Interactions of histatin 5 and histatin 5-derived peptides with liposome membranes: surface effects, translocation and permeabilization

Influence of Immobilization Strategies on the Antibacterial Properties of Antimicrobial Peptide-Chitosan Coatings

It is key to fight bacterial adhesion to prevent biofilm establishment on biomaterials. Surface immobilization of antimicrobial peptides (AMP) is a promising strategy to avoid bacterial colonization. This work aimed to investigate whether the direct surface immobilization of Dhvar5, an AMP with head-to-tail amphipathicity, would improve the antimicrobial activity of chitosan ultrathin coatings. The peptide was grafted by copper-catalyzed azide-alkyne cycloaddition (CuAAC) chemistry by either its C- or N- terminus to assess the influence of peptide orientation on surface properties and antimicrobial activity. These features were compared with those of coatings fabricated using previously described Dhvar5-chitosan conjugates (immobilized in bulk). The peptide was chemoselectively immobilized onto the coating by both termini. Moreover, the covalent immobilization of Dhvar5 by either terminus enhanced the antimicrobial effect of the chitosan coating by decreasing colonization by both Gram-positive (Staphylococcus aureus, Staphylococcus epidermidis) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria. Relevantly, the antimicrobial performance of the surface on Gram-positive bacteria depended on how Dhvar5-chitosan coatings were produced. An antiadhesive effect was observed when the peptide was grafted onto prefabricated chitosan coatings (film), and a bactericidal effect was exhibited when coatings were prepared from Dhvar5-chitosan conjugates (bulk). This antiadhesive effect was not due to changes in surface wettability or protein adsorption but rather depended on variations in peptide concentration, exposure, and surface roughness. Results reported in this study show that the antibacterial potency and effect of immobilized AMP vary greatly with the immobilization procedure. Overall, independently of the fabrication protocol and mechanism of action, Dhvar5-chitosan coatings are a promising strategy for the development of antimicrobial medical devices, either as an antiadhesive or contact-killing surface.

GPCR/endocytosis/ERK signaling/S2R is involved in the regulation of the internalization, mitochondria-targeting and -activating properties of human salivary histatin 1

Human salivary histatin 1 (Hst1) exhibits a series of cell-activating properties, such as promoting cell spreading, migration, and metabolic activity. We recently have shown that fluorescently labeled Hst1 (F-Hst1) targets and activates mitochondria, presenting an important molecular mechanism. However, its regulating signaling pathways remain to be elucidated. We investigated the influence of specific inhibitors of G protein-coupled receptors (GPCR), endocytosis pathways, extracellular signal-regulated kinases 1/2 (ERK1/2) signaling, p38 signaling, mitochondrial respiration and Na+/K+-ATPase activity on the uptake, mitochondria-targeting and -activating properties of F-Hst1. We performed a siRNA knockdown (KD) to assess the effect of Sigma-2 receptor (S2R) /Transmembrane Protein 97 (TMEM97)—a recently identified target protein of Hst1. We also adopted live cell imaging to monitor the whole intracellular trafficking process of F-Hst1. Our results showed that the inhibition of cellular respiration hindered the internalization of F-Hst1. The inhibitors of GPCR, ERK1/2, phagocytosis, and clathrin-mediated endocytosis (CME) as well as siRNA KD of S2R/TMEM97 significantly reduced the uptake, which was accompanied by the nullification of the promoting effect of F-Hst1 on cell metabolic activity. Only the inhibitor of CME and KD of S2R/TMEM97 significantly compromised the mitochondria-targeting of Hst1. We further showed the intracellular trafficking and targeting process of F-Hst1, in which early endosome plays an important role. Overall, phagocytosis, CME, GPCR, ERK signaling, and S2R/TMEM97 are involved in the internalization of Hst1, while only CME and S2R/TMEM97 are critical for its subcellular targeting. The inhibition of either internalization or mitochondria-targeting of Hst1 could significantly compromise its mitochondria-activating property.

Tetrahedral DNA nanostructure improves transport efficiency and anti-fungal effect of histatin 5 against Candida albicans

OBJECTIVES

Anti-microbial peptides (AMPs) have been comprehensively investigated as a novel alternative to traditional antibiotics against microorganisms. Meanwhile, Tetrahedral DNA nanostructures (TDNs) have gained attention in the field of biomedicine for their premium biological effects and transportation efficiency as delivery vehicles. Hence, in this study, TDN/Histatin 5 (His-5) was synthesized and the transport efficiency and anti-fungal effect were measured to evaluate the promotion of His-5 modified by TDNs.

