Anticandida activity is retained in P-113, a 12-amino-acid fragment of histatin 5

Secreted Aspartic Proteinases: Key Factors in Candida Infections and Host-Pathogen Interactions

Extracellular proteases are key factors contributing to the virulence of pathogenic fungi from the genus Candida. Their proteolytic activities are crucial for extracting nutrients from the external environment, degrading host defenses, and destabilizing the internal balance of the human organism. Currently, the enzymes most frequently described in this context are secreted aspartic proteases (Saps). This review comprehensively explores the multifaceted roles of Saps, highlighting their importance in biofilm formation, tissue invasion through the degradation of extracellular matrix proteins and components of the coagulation cascade, modulation of host immune responses via impairment of neutrophil and monocyte/macrophage functions, and their contribution to antifungal resistance. Additionally, the diagnostic challenges associated with Candida infections and the potential of Saps as biomarkers were discussed. Furthermore, we examined the prospects of developing vaccines based on Saps and the use of protease inhibitors as adjunctive therapies for candidiasis. Given the complex biology of Saps and their central role in Candida pathogenicity, a multidisciplinary approach may pave the way for innovative diagnostic strategies and open new opportunities for innovative clinical interventions against candidiasis.

Searching for Conjugates as New Structures for Antifungal Therapies

The progressive increase in fungal infections and the decrease in the effectiveness of current therapy explain research on new drugs. The synthesis of compounds with proven antifungal activity, favorable physicochemical and pharmacokinetic properties affecting their pharmaceutical availability and bioavailability, and limiting or eliminating side effects has become the goal of many studies. The publication describes the directions of searching for new compounds with antifungal activity, focusing on conjugates. The described modifications include, among others, azoles or amphotericin B in combination with fatty acids, polysaccharides, proteins, and synthetic polymers. The benefits of these combinations in terms of activity, mechanism of action, and bioavailability were indicated. The possibilities of creating or using nanoparticles, "umbrella" conjugates, siderophores (iron-chelating compounds), and monoclonal antibodies were also presented. Taking into account the role of vaccinations in prevention, the scope of research related to developing a vaccine protecting against fungal infections was also indicated.

Anti-Microbial Activities of Mussel-Derived Recombinant Proteins against Gram-Negative Bacteria

Many anti-microbial peptides (AMPs) and pro-apoptotic peptides are considered as novel anti-microbial agents, distinguished by their different characteristics. Nevertheless, AMPs exhibit certain limitations, including poor stability and potential toxicity, which hinder their suitability for applications in pharmaceutics and medical devices. In this study, we used recombinant mussel adhesive protein (MAP) as a robust scaffold to overcome these limitations associated with AMPs. Mussel adhesive protein fused with functional peptides (MAP-FPs) was used to evaluate anti-microbial activities, minimal inhibitory concentration (MIC), and time-kill kinetics (TKK) assays against six of bacteria strains. MAP and MAP-FPs were proved to have an anti-microbial effect with MIC of 4 or 8 µM against only Gram-negative bacteria strains. All tested MAP-FPs killed four different Gram-negative bacteria strains within 180 min. Especially, MAP-FP-2 and -5 killed three Gram-negative bacteria strain, including E. coli, S. typhimurium, and K. pneumoniae, within 10 min. A cytotoxicity study using Vero and HEK293T cells indicated the safety of MAP and MAP-FP-2 and -3. Thermal stability of MAP-FP-2 was also validated by HPLC analysis at an accelerated condition for 4 weeks. This study identified that MAP-FPs have novel anti-microbial activity, inhibiting the growth and rapidly killing Gram-negative bacteria strains with high thermal stability and safety.

Activatable prodrug for controlled release of an antimicrobial peptide via the proteases overexpressed in Candida albicans and Porphyromonas gingivalis

Candida albicans and Porphyromonas gingivalis are prevalent in the subgingival area where the frequency of fungal colonization increases with periodontal disease. Candida's transition to a pathogenic state and its interaction with P. gingivalis exacerbate periodontal disease severity. However, current treatments for these infections differ, and combined therapy remains unexplored. This work is based on an antimicrobial peptide that is therapeutic and induces a color change in a nanoparticle reporter. Methods: We built and characterized two enzyme-activatable prodrugs to treat and detect C. albicans and P. gingivalis via the controlled release of the antimicrobial peptide. The zwitterionic prodrug quenches the antimicrobial peptide's activity until activation by a protease inherent to the pathogens (SAP9 for C. albicans and RgpB for P. gingivalis). The toxicity of the intact prodrugs was evaluated against fungal, bacterial, and mammalian cells. Therapeutic efficacy was assessed through microscopy, disk diffusion, and viability assays, comparing the prodrug to the antimicrobial peptide alone. Finally, we developed a colorimetric detection system based on the aggregation of plasmonic nanoparticles. Results: The intact prodrugs showed negligible toxicity to cells absent a protease trigger. The therapeutic impact of the prodrugs was comparable to that of the antimicrobial peptide alone, with a minimum inhibitory concentration of 3.1 - 16 µg/mL. The enzymatic detection system returned a detection limit of 10 nM with gold nanoparticles and 3 nM with silver nanoparticles. Conclusion: This approach offers a convenient and selective protease sensing and protease-induced treatment mechanism based on bioinspired antimicrobial peptides.

Antimicrobial peptide LL-37 disrupts plasma membrane and calcium homeostasis in Candida albicans via the Rim101 pathway

IMPORTANCE

Candida albicans is a major human fungal pathogen, and antimicrobial peptides are key components of innate immunity. Studying the interplay between C. albicans and human antimicrobial peptides would enhance a better understanding of pathogen-host interactions. Moreover, potential applications of antimicrobial peptides in antifungal therapy have aroused great interest. This work explores new mechanisms of LL-37 against C. albicans and reveals the complex connection among calcium homeostasis, oxidative stress, signaling, and possibly organelle interaction. Notably, these findings support the possible use of antimicrobial peptides to prevent and treat fungal infections.

Activatable prodrug for controlled release of an antimicrobial peptide via the proteases overexpressed in Candida albicans and Porphyromonas gingivalis

We report the controlled release of an antimicrobial peptide using enzyme-activatable prodrugs to treat and detect Candida albicans and Porphyromonas gingivalis . Our motivation lies in the prevalence of these microorganisms in the subgingival area where the frequency of fungal colonization increases with periodontal disease. This work is based on an antimicrobial peptide that is both therapeutic and induces a color change in a nanoparticle reporter. This antimicrobial peptide was then built into a zwitterionic prodrug that quenches its activity until activation by a protease inherent to these pathogens of interest: SAP9 or RgpB for C. albicans and P. gingivalis , respectively. We first confirmed that the intact zwitterionic prodrug has negligible toxicity to fungal, bacterial, and mammalian cells absent a protease trigger. Next, the therapeutic impact was assessed via disk diffusion and viability assays and showed a minimum inhibitory concentration of 3.1 - 16 µg/mL, which is comparable to the antimicrobial peptide alone (absent integration into prodrug). Finally, the zwitterionic design was exploited for colorimetric detection of C. albicans and P. gingivalis proteases. When the prodrugs were cleaved, the plasmonic nanoparticles aggregated causing a color change with a limit of detection of 10 nM with gold nanoparticles and 3 nM with silver nanoparticles. This approach has value as a convenient and selective protease sensing and protease-induced treatment mechanism based on bioinspired antimicrobial peptides.

