Structural characterization and antimicrobial activity of the Zn(II) complex with P113 (demegen), a derivative of histatin 5

Antimicrobial Peptides: The Game-Changer in the Epic Battle Against Multidrug-Resistant Bacteria

The rapid progress of antibiotic resistance among bacteria has prompted serious medical concerns regarding how to manage multidrug-resistant (MDR) bacterial infections. One emerging strategy to combat antibiotic resistance is the use of antimicrobial peptides (AMPs), which are amino acid chains that act as broad-spectrum antimicrobial molecules and are essential parts of the innate immune system in mammals, fungi, and plants. AMPs have unique antibacterial mechanisms that offer benefits over conventional antibiotics in combating drug-resistant bacterial infections. Currently, scientists have conducted multiple studies on AMPs for combating drug-resistant bacterial infections and found that AMPs are a promising alternative to conventional antibiotics. On the other hand, bacteria can develop several tactics to resist and bypass the effect of AMPs. Therefore, it is like a battle between the bacterial community and the AMPs, but who will win? This review provides thorough insights into the development of antibiotic resistance as well as detailed information about AMPs in terms of their history and classification. Furthermore, it addresses the unique antibacterial mechanisms of action of AMPs, how bacteria resist these mechanisms, and how to ensure AMPs win this battle. Finally, it provides updated information about FDA-approved AMPs and those that were still in clinical trials. This review provides vital information for researchers for the development and therapeutic application of novel AMPs for drug-resistant bacterial infections.

Zinc Binding Inhibits Cellular Uptake and Antifungal Activity of Histatin-5 in Candida albicans

Histatin-5 (Hist-5) is a polycationic, histidine-rich antimicrobial peptide with potent antifungal activity against the opportunistic fungal pathogen Candida albicans. Hist-5 can bind metals in vitro, and metals have been shown to alter the fungicidal activity of the peptide. Previous reports on the effect of Zn²⁺ on Hist-5 activity have been varied and seemingly contradictory. Here, we present data elucidating the dynamic role Zn²⁺ plays as an inhibitory switch to regulate Hist-5 fungicidal activity. A novel fluorescently labeled Hist-5 peptide (Hist-5*) was developed to visualize changes in internalization and localization of the peptide as a function of metal availability in the growth medium. Hist-5* was verified for use as a model peptide and retained antifungal activity and mode of action similar to native Hist-5. Cellular growth assays showed that Zn²⁺ had a concentration-dependent inhibitory effect on Hist-5 antifungal activity. Imaging by confocal microscopy revealed that equimolar concentrations of Zn²⁺ kept the peptide localized along the cell periphery rather than internalizing, thus preventing cytotoxicity and membrane disruption. However, the Zn-induced decrease in Hist-5 activity and uptake was rescued by decreasing the Zn²⁺ availability upon addition of a metal chelator EDTA or S100A12, a Zn-binding protein involved in the innate immune response. These results lead us to suggest a model wherein commensal C. albicans may exist in harmony with Hist-5 at concentrations of Zn²⁺ that inhibit peptide internalization and antifungal activity. Activation of host immune processes that initiate Zn-sequestering mechanisms of nutritional immunity could trigger Hist-5 internalization and cell killing.

Mycobacterial Membranes as Actionable Targets for Lipid-Centric Therapy in Tuberculosis

Infectious diseases remain significant health concerns worldwide, and resistance is particularly common in patients with tuberculosis caused by Mycobacterium tuberculosis. The development of anti-infectives with novel modes of action may help overcome resistance. In this regard, membrane-active agents, which modulate membrane components essential for the survival of pathogens, present attractive antimicrobial agents. Key advantages of membrane-active compounds include their ability to target slow-growing or dormant bacteria and their favorable pharmacokinetics. Here, we comprehensively review recent advances in the development of membrane-active chemotypes that target mycobacterial membranes and discuss clinically relevant membrane-active antibacterial agents that have shown promise in counteracting bacterial infections. We discuss the relationship between the membrane properties and the synthetic requirements within the chemical scaffold, as well as the limitations of current membrane-active chemotypes. This review will lay the chemical groundwork for the development of membrane-active antituberculosis agents and will foster the discovery of more effective antitubercular agents.

Metal Binding Antimicrobial Peptides in Nanoparticle Bio-functionalization: New Heights in Drug Delivery and Therapy

Peptides are considered very important due to the diversity expressed through their amino acid sequence, structure variation, large spectrum, and their essential role in biological systems. Antimicrobial peptides (AMPs) emerged as a potent tool in therapy owing to their antimicrobial properties but also their ability to trespass the membranes, specificity, and low toxicity. They comprise a variety of peptides from which specific amino acid-rich peptides are of interest to the current review due to their features in metal interaction and cell penetration. Histidine-rich peptides such as Histatins belong to the metal binding salivary residing peptides with efficient antibacterial, antifungal, and wound-healing activities. Furthermore, their ability to activate in acidic environment attracted the attention to their potential in therapy. The current review covers the current knowledge about AMPs and critically assess the potential of associating with metal ions both structurally and functionally. This review provides interesting hints for the advantages provided by AMPs and metal ions in biomedicine, making use of their direct properties in brain diseases therapy or in the creation of new bio-functionalized nanoparticles for cancer diagnosis and treatment.

Rediscovery of antimicrobial peptides as therapeutic agents

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

A secreted fungal histidine- and alanine-rich protein regulates metal ion homeostasis and oxidative stress

Like pathogens, beneficial endophytic fungi secrete effector proteins to promote plant colonization, for example, through perturbation of host immunity. The genome of the root endophyte Serendipita indica encodes a novel family of highly similar, small alanine- and histidine-rich proteins, whose functions remain unknown. Members of this protein family carry an N-terminal signal peptide and a conserved C-terminal DELD motif. Here we report on the functional characterization of the plant-responsive DELD family protein Dld1 using a combination of structural, biochemical, biophysical and cytological analyses. The crystal structure of Dld1 shows an unusual, monomeric histidine zipper consisting of two antiparallel coiled-coil helices. Similar to other histidine-rich proteins, Dld1 displays varying affinity to different transition metal ions and undergoes metal ion- and pH-dependent unfolding. Transient expression of mCherry-tagged Dld1 in barley leaf and root tissue suggests that Dld1 localizes to the plant cell wall and accumulates at cell wall appositions during fungal penetration. Moreover, recombinant Dld1 enhances barley root colonization by S. indica, and inhibits H₂ O₂ -mediated radical polymerization of 3,3'-diaminobenzidine. Our data suggest that Dld1 has the potential to enhance micronutrient accessibility for the fungus and to interfere with oxidative stress and reactive oxygen species homeostasis to facilitate host colonization.

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.

Constructing an Antibiofouling and Mineralizing Bioactive Tooth Surface to Protect against Decay and Promote Self-Healing

Numerous methods have been investigated to manage dental caries, one of the top three diseases threatening human health as reported by the World Health Organization. An innovative strategy was proposed to prevent dental caries and achieve self-healing of the decayed tooth, and a novel bioactive peptide was designed and synthesized to construct an antibiofouling and mineralizing dual-bioactive tooth surface. Compared to its original endogenous peptide, the synthesized bioactive peptide showed statistically significantly higher binding affinity to the tooth surface, stronger suppression of demineralization, and a certain promotion of tooth remineralization. The abilities of the peptide to inhibit Streptococcus mutans (S. mutans) biofilm formation and S. mutans adhesion on the tooth surface were not affected after synthesis. Biocompatibility tests revealed the safety of the synthesized bioactive peptide. Interaction mechanisms between the synthesized bioactive peptide and tooth surface were also explained by molecular dynamic simulation analysis. In summary, the synthesized bioactive peptide could be applied safely to prevent dental caries and effectively induce in situ self-healing remineralization for treatment of the decayed tooth.

Antimicrobial peptide–metal ion interactions – a potential way of activity enhancement

We discuss the potential correlation between the antimicrobial peptide–metal binding mode, structure, thermodynamics and mode of action.

Specific Histidine Residues Confer Histatin Peptides with Copper-Dependent Activity against Candida albicans

The histidine-rich salivary peptides of the histatin family are known to bind copper (Cu) and other metal ions in vitro; however, the details of these interactions are poorly understood, and their implications for in vivo antifungal activity have not been established. Here, we show that the availability of Cu during exposure of Candida albicans to histatin-5 (Hist-5) modulates its antifungal activity. Antifungal susceptibility testing revealed that co-treatment of Hist-5 with Cu improved the EC₅₀ from ∼5 to ∼1 μM, whereas co-treatment with a high-affinity Cu-specific chelator abrogated antifungal activity. Spectrophotometric titrations revealed two previously unrecognized Cu(I)-binding sites with apparent K_d values at pH 7.4, ∼20 nM, and confirmed a high-affinity Cu(II)-binding site at the Hist-5 N-terminus with an apparent K_d of ∼8 pM. Evaluation of a series of His-to-Ala full-length and truncated Hist-5 peptides identified adjacent His residues (bis-His) as critical anchors for Cu(I) binding, with the presence of a third ligand revealed by X-ray absorption spectroscopy. On their own, the truncated peptides were ineffective at inhibiting the growth of C. albicans, but treatment with supplemental Cu resulted in EC₅₀ values down to ∼5 μM, approaching that of full-length Hist-5. The efficacy of the peptides depended on an intact bis-His site and correlated with Cu(I) affinity. Together, these results establish new structure-function relationships linking specific histidine residues with Cu binding affinity and antifungal activity and provide further evidence of the involvement of metals in modulating the biological activity of these antifungal peptides.

Antibacterial Peptides: Opportunities for the Prevention and Treatment of Dental Caries

Dental caries is a multifactorial disease that is a growing and costly global health concern. The onset of disease is a consequence of an ecological imbalance within the dental plaque biofilm that favors specific acidogenic and aciduric caries pathogens, namely Streptococcus mutans and Streptococcus sobrinus. It is now recognized by the scientific and medical community that it is neither possible nor desirable to totally eliminate dental plaque. Conversely, the chemical biocides most commonly used for caries prevention and treatment indiscriminately attack all plaque microorganisms. These treatments also suffer from other drawbacks such as bad taste, irritability, and staining. Furthermore, the public demand for safe and natural personal hygiene products continues to rise. Therefore, there are opportunities that exist to develop new strategies for the treatment of this disease. As an alternative to conventional antibiotics, antibacterial peptides have been explored greatly over the last three decades for many different therapeutic uses. There are currently tens of hundreds of antibacterial peptides characterized across the evolutionary spectrum, and among these, many demonstrate physical and/or biological properties that may be suitable for a more targeted approach to the selective control or elimination of putative caries pathogens. Additionally, many peptides, such as nisin, are odorless, colorless, and tasteless and do not cause irritation or staining. This review focuses on antibacterial peptides for their potential role in the treatment and prevention of dental caries and suggests candidates that need to be explored further. Practical considerations for the development of antibacterial peptides as oral treatments are also discussed.

Fabrication of innovative ZnO nanoflowers showing drastic biological activity

The present article highlights a facile approach towards the synthesis of ZnO nanoflowers using designed single molecular precursors (1 and 2) at room temperature. The relative biological activities of 1, 2 and ZnO nanoflowers have also been demonstrated.

Varying structural motifs in the salen based metal complexes of Co(ii), Ni(ii) and Cu(ii): synthesis, crystal structures, molecular dynamics and biological activities

Four new metal complexes which demonstrates varying structural motifs from monomeric to dimeric to tetrameric complexes by slightly altering the reaction conditions and their biological applications.

Iron binding modulates candidacidal properties of salivary histatin 5

Salivary protein histatin 5 (Hst 5) is fungicidal toward Candida albicans, the causative agent of oropharyngeal candidiasis. However, its activity in saliva is compromised by salivary protease-mediated degradation and interaction with salivary salts. Hst 5 has also been shown to bind various metals in saliva-namely, Zn, Cu, and Ni. Surprisingly, interactions of Hst 5 with Fe have not been studied, although iron is one of the most abundant metals present in saliva. Using circular dichroism, we show that Hst 5 can bind up to 10 equivalents of iron as measured by loss of its alpha-helical secondary structure that is normally observed for it in trifluoroethylene. A significant decrease in the candidacidal ability of Hst 5 was observed upon iron binding, with increasing iron concentrations being inversely proportional to Hst 5 killing activity. Binding assays showed that the decrease in killing was likely a result of reduced binding (10-fold reduction) of Fe-Hst 5 to C. albicans cells. Protease stability analysis showed that Fe-Hst 5 was completely resistant to trypsin digestion. In contrast, zinc binding had limited effects on Hst 5 fungicidal activity or protease susceptibility. RNA sequencing results identified changes in iron uptake genes in Hst 5-treated C. albicans cells. Our findings thus suggest that consequences of Hst 5 binding iron not only affect candidacidal ability and proteolyic stability of Hst 5, but may also contribute to a novel killing mechanism involving interference with cellular iron metabolism.

Additivity and synergy between an antimicrobial peptide and inhibitory ions

Recently we described the pH dependence of activity for a family of cationic antimicrobial peptides (CAMPs) selected from a combinatorial library. In the current work we report on the effects of toxic ions (Cu(2+), Zn(2+), and F(-)) and the chelator EDTA on the activity profiles of one member of this family, the 12-residue cationic antimicrobial peptide *ARVA, against a panel of microorganisms. All four ions exhibited either synergy or additivity with *ARVA for all organisms tested with the exception of *ARVA combined with NaF against Candida albicans which exhibited indifference. CuCl2 and ZnCl2 exhibited synergy with *ARVA against both the Gram negative Pseudomonas aeruginosa and the Gram positive Staphylococcus aureus as well as strong additivity against Escherichia coli at submillimolar concentrations. The chelator EDTA was synergistic with *ARVA against the two Gram negative organisms but showed only simple additivity with S. aureus and C. albicans despite their much lower MICs with EDTA. This effect may be related to the known differences in the divalent ion binding properties of the Gram negative LPS layer as compared to the peptidoglycan layer of the Gram positive organism. Unlike the other ions, NaF showed only additivity or indifference when combined with *ARVA and required much higher concentrations for activity. The yeast C. albicans did not show synergy or strong additivity with any of the inhibitory compounds tested. The effects of toxic ions and chelators observed here have important implications for applications using CAMPs and for the design of novel formulations involving CAMPs. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Saliva-microbe interactions and salivary gland dysfunction

Adequate salivary secretion is crucial to both oral and general health, since it provides a complex milieu for support of the microbial populations of the mouth, while at the same time containing antimicrobial products that help control these microbial populations. This paper summarizes several aspects of salivary component function, gland secretion mechanisms, and immunopathogenesis as related to oral health and disease. Salivary components mediate microbial attachment to oral surfaces, and also interact with planktonic microbial surfaces to facilitate agglutination and elimination of pathogens from the oral cavity. Adhesive interactions are often mediated by lectin-like bacterial proteins that bind to glycan motifs on salivary glycoproteins. An important salivary antimicrobial protein is histatin 5 (Hst 5), which shows potent and selective antifungal activity and also susceptibility to proteolytic degradation. Coupling of Hst 5 with the carrier molecule spermidine significantly enhanced killing of C. albicans and resistance to proteolytic degradation, compared with the parent peptide. Loss of salivary secretion may be caused by disorders such as Sjögren's syndrome (SS) or ectodermal dysplasia, or may be a side-effect of radiation therapy. Two new approaches to the treatment of salivary gland dysfunction include the use of resolvins and the creation of differentiated acinar structures to construct an artificial salivary gland. B-cells contribute to the pathogenesis of SS by releasing cytokines and autoantibodies and by influencing T-cell differentiation. CXCL13, a potent B-cell chemokine associated with autoimmune diseases, is elevated locally and systemically in SS and may represent a novel biomarker or therapeutic target in the management and treatment of SS.

Antimicrobial peptides for therapeutic applications: a review

Antimicrobial peptides (AMPs) have been considered as potential therapeutic sources of future antibiotics because of their broad-spectrum activities and different mechanisms of action compared to conventional antibiotics. Although AMPs possess considerable benefits as new generation antibiotics, their clinical and commercial development still have some limitations, such as potential toxicity, susceptibility to proteases, and high cost of peptide production. In order to overcome those obstacles, extensive efforts have been carried out. For instance, unusual amino acids or peptido-mimetics are introduced to avoid the proteolytic degradation and the design of short peptides retaining antimicrobial activities is proposed as a solution for the cost issue. In this review, we focus on small peptides, especially those with less than twelve amino acids, and provide an overview of the relationships between their three-dimensional structures and antimicrobial activities. The efforts to develop highly active AMPs with shorter sequences are also described.