MATERIALS AND METHODS

Tetrahedral DNA nanostructures/His-5 complex was prepared via electrostatic attraction and characterized by transmission electron microscopy (TEM), polyacrylamide gel electrophoresis (PAGE), dynamic light scattering (DLS) and electrophoretic light scattering (ELS). The anti-fungal effect of the TDN/His-5 complex was evaluated by determining the growth curve and colony-forming units of C. albicans. The morphological transformation of C. albicans was observed by light microscope and scanning electron microscope (SEM). Immunofluorescence was performed, and potassium efflux was detected to mechanistically demonstrate the efficacy of TDN/His-5.

RESULTS

The results showed that Histatin 5 modified by TDNs had preferable stability in serum and was effectively transported into C. albicans, leading to the increased formation of intracellular reactive oxygen species, higher potassium efflux and enhanced anti-fungal effect against C. albicans.

CONCLUSIONS

Our study showed that TDN/His-5 was synthesized successfully. And by the modification of TDNs, His-5 showed increased transport efficiency and improved anti-fungal effect.

A Synergic Potential of Antimicrobial Peptides against Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) is the pathogenic agent responsible for the bacterial canker of kiwifruit (BCK) leading to major losses in kiwifruit productions. No effective treatments and measures have yet been found to control this disease. Despite antimicrobial peptides (AMPs) having been successfully used for the control of several pathogenic bacteria, few studies have focused on the use of AMPs against Psa. In this study, the potential of six AMPs (BP100, RW-BP100, CA-M, 3.1, D4E1, and Dhvar-5) to control Psa was investigated. The minimal inhibitory and bactericidal concentrations (MIC and MBC) were determined and membrane damaging capacity was evaluated by flow cytometry analysis. Among the tested AMPs, the higher inhibitory and bactericidal capacity was observed for BP100 and CA-M with MIC of 3.4 and 3.4-6.2 µM, respectively and MBC 3.4-10 µM for both. Flow cytometry assays suggested a faster membrane permeation for peptide 3.1, in comparison with the other AMPs studied. Peptide mixtures were also tested, disclosing the high efficiency of BP100:3.1 at low concentration to reduce Psa viability. These results highlight the potential interest of AMP mixtures against Psa, and 3.1 as an antimicrobial molecule that can improve other treatments in synergic action.

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.

Bio- and Nanotechnology as the Key for Clinical Application of Salivary Peptide Histatin: A Necessary Advance

Candida albicans is a common microorganism of human’s microbiota and can be easily found in both respiratory and gastrointestinal tracts as well as in the genitourinary tract. Approximately 30% of people will be infected by C. albicans during their lifetime. Due to its easy adaptation, this microorganism started to present high resistance to antifungal agents which is associated with their indiscriminate use. There are several reports of adaptive mechanisms that this species can present. Some of them are intrinsic alteration in drug targets, secretion of extracellular enzymes to promote host protein degradation and efflux receptors that lead to a diminished action of common antifungal and host’s innate immune response. The current review aims to bring promising alternatives for the treatment of candidiasis caused mainly by C. albicans. One of these alternatives is the use of antifungal peptides (AFPs) from the Histatin family, like histatin-5. Besides that, our focus is to show how nanotechnology can allow the application of these peptides for treatment of this microorganism. In addition, our intention is to show the importance of nanoparticles (NPs) for this purpose, which may be essential in the near future.

Peptide Self-Assembly Is Linked to Antibacterial, but Not Antifungal, Activity of Histatin 5 Derivatives

Antimicrobial peptides are important modulators of host defense against bacterial, fungal, and viral pathogens in humans and other multicellular organisms. Two converging paradigms point to a link between antimicrobial peptides that self-assemble into amyloid-like nanoassemblies and classical amyloidogenic peptides that often have potent broad-spectrum antimicrobial activity, suggesting that antimicrobial and amyloidogenic peptides may represent two sides of the same coin. Here, we asked if the ability of an antifungal peptide to self-assemble affects its antifungal or antibacterial activity. We found that modifications of classical antifungal peptide derivative allowed it to self-assemble and did not alter its antifungal activity, and yet self-assembly substantially increased the antibacterial activity of the peptide. These results support the idea that peptide self-assembly can enhance antibacterial activities and emphasize a distinction between the action of antifungal peptides and that of antibacterial peptides. Accordingly, we suggest that the possible generality of this distinction should be widely tested.