Abstract Figure

Ceragenins exhibit bactericidal properties that are independent of the ionic strength in the environment mimicking cystic fibrosis sputum

The purpose of the work was to investigate the impact of sodium chloride (NaCl) on the antimicrobial efficacy of ceragenins (CSAs) and antimicrobial peptides (AMPs) against bacterial and fungal pathogens associated with cystic fibrosis (CF) lung infections. CF-associated bacterial (Pseudomonas aeruginosa, Ochrobactrum spp., and Staphylococcus aureus), and fungal pathogens (Candida albicans, and Candida tropicalis) were used as target organisms for ceragenins (CSA-13 and CSA-131) and AMPs (LL-37 and omiganan). Susceptibility to the tested compounds was assessed using minimal inhibitory concentrations (MICs) and bactericidal concentrations (MBCs), as well as by colony counting assays in CF sputum samples supplemented with various concentrations of NaCl. Our results demonstrated that ceragenins exhibit potent antimicrobial activity in CF sputum regardless of the NaCl concentration when compared to LL-37 and omiganan. Given the broad-spectrum antimicrobial activity of ceragenins in the microenvironments mimicking the airways of CF patients, ceragenins might be promising agents in managing CF disease.

Benzo[g]quinazolines as antifungal against candidiasis: Screening, molecular docking, and QSAR investigations

Candida albicans, an opportunistic pathogen, is the most common type of fungus and represents a substantial source of human invasive disease (nosocomial infection). This category of fungi are part of our microbiota, and given the appropriate environmental conditions, it has the potential to cause both superficial and systemic infections. There is a soaring resistance against the available anticandidal agents. The purpose of this research is to investigate the activity of certain previously synthesized benzo[g]quinazolines against C. albicans in vitro by using the cup-plate diffusion method. There was a marked difference in the effectiveness of the target compounds 1-6 against the sample of C. albicans that was tested. Benzo[g]quinazolines 1 (inhibition zone = 20 mm) and 2 (inhibition zone = 22 mm) had good effects in comparison to fluconazole (inhibition zone = 26 mm). A docking study was conducted between benzo[g]quinazolines 1-6 and Candida spp. CYP51 to establish the binding mode compared with fluconazole and VT-1161 (oteseconazole) as reference medicines, and it was determined that binding at the active site of Candida spp. CYP51 occurred in the same manner. Quantitative structure-activity relationship (QSAR) investigation was performed to further characterize the identified anticandidal agents and recognize the major regulatory components governing such activity. In future studies, the benzo[g]quinazoline scaffold could serve as a model for the design and development of novel derivatives with antifungal potential.

Quantifying the Antifungal Activity of Peptides Against Candida albicans

Traditional methods for performing antifungal susceptibility testing for Candida albicans are time-consuming and lack quantitative results. For example, a common approach relies on plating cells treated with different concentrations of antifungal molecules on agar plates and then counting the colonies to determine the relationship between molecule concentration and growth inhibition. This method requires many plates and substantial time to count the colonies. Another common approach eliminates the plates and counting of colonies by visually inspecting cultures treated with antifungal agents to identify the minimum concentration required to inhibit growth; however, visual inspection produces only qualitative results, and information on growth at subinhibitory concentrations is lost. This protocol describes a method for measuring the susceptibility of C. albicans to antifungal peptides. By relying on optical density measurements of cultures, the method reduces the time and materials needed to obtain quantitative results on culture growth at different peptide concentrations. The incubation of the fungus with peptides is performed in a 96-well plate using an appropriate buffer, with controls representing no growth inhibition and complete growth inhibition. Following the incubation with the peptide, the resulting cell suspensions are diluted to reduce peptide activity and then grown overnight. After overnight growth, the optical density of each well is measured and compared to the positive and negative controls to calculate the resulting growth inhibition at each peptide concentration. The results using this assay are comparable to the results using the traditional method of plating the cultures on agar plates, but this protocol reduces plastic waste and the time spent on counting colonies. Although the applications of this protocol have focused on antifungal peptides, the method will also be applicable to testing other molecules with known or suspected antifungal activity.

An injectable multifunctional thermo-sensitive chitosan-based hydrogel for periodontitis therapy

Bacteria are recognized as the driving factors of periodontitis. However, excessive reactive oxygen species (ROS) can harm periodontal tissue while also causing an uncontrolled inflammatory response. Hence, eliminating excessive ROS and blocking ROS-induced abnormal inflammatory response by antioxidants are achieving remarkable results in periodontitis therapy. Moreover, influenced by the deep and irregular periodontal pockets, injectable thermo-sensitive chitosan-based hydrogels have attracted a lot of attention. This study aimed to formulate an antibacterial and antioxidant therapeutic regimen by incorporating antimicrobial peptides (Nal-P-113) and/or antioxidants (polydopamine nanoparticles, PDNPs) into chitosan-based hydrogels. The hydrogel was characterized in vitro and finally examined in rats using the experimental periodontitis model. The release kinetics showed that the hydrogel could stably release Nal-P-113 and PDNPs for up to 13 days. The scavenging activity of the hydrogel against DPPH was about 80 % and the antibacterial ratio against Streptococcus gordonii (S. gordonii), Fusobacterium nucleatum (F. nucleatum) and Porphyromonas gingivalis (P. gingivalis) was about 99 %. Importantly, it was examined that the hydrogel had the ability to prevent periodontal tissue damage. Thus, chitosan-based hydrogels may provide a basis for designing multifunctional local drug delivery biomaterials for the treatment of periodontitis.

Dental Materials for Oral Microbiota Dysbiosis: An Update

The balance or dysbiosis of the microbial community is a major factor in maintaining human health or causing disease. The unique microenvironment of the oral cavity provides optimal conditions for colonization and proliferation of microbiota, regulated through complex biological signaling systems and interactions with the host. Once the oral microbiota is out of balance, microorganisms produce virulence factors and metabolites, which will cause dental caries, periodontal disease, etc. Microbial metabolism and host immune response change the local microenvironment in turn and further promote the excessive proliferation of dominant microbes in dysbiosis. As the product of interdisciplinary development of materials science, stomatology, and biomedical engineering, oral biomaterials are playing an increasingly important role in regulating the balance of the oral microbiome and treating oral diseases. In this perspective, we discuss the mechanisms underlying the pathogenesis of oral microbiota dysbiosis and introduce emerging materials focusing on oral microbiota dysbiosis in recent years, including inorganic materials, organic materials, and some biomolecules. In addition, the limitations of the current study and possible research trends are also summarized. It is hoped that this review can provide reference and enlightenment for subsequent research on effective treatment strategies for diseases related to oral microbiota dysbiosis.

