crocodylian β-defensin variant from Alligator mississippiensis with antimicrobial and antibiofilm activity

β-defensin host defense peptides are important components of the innate immune system of vertebrates. Although evidence of their broad antimicrobial, antibiofilm and immunomodulatory activities in mammals have been presented, β-defensins from other vertebrate species, like crocodylians, remain largely unexplored. In this study, five new crocodylian β-defensin variants from Alligator mississippiensis and Crocodylus porosus were selected for synthesis and characterization based on their charge and hydrophobicity values. Linear peptides were synthesized, folded, purified and then evaluated for their antimicrobial and antibiofilm activities against the bacterial pathogens, Salmonella enterica serovar Typhimurium, Staphylococcus aureus, Enterobacter cloacae and Acinetobacter baumannii. The Am23SK variant (SCRFSGGYCIWNWERCRSGHFLVALCPFRKRCCK) from A. mississippiensis displayed promising activity against both planktonic cells and bacterial biofilms, outperforming the human β-defensin 3 under the experimental conditions. Moreover, Am23SK exhibited no cytotoxicity towards mammalian cells and exerted immunomodulatory effects in vitro, moderately suppressing the production of proinflammatory mediators from stimulated human bronchial epithelial cells. Overall, our results have expanded the activity landscape of crocodylian and reptilian β-defensin in general.

Immune Cell Degranulation in Fungal Host Defence

Humans have developed complex immune systems that defend against invading microbes, including fungal pathogens. Many highly specialized cells of the immune system share the ability to store antimicrobial compounds in membrane bound organelles that can be immediately deployed to eradicate or inhibit growth of invading pathogens. These membrane-bound organelles consist of secretory vesicles or granules, which move to the surface of the cell, where they fuse with the plasma membrane to release their contents in the process of degranulation. Lymphocytes, macrophages, neutrophils, mast cells, eosinophils, and basophils all degranulate in fungal host defence. While anti-microbial secretory vesicles are shared among different immune cell types, information about each cell type has emerged independently leading to an uncoordinated and confusing classification of granules and incomplete description of the mechanism by which they are deployed. While there are important differences, there are many similarities in granule morphology, granule content, stimulus for degranulation, granule trafficking, and release of granules against fungal pathogens. In this review, we describe the similarities and differences in an attempt to translate knowledge from one immune cell to another that may facilitate further studies in the context of fungal host defence.

Comparison of a Short Linear Antimicrobial Peptide with Its Disulfide-Cyclized and Cyclotide-Grafted Variants against Clinically Relevant Pathogens

According to the World Health Organization (WHO) the development of resistance against antibiotics by microbes is one of the most pressing health concerns. The situation will intensify since only a few pharmacological companies are currently developing novel antimicrobial compounds. Discovery and development of novel antimicrobial compounds with new modes of action are urgently needed. Antimicrobial peptides (AMPs) are known to be able to kill multidrug-resistant bacteria and, therefore, of interest to be developed into antimicrobial drugs. Proteolytic stability and toxicities of these peptides are challenges to overcome, and one strategy frequently used to address stability is cyclization. Here we introduced a disulfide-bond to cyclize a potent and nontoxic 9mer peptide and, in addition, as a proof-of-concept study, grafted this peptide into loop 6 of the cyclotide MCoTI-II. This is the first time an antimicrobial peptide has been successfully grafted onto the cyclotide scaffold. The disulfide-cyclized and grafted cyclotide showed moderate activity in broth and strong activity in 1/5 broth against clinically relevant resistant pathogens. The linear peptide showed superior activity in both conditions. The half-life time in 100% human serum was determined, for the linear peptide, to be 13 min, for the simple disulfide-cyclized peptide, 9 min, and, for the grafted cyclotide 7 h 15 min. The addition of 10% human serum led to a loss of antimicrobial activity for the different organisms, ranging from 1 to >8-fold for the cyclotide. For the disulfide-cyclized version and the linear version, activity also dropped to different degrees, 2 to 18-fold, and 1 to 30-fold respectively. Despite the massive difference in stability, the linear peptide still showed superior antimicrobial activity. The cyclotide and the disulfide-cyclized version demonstrated a slower bactericidal effect than the linear version. All three peptides were stable at high and low pH, and had very low hemolytic and cytotoxic activity.

Design and Pharmacodynamics of Recombinant Fungus Defensin NZL with Improved Activity against Staphylococcus hyicus In Vitro and In Vivo

Staphylococcus hyicus is recognized as a leading pathogen of exudative epidermitis in modern swine industry. Antimicrobial peptides are attractive candidates for development as potential therapeutics to combat the serious threats of the resistance of S. hyicus. In this study, a series of derivatives were designed based on the NZ2114 template with the aim of obtaining peptides with more potent antimicrobial activity through changing net positive charge or hydrophobicity. Among them, a variant designated as NZL was highly expressed in Pichia pastoris (P. pastoris) with total secreted protein of 1505 mg/L in a 5-L fermenter and exhibited enhanced antimicrobial activity relative to parent peptide NZ2114. Additionally, NZL could kill over 99% of S. hyicus NCTC10350 in vitro within 8 h and in Hacat cells. The results of membrane permeabilization assay, morphological observations, peptide localization assay showed that NZL had potent activity against S. hyicus, which maybe kill S. hyicus through action on the cell wall. NZL also showed an effective therapy in a mouse peritonitis model caused by S. hyicus, superior to NZ2114 or ceftriaxone. Overall, these findings can contribute to explore a novel potential candidate against S. hyicus infections.

Promising role of defensins peptides as therapeutics to combat against viral infection

Antimicrobial peptides (AMPs) are ubiquitously present small peptides, which play a critical function in the innate immune system. The defensin class of AMPs represented an evolutionarily ancient family containing cationic cysteine residue and frequently expressed in epithelial or neutrophils cells. It plays myriad functions in host innate immune responses against various infection. Defensin has a broad spectrum of antimicrobial activities, including anti-bacteria, anti-viruses (AVPs), anti-fungi, anti-cancers, and also overcoming bacterial drug resistance. In this review, we compiled the progress on defensin, particularly incorporating the mechanism of action, their application as an antiviral agent, prospects in different areas, and limitations to be solved as an antiviral peptide. Defensins were explored, in particular, their capacity to stimulate innate and adaptive immunity by trigging as anti-coronavirus (COVID-19) peptides. The present review summarised its immunomodulatory and immunoenhancing properties and predominantly focused on its promising therapeutic adjuvant choices for combat against viral infection.

Physiological Functions of Bacterial "Multidrug" Efflux Pumps

Bacterial multidrug efflux pumps have come to prominence in human and veterinary pathogenesis because they help bacteria protect themselves against the antimicrobials used to overcome their infections. However, it is increasingly realized that many, probably most, such pumps have physiological roles that are distinct from protection of bacteria against antimicrobials administered by humans. Here we undertake a broad survey of the proteins involved, allied to detailed examples of their evolution, energetics, structures, chemical recognition, and molecular mechanisms, together with the experimental strategies that enable rapid and economical progress in understanding their true physiological roles. Once these roles are established, the knowledge can be harnessed to design more effective drugs, improve existing microbial production of drugs for clinical practice and of feedstocks for commercial exploitation, and even develop more sustainable biological processes that avoid, for example, utilization of petroleum.

Molecular engineering of antimicrobial peptides: microbial targets, peptide motifs and translation opportunities

The global public health threat of antimicrobial resistance has led the scientific community to highly engage into research on alternative strategies to the traditional small molecule therapeutics. Here, we review one of the most popular alternatives amongst basic and applied research scientists, synthetic antimicrobial peptides. The ease of peptide chemical synthesis combined with emerging engineering principles and potent broad-spectrum activity, including against multidrug-resistant strains, has motivated intense scientific focus on these compounds for the past decade. This global effort has resulted in significant advances in our understanding of peptide antimicrobial activity at the molecular scale. Recent evidence of molecular targets other than the microbial lipid membrane, and efforts towards consensus antimicrobial peptide motifs, have supported the rise of molecular engineering approaches and design tools, including machine learning. Beyond molecular concepts, supramolecular chemistry has been lately added to the debate; and helped unravel the impact of peptide self-assembly on activity, including on biofilms and secondary targets, while providing new directions in pharmaceutical formulation through taking advantage of peptide self-assembled nanostructures. We argue that these basic research advances constitute a solid basis for promising industry translation of rationally designed synthetic peptide antimicrobials, not only as novel drugs against multidrug-resistant strains but also as components of emerging antimicrobial biomaterials. This perspective is supported by recent developments of innovative peptide-based and peptide-carrier nanobiomaterials that we also review.

Rediscovery of antimicrobial peptides as therapeutic agents

In recent years, the occurrence of antibiotic-resistant pathogens is increasing rapidly. There is growing concern as the development of antibiotics is slower than the increase in the resistance of pathogenic bacteria. Antimicrobial peptides (AMPs) are promising alternatives to antibiotics. Despite their name, which implies their antimicrobial activity, AMPs have recently been rediscovered as compounds having antifungal, antiviral, anticancer, antioxidant, and insecticidal effects. Moreover, many AMPs are relatively safe from toxic side effects and the generation of resistant microorganisms due to their target specificity and complexity of the mechanisms underlying their action. In this review, we summarize the history, classification, and mechanisms of action of AMPs, and provide descriptions of AMPs undergoing clinical trials. We also discuss the obstacles associated with the development of AMPs as therapeutic agents and recent strategies formulated to circumvent these obstacles.

Antimicrobial Peptides From Lycosidae (Sundevall, 1833) Spiders

Antimicrobial peptides (AMPs) have been found in all organism taxa and may play an essential role as a host defense system. AMPs are organized in various conformations, such as linear peptides, disulfide bond-linked peptides, backbone-linked peptides and circular peptides. AMPs apparently act primarily on the plasma membrane, although an increasing number of works have shown that they may also target various intracellular sites. Spider venoms are rich sources of biomolecules that show several activities, including modulation or blockage of ion channels, anti-insect, anti-cancer, antihypertensive and antimicrobial activities, among others. In spider venoms from the Lycosidae family there are many linear AMPs with a wide range of activities against several microorganisms. Due to these singular activities, some Lycosidae AMPs have been modified to improve or decrease desirable or undesirable effects, respectively. Such modifications, especially with the aim of increasing their antibiotic activity, have led to the filing of many patent applications. This review explores the abundance of Lycosidae venom AMPs and some of their derivatives, and their use as new drug models.

Antibacterial activity of human defensins against Staphylococcus aureus and Escherichia coli

BACKGROUND

The global problem of antibiotic resistance requires the search for and development of new methods of treatment. One of the promising strategies is the use of low doses of antimicrobial peptides, in particular, human defensins HNP-1, hBD-1, and hBD-3, in combination with antibacterial drugs already used in clinical practice. This approach may be used to increase the effectiveness of conventional antibiotics. However, this requires thorough study of the effectiveness of defensins in combination with antibiotics against a large number of bacterial strains with known phenotypes of antibiotic resistance. The aim of this work was to study the antibacterial effect of HNP-1, hBD-1 and hBD-3 in combination with rifampicin or amikacin against clinical isolates of Staphylococcus aureus (n = 27) and Escherichia coli (n = 24) collected from hospitalized patients.

METHODS

The standard checkerboard assay was used to determine minimum inhibitory concentrations (MICs) of antimicrobials. The combined microbicidal effects of two substances (defensin + conventional antibiotic) were assessed by the fractional inhibitory concentration index (FICI).

RESULTS

The highest anti-staphylococcal activity (including methicillin-resistant strains) among defensins was demonstrated by hBD-3 that had MIC of 1 (0.5-4) mg/L (hereinafter, MIC values are presented as median and interquartile range). The MIC of HNP-1 against S. aureus was 4 (2-8) mg/L; the MIC of hBD-1 was 8 (4-8) mg/L. Against E. coli, the most effective was also found to be hBD-3 that had MIC of 4 (4-8) mg/L; the MIC of HNP-1 was 12 (4-32) mg/L. The combinations of HNP-1 + rifampicin and hBD-3 + rifampicin demonstrated synergistic effects against S. aureus. Against E. coli, combinations of HNP-1 + amikacin and hBD-3 + amikacin also showed synergy of action.

Sending signals - The microbiota's contribution to intestinal epithelial homeostasis

The intestine is inhabited by a diverse range of microorganisms, which requires the host to employ numerous barrier measures to prevent bacterial invasion. However, the intestinal microbiota additionally acts symbiotically with host cells to maintain epithelial barrier function, and perturbation to this interaction plays a pivotal role in intestinal pathogenesis. In this review, we highlight current findings of how the intestinal microbiota influences host intestinal epithelial cells. In particular, we review the roles of numerous microbial-derived products as well as mechanisms by which these microbial products influence the regulation of intestinal epithelial population dynamics and barrier function.