The rise of multidrug-resistant pathogens has awakened interest in new drug candidates such as antimicrobial peptides and their derivatives. Recent work suggests that some antimicrobial peptides have the ability to self-assemble into ordered amyloid-like nanostructures which facilitate their antibacterial activity. Here, we evaluate a histatin-based antimicrobial peptide, and its self-assembling derivative, in the interplay between self-assembly, membrane interactions, and antibacterial and antifungal activities. We demonstrate substantial membrane targeting by both peptides, as well as mechanistic insights into this mode of action, which correlates to their antifungal activity and is not affected by their self-assembling state. The ability to self-assemble does, however, significantly affect peptide antibacterial activity against both Gram-negative and Gram-positive bacteria. These results are surprising and hint at important distinctions between antifungal and antibacterial peptide activities in prokaryotes and eukaryotic microbes.

IMPORTANCE Antimicrobial peptides are important modulators of host defense against bacterial, fungal, and viral pathogens in humans and other multicellular organisms. Two converging paradigms point to a link between antimicrobial peptides that self-assemble into amyloid-like nanoassemblies and classical amyloidogenic peptides that often have potent broad-spectrum antimicrobial activity, suggesting that antimicrobial and amyloidogenic peptides may represent two sides of the same coin. Here, we asked if the ability of an antifungal peptide to self-assemble affects its antifungal or antibacterial activity. We found that modifications of classical antifungal peptide derivative allowed it to self-assemble and did not alter its antifungal activity, and yet self-assembly substantially increased the antibacterial activity of the peptide. These results support the idea that peptide self-assembly can enhance antibacterial activities and emphasize a distinction between the action of antifungal peptides and that of antibacterial peptides. Accordingly, we suggest that the possible generality of this distinction should be widely tested.

Salivary Histatin 1 and 2 Are Targeted to Mitochondria and Endoplasmic Reticulum in Human Cells

Human salivary histatin 1 (Hst1) and Hst2 exhibit a series of cell-activating properties (e.g., promoting adhesion, spreading, migration and metabolic activity of mammalian cells). In contrast, Hst5 shows an anti-fungal property but no cell-activating properties. Previous findings suggest that their uptake and association with subcellular targets may play a determinant role in their functions. In this study, we studied the uptake dynamics and subcellular targets of Hst1, Hst2 and Hst5 in epithelial cells (HO1N1 human buccal carcinoma epithelial cell line). Confocal laser scanning microscopy (CLSM) revealed that fluorescently labeled Hst1 (F-Hst1) was taken up into the intracellular space of epithelial cells. Then, 60 min post-incubation, the total fluorescence of cell-associated F-Hst1, as measured using flow cytometry, was significantly higher compared to those of F-Hst2 and F-Hst5. In contrast, virtually no association occurred using the negative control—scrambled F-Hst1 (F-Hst^scr). CLSM images revealed that F-Hst1, 2 and 5 co-localized with mitotracker^TM-labeled mitochondria. In addition, F-Hst1 and F-Hst2 but neither F-Hst5 nor F-Hst1^scr co-localized with the ER-tracker^TM-labeled endoplasmic reticulum. No co-localization of Hst1, 2 and 5 with lysosomes or the Golgi apparatus was observed. Furthermore, Hst1 and Hst2 but not Hst5 or Hst1^scr significantly promoted the metabolic activity of both human epithelial cell lines, HaCaT human keratinocytes and primary human gingival fibroblasts.

Anionic food color tartrazine enhances antibacterial efficacy of histatin-derived peptide DHVAR4 by fine-tuning its membrane activity

Here it is demonstrated how some anionic food additives commonly used in our diet, such as tartrazine (TZ), bind to DHVAR4, an antimicrobial peptide (AMP) derived from oral host defense peptides, resulting in significantly fostered toxic activity against both Gram-positive and Gram-negative bacteria, but not against mammalian cells. Biophysical studies on the DHVAR4-TZ interaction indicate that initially large, positively charged aggregates are formed, but in the presence of lipid bilayers, they rather associate with the membrane surface. In contrast to synergistic effects observed for mixed antibacterial compounds, this is a principally different mechanism, where TZ directly acts on the membrane-associated AMP promoting its biologically active helical conformation. Model vesicle studies show that compared to dye-free DHVAR4, peptide-TZ complexes are more prone to form H-bonds with the phosphate ester moiety of the bilayer head-group region resulting in more controlled bilayer fusion mechanism and concerted severe cell damage. AMPs are considered as promising compounds to combat formidable antibiotic-resistant bacterial infections; however, we know very little on their in vivo actions, especially on how they interact with other chemical agents. The current example illustrates how food dyes can modulate AMP activity, which is hoped to inspire improved therapies against microbial infections in the alimentary tract. Results also imply that the structure and function of natural AMPs could be manipulated by small compounds, which may also offer a new strategic concept for the future design of peptide-based antimicrobials.

Activity and characterization of a pH-sensitive antimicrobial peptide

Antimicrobial peptides (AMPs) have been an area of great interest, due to the high selectivity of these molecules toward bacterial targets over host cells and the limited development of bacterial resistance to these molecules throughout evolution. Previous work showed that when Histidine was incorporated into the peptide C18G it lost antimicrobial activity. The role of pH on activity and biophysical properties of the peptide was investigated to explain this phenomenon. Minimal inhibitory concentration (MIC) results demonstrated that decreased media pH increased antimicrobial activity. Trichloroethanol (TCE) quenching and red-edge excitation spectroscopy (REES) showed a clear pH dependence on peptide aggregation in solution. Trp fluorescence was used to monitor binding to lipid vesicles and demonstrated the peptide binds to anionic bilayers at all pH values tested, however, binding to zwitterionic bilayers was enhanced at pH 7 and 8 (above the His pKa). Dual Quencher Analysis (DQA) confirmed the peptide inserted more deeply in PC:PG and PE:PG membranes, but could insert into PC bilayers at pH conditions above the His pKa. Bacterial membrane permeabilization assays which showed enhanced membrane permeabilization at pH 5 and 6 but vesicle leakage assays indicate enhanced permeabilization of PC and PC:PG bilayers at neutral pH. The results indicate the ionization of the His side chain affects the aggregation state of the peptide in solution and the conformation the peptide adopts when bound to bilayers, but there are likely more subtle influences of lipid composition and properties that impact the ability of the peptide to form pores in membranes.

Antimicrobial coatings prepared from Dhvar-5-click-grafted chitosan powders

Antimicrobial peptides (AMP) are powerful components of the innate immune system, as they display wide activity spectrum and low tendency to induce pathogen resistance. Hence, the development of AMP-based coatings is a very promising strategy to prevent biomaterials-associated infections. This work aims to investigate if Dhvar-5-chitosan conjugates, previously synthesized by us via azide-alkyne "click" reaction, can be applied as antimicrobial coatings. Ultrathin coatings were prepared by spin coater after dissolving Dhvar-5-chitosan conjugate powder in aqueous acetic acid. Peptide orientation and exposure from the surface was confirmed by ellipsometry and contact angle measurements. Bactericidal activity was evaluated against Staphylococcus epidermidis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa, the most prevalent pathogens in implant-associated infections. Results showed that Dhvar-5-chitosan coatings displayed bactericidal effect. Moreover, since Dhvar-5 has head-to-tail amphipathicity, it was clear that the bactericidal potency was dependent on which domain of the peptide (cationic or hydrophobic) was exposed. In this context, Dhvar-5 immobilized through its C-terminus (exposing its hydrophobic end) presented higher antimicrobial activity against Gram-positive bacteria and reduced adhesion of Gram-negative bacteria. This orientation-dependent antimicrobial activity was further corroborated by the anti-biofilm assay, as covalent immobilization of Dhvar-5 through its C-terminus provided anti-biofilm properties to the chitosan thin film. Immobilization of Dhvar-5 showed no cytotoxic effect against HFF-1 cells, as both metabolic activity and cell morphology were similar to control. In conclusion, Dhvar-5-chitosan coatings are promising antimicrobial surfaces without cytotoxic effects against human cells. STATEMENT OF SIGNIFICANCE: AMP-tethering onto ground biomaterial is still a poorly explored strategy in research. In this work, AMP-tethered ground chitosan is used to produce highly antibacterial ultrathin films. Powdered AMP-tethered chitosan appears as an alternative solution for antimicrobial devices production, as it is suitable for large scale production, being easier to handle for fabrication of different coatings and materials with antimicrobial properties and without inducing toxicity.