Efficiency of Antimicrobial Peptides Against Multidrug-Resistant Staphylococcal Pathogens

Antibiotics play a vital role in saving millions of lives from fatal infections; however, the inappropriate use of antibiotics has led to the emergence and propagation of drug resistance worldwide. Multidrug-resistant bacteria represent a significant challenge to treating infections due to the limitation of available antibiotics, necessitating the investigation of alternative treatments for combating these superbugs. Under such circumstances, antimicrobial peptides (AMPs), including human-derived AMPs and bacteria-derived AMPs (so-called bacteriocins), are considered potential therapeutic drugs owing to their high efficacy against infectious bacteria and the poor ability of these microorganisms to develop resistance to them. Several staphylococcal species including Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus haemolyticus, and Staphylococcus saprophyticus are commensal bacteria and known to cause many opportunistic infectious diseases. Methicillin-resistant Staphylococci, especially methicillin-resistant S. aureus (MRSA), are of particular concern among the critical multidrug-resistant infectious Gram-positive pathogens. Within the past decade, studies have reported promising AMPs that are effective against MRSA and other methicillin-resistant Staphylococci. This review discusses the sources and mechanisms of AMPs against staphylococcal species, as well as their potential to become chemotherapies for clinical infections caused by multidrug-resistant staphylococci.

Delineating the Mechanism of Action of a Protease Resistant and Salt Tolerant Synthetic Antimicrobial Peptide against Pseudomonas aeruginosa

Rapidly growing antimicrobial resistance (AMR) against antibiotics has propelled the development of synthetic antimicrobial peptides (AMPs) as potential antimicrobial agents. An antimicrobial peptide Nle-Dab-Trp-Nle-Dab-Dab-Nle-CONH₂ (P36; Nle = norleucine, Dab = diaminobutyric acid, Trp = tryptophan) potent against Pseudomonas aeruginosa (P. aeruginosa) has been developed in the present study. Rational design strategy adopted in this study led to the improvisation of the therapeutic qualities such as activity, salt tolerance, cytotoxicity, and protease resistance of the template peptide P4, which was earlier reported from our group. P36 exhibited salt tolerant antimicrobial potency against P. aeruginosa, along with very low cytotoxicity against mammalian cell lines. P36 was found to be nonhemolytic and resistant toward protease degradation which qualified it as a potent antimicrobial agent. We have investigated the mechanism of action of this molecule in detail using several experimental techniques (spectroscopic, biophysical, and microscopic) and molecular dynamics simulations. P36 was a membrane active AMP with membrane destabilization and deformation abilities, leading to leakage of the intracellular materials and causing eventual cell death. The interaction between P36 and the microbial membrane/membrane mimics was primarily driven by electrostatics. P36 was unstructured in water and upon binding to the microbial membrane mimic SDS, suggesting no influence of secondary structure on its antimicrobial potency. Positive charge, optimum hydrophobic-hydrophilic balance, and chain length remained the most important concerns to be addressed while designing small cationic antimicrobial peptides.

Strategy to Enhance Anticancer Activity and Induced Immunogenic Cell Death of Antimicrobial Peptides by Using Non-Nature Amino Acid Substitutions

There is an urgent and imminent need to develop new agents to fight against cancer. In addition to the antimicrobial and anti-inflammatory activities, many antimicrobial peptides can bind to and lyse cancer cells. P-113, a 12-amino acid clinically active histatin-rich peptide, was found to possess anti-Candida activities but showed poor anticancer activity. Herein, anticancer activities and induced immunogenic cancer cell death of phenylalanine-(Phe-P-113), β-naphthylalanine-(Nal-P-113), β-diphenylalanine-(Dip-P-113), and β-(4,4′-biphenyl)alanine-(Bip-P-113) substituted P-113 were studied. Among these peptides, Nal-P-113 demonstrated the best anticancer activity and caused cancer cells to release potent danger-associated molecular patterns (DAMPs), such as reactive oxygen species (ROS), cytochrome c, ATP, and high-mobility group box 1 (HMGB1). These results could help in developing antimicrobial peptides with better anticancer activity and induced immunogenic cell death in therapeutic applications.

Helical structure motifs made searchable for functional peptide design

The systematic design of functional peptides has technological and therapeutic applications. However, there is a need for pattern-based search engines that help locate desired functional motifs in primary sequences regardless of their evolutionary conservation. Existing databases such as The Protein Secondary Structure database (PSS) no longer serves the community, while the Dictionary of Protein Secondary Structure (DSSP) annotates the secondary structures when tertiary structures of proteins are provided. Here, we extract 1.7 million helices from the PDB and compile them into a database (Therapeutic Peptide Design database; TP-DB) that allows queries of compounded patterns to facilitate the identification of sequence motifs of helical structures. We show how TP-DB helps us identify a known purification-tag-specific antibody that can be repurposed into a diagnostic kit for Helicobacter pylori. We also show how the database can be used to design a new antimicrobial peptide that shows better Candida albicans clearance and lower hemolysis than its template homologs. Finally, we demonstrate how TP-DB can suggest point mutations in helical peptide blockers to prevent a targeted tumorigenic protein-protein interaction. TP-DB is made available at http://dyn.life.nthu.edu.tw/design/ .

Envisaging Antifungal Potential of Histatin 5: A Physiological Salivary Peptide

Fungi are reported to cause a range of superficial to invasive human infections. These often result in high morbidity and at times mortality. Conventional antifungal agents though effective invariably exhibit drug interactions, treatment-related toxicity, and fail to elicit significant effect, thus indicating a need to look for suitable alternatives. Fungi thrive in humid, nutrient-enriched areas. Such an environment is well-supported by the oral cavity. Despite this, there is a relatively low incidence of severe oral and periodontal fungal infections, attributed to the presence of antimicrobial peptides hosted by saliva, viz. histatin 5 (Hstn 5). It displays fungicidal activity against a variety of fungi including Candida albicans, Candida glabrata, Candida krusei, Cryptococcus neoformans, and unicellular yeast-like Saccharomyces cerevisiae. Candida albicans alone accounts for about 70% of all global fungal infections including periodontal disease. This review intends to discuss the scope of Hstn 5 as a novel recourse for the control of fungal infections.

Thrombin-Derived C-Terminal Peptide Reduces Candida-Induced Inflammation and Infection In Vitro and In Vivo

Infections due to the opportunistic fungus Candida have been on the rise in the last decades, especially in immunocompromised individuals and hospital settings. Unfortunately, the treatments available today are limited. Thrombin-derived C-terminal peptide (TCP-25) is an antimicrobial peptide (AMP) with antibacterial and immunomodulatory effects. In this work, we, for the first time, demonstrate the ability of TCP-25 ability to counteract Candidain vitro and in vivo. Using a combination of viable count assay (VCA), radial diffusion assay (RDA), and fluorescence and transmission electron microscopy analyses, TCP-25 was found to exert a direct fungicidal activity. An inhibitory activity of TCP-25 on NF-κB activation induced by both zymosan alone and heat-killed C. albicans was demonstrated in vitro using THP-1 cells, and in vivo using NF-κB reporter mice. Moreover, the immunomodulatory property of TCP-25 was further substantiated in vitro by analyzing cytokine responses in human blood stimulated with zymosan, and in vivo employing a zymosan-induced peritonitis model in C57BL/6 mice. The therapeutic potential of TCP-25 was demonstrated in mice infected with luminescent C. albicans. Finally, the binding between TCP-25 and zymosan was investigated using circular dichroism spectroscopy and intrinsic fluorescence analysis. Taken together, our results show that TCP-25 has a dual function by inhibiting Candida as well as the associated zymosan-induced inflammation. The latter function is accompanied by a change in secondary structure upon binding to zymosan. TCP-25, therefore, shows promise as a novel drug candidate against Candida infections.

Candida albicans Sfp1 Is Involved in the Cell Wall and Endoplasmic Reticulum Stress Responses Induced by Human Antimicrobial Peptide LL-37

Candida albicans is a commensal fungus of humans but can cause infections, particularly in immunocompromised individuals, ranging from superficial to life-threatening systemic infections. The cell wall is the outermost layer of C. albicans that interacts with the host environment. Moreover, antimicrobial peptides (AMPs) are important components in innate immunity and play crucial roles in host defense. Our previous studies showed that the human AMP LL-37 binds to the cell wall of C. albicans, alters the cell wall integrity (CWI) and affects cell adhesion of this pathogen. In this study, we aimed to further investigate the molecular mechanisms underlying the C. albicans response to LL-37. We found that LL-37 causes cell wall stress, activates unfolded protein response (UPR) signaling related to the endoplasmic reticulum (ER), induces ER-derived reactive oxygen species and affects protein secretion. Interestingly, the deletion of the SFP1 gene encoding a transcription factor reduced C. albicans susceptibility to LL-37, which is cell wall-associated. Moreover, in the presence of LL-37, deletion of SFP1 attenuated the UPR pathway, upregulated oxidative stress responsive (OSR) genes and affected bovine serum albumin (BSA) degradation by secreted proteases. Therefore, these findings suggested that Sfp1 positively regulates cell wall integrity and ER homeostasis upon treatment with LL-37 and shed light on pathogen-host interactions.

Histatin 5 Metallopeptides and Their Potential against Candida albicans Pathogenicity and Drug Resistance

Usually caused by Candida albicans, buccal candidiasis begins with the morphological transition between yeast and hyphal cells. Over time and without the correct treatment, it can be disseminated through the bloodstream becoming a systemic infection with high mortality rates. C. albicans already shows resistance against antifungals commonly used in treatments. Therefore, the search for new drugs capable of overcoming antifungal resistance is essential. Histatin 5 (Hst5) is an antimicrobial peptide of the Histatin family, that can be found naturally in human saliva. This peptide presents high antifungal activity against C. albicans. However, Hst5 action can be decreased for interaction with enzymes and metal ions present in the oral cavity. The current work aims to bring a brief review of relevant aspects of the pathogenesis and resistance mechanisms already reported for C. albicans. In addition, are also reported here the main immune responses of the human body and the most common antifungal drugs. Finally, the most important aspects regarding Histatin 5 and the benefits of its interaction with metals are highlighted. The intention of this review is to show the promising use of Hst5 metallopeptides in the development of effective drugs.

The multifaceted nature of antimicrobial peptides: current synthetic chemistry approaches and future directions

Bacterial infections caused by 'superbugs' are increasing globally, and conventional antibiotics are becoming less effective against these bacteria, such that we risk entering a post-antibiotic era. In recent years, antimicrobial peptides (AMPs) have gained significant attention for their clinical potential as a new class of antibiotics to combat antimicrobial resistance. In this review, we discuss several facets of AMPs including their diversity, physicochemical properties, mechanisms of action, and effects of environmental factors on these features. This review outlines various chemical synthetic strategies that have been applied to develop novel AMPs, including chemical modifications of existing peptides, semi-synthesis, and computer-aided design. We will also highlight novel AMP structures, including hybrids, antimicrobial dendrimers and polypeptides, peptidomimetics, and AMP-drug conjugates and consider recent developments in their chemical synthesis.

Antimicrobial Activity of the Peptide LfcinB15 against Candida albicans

Lactoferricin (Lfcin) is an amphipathic, cationic peptide derived from proteolytic cleavage of the N-lobe of lactoferrin (Lf). Lfcin and its derivatives possess broad-spectrum antibacterial and antifungal activities. However, unlike their antibacterial functions, the modes of action of Lfcin and its derivatives against pathogenic fungi are less well understood. In this study, the mechanisms of LfcinB15, a derivative of bovine Lfcin, against Candida albicans were, therefore, extensively investigated. LfcinB15 exhibited inhibitory activity against planktonic cells, biofilm cells, and clinical isolates of C. albicans and non-albicans Candida species. We further demonstrated that LfcinB15 is localized on the cell surface and vacuoles of C. albicans cells. Moreover, LfcinB15 uses several different methods to kill C. albicans, including disturbing the cell membrane, inducing reactive oxygen species (ROS) generation, and causing mitochondrial dysfunction. Finally, the Hog1 and Mkc1 mitogen-activated protein kinases were both activated in C. albicans cells in response to LfcinB15. These findings help us to obtain more insight into the complex mechanisms used by LfcinB15 and other Lfcin-derived peptides to fight fungal pathogens.

A proline rich protein from the gingival seal around teeth exhibits antimicrobial properties against Porphyromonas gingivalis

The gingival seal around teeth prevents bacteria from destroying the tooth-supporting tissues and disseminating throughout the body. Porphyromonas gingivalis, a major periodontopathogen, degrades components of the specialized extracellular matrix that mediates attachment of the gingiva to the tooth. Of these, secretory calcium-binding phosphoprotein proline-glutamine rich 1 (SCPPPQ1) protein has a distinctive resistance to degradation, suggesting that it may offer resistance to bacterial attack. In silico analysis of its amino acid sequence was used to explore its molecular characteristics and to predict its two- and three-dimensional structure. SCPPPQ1 exhibits similarities with both proline-rich and cationic antimicrobial proteins, suggesting a putative antimicrobial potential. A combination of imaging approaches showed that incubation with 20 μM of purified SCPPPQ1 decrease bacterial number (p < 0.01). Fluorescence intensity decreased by 70% following a 2 h incubation of Porphyromonas gingivalis with the protein. Electron microscopy analyses revealed that SCPPPQ1 induced bacterial membrane disruption and breaches. While SCPPPQ1 has no effect on mammalian cells, our results suggest that it is bactericidal to Porphyromonas gingivalis, and that this protein, normally present in the gingival seal, may be exploited to maintain a healthy seal and prevent systemic dissemination of bacteria.

Lactoferrin-Derived Peptide Lactofungin Is Potently Synergistic with Amphotericin B

Lactoferrin (LF) is an iron-binding glycoprotein with broad-spectrum antimicrobial activity. Previously, we discovered that LF synergistically enhanced the antifungal efficacy of amphotericin B (AMB) across a variety of yeast species and subsequently hypothesized that this synergy was enhanced by the presence of small peptides derived from the whole LF molecule. In this study, LF was digested with pepsin under a range of conditions. The resulting hydrolysates exhibited enhanced synergy with AMB compared to its synergy with undigested LF. Samples were analyzed using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry, and 14 peptides were identified. The sequences of these peptides were predicted by matching their molecular weights to those of a virtual digest with pepsin. The relative intensities of predicted peptides in each hydrolysate were compared with the activity of the hydrolysate, and the structural and physicochemical properties of the peptides were assessed. From this, a 30-residue peptide was selected for synthesis and dubbed lactofungin (LFG). Pure LFG was highly synergistic with AMB, outperforming native LF in all fungal species tested. With potential for further structural and chemical improvements, LFG is an excellent lead for development as an antifungal adjuvant.

Innate Inspiration: Antifungal Peptides and Other Immunotherapeutics From the Host Immune Response

The purpose of this review is to describe antifungal therapeutic candidates in preclinical and clinical development derived from, or directly influenced by, the immune system, with a specific focus on antimicrobial peptides (AMP). Although the focus of this review is AMP with direct antimicrobial effects on fungi, we will also discuss compounds with direct antifungal activity, including monoclonal antibodies (mAb), as well as immunomodulatory molecules that can enhance the immune response to fungal infection, including immunomodulatory AMP, vaccines, checkpoint inhibitors, interferon and colony stimulating factors as well as immune cell therapies. The focus of this manuscript will be a non-exhaustive review of antifungal compounds in preclinical and clinical development that are based on the principles of immunology and the authors acknowledge the incredible amount of in vitro and in vivo work that has been conducted to develop such therapeutic candidates.

Zinc Binding by Histatin 5 Promotes Fungicidal Membrane Disruption in C. albicans and C. glabrata

Histatin 5 (Hst 5) is an antimicrobial peptide produced in human saliva with antifungal activity for opportunistic pathogen Candida albicans. Hst 5 binds to multiple cations including dimerization-inducing zinc (Zn2+), although the function of this capability is incompletely understood. Hst 5 is taken up by C. albicans and acts on intracellular targets under metal-free conditions; however, Zn2+ is abundant in saliva and may functionally affect Hst 5. We hypothesized that Zn2+ binding would induce membrane-disrupting pores through dimerization. Through the use of Hst 5 and two derivatives, P113 (AA 4-15 of Hst 5) and Hst 5ΔMB (AA 1-3 and 15-19 mutated to Glu), we determined that Zn2+ significantly increases killing activity of Hst 5 and P113 for both C. albicans and Candida glabrata. Cell association assays determined that Zn2+ did not impact initial surface binding by the peptides, but Zn2+ did decrease cell association due to active peptide uptake. ATP efflux assays with Zn2+ suggested rapid membrane permeabilization by Hst 5 and P113 and that Zn2+ affinity correlates to higher membrane disruption ability. High-performance liquid chromatography (HPLC) showed that the higher relative Zn2+ affinity of Hst 5 likely promotes dimerization. Together, these results suggest peptide assembly into fungicidal pore structures in the presence of Zn2+, representing a novel mechanism of action that has exciting potential to expand the list of Hst 5-susceptible pathogens.

Online capillary electrophoresis - mass spectrometry analysis of histatin-5 and its degradation products

Histatin-5 (Hst-5) is a human salivary peptide with antibacterial and antifungal activities. Thorough characterization and reliable quantification of Hst-5 and its degradation products are essential for understanding the Hst-5 degradation pathway. Due to the highly basic and strong cationic nature of the Hst-5 peptide, the quantitative analysis of Hst-5 and its degradation forms by online mass spectrometry remains challenging. Here, we adopt a recently developed electrokinetically pumped sheath liquid capillary electrophoresis - mass spectrometry (CE-MS) coupling technology, and successfully apply it for the analysis of Hst-5 and its degradation products. Our CE-MS method is demonstrated to be robust and quantitative. This novel analytical platform is reproducible and free of sample carryover. The efficacy of this method is demonstrated with a kinetic study of Hst-5 degradation by Sap9, a secreted aspartic peptidase. Our work demonstrates the potential of online CE-MS as a powerful approach for characterizing highly basic peptides.

Antimicrobial Susceptibility Testing of Antimicrobial Peptides to Better Predict Efficacy

During the development of antimicrobial peptides (AMP) as potential therapeutics, antimicrobial susceptibility testing (AST) stands as an essential part of the process in identification and optimisation of candidate AMP. Standard methods for AST, developed almost 60 years ago for testing conventional antibiotics, are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to AMP. Without careful consideration of the parameters comprising AST there is a risk of failing to identify novel antimicrobials at a time when antimicrobial resistance (AMR) is leading the planet toward a post-antibiotic era. More physiologically/clinically relevant AST will allow better determination of the preclinical activity of drug candidates and allow the identification of lead compounds. An important consideration is the efficacy of AMP in biological matrices replicating sites of infection, e.g., blood/plasma/serum, lung bronchiolar lavage fluid/sputum, urine, biofilms, etc., as this will likely be more predictive of clinical efficacy. Additionally, specific AST for different target microorganisms may help to better predict efficacy of AMP in specific infections. In this manuscript, we describe what we believe are the key considerations for AST of AMP and hope that this information can better guide the preclinical development of AMP toward becoming a new generation of urgently needed antimicrobials.

Antifungal and Antibiofilm Activities and the Mechanism of Action of Repeating Lysine-Tryptophan Peptides against Candida albicans

The rapid increase in the emergence of antifungal-resistant Candida albicans strains is becoming a serious health concern. Because antimicrobial peptides (AMPs) may provide a potential alternative to conventional antifungal agents, we have synthesized a series of peptides with a varying number of lysine and tryptophan repeats (KWn-NH2). The antifungal activity of these peptides increased with peptide length, but only the longest KW5 peptide displayed cytotoxicity towards a human keratinocyte cell line. The KW4 and KW5 peptides exhibited strong antifungal activity against C. albicans, even under conditions of high-salt and acidic pH, or the addition of fungal cell wall components. Moreover, KW4 inhibited biofilm formation by a fluconazole-resistant C. albicans strain. Circular dichroism and fluorescence spectroscopy indicated that fungal liposomes could interact with the longer peptides but that they did not release the fluorescent dye calcein. Subsequently, fluorescence assays with different dyes revealed that KW4 did not disrupt the membrane integrity of intact fungal cells. Scanning electron microscopy showed no changes in fungal morphology, while laser-scanning confocal microscopy indicated that KW4 can localize into the cytosol of C. albicans. Gel retardation assays revealed that KW4 can bind to fungal RNA as a potential intracellular target. Taken together, our data indicate that KW4 can inhibit cellular functions by binding to RNA and DNA after it has been translocated into the cell, resulting in the eradication of C. albicans.

The Interactions between the Antimicrobial Peptide P-113 and Living Candida albicans Cells Shed Light on Mechanisms of Antifungal Activity and Resistance

In the absence of proper immunity, such as in the case of acquired immune deficiency syndrome (AIDS) patients, Candida albicans, the most common human fungal pathogen, may cause mucosal and even life-threatening systemic infections. P-113 (AKRHHGYKRKFH), an antimicrobial peptide (AMP) derived from the human salivary protein histatin 5, shows good safety and efficacy profiles in gingivitis and human immunodeficiency virus (HIV) patients with oral candidiasis. However, little is known about how P-113 interacts with Candida albicans or its degradation by Candida-secreted proteases that contribute to the fungi’s resistance. Here, we use solution nuclear magnetic resonance (NMR) methods to elucidate the molecular mechanism of interactions between P-113 and living Candida albicans cells. Furthermore, we found that proteolytic cleavage of the C-terminus prevents the entry of P-113 into cells and that increasing the hydrophobicity of the peptide can significantly increase its antifungal activity. These results could help in the design of novel antimicrobial peptides that have enhanced stability in vivo and that can have potential therapeutic applications.

Antifungal Peptides as Therapeutic Agents

Fungi have been used since ancient times in food and beverage-making processes and, more recently, have been harnessed for the production of antibiotics and in processes of relevance to the bioeconomy. Moreover, they are starting to gain attention as a key component of the human microbiome. However, fungi are also responsible for human infections. The incidence of community-acquired and nosocomial fungal infections has increased considerably in recent decades. Antibiotic resistance development, the increasing number of immunodeficiency- and/or immunosuppression-related diseases and limited therapeutic options available are triggering the search for novel alternatives. These new antifungals should be less toxic for the host, with targeted or broader antimicrobial spectra (for diseases of known and unknown etiology, respectively) and modes of actions that limit the potential for the emergence of resistance among pathogenic fungi. Given these criteria, antimicrobial peptides with antifungal properties, i.e., antifungal peptides (AFPs), have emerged as powerful candidates due to their efficacy and high selectivity. In this review, we provide an overview of the bioactivity and classification of AFPs (natural and synthetic) as well as their mode of action and advantages over current antifungal drugs. Additionally, natural, heterologous and synthetic production of AFPs with a view to greater levels of exploitation is discussed. Finally, we evaluate the current and potential applications of these peptides, along with the future challenges relating to antifungal treatments.

The interaction Between Carbohydrates and the Antimicrobial Peptide P-113Tri is Involved in the Killing of Candida albicans

The emergence of drug resistance to Candida albicans is problematic in the clinical setting. Therefore, developing new antifungal drugs is in high demand. Our previous work indicated that the antimicrobial peptide P-113Tri exhibited higher antifungal activity against planktonic cells, biofilm cells, and clinical isolates of Candida species compared to its parental peptide P-113. In this study, we further investigated the difference between these two peptides in their mechanisms against C. albicans. Microscopic examination showed that P-113 rapidly gained access to C. albicans cells. However, most of the P-113Tri remained on the cell surface. Moreover, using a range of cell wall-defective mutants and competition assays, the results indicated that phosphomannan and N-linked mannan in the cell wall are important for peptide binding to C. albicans cells. Furthermore, the addition of exogenous phosphosugars reduced the efficacy of the peptide, suggesting that negatively charged phosphosugars also contributed to the peptide binding to the cell wall polysaccharides. Finally, using a glycan array, P-113Tri, but not P-113, can bind to other glycans commonly present on other microbial and mammalian cells. Together, these results suggest that P-113 and P-113Tri have fundamental differences in their interaction with C. albicans and candidacidal activities.

Effects of histatin 5 modifications on antifungal activity and kinetics of proteolysis

Histatin 5 (Hst-5) is an antimicrobial peptide with strong antifungal activity against Candida albicans, an opportunistic pathogen that is a common cause of oral thrush. The peptide is natively secreted by human salivary glands and shows promise as an alternative therapeutic against infections caused by C. albicans. However, Hst-5 can be cleaved and inactivated by a family of secreted aspartic proteases (Saps) produced by C. albicans. Single-residue substitutions can significantly affect the proteolytic resistance of Hst-5 to Saps and its antifungal activity; the K17R substitution increases resistance to proteolysis, while the K11R substitution enhances antifungal activity. In this work, we showed that the positive effects of these two single-residue modifications can be combined in a single peptide, K11R-K17R, with improved proteolytic resistance and antifungal activity. We also investigated the effect of additional single-residue substitutions, with a focus on the effect of addition or removal of negatively charged residues, and found Sap-dependent effects on degradation. Both single- and double-substitutions affected the kinetics of proteolytic degradation of the intact peptide and of the fragments formed during degradation. Our results demonstrate the importance of considering proteolytic stability and not just antimicrobial activity when designing peptides for potential therapeutic applications.

Direct assessment of the antioxidant property of salivary histatin

Histatin, a salivary protein, affects oral homeostasis through preservation of tooth integrity and protection against caries and fungal infections. However, the effects of histatin in the generation of oxidative stress induced by reactive oxygen species and in the oral cavity remain unclear. In this study, the effects of histatin on direct reactive oxygen species scavenging activity were examined using electron spin resonance. We demonstrated, for the first time, that histatin exhibits antioxidant activity against hydroxyl radicals generated by Fenton's reaction by metal chelation or binding. The direct antioxidant effects of histatin, along with its antimicrobial activity, may be important in the oral protection of salivary proteins.

Novel mitochondrial complex I-inhibiting peptides restrain NADH dehydrogenase activity

The emergence of drug-resistant fungal pathogens is becoming increasingly serious due to overuse of antifungals. Antimicrobial peptides have potent activity against a broad spectrum of pathogens, including fungi, and are considered a potential new class of antifungals. In this study, we examined the activities of the newly designed peptides P-113Du and P-113Tri, together with their parental peptide P-113, against the human fungal pathogen Candida albicans. The results showed that these peptides inhibit mitochondrial complex I, specifically NADH dehydrogenase, of the electron transport chain. Moreover, P-113Du and P-113Tri also block alternative NADH dehydrogenases. Currently, most inhibitors of the mitochondrial complex I are small molecules or artificially-designed antibodies. Here, we demonstrated novel functions of antimicrobial peptides in inhibiting the mitochondrial complex I of C. albicans, providing insight in the development of new antifungal agents.

Peptide and non-peptide mimetics as potential therapeutics targeting oral bacteria and oral biofilms

The development of the oral biofilm requires a complex series of interactions between host tissues and the colonizing bacteria as well as numerous interspecies interactions between the organisms themselves. Disruption of normal host-microbe homoeostasis in the oral cavity can lead to a dysbiotic microbial community that contributes to caries or periodontal disease. A variety of approaches have been pursued to develop novel potential therapeutics that are active against the oral biofilm and/or target specific oral bacteria. The structure and function of naturally occurring antimicrobial peptides from oral tissues and secretions as well as external sources such as frog skin secretions have been exploited to develop numerous peptide mimetics and small molecule peptidomimetics that show improved antimicrobial activity, increased stability and other desirable characteristics relative to the parent peptides. In addition, a rational and minimalist approach has been developed to design small artificial peptides with amphipathic α-helical properties that exhibit potent antibacterial activity. Furthermore, with an increased understanding of the molecular mechanisms of beneficial and/or antagonistic interspecies interactions that contribute to the formation of the oral biofilm, new potential targets for therapeutic intervention have been identified and both peptide-based and small molecule mimetics have been developed that target these key components. Many of these mimetics have shown promising results in in vitro and pre-clinical testing and the initial clinical evaluation of several novel compounds has demonstrated their utility in humans.

Human salivary proteins and their peptidomimetics: Values of function, early diagnosis, and therapeutic potential in combating dental caries

Saliva contains a large number of proteins that play various crucial roles to maintain the oral health and tooth integrity. This oral fluid is proposed to be one of the most important host factors, serving as a special medium for monitoring aspects of microorganisms, diet and host susceptibility involved in the caries process. Extensive salivary proteomic and peptidomic studies have resulted in considerable advances in the field of biomarkers discovery for dental caries. These salivary biomarkers may be exploited for the prediction, diagnosis, prognosis and treatment of dental caries, many of which could also provide the potential templates for bioactive peptides used for the biomimetic management of dental caries, rather than repairing caries lesions with artificial materials. A comprehensive understanding of the biological function of salivary proteins as well as their derived biomimetic peptides with promising potential against dental caries has been long awaited. This review overviewed a collection of current literature and addressed the majority of different functions of salivary proteins and peptides with their potential as functional biomarkers for caries risk assessment and clinical prospects for the anti-caries application.

Dependence on size and shape of non-nature amino acids in the enhancement of lipopolysaccharide (LPS) neutralizing activities of antimicrobial peptides

HYPOTHESIS

Release of lipopolysaccharides (LPS) from bacteria into bloodstream may cause serious unwanted stimulation of the host immune system. P-113 is a clinically active histidine-rich antimicrobial peptide. Nal-P-113, a β-naphthylalanine-substituted P-113, is salt-resistant but has limited LPS neutralizing activity. We suspected the size and shape of the non-natural bulky amino acid may affect its LPS neutralizing activity. Herein, antimicrobial, LPS neutralizing, and antiproteolytic effects of phenylalanine- (Phe-P-113), β-naphthylalanine- (Nal-P-113), β-diphenylalanine- (Dip-P-113), and β-(4,4'-biphenyl)alanine- (Bip-P-113) substituted P-113 were studied.

EXPERIMENTS

Structure-activity relationships of P-113, Phe-P-113, Nal-P-113, Dip-P-113, and Bip-P-113 were evaluated using antimicrobial activity assays, serum proteolytic assays, peptide-induced permeabilization of large unilamellar vesicles, zeta potential measurements, dynamic light scattering measurement of LPS aggregation, and Limulus amebocyte lysate assays for measuring LPS neutralization. In vitro and in vivo LPS neutralizing activities were further confirmed by LPS-induced inflammation inhibition in an endotoxemia mouse model.

FINDINGS

Bip-P-113 and Dip-P-113 had the longest and widest non-nature amino acids, respectively. Bip-P-113 enhanced salt resistance, serum proteolytic stability, peptide-induced permeabilization, zeta potential measurements, LPS aggregation, and in vitro and in vivo LPS neutralizing activities. These results could help design novel antimicrobial peptides that have enhanced stability in vivo and that can have potential therapeutic applications.

Expression in Escherichia coli of novel recombinant hybrid antimicrobial peptide AL32-P113 with enhanced antimicrobial activity in vitro

Antibiotic-resistant pathogens have become a major public health problem worldwide. New discoveries and strategies as regards antibiotic drug development are urgently in need for curing infected patients. Antimicrobial peptides (AMPs) are short cationic peptides that play important roles in innate immune system with a broad spectrum of antimicrobial activity. Recently, hybrid AMPs have been reported to increase antimicrobial activity, stability, and in vivo half-life. In the present study, a gene encoding for AL32-P113 hybrid peptide consisting of two truncated active forms of human LL-37 and histatin-5 (Hst-5) was commercially constructed, cloned into pTXB-1 commercial plasmid, and expressed in E. coli BL21 (DE3). To increase the yield of target protein expression, IPTG concentration, time and temperature were optimized. The results indicate that AL32-P113-intein fusion protein with 33.7 kDa was expressed mostly in inclusion form and estimated to be 20% of the total protein. After chitin affinity purification, 5.7-kDa of AL32-P113 peptide was separated with an average concentration of 12.1 mg per litre of bacterial culture and over 86% purity. The minimum inhibitory concentration (MIC) was evaluated for antimicrobial activity determination of recombinant AL32-P113 compared to synthetic peptides, LL-37, Hst-5, and L31-P113. The results implied that both hybrid peptides exhibited potent antimicrobial activity against gram-negative bacteria and yeast cells whereas the L31-P113 peptide possessed approximately four times greater antimicrobial activity in gram-positive bacteria than parent LL-37. An increasing of undesired hemolysis of these hybrid peptides toward human red cells was also observed when red blood cell hemolytic assay was performed. Several factors including charge and secondary structure predicted by public software were utilized for explanation of the antimicrobial potency of both hybrid peptides. This study proved that hybrid peptides show broader and more potent antimicrobial ability against pathogens and they could be applied as a therapeutic approach for topical treatment of microbial infection in the future.

High Level Expression and Purification of the Clinically Active Antimicrobial Peptide P-113 in Escherichia coli

P-113, which was originally derived from the human saliva protein histatin 5, is a histidine-rich antimicrobial peptide with the sequence AKRHHGYKRKFH. P-113 is currently undergoing phase II clinical trial as a pharmaceutical agent to fight against fungal infections in HIV patients with oral candidiasis. Previously, we developed a new procedure for the high-yield expression and purification of hG31P, an analogue and antagonist of human CXCL8. Moreover, we have successfully removed lipopolysaccharide (LPS, endotoxin) associated with hG31P in the expression with Escherichia coli. In this paper, we have used hG31P as a novel fusion protein for the expression and purification of P-113. The purity of the expressed P-113 is more than 95% and the yield is 4 mg P-113 per liter of E. coli cell culture in Luria-Bertani (LB) medium. The antimicrobial activity of the purified P-113 was tested. Furthermore, we used circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy to study the structural properties of P-113. Our results indicate that using hG31P as a fusion protein to obtain large quantities of P-113 is feasible and is easy to scale up for commercial production. An effective way of producing enough P-113 for future clinical studies is evident in this study.

Design, Synthesis and Evaluation of Branched RRWQWR-Based Peptides as Antibacterial Agents Against Clinically Relevant Gram-Positive and Gram-Negative Pathogens

Multidrug resistance of pathogenic bacteria has become a public health crisis that requires the urgent design of new antibacterial drugs such as antimicrobial peptides (AMPs). Seeking to obtain new, lactoferricin B (LfcinB)-based synthetic peptides as viable early-stage candidates for future development as AMPs against clinically relevant bacteria, we designed, synthesized and screened three new cationic peptides derived from bovine LfcinB. These peptides contain at least one RRWQWR motif and differ by the copy number (monomeric, dimeric or tetrameric) and structure (linear or branched) of this motif. They comprise a linear palindromic peptide (RWQWRWQWR), a dimeric peptide (RRWQWR)₂KAhx and a tetrameric peptide (RRWQWR)₄K₂Ahx₂C₂. They were screened for antibacterial activity against Enterococcus faecalis (ATCC 29212 and ATCC 51575 strains), Pseudomonas aeruginosa (ATCC 10145 and ATCC 27853 strains) and clinical isolates of two Gram-positive bacteria (Enterococcus faecium and Staphylococcus aureus) and two Gram-negative bacteria (Klebsiella pneumoniae and Pseudomonas aeruginosa). All three peptides exhibited greater activity than did the reference peptide, LfcinB (17-31), which contains a single linear RRWQWR motif. Against the ATCC reference strains, the three new peptides exhibited minimum inhibitory concentration (MIC₅₀) values of 3.1-198.0 μM and minimum bactericidal concentration (MBC) values of 25-200 μM, and against the clinical isolates, MIC₅₀ values of 1.6-75.0 μM and MBC values of 12.5-100 μM. However, the tetrameric peptide was also found to be strongly hemolytic (49.1% at 100 μM). Scanning Electron Microscopy (SEM) demonstrated that in the dimeric and tetrameric peptides, the RRWQWR motif is exposed to the pathogen surface. Our results may inform the design of new, RRWQWR-based AMPs.

Engineering improved variants of the antifungal peptide histatin 5 with reduced susceptibility to Candida albicans secreted aspartic proteases and enhanced antimicrobial potency

Candida albicans is an opportunistic fungal pathogen and a commensal organism that commonly colonizes mucosal surfaces, including those inside the human mouth. To help control C. albicans, human saliva contains the antifungal peptide histatin 5 (Hst-5), which has strong antifungal activity against C. albicans. However, the pathogen produces secreted aspartic proteases (Saps) that cleave Hst-5 at lysine residues and eliminate its antifungal properties. We designed variants of Hst-5 with its lysine residues substituted with arginine or leucine to evaluate the effect on proteolysis by Saps. We found site-, residue-, and Sap-dependent effects from single amino acid substitutions. The K17R and K17L modifications led to dramatic results, with over 77% and 100% intact peptide remaining after incubation with Sap9 and Sap2, respectively, compared to 47% and 61% of Hst-5. This decrease in proteolysis was accompanied by a reduction in cleavage on the C-terminal side of K17, suggesting the Saps prefer lysine at K17 for cleavage. Incubation with C. albicans cells and culture supernatant corroborated the results with purified Saps and highlighted their biological relevance. The modifications to Hst-5 do not diminish the antifungal activity of Hst-5, and, in fact, the K17R, K17L, and K11R peptides retained significantly more antifungal activity after treatment with Saps than Hst-5. Our results indicate that single amino acid modifications drastically impact both proteolysis at the modification sites and the overall level of proteolysis of the peptide, demonstrating the potential of designing peptides for resistance to proteolysis as a means for improving therapeutic efficacy.

Structural Characterization of Histatin 5-Spermidine Conjugates: A Combined Experimental and Theoretical Study

Histatin 5 (Hst5) is a naturally occurring antimicrobial peptide that acts as the first line of defense against oral candidiasis. It has been shown that conjugation of the active Hst5 fragment, Hst5_4-15, and the polyamine spermidine (Spd) improves the candidacidal effect. Knowledge about the structure of these conjugates is, however, very limited. Thus, the aim of this study was to characterize the structural properties of the Hst5_4-15-Spd conjugates by performing atomistic molecular dynamics simulations in combination with small-angle X-ray scattering. It was shown that the Hst5_4-15-Spd conjugates adopt extended and slightly rigid random coil conformations without any secondary structure in aqueous solution. It is hypothesized that the increased fungal killing potential of Hst5_4-15-Spd, in comparison with the Spd-Hst5_4-15 conjugate, is due to the more extended conformations of the former, which cause the bonded Spd molecule to be more accessible for recognition by polyamine transporters in the cell.

Role of β-naphthylalanine end-tags in the enhancement of antiendotoxin activities: Solution structure of the antimicrobial peptide S1-Nal-Nal in complex with lipopolysaccharide

Lipopolysaccharide (LPS, endotoxin) is the major component of Gram-negative bacterial outer surface membrane. LPS released from bacteria into bloodstream during infection may cause serious unwanted stimulation of host's immune system and lead to septic shock of the patient. Recently, we have developed a strategy to increase salt resistance and LPS neutralization of short antimicrobial peptides by adding β-naphthylalanine end-tags to their termini. Herein, correlations between membrane immersion depth, orientation, and antiendotoxin activities of the antimicrobial peptides S1 and S1-Nal-Nal have been investigated via solution structure, paramagnetic resonance enhancement, and saturation transfer difference NMR studies. Unlike the parent peptide S1, S1-Nal-Nal rotated its two terminal β-naphthylalanine residues into the hydrophobic lipid A motif of LPS micelles. The LPS-induced inflammation may then be prohibited by the blocked lipid A motif.

Antimicrobial Peptides: Mechanisms of Action and Resistance

More than 40 antimicrobial peptides and proteins (AMPs) are expressed in the oral cavity. These AMPs have been organized into 6 functional groups, 1 of which, cationic AMPs, has received extensive attention in recent years for their promise as potential antibiotics. The goal of this review is to describe recent advances in our understanding of the diverse mechanisms of action of cationic AMPs and the bacterial resistance against these peptides. The recently developed peptide GL13K is used as an example to illustrate many of the discussed concepts. Cationic AMPs typically exhibit an amphipathic conformation, which allows increased interaction with negatively charged bacterial membranes. Peptides undergo changes in conformation and aggregation state in the presence of membranes; conversely, lipid conformation and packing can adapt to the presence of peptides. As a consequence, a single peptide can act through several mechanisms depending on the peptide's structure, the peptide:lipid ratio, and the properties of the lipid membrane. Accumulating evidence shows that in addition to acting at the cell membrane, AMPs may act on the cell wall, inhibit protein folding or enzyme activity, or act intracellularly. Therefore, once a peptide has reached the cell wall, cell membrane, or its internal target, the difference in mechanism of action on gram-negative and gram-positive bacteria may be less pronounced than formerly assumed. While AMPs should not cause widespread resistance due to their preferential attack on the cell membrane, in cases where specific protein targets are involved, the possibility exists for genetic mutations and bacterial resistance. Indeed, the potential clinical use of AMPs has raised the concern that resistance to therapeutic AMPs could be associated with resistance to endogenous host-defense peptides. Current evidence suggests that this is a rare event that can be overcome by subtle structural modifications of an AMP.