Prokaryotic selectivity and LPS-neutralizing activity of short antimicrobial peptides designed from the human antimicrobial peptide LL-37

Antimicrobial peptides in bone regeneration: mechanism and potential

INTRODUCTION

Antimicrobial peptides (AMPs) are small-molecule peptides with a unique antimicrobial mechanism. Other notable biological activities of AMPs, including anti-inflammatory, angiogenesis, and bone formation effects, have recently received widespread attention. These remarkable bioactivities, combined with the unique antimicrobial mechanism of action of AMPs, have led to their increasingly important role in bone regeneration.

AREAS COVERED

In this review, on the one hand, we aimed to summarize information about the AMPs that are currently used for bone regeneration by reviewing published literature in the PubMed database. On the other hand, we also highlight some AMPs with potential roles in bone regeneration and their possible mechanisms of action.

EXPERT OPINION

The translation of AMPs to the clinic still faces many problems, but their unique antimicrobial mechanisms and other conspicuous biological activities suggest great potential. An in-depth understanding of the structure and mechanism of action of AMPs will help us to subsequently combine AMPs with different carrier systems and perform structural modifications to reduce toxicity and achieve stable release, which may be a key strategy for facilitating the translation of AMPs to the clinic.

KR-12 Derivatives Endow Nanocellulose with Antibacterial and Anti-Inflammatory Properties: Role of Conjugation Chemistry

This work combines the wound-healing-related properties of the host defense peptide KR-12 with wood-derived cellulose nanofibrils (CNFs) to obtain bioactive materials, foreseen as a promising solution to treat chronic wounds. Amine coupling through carbodiimide chemistry, thiol-ene click chemistry, and Cu(I)-catalyzed azide-alkyne cycloaddition were investigated as methods to covalently immobilize KR-12 derivatives onto CNFs. The effects of different coupling chemistries on the bioactivity of the KR12-CNF conjugates were evaluated by assessing their antibacterial activities against Escherichia coli and Staphylococcus aureus. Potential cytotoxic effects and the capacity of the materials to modulate the inflammatory response of lipopolysaccharide (LPS)-stimulated RAW 245.6 macrophages were also investigated. The results show that KR-12 endowed CNFs with antibacterial activity against E. coli and exhibited anti-inflammatory properties and those conjugated by thiol-ene chemistry were the most bioactive. This finding is attributed to a favorable peptide conformation and accessibility (as shown by molecular dynamics simulations), driven by the selective chemistry and length of the linker in the conjugate. The results represent an advancement in the development of CNF-based materials for chronic wound care. This study provides new insights into the effect of the conjugation chemistry on the bioactivity of immobilized host defense peptides, which we believe to be of great value for the use of host defense peptides as therapeutic agents.

Yeast Expressed Hybrid Peptide CLP Abridged Pro-Inflammatory Cytokine Levels by Endotoxin Neutralization

The aim of this study was to apply a strategy to express a recombinant CLP peptide and explore its application as a product derived from natural compounds. The amphiphilic CLP peptide was hybridized from three parent peptides (CM4, LL37, and TP5) and was considered to have potent endotoxin-neutralizing activity with minimal cytotoxic and hemolytic activity. To achieve high secretion expression, an expression vector of pPICZαA-HSA-CLP was constructed by the golden gate cloning strategy before being transformed into Pichia pastoris and integrated into the genome. The recombinant CLP was purified through the Ni-NTA affinity chromatography and analyzed by SDS-PAGE and mass spectrometry. The Limulus amebocyte lysate (LAL) test exhibited that the hybrid peptide CLP inhibited lipopolysaccharides (LPS) in a dose-dependent manner and was significantly (p < 0.05) more efficient compared to the parent peptides. In addition, it essentially diminished (p < 0.05) the levels of nitric oxide and pro-inflammatory cytokines (including TNF-α, IL6, and IL-1β) in LPS-induced mouse RAW264.7 macrophages. As an attendant to the control and the parental peptide LL37, the number of LPS-induced apoptotic cells was diminished compared to the control parental peptide LL37 (p < 0.05) with the treatment of CLP. Consequently, we concluded that the hybrid peptide CLP might be used as a therapeutic agent.

Antimicrobial Peptides and Cationic Nanoparticles: A Broad-Spectrum Weapon to Fight Multi-Drug Resistance Not Only in Bacteria

In the last few years, antibiotic resistance and, analogously, anticancer drug resistance have increased considerably, becoming one of the main public health problems. For this reason, it is crucial to find therapeutic strategies able to counteract the onset of multi-drug resistance (MDR). In this review, a critical overview of the innovative tools available today to fight MDR is reported. In this direction, the use of membrane-disruptive peptides/peptidomimetics (MDPs), such as antimicrobial peptides (AMPs), has received particular attention, due to their high selectivity and to their limited side effects. Moreover, similarities between bacteria and cancer cells are herein reported and the hypothesis of the possible use of AMPs also in anticancer therapies is discussed. However, it is important to take into account the limitations that could negatively impact clinical application and, in particular, the need for an efficient delivery system. In this regard, the use of nanoparticles (NPs) is proposed as a potential strategy to improve therapy; moreover, among polymeric NPs, cationic ones are emerging as promising tools able to fight the onset of MDR both in bacteria and in cancer cells.

Oral antimicrobial peptide-EGCG nanomedicines for synergistic treatment of ulcerative colitis

The pathogenesis of ulcerative colitis (UC) is associated with severe inflammation, damaged colonic barriers, increased oxidative stress, and intestinal dysbiosis. The majority of current medications strive to alleviate inflammation but fail to target additional disease pathologies. Addressing multiple symptoms using a single 'magic bullet' remains a challenge. To overcome this, a smart epigallocatechin-3-gallate (EGCG)-loaded silk fibroin-based nanoparticle (NP) with the surface functionalization of antimicrobial peptides (Cathelicidin-BF, CBF) was constructed, which were internalized by Colon-26 cells and RAW 264.7 macrophages with high efficiencies. These CBF-EGCG-NPs efficiently restored colonic epithelial barriers by relieving oxidative stress and promoting epithelium migration. They also alleviated immune responses through the downregulation of pro-inflammatory factors, upregulation of anti-inflammatory factors, M2 macrophage polarization, and lipopolysaccharide (LPS) elimination. Interestingly, oral administration of hydrogel (chitosan/alginate)-embedding CBF-EGCG-NPs could not only retard progression and treat UC, but also modulate intestinal microbiota by increasing their overall diversity and richness and augmenting the abundance of beneficial bacteria (e.g., Firmicutes and Lactobacillaceae). Our work provides a "many birds with one stone" strategy for addressing UC symptoms using a single NP-based oral platform that targets immune microenvironment modulation, LPS clearance, and microbial remodeling.

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

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

Synergistic Action of Antimicrobial Lung Proteins against Klebsiella pneumoniae

As key components of innate immunity, lung antimicrobial proteins play a critical role in warding off invading respiratory pathogens. Lung surfactant protein A (SP-A) exerts synergistic antimicrobial activity with the N-terminal segment of the SP-B proprotein (SP-B^N) against Klebsiella pneumoniae K2 in vivo. However, the factors that govern SP-A/SP-B^N antimicrobial activity are still unclear. The aim of this study was to identify the mechanisms by which SP-A and SP-B^N act synergistically against K. pneumoniae, which is resistant to either protein alone. The effect of these proteins on K. pneumoniae was studied by membrane permeabilization and depolarization assays and transmission electron microscopy. Their effects on model membranes of the outer and inner bacterial membranes were analyzed by differential scanning calorimetry and membrane leakage assays. Our results indicate that the SP-A/SP-B^N complex alters the ultrastructure of K. pneumoniae by binding to lipopolysaccharide molecules present in the outer membrane, forming packing defects in the membrane that may favor the translocation of both proteins to the periplasmic space. The SP-A/SP-B^N complex depolarized and permeabilized the inner membrane, perhaps through the induction of toroidal pores. We conclude that the synergistic antimicrobial activity of SP-A/SP-B^N is based on the capability of this complex, but not either protein alone, to alter the integrity of bacterial membranes.

Membrane Association Modes of Natural Anticancer Peptides: Mechanistic Details on Helicity, Orientation, and Surface Coverage

Anticancer peptides (ACPs) could potentially offer many advantages over other cancer therapies. ACPs often target cell membranes, where their surface mechanism is coupled to a conformational change into helical structures. However, details on their binding are still unclear, which would be crucial to reach progress in connecting structural aspects to ACP action and to therapeutic developments. Here we investigated natural helical ACPs, Lasioglossin LL-III, Macropin 1, Temporin-La, FK-16, and LL-37, on model liposomes, and also on extracellular vesicles (EVs), with an outer leaflet composition similar to cancer cells. The combined simulations and experiments identified three distinct binding modes to the membranes. Firstly, a highly helical structure, lying mainly on the membrane surface; secondly, a similar, yet only partially helical structure with disordered regions; and thirdly, a helical monomeric form with a non-inserted perpendicular orientation relative to the membrane surface. The latter allows large swings of the helix while the N-terminal is anchored to the headgroup region. These results indicate that subtle differences in sequence and charge can result in altered binding modes. The first two modes could be part of the well-known carpet model mechanism, whereas the newly identified third mode could be an intermediate state, existing prior to membrane insertion.

The Impact of Lung Proteases on Snake-Derived Antimicrobial Peptides

Respiratory infections are a leading cause of global morbidity and mortality and are of significant concern for individuals with chronic inflammatory lung diseases. There is an urgent need for novel antimicrobials. Antimicrobial peptides (AMPs) are naturally occurring innate immune response peptides with therapeutic potential. However, therapeutic development has been hindered by issues with stability and cytotoxicity. Availing of direct drug delivery to the affected site, for example the lung, can reduce unwanted systemic side effects and lower the required dose. As cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD) lungs typically exhibit elevated protease levels, the aim of this study was to assess their impact on snake-derived AMPs. Peptide cleavage was determined using SDS-PAGE and antimicrobial and anti-inflammatory activities of neutrophil elastase (NE)-incubated peptides were assessed using a radial diffusion assay (RDA) and an in vitro LPS-induced inflammation model, respectively. Although the snake-derived AMPs were found to be susceptible to cleavage by lung proteases including NE, several retained their function following NE-incubation. This facilitated the design of novel truncated derivatives that retained functionality following NE incubation. Snake-derived AMPs are tractable candidate treatments for use in environments that feature elevated NE levels, such as the CF airways.

Short and Robust Anti-Infective Lipopeptides Engineered Based on the Minimal Antimicrobial Peptide KR12 of Human LL-37

This study aims to push the frontier of the engineering of human cathelicidin LL-37, a critical antimicrobial innate immune peptide that wards off invading pathogens. By sequential truncation of the smallest antibacterial peptide (KR12) of LL-37 and conjugation with fatty acids, with varying chain lengths, a library of lipopeptides is generated. These peptides are subjected to antibacterial activity and hemolytic assays. Candidates (including both forms made of l- and d-amino acids) with the optimal cell selectivity are subsequently fed to the second layer of in vitro filters, including salts, pH, serum, and media. These practices lead to the identification of a miniature LL-37 like peptide (d-form) with selectivity, stability, and robust antimicrobial activity in vitro against both Gram-positive and negative bacteria. Proteomic studies reveal far fewer serum proteins that bind to the d-form than the l-form peptide. C10-KR8d targets bacterial membranes to become helical, making it difficult for bacteria to develop resistance in a multiple passage experiment. In vivo, C10-KR8d is able to reduce bacterial burden of methicillin-resistant Staphylococcus aureus (MRSA) USA300 LAC in neutropenic mice. In addition, this designer peptide prevents bacterial biofilm formation in a catheter-associated mouse model. Meanwhile, C10-KR8d also recruits cytokines to the vicinity of catheters to clear infection. Thus, based on the antimicrobial region of LL-37, this study succeeds in identifying the smallest anti-infective peptide C10-KR8d with both robust antimicrobial, antibiofilm, and immune modulation activities.

Overview of Host Defense Peptides and Their Applications for Plastic and Reconstructive Surgeons

BACKGROUND

Host defense peptides are a family of endogenous short peptides that are found in all living beings and play a critical role in innate immunity against infection.

METHODS

A nonsystematic review of host defense peptides was conducted with specific interest in properties and applications relevant to plastic and reconstructive surgery.

RESULTS

In addition to their direct antimicrobial actions against pathogens, including multidrug-resistant bacteria, they also demonstrate important functions in immunomodulation, tumor cell lysis, and tissue regeneration. These properties have made them a topic of clinical interest for plastic surgeons because of their potential applications as novel antibiotics, wound healing medications, and cancer therapies. The rising clinical interest has led to a robust body of literature describing host defense peptides in great depth and breadth. Numerous mechanisms have been observed to explain their diverse functions, which rely on specific structural characteristics. However, these peptides remain mostly experimental, with limited translation to clinical practice because of numerous failures to achieve acceptable results in human trials.

CONCLUSIONS

Despite the broad ranging potential of these peptides for use in the field of plastic and reconstructive surgery, they are rarely discussed in the literature or at scientific meetings. In this review, the authors provide a summary of the background, structure, function, bacterial resistance, and clinical applications of host defense peptides with the goal of stimulating host defense peptide-based innovation within the field of plastic and reconstructive surgery.

Two Tachykinin-Related Peptides with Antimicrobial Activity Isolated from Triatoma infestans Hemolymph

Antimicrobial peptides and proteins (AMPs) are molecules that can interact with microbial cells and lead to membrane disruption or intracellular molecule interactions and death. Several molecules with antimicrobial effects also present other biological activities. One such protein group representing the duplicity of activities is the tachykinin family. Tachykinins (TKs) form a family of neuropeptides in vertebrates with a consensus C-terminal region (F-X-G-Y-R-NH2). Invertebrate TKs and TK-related peptides (TKRPs) are subfamilies found in invertebrates that present high homology with TKs and have similar biological effects. Several of these molecules have already been described but reports of TKRP in Hemiptera species are limited. By analyzing the Triatoma infestans hemolymph by reversed-phase high-performance liquid chromatography, biological assays, and mass spectrometry, two antimicrobial molecules were isolated and identified as TKRPs, which we named as TRP1-TINF and TRP2-TINF (tachykinin-related peptides I and II from T. infestans). TRP1-TINF is a random secondary structure peptide with 9 amino acid residues. It is susceptible to aminopeptidases degradation and is active mainly against Micrococcus luteus (32 μM). TRP2-TINF is a 10-amino acid peptide with a 310 helix secondary structure and is susceptible to carboxypeptidases degradation. It has major antimicrobial activity against both Pseudomonas aeruginosa and Escherichia coli (45 μM). Neither molecule is toxic to human erythrocytes and both present minor toxicity toward Vero cells at a concentration of 1000 μM. As the first description of TKRPs with antimicrobial activity in T. infestans, this work contributes to the wider comprehension of the insects’ physiology and describes pharmacological relevant molecules.

Design of a new pH-activatable cell-penetrating peptide for drug delivery into tumor cells

Cell-penetrating peptides (CPPs) have been considered as potential drug delivery vectors due to their remarkable membrane translocation capacity. However, lack of specificity and extreme systemic toxicity hamper their successful application for drug delivery. Here, we designed a new pH-activatable CPP, LHHLLHHLHHLLHH-NH₂ (LH), by substitution of all lysines and two leucines of LKKLLKLLKKLLKL-NH₂ (LK) with histidines. As expected, histidine-rich LH could be activated and penetrate into cells at pH 6.0, whereas its membrane transduction activity could be shielded at pH 7.4. In contrast, LK showed no obviously different cellular uptake at both pH conditions. Importantly, LH was significantly less cytotoxicity compared with LK at both pH values, suggesting a better safety for further application. In addition, after conjugation of camptothecin (CPT) with LH, this conjugate displayed remarkably pH-dependent antitumor activity than free CPT and LK-CPT. This study provides a new tumor pH-responsive CPP with low toxicity for selective anticancer drug delivery.

Immunomodulatory Functions of the Human Cathelicidin LL-37 (aa 13-31)-Derived Peptides are Associated with Predicted α-Helical Propensity and Hydrophobic Index

The anti-endotoxin activity of the cationic peptide LL-37 and its derivative IG-19 is attributed to electrostatic interaction of the peptides’ positive charge with negatively charged bacterial lipopolysaccharides (LPS), and in part to the alteration of intracellular mechanisms independent of peptide binding to LPS. We examined the immunomodulatory responses induced by IG-19 and four IG-19-derived scrambled peptides (IG-19a–d), in the presence and absence of LPS, in macrophages and peripheral blood-derived mononuclear cells. All peptides had identical net charge (+5) and amino acid composition, but different hydrophobicity and α-helical propensity. Peptide IG-19 suppressed LPS-induced cytokine/chemokine production by >90%, IG-19a and IG-19b suppressed it by 40–50%, and IG-19c and IG-19d did not suppress cytokine/chemokine production at all. In silico prediction algorithms and the peptide retention time (RT) on a C18 RP HPLC column indicated a linear association between α-helical propensity and hydrophobicity with the ability of the peptides to inhibit LPS-induced responses. Peptide RT exhibited a significant correlation (>70%) between the suppression of LPS-induced cytokine/chemokine production and peptide-induced production of the anti-inflammatory cytokine IL-1RA. These results indicate that RT on a C18 column can be used as a predictor for the immunomodulatory functions of cationic peptides. Overall, we demonstrated that the immunomodulatory functions of LL-37-derived peptides with identical positive charge and amino acid composition are directly associated with the predicted α-helical propensity and hydrophobicity of the peptides.

A Novel Peptide Ameliorates LPS-Induced Intestinal Inflammation and Mucosal Barrier Damage via Its Antioxidant and Antiendotoxin Effects

Intestinal inflammation is an inflammatory disease resulting from immune dysregulation in the gut. It can increase the risk of enteric cancer, which is a common malignancy globally. As a new class of anti-inflammatory agents, native peptides have potential for use in the treatment of several intestinal inflammation conditions; however, their potential cytotoxicity and poor anti-inflammatory activity and stability have prevented their development. Hybridization has been proposed to overcome this problem. Thus, in this study, we designed a hybrid peptide (LL-37-TP5, LTP) by combing the active centre of LL-37 (13-36) with TP5. The half-life and cytotoxicity were tested in vitro, and the hybrid peptide showed a longer half-life and lower cytotoxicity than its parental peptides. We also detected the anti-inflammatory effects and mechanisms of LTP on Lipopolysaccharide (LPS)-induced intestinal inflammation in murine model. The results showed that LTP effectively prevented LPS-induced weight loss, impairment of intestinal tissues, leukocyte infiltration, and histological evidence of inflammation. Additionally, LTP decreased the levels of tumour necrosis factor-alpha, interferon-gamma, and interleukin-6; increased the expression of zonula occludens-1 and occludin; and reduced permeability in the jejunum of LPS-treated mice. Notably, LTP appeared to be more potent than the parental peptides LL-37 and TP5. The anti-inflammatory effects of LTP may be associated with the neutralization of LPS, inhibition of oxidative stress, and inhibition of the NF-κB signalling pathway. The findings of this study suggest that LTP might be an effective therapeutic agent for treating intestinal inflammation.

Design and Development of a Novel Peptide for Treating Intestinal Inflammation

Intestinal inflammatory disorders, such as inflammatory bowel disease (IBD), are associated with increased pro-inflammatory cytokine secretion in the intestines. Furthermore, intestinal inflammation increases the risk of enteric cancer, which is a common malignancy globally. Native anti-inflammatory peptides are a class of anti-inflammatory agents that could be used in the treatment of several intestinal inflammation conditions. However, potential cytotoxicity, and poor anti-inflammatory activity have prevented their development as anti-inflammatory agents. Therefore, in this study, we designed and developed a novel hybrid peptide for the treatment of intestinal inflammation. Eight hybrid peptides were designed by combining the active centers of antimicrobial peptides, including LL-37 (13-36), YW12D, innate defense regulator 1, and cathelicidin 2 (1-13) with thymopentin or the active center of thymosin alpha 1 (Tα1) (17-24). The hybrid peptide, LL-37-Tα1 (LTA), had improved anti-inflammatory activity with minimal cytotoxicity. LTA was screened by molecule docking and in vitro experiments. Likewise, its anti-inflammatory effects and mechanisms were also evaluated using a lipopolysaccharide (LPS)-induced intestinal inflammation murine model. The results showed that LTA prevented LPS-induced impairment in the jejunum epithelium tissues and infiltration of leukocytes, which are both histological markers of inflammation. Additionally, LTA decreased the levels of tumor necrosis factor-alpha, interferon-gamma, interleukin-6, and interleukin-1β. LTA increased the expression of zonula occludens-1 and occludin, and reduced permeability and apoptosis in the jejunum of LPS-treated mice. Additionally, its anti-inflammatory effect is associated with neutralizing LPS, binding to the Toll-like receptor 4-myeloid differentiation factor 2 (TLR4/MD-2) complex, and modulating the nuclear factor-kappa B signal transduction pathway. The findings of this study suggest that LTA may be an effective therapeutic agent in the treatment of intestinal inflammation.

Expression and Purification of Hybrid LL-37Tα1 Peptide in Pichia pastoris and Evaluation of Its Immunomodulatory and Anti-inflammatory Activities by LPS Neutralization

This study pertains to the new approach for the development of hybrid peptide LL-37Tα1 and its biomedical applications. A linear cationic hybrid peptide, LL-37Tα1 was derived from two parental peptides (LL-37 and Tα1) recognized as potent anti-endotoxin without any hemolytic or cytotoxic activity. We successfully cloned the gene of hybrid peptide LL-37Tα1 in PpICZαA vector and expressed in the Pichia pastoris. The recombinant peptide was purified by Ni-affinity column and reverse-phase high performance liquid chromatography (RP-HPLC) with an estimated molecular mass of 3.9 kDa as determined by SDS-PAGE and mass spectrometry. We analyzed the LPS neutralization by limulus amebocyte lysate (LAL) activity and the results indicate that the hybrid peptide LL-37Tα1 directly binds endotoxin and significantly (p < 0.05) neutralizes the effect of LPS in a dose-dependent manner. Lactate dehydrogenase (LDH) assay revealed that LL-37Tα1 successfully reduces the LPS-induced cytotoxicity in mouse RAW264.7 macrophages. Moreover, it significantly (p < 0.05) decreased the levels of nitric oxide, proinflammatory cytokines including TNF-α, IL-6, IL-1β, and diminished the number of apoptotic cells in LPS-stimulated mouse RAW264.7 macrophages. Our results suggest that the P. pastoris expression system is cost-effective for commercial production of the immunomodulatory and anti-inflammatory hybrid peptide (IAHP) LL-37Tα1 and the peptide may serve as effective anti-endotoxin/anti-inflammatory agent with minimal cytotoxicity.

Sarconesin II, a New Antimicrobial Peptide Isolated from Sarconesiopsis magellanica Excretions and Secretions

Antibiotic resistance is at dangerous levels and increasing worldwide. The search for new antimicrobial drugs to counteract this problem is a priority for health institutions and organizations, both globally and in individual countries. Sarconesiopsis magellanica blowfly larval excretions and secretions (ES) are an important source for isolating antimicrobial peptides (AMPs). This study aims to identify and characterize a new S. magellanica AMP. RP-HPLC was used to fractionate ES, using C18 columns, and their antimicrobial activity was evaluated. The peptide sequence of the fraction collected at 43.7 min was determined by mass spectrometry (MS). Fluorescence and electronic microscopy were used to evaluate the mechanism of action. Toxicity was tested on HeLa cells and human erythrocytes; physicochemical properties were evaluated. The molecule in the ES was characterized as sarconesin II and it showed activity against Gram-negative (Escherichia coli MG1655, Pseudomonas aeruginosa ATCC 27853, P. aeruginosa PA14) and Gram-positive (Staphylococcus aureus ATCC 29213, Micrococcus luteus A270) bacteria. The lowest minimum inhibitory concentration obtained was 1.9 μM for M. luteus A270; the AMP had no toxicity in any cells tested here and its action in bacterial membrane and DNA was confirmed. Sarconesin II was documented as a conserved domain of the ATP synthase protein belonging to the Fli-1 superfamily. The data reported here indicated that peptides could be alternative therapeutic candidates for use in infections against Gram-negative and Gram-positive bacteria and eventually as a new resource of compounds for combating multidrug-resistant bacteria.

Design of Antimicrobial Peptides: Progress Made with Human Cathelicidin LL-37

The incorporation of the innate immune system into humans is essential for survival and health due to the rapid replication of invading microbes and the delayed action of the adaptive immune system. Antimicrobial peptides are important components of human innate immunity. Over 100 such peptides have been identified in various human tissues. Human cathelicidin LL-37 is best studied, and there has been a growing interest in designing new peptides based on LL-37. This chapter describes the alternative processing of the human cathelicidin precursor, protease digestion, and lab cutting of LL-37. Both a synthetic peptide library and structure-based design are utilized to identify the active regions. Although challenging, the determination of the 3D structure of LL-37 enabled the identification of the core antimicrobial region. The minimal region of LL-37 can be function-dependent. We discuss the design and potential applications of LL-37 into antibacterial, antibiofilm, antiviral, antifungal, immune modulating, and anticancer peptides. LL-37 has been engineered into 17BIPHE2, a stable, selective, and potent antimicrobial, antibiofilm, and anticancer peptide. Both 17BIPHE2 and SAAP-148 can eliminate the ESKAPE pathogens and show topical in vivo antibiofilm efficacy. Also discussed are other application strategies, including peptide formulation, antimicrobial implants, and peptide-inducing factors such as vitamin D and sunlight. Finally, we summarize what we learned from peptide design based on human LL-37.

Sarconesin: Sarconesiopsis magellanica Blowfly Larval Excretions and Secretions With Antibacterial Properties

Larval therapy (LT) is an alternative treatment for healing chronic wounds; its action is based on debridement, the removal of bacteria, and stimulating granulation tissue. The most important mechanism when using LT for combating infection depends on larval excretions and secretions (ES). Larvae are protected against infection by a spectrum of antimicrobial peptides (AMPs); special interest in AMPs has also risen regarding understanding their role in wound healing since they degrade necrotic tissue and kill different bacteria during LT. Sarconesiopsis magellanica (Diptera: Calliphoridae) is a promising medically-important necrophagous fly. This article reports a small AMP being isolated from S. magellanica ES products for the first time; these products were obtained from third-instar larvae taken from a previously-established colony. ES were fractionated by RP-HPLC using C18 columns for the first analysis; the products were then lyophilised and their antimicrobial activity was characterized by incubation with different bacterial strains. These fractions' primary sequences were determined by mass spectrometry and de novo sequencing; five AMPs were obtained, the Sarconesin fraction was characterized and antibacterial activity was tested in different concentrations with minimum inhibitory concentrations starting at 1.2 μM. Potent inhibitory activity was shown against Gram-negative (Escherichia coli D31, E. coli DH5α, Salmonella enterica ATCC 13314, Pseudomonas aeruginosa 27853) and Gram-positive (Staphylococcus aureus ATCC 29213, S. epidermidis ATCC 12228, Micrococcus luteus A270) bacteria. Sarconesin has a significant similarity with Rho-family GTPases which are important in organelle development, cytoskeletal dynamics, cell movement, and wound repair. The data reported here indicated that Sarconesin could be an alternative candidate for use in therapeutics against Gram-negative and Gram-positive bacterial infections. Our study describes one peptide responsible for antibacterial activity when LT is being used. The results shown here support carrying out further experiments aimed at validating S. magellanica AMPs as novel resources for combating antibacterial resistance.

Antimicrobial peptides with selective antitumor mechanisms: prospect for anticancer applications

In the last several decades, there have been significant advances in anticancer therapy. However, the development of resistance to cancer drugs and the lack of specificity related to actively dividing cells leading to toxic side effects have undermined these achievements. As a result, there is considerable interest in alternative drugs with novel antitumor mechanisms. In addition to the recent approach using immunotherapy, an effective but much cheaper therapeutic option of pharmaceutical drugs would still provide the best choice for cancer patients as the first line treatment. Ribosomally synthesized cationic antimicrobial peptides (AMPs) or host defense peptides (HDP) display broad-spectrum activity against bacteria based on electrostatic interactions with negatively charged lipids on the bacterial surface. Because of increased proportions of phosphatidylserine (negatively charged) on the surface of cancer cells compared to normal cells, cationic amphipathic peptides could be an effective source of anticancer agents that are both selective and refractory to current resistance mechanisms. We reviewed herein the prospect for AMP application to cancer treatment, with a focus on modes of action of cationic AMPs.

Perspectives for clinical use of engineered human host defense antimicrobial peptides

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

Structural remodeling and oligomerization of human cathelicidin on membranes suggest fibril-like structures as active species

Antimicrobial peptides as part of the mammalian innate immune system target and remove major bacterial pathogens, often through irreversible damage of their cellular membranes. To explore the mechanism by which the important cathelicidin peptide LL-37 of the human innate immune system interacts with membranes, we performed biochemical, biophysical and structural studies. The crystal structure of LL-37 displays dimers of anti-parallel helices and the formation of amphipathic surfaces. Peptide-detergent interactions introduce remodeling of this structure after occupation of defined hydrophobic sites at the dimer interface. Furthermore, hydrophobic nests are shaped between dimer structures providing another scaffold enclosing detergents. Both scaffolds underline the potential of LL-37 to form defined peptide-lipid complexes in vivo. After adopting the activated peptide conformation LL-37 can polymerize and selectively extract bacterial lipids whereby the membrane is destabilized. The supramolecular fibril-like architectures formed in crystals can be reproduced in a peptide-lipid system after nanogold-labelled LL-37 interacted with lipid vesicles as followed by electron microscopy. We suggest that these supramolecular structures represent the LL-37-membrane active state. Collectively, our study provides new insights into the fascinating plasticity of LL-37 demonstrated at atomic resolution and opens the venue for LL-37-based molecules as novel antibiotics.

Cationic peptides from peptic hydrolysates of rice endosperm protein exhibit antimicrobial, LPS-neutralizing, and angiogenic activities

In this study, we hydrolyzed rice endosperm protein (REP) with pepsin and generated 20 fractions containing multifunctional cationic peptides with varying isoelectric point (pI) values using ampholyte-free isoelectric focusing (autofocusing). Subsequently, we determined antimicrobial activities of each fraction against the pathogens Prophyromonas gingivalis, Propionibacterium acnes, Streptocossus mutans, and Candida albicans. Fractions 18, 19, and 20 had pI values greater than 12 and exhibited antimicrobial activity against P. gingivalis, P. acnes, and C. albicans, but not against S. mutans. In further experiments, we purified and identified cationic peptides from fractions 18, 19, and 20 using reversed-phase high-performance liquid chromatography and matrix-assisted laser/desorption ionization-time-of-flight mass spectroscopy. We also chemically synthesized five identified peptides (RSVSKSR, RRVIEPR, ERFQPMFRRPG, RVRQNIDNPNRADTYNPRAG, and VVRRVIEPRGLL) with pI values greater than 10.5 and evaluated antimicrobial, lipopolysaccharide (LPS)-neutralizing, and angiogenic activities. Among these synthetic peptides, only VVRRVIEPRGLL exhibited antimicrobial activity against P. gingivalis, with an IC₅₀ value of 87μM. However, all five cationic peptides exhibited LPS-neutralizing and angiogenic activities with little or no hemolytic activity against mammalian red blood cells at functional concentrations. These present data show dual or multiple functions of the five identified cationic peptides with little or no hemolytic activity. Therefore, fractions containing cationic peptides from REP hydrolysates have the potential to be used as dietary supplements and functional ingredients in food products.

Tryptophan-Containing Cyclic Decapeptides with Activity against Plant Pathogenic Bacteria

A library of 66 cyclic decapeptides incorporating a Trp residue was synthesized on solid phase and screened against the phytopathogenic bacteria Pseudomonas syringae pv. syringae, Xanthomonas axonopodis pv. vesicatoria, and Erwinia amylovora. The hemolytic activity of these peptides was also evaluated. The results obtained were compared with those of a collection of Phe analogues previously reported. The analysis of the data showed that the presence of the Trp improved the antibacterial activity against these three pathogens. In particular, 40 to 46 Trp analogues displayed lower minimum inhibitory concentration (MIC) values than their corresponding Phe counterparts. Interestingly, 26 Trp-containing sequences exhibited MIC of 0.8 to 3.1 μM against X. axonopodis pv. vesicatoria, 21 peptides MIC of 1.6 to 6.2 μM against P. syringae pv. syringae and six peptides MIC of 6.2 to 12.5 μM against E. amylovora. Regarding the hemolysis, in general, Trp derivatives displayed a percentage of hemolysis comparable to that of their Phe analogues. Notably, 49 Trp-containing cyclic peptides showed a hemolysis ≤ 20% at 125 μM. The peptides with the best biological activity profile were c(LKKKLWKKLQ) (BPC086W) and c(LKKKKWLLKQ) (BPC108W), which displayed MIC values ranging from 0.8 to 12.5 μM and a hemolysis ≤ 8% at 125 μM. Therefore, it is evident that these Trp sequences constitute promising candidates for the development of new agents for use in plant protection.

Identification and characterization of multifunctional cationic and amphipathic peptides from soybean proteins

In this study, we identified and chemically synthesized three cationic and amphipathic peptides (Glycinin-17, BCAS-16, and BCBS-11) from soybean proteins. These peptides had high isoelectric points, high positive net charges, and included multiple hydrophobic amino acids. Subsequently, we identified multiple functions of these peptides, including antimicrobial, lipopolysaccharide-neutralizing, and angiogenic activities, and examined their cytotoxic activities against mammalian red blood cells. Glycinin-17, BCAS-16, and BCBS-11 exhibited antimicrobial activity against Porphyromonas gingivalis and Candida albicans whereas Glycinin-17 did not possess antimicrobial effects on Propionibacterium acnes and Streptococcus mutans. Membrane-depolarization assays and flow cytometric analyses showed that the antimicrobial properties of Glycinin-17, BCAS-16, and BCBS-11 against P. gingivalis, P. acnes, and S. mutans were dependent on membrane-disrupting potential. In contrast, major antimicrobial activities of these peptides against C. albicans were dependent on interactions with targets other than cell membranes. Furthermore, chromogenic Limulus amebocyte lysate assays showed that 50% effective concentrations (EC50 , 0.12-0.31 μM) of these three peptides neutralize LPS with similar potency (EC50 : 0.11 μM) to that of polymyxin B. Moreover, tube-formation assays in human umbilical vein endothelial cells showed similar angiogenic activities of the three peptides as that following treatment with LL-37. Although BCAS-16 exhibited hemolytic activity, the rate of hemolysis for Glycinin-17 and BCBS-11 in the presence of 500-μM Glycinin-17 and BCBS-11 was less than 2%. These results demonstrate that cationic and amphipathic peptides from soybean proteins, particularly Glycinin-17 and BCBS-11, have potential as multifunctional ingredients for healthcare applications.

Antibacterial Actions of Glycinin Basic Peptide against Escherichia coli

Glycinin basic peptide (GBP) is an antibacterial ingredient that occurs naturally in the basic parts of soybean glycinin. The antibacterial actions of GBP against Escherichia coli ATCC 8739 were investigated in this study. The minimum inhibitory concentration of GBP against E. coli was 200 μg/mL. The exposure of E. coli cells to GBP induced significant cell damage and inactivated intracellular esterases (stressed and dead cells, 70.9% ± 0.04 for 200 μg/mL of GBP and 91.9% ± 0.06 for 400 μg/mL of GBP), as determined through dual staining in flow cytometry. GBP resulted in the exposure of phosphatidylserine in E. coli cells. The analyses of flow cytometry-manifested GBP treatment led to the shrinkage of the cell surface and the complication of cell granularity. The observations in transmission electron microscopy demonstrated that 400 μg/mL of GBP severely disrupted the membrane integrity, resulting in ruptures or pores in the membrane, outflows of intracellular contents, or aggregation of the cytoplasm. Release of alkaline phosphatase, lipopolysaccharide, and reducing sugar further verified that the membrane damage was due to GBP. In addition, GBP treatment changed the helicity and base staking of DNA, as determined by circular dichroism spectroscopy. These results showed that GBP had strong antibacterial activity against E. coli via membrane damage and DNA perturbation. Additionally, GBP exhibited no cytotoxicity on the viability of human embryonic kidney cells. Thus, GBP may be a promising candidate as a natural antibacterial agent.

Characterization and production of multifunctional cationic peptides derived from rice proteins

Food proteins have been identified as a source of bioactive peptides. These peptides are inactive within the sequence of the parent protein and must be released during gastrointestinal digestion, fermentation, or food processing. Of bioactive peptides, multifunctional cationic peptides are more useful than other peptides that have specific activity in promotion of health and/or the treatment of diseases. We have identified and characterized cationic peptides from rice enzymes and proteins that possess multiple functions, including antimicrobial, endotoxin-neutralizing, arginine gingipain-inhibitory, and/or angiogenic activities. In particular, we have elucidated the contribution of cationic amino acids (arginine and lysine) in the peptides to their bioactivities. Further, we have discussed the critical parameters, particularly proteinase preparations and fractionation or purification, in the enzymatic hydrolysis process for producing bioactive peptides from food proteins. Using an ampholyte-free isoelectric focusing (autofocusing) technique as a tool for fractionation, we successfully prepared fractions containing cationic peptides with multiple functions.

Chicken cathelicidins as potent intrinsically disordered biocides with antimicrobial activity against infectious pathogens

This study was performed to identify the expression patterns of the cathelicidin genes in a local chicken breed and to evaluate the antimicrobial activities of the cathelicidin peptides against pathogenic bacteria. This analysis revealed that the coding regions of CATH-1, -2, and -3 genes contain 447 bp, 465 bp, and 456 bp, respectively, and encode proteins of 148, 154, 151 amino acids, respectively. The complete amino acid sequences of the cathelicidin peptides are similar to those found in Meleagris gallopavo, Phasianus colchicus, and Coturnix coturnix, and show high sequence identity to their Columba livia and Anas platyrhynchos counterparts. In contrast, these avian peptides shared a very low sequence identity with the mammalian cathelicidins. The analysis further revealed that the cathelicidin genes are expressed in various organ and tissues. We also show that the CATH peptides 1, 2, 3 and their amide-modified structures possess potent antimicrobial activities against both Gram-positive and Gram-negative pathogens, with these bacteria being affected to different extents. The antimicrobial activities of the peptides are slightly lower than those of their amide analogs. Computational analysis revealed that pre-pro-cathelicidins are hybrid proteins that contain ordered domains and functional intrinsically disordered regions. Furthermore, high structural and sequence variability of mature cathelicidins is a strong indication of their rather disordered nature. It is likely that intrinsic disorder is needed for the multifarious functionality of these antimicrobial peptides. Our analyses indicated that cathelicidin peptides require further study to better understand their full potentials in the treatment of diseases in both humans and animals. The data obtained for synthetic avian peptides will help elucidating of their potential applications in the pharmaceutical industry.

Structural basis for endotoxin neutralization by the eosinophil cationic protein

Acute infection by Gram-negative pathogens can induce an exacerbated immune response that leads to lethal septic shock syndrome. Bacterial lipopolysaccharide (LPS) is a major pathogen-associated molecular pattern molecule that can initiate massive and lethal immune system stimulation. Therefore, the development of new and effective LPS-neutralizing agents is a top priority. The eosinophil cationic protein (ECP) is an antimicrobial protein secreted in response to infection, with a remarkable affinity for LPS. In the present study, we demonstrate that ECP is able to neutralize bacterial LPS and inhibit tumor necrosis factor-α production in human macrophages. We also characterized ECP neutralizing activity using progressively truncated LPS mutants, and conclude that the polysaccharide moiety and lipid A portions are required for LPS-mediated neutralization. In addition, we mapped the structural determinants required for the ECP-LPS interaction by nuclear magnetic resonance. Our results show that ECP is able to neutralize LPS and therefore opens a new route for developing novel therapeutic agents based on the ECP structural scaffolding.

Breaking the Spell: Combating Multidrug Resistant 'Superbugs'

Multidrug-resistant (MDR) bacteria have become a severe threat to community wellbeing. Conventional antibiotics are getting progressively more ineffective as a consequence of resistance, making it imperative to realize improved antimicrobial options. In this review we emphasized the microorganisms primarily reported of being resistance, referred as ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa, and Enterobacteriaceae) accentuating their capacity to "escape" from routine antimicrobial regimes. The upcoming antimicrobial agents showing great potential and can serve as alternative therapeutic options are discussed. We also provided succinct overview of two evolving technologies; specifically network pharmacology and functional genomics profiling. Furthermore, In vivo imaging techniques can provide novel targets and a real time tool for potential lead molecule assessment. The employment of such approaches at prelude of a drug development process, will enables more informed decisions on candidate drug selection and will maximize or predict therapeutic potential before clinical testing.

Endotoxin-neutralizing activity and mechanism of action of a cationic α-helical antimicrobial octadecapeptide derived from α-amylase of rice

We have previously reported that AmyI-1-18, an octadecapeptide derived from α-amylase (AmyI-1) of rice, is a novel cationic α-helical peptide that exhibited antimicrobial activity against human pathogens, including Porphyromonas gingivalis, Pseudomonas aeruginosa, Propionibacterium acnes, Streptococcus mutans, and Candida albicans. In this study, to further investigate the potential functions of AmyI-1-18, we examined its inhibitory ability against the endotoxic activities of lipopolysaccharides (LPSs, smooth and Rc types) and lipid A from Escherichia coli. AmyI-1-18 inhibited the production of endotoxin-induced nitric oxide (NO), an inflammatory mediator, in mouse macrophages (RAW264) in a concentration-dependent manner. The results of a chromogenic Limulus amebocyte lysate assay illustrated that the ability [50% effective concentration (EC50): 0.17 μM] of AmyI-1-18 to neutralize lipid A was similar to its ability (EC50: 0.26 μM) to neutralize LPS, suggesting that AmyI-1-18 specifically binds to the lipid A moiety of LPS. Surface plasmon resonance analysis of the interaction between AmyI-1-18 and LPS or lipid A also suggested that AmyI-1-18 directly binds to the lipid A moiety of LPS because the dissociation constant (KD) of AmyI-1-18 with lipid A is 5.6×10(-10) M, which is similar to that (4.3×10(-10) M) of AmyI-1-18 with LPS. In addition, AmyI-1-18 could block the binding of LPS-binding protein to LPS, although its ability was less than that of polymyxin B. These results suggest that AmyI-1-18 expressing antimicrobial and endotoxin-neutralizing activities is useful as a safe and potent host defense peptide against pathogenic Gram-negative bacteria in many fields of healthcare.

Inhibitory Effects of Antimicrobial Peptides on Lipopolysaccharide-Induced Inflammation

Antimicrobial peptides (AMPs) are usually small molecule peptides, which display broad-spectrum antimicrobial activity, high efficiency, and stability. For the multiple-antibiotic-resistant strains, AMPs play a significant role in the development of novel antibiotics because of their broad-spectrum antimicrobial activities and specific antimicrobial mechanism. Besides broad-spectrum antibacterial activity, AMPs also have anti-inflammatory activity. The neutralization of lipopolysaccharides (LPS) plays a key role in anti-inflammatory action of AMPs. On the one hand, AMPs can readily penetrate the cell wall barrier by neutralizing LPS to remove Gram-negative bacteria that can lead to infection. On the contrary, AMPs can also inhibit the production of biological inflammatory cytokines to reduce the inflammatory response through neutralizing circulating LPS. In addition, AMPs also modulate the host immune system by chemotaxis of leukocytes, to promote immune cell proliferation, epithelialization, and angiogenesis and thus play a protective role. This review summarizes some recent researches about anti-inflammatory AMPs, with a focus on the interaction of AMPs and LPS on the past decade.

Antimicrobial Peptide LL-37 Is Both a Substrate of Cathepsins S and K and a Selective Inhibitor of Cathepsin L

Lung cysteine cathepsins B, K, L, and S contribute to physiological and pathological processes including degradation of antimicrobial peptides/proteins (AMPs) such as surfactant protein SP-A, lactoferrin, secretory leukocyte peptidase inhibitor, and beta-defensins-2 and -3. Substantial amounts of uncleaved LL-37, a 37-mer cationic AMP, were observed in the sputum of patients with cystic fibrosis (CF). Nevertheless LL-37 was degraded after prolonged incubation in CF sputum, and the hydrolysis was blocked by E-64, a selective inhibitor of cysteine proteases. Cathepsins K and S, expressed in human alveolar macrophages, thoroughly hydrolyzed LL-37 in vitro, whereas it competitively inhibited cathepsin L (Ki = 150 nM). Cleavage of LL-37 by cathepsins S and K impaired its antimicrobial activity against Pseudomonas aeruginosa and Staphylococcus aureus, in a time- and concentration-dependent manner. The exchange of residues 67 and 205 in the S2 pockets of cathepsins L (Leu67Tyr/Ala205Leu) and K (Tyr67Leu/Leu205Ala) switched the specificity of these mutants toward LL-37. Molecular modeling suggested that LL-37 interacted with the active site of cathepsin L in both forward (i.e., substrate-like) and reverse orientations with similar binding energies. Our data support the hypothesis that cysteine cathepsins modulate the innate immunity response by degrading distinct and representative members of the AMP family.

Strategies employed in the design and optimization of synthetic antimicrobial peptide amphiphiles with enhanced therapeutic potentials

Antimicrobial peptides (AMPs) which predominantly act via membrane active mechanisms have emerged as an exciting class of antimicrobial agents with tremendous potential to overcome the global epidemic of antibiotics-resistant infections. The first generation of AMPs derived from natural sources as diverse as plants, insects and humans has provided a wealth of compositional and structural information to design novel synthetic AMPs with enhanced antimicrobial potencies and selectivities, reduced cost of production due to shorter sequences and improved stabilities under physiological conditions. In this review, we will first discuss the common strategies employed in the design and optimization of synthetic AMPs, followed by highlighting the various approaches utilized to enhance the therapeutic potentials of designed AMPs under physiological conditions. Lastly, future perspectives on the development of improved AMPs for therapeutic applications will be presented.