In vivo efficacy of tobramycin-loaded synthetic calcium phosphate beads in a rabbit model of staphylococcal osteomyelitis

Background

Osteomyelitis is an acute or chronic inflammatory process of the bone following infection with pyogenic organisms like Staphylococcus aureus. Tobramycin (TOB) is a promising aminoglycoside antibiotic used to treat various bacterial infections, including S. aureus. The aim of this study was to investigate the efficacy of tobramycin-loaded calcium phosphate beads (CPB) in a rabbit osteomyelitis model.

Methods

Tobramycin (30 mg/mL) was incorporated into CPB by dipping method and the efficacy of TOB-loaded CPB was studied in a rabbit osteomyelitis model. For juxtaposition, CPB with and without TOB were prepared. Twenty-five New Zealand white rabbits were grouped (n = 5) as sham (group 1), TOB-loaded CPB without S. aureus (group 2), S. aureus only (group 3), S. aureus + CPB (group 4), and S. aureus + TOB-loaded CPB (group 5). Groups infected with S. aureus followed by CPB implantation were immediately subjected to surgery at the mid-shaft of the tibia. After 28 days post-surgery, all rabbits were euthanized and the presence or absence of chronic osteomyelitis and the extent of architectural destruction of the bone were assessed by radiology, bacteriology and histological studies.

Results

Tobramycin-loaded CPB group potentially inhibited the growth of S. aureus causing 3.2 to 3.4 log₁₀ reductions in CFU/g of bone tissue compared to the controls. Untreated groups infected with S. aureus showed signs of chronic osteomyelitis with abundant bacterial growth and alterations in bone architecture. The sham group and TOB-loaded CPB group showed no evidence of bacterial growth.

Conclusions

TOB-incorporated into CPB for local bone administration was proven to be more successful in increasing the efficacy of TOB in this rabbit osteomyelitis model and hence could represent a good alternative to other formulations used in the treatment of osteomyelitis.

Electronic supplementary material

The online version of this article (10.1186/s12941-018-0296-3) contains supplementary material, which is available to authorized users.

A Label-Free Fluorescent Array Sensor Utilizing Liposome Encapsulating Calcein for Discriminating Target Proteins by Principal Component Analysis

A new fluorescent arrayed biosensor has been developed to discriminate species and concentrations of target proteins by using plural different phospholipid liposome species encapsulating fluorescent molecules, utilizing differences in permeation of the fluorescent molecules through the membrane to modulate liposome-target protein interactions. This approach proposes a basically new label-free fluorescent sensor, compared with the common technique of developed fluorescent array sensors with labeling. We have confirmed a high output intensity of fluorescence emission related to characteristics of the fluorescent molecules dependent on their concentrations when they leak from inside the liposomes through the perturbed lipid membrane. After taking an array image of the fluorescence emission from the sensor using a CMOS imager, the output intensities of the fluorescence were analyzed by a principal component analysis (PCA) statistical method. It is found from PCA plots that different protein species with several concentrations were successfully discriminated by using the different lipid membranes with high cumulative contribution ratio. We also confirmed that the accuracy of the discrimination by the array sensor with a single shot is higher than that of a single sensor with multiple shots.

Membrane-Active Epithelial Keratin 6A Fragments (KAMPs) Are Unique Human Antimicrobial Peptides with a Non-αβ Structure

Antibiotic resistance is a pressing global health problem that threatens millions of lives each year. Natural antimicrobial peptides and their synthetic derivatives, including peptoids and peptidomimetics, are promising candidates as novel antibiotics. Recently, the C-terminal glycine-rich fragments of human epithelial keratin 6A were found to have bactericidal and cytoprotective activities. Here, we used an improved 2-dimensional NMR method coupled with a new protocol for structural refinement by low temperature simulated annealing to characterize the solution structure of these kerain-derived antimicrobial peptides (KAMPs). Two specific KAMPs in complex with membrane mimicking sodium dodecyl sulfate (SDS) micelles displayed amphipathic conformations with only local bends and turns, and a central 10-residue glycine-rich hydrophobic strip that is central to bactericidal activity. To our knowledge, this is the first report of non-αβ structure for human antimicrobial peptides. Direct observation of Staphylococcus aureus and Pseudomonas aeruginosa by scanning and transmission electron microscopy showed that KAMPs deformed bacterial cell envelopes and induced pore formation. Notably, in competitive binding experiments, KAMPs demonstrated binding affinities to LPS and LTA that did not correlate with their bactericidal activities, suggesting peptide-LPS and peptide-LTA interactions are less important in their mechanisms of action. Moreover, immunoprecipitation of KAMPs-bacterial factor complexes indicated that membrane surface lipoprotein SlyB and intracellular machineries NQR sodium pump and ribosomes are potential molecular targets for the peptides. Results of this study improve our understanding of the bactericidal function of epithelial cytokeratin fragments, and highlight an unexplored class of human antimicrobial peptides, which may serve as non-αβ peptide scaffolds for the design of novel peptide-based antibiotics.

Dhvar5 antimicrobial peptide (AMP) chemoselective covalent immobilization results on higher antiadherence effect than simple physical adsorption

Bacterial colonization and subsequent biofilm formation is still one of the major problems associated with medical devices. Antimicrobial peptides (AMP) immobilization onto biomaterials surface is a promising strategy to avoid bacterial colonization. However, a correct peptide orientation and exposure from the surface is essential to maintain AMP antimicrobial activity. This work aims to evaluate the effect of the immobilization on antibacterial activity of Dhvar5 (LLLFLLKKRKKRKY), an AMP with a head-to-tail amphipathicity. Dhvar5 was linked to thin chitosan coatings in i) a controlled orientation and exposure, testing covalent immobilization of its N- or C-terminus and using spacers with different lengths and flexibilities or in ii) a random orientation by physical adsorption. Chitosan coating was chosen due to its antimicrobial properties and readiness to be functionalized. Surface characterization demonstrated the chemoselective immobilization of the peptide with different spacers in a similar concentration (∼2 ng/cm2). Efficacy assays demonstrated that covalent immobilization of Dhvar5 exposing its cationic end, improves the chitosan coating antimicrobial effect by decreasing Methicillin-resistant Staphylococcus aureus (MRSA) colonization. This effect was enhanced when longer spacers were used independently of their flexibility. In opposite, immobilized Dhvar5 exposing its hydrophobic end has no effect on bacterial adhesion to chitosan, and when adsorbed in a random orientation even induces bacterial adhesion to chitosan coating.

Histatin 5-spermidine conjugates have enhanced fungicidal activity and efficacy as a topical therapeutic for oral candidiasis

Oropharyngeal candidiasis (OPC) is caused by the opportunistic fungi Candida albicans and is prevalent in immunocompromised patients, individuals with dry mouth, or patients with prolonged antibiotic therapies that reduce oral commensal bacteria. Human salivary histatins, including histatin 5 (Hst 5), are small cationic proteins that are the major source of fungicidal activity of saliva. However, Hsts are rapidly degraded in vivo, limiting their usefulness as therapeutic agents despite their lack of toxicity. We constructed a conjugate peptide using spermidine (Spd) linked to the active fragment of Hst 5 (Hst 54-15), based upon our findings that C. albicans spermidine transporters are required for Hst 5 uptake and fungicidal activity. We found that Hst 54-15-Spd was significantly more effective in killing C. albicans and Candida glabrata than Hst 5 alone in both planktonic and biofilm growth and that Hst 54-15-Spd retained high activity in both serum and saliva. Hst 54-15-Spd was not bactericidal against streptococcal oral commensal bacteria and had no hemolytic activity. We tested the effectiveness of Hst 54-15-Spd in vivo by topical application to tongue surfaces of immunocompromised mice with OPC. Mice treated with Hst 54-15-Spd had significant clearance of candidal tongue lesions macroscopically, which was confirmed by a 3- to 5-log fold reduction of C. albicans colonies recovered from tongue tissues. Hst 54-15-Spd conjugates are a new class of peptide-based drugs with high selectivity for fungi and potential as topical therapeutic agents for oral candidiasis.

Peptides: β-cyclodextrin inclusion compounds as highly effective antimicrobial and anti-epithelial proliferation agents

BACKGROUND

The use of antimicrobial peptides (AMPs) as therapeutic agents for periodontal infections has great advantages, such as broad spectrum of action, low toxicity, and limited bacterial resistance. However, their practical use is limited because of the large amount of peptide required to exercise the microbicidal function.

METHODS

LyeTxI, LL37f, and KR12 cationic peptides were prepared with β-cyclodextrin (βCD) at 1:1 molar ratios. The susceptibility of Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans, and Fusobacterium nucleatum were assessed in anaerobic conditions. Cytotoxicity assays were performed using osteoblast and Caco-2 epithelial cells, and hemolytic activity was assessed on rabbit erythrocytes at an absorbance of 414 nm. Parameters of surface roughness and electrical charge were established by atomic force microscopy and zeta (ζ) potential, respectively.

RESULTS

AMP/βCDs drastically decreased the peptide concentration required for activity against the bacteria tested. Moreover, AMPs associated with βCD were able to modify cell-surface parameters, such as roughness and ζ potential. On the other hand, AMP/βCD did not alter the degree of hemolysis induced by the pure AMPs. The effective concentration at half-maximum values of the peptides and compounds on osteoblasts were greater than the concentrations required to achieve inhibition of bacterial growth in all the species tested. AMP/βCDs inhibited the proliferation of Caco-2 epithelial cells in a more efficient manner than AMPs alone.

CONCLUSION

AMP/βCD compounds more effectively inhibit periodontopathogenic bacteria than AMPs alone, with the additional ability of inhibiting the proliferation of epithelial cells at concentrations that are non-cytotoxic for osteoblasts and erythrocytes.

Chimerization of lactoferricin and lactoferrampin peptides strongly potentiates the killing activity against Candida albicans

Bovine lactoferrin harbors 2 antimicrobial sequences (LFcin and LFampin), situated in close proximity in the N1-domain. To mimic their semi parallel configuration we have synthesized a chimeric peptide (LFchimera) in which these sequences are linked in a head-to-head fashion to the α- and ε-amino group, respectively, of a single lysine. In line with previously described bactericidal effects, this peptide was also a stronger candidacidal agent than the antimicrobial peptides LFcin17-30 and LFampin265-284, or a combination of these 2. Conditions that strongly reduced the candidacidal activities of LFcin17-30 and LFampin265-284, such as high ionic strength and energy depletion, had little influence on the activity of LFchimera. Freeze-fracture electron microscopy showed that LFchimera severely affected the membrane morphology, resulting in disintegration of the membrane bilayer and in an efflux of small and high molecular weight molecules such as ATP and proteins. The differential effects displayed by the chimeric peptide and a mixture of its constituent peptides clearly demonstrate the synergistic effect of linking these peptides in a fashion that allows a similar spatial arrangement as in the parent protein, suggesting that in bovine lactoferrrin the corresponding fragments act in concert in its candidacidal activity.

Histatin 5 uptake by Candida albicans utilizes polyamine transporters Dur3 and Dur31 proteins

Histatin 5 (Hst 5) is a salivary gland-secreted cationic peptide with potent fungicidal activity against Candida albicans. Hst 5 kills fungal cells following intracellular translocation, although its selective transport mechanism is unknown. C. albicans cells grown in the presence of polyamines were resistant to Hst 5 due to reduced intracellular uptake, suggesting that this cationic peptide may enter candidal cells through native yeast polyamine transporters. Based upon homology to known Saccharomyces cerevisiae polyamine permeases, we identified six C. albicans Dur polyamine transporter family members and propose a new nomenclature. Gene deletion mutants were constructed for C. albicans polyamine transporters Dur3, Dur31, Dur33, Dur34, and were tested for Hst 5 sensitivity and uptake of spermidine. We found spermidine uptake and Hst 5 mediated killing were decreased significantly in Δdur3, Δdur31, and Δdur3/Δdur31 strains; whereas a DUR3 overexpression strain increased Hst 5 sensitivity and higher spermidine uptake. Treatment of cells with a spermidine synthase inhibitor increased spermidine uptake and Hst 5 killing, whereas protonophores and cold treatment reduced spermidine uptake. Inhibition assays showed that Hst 5 is a competitive analog of spermidine for uptake into C. albicans cells, and that Hst 5 Ki values were increased by 80-fold in Δdur3/Δdur31 cells. Thus, Dur3p and Dur31p are preferential spermidine transporters used by Hst 5 for its entry into candidal cells. Understanding of polyamine transporter-mediated internalization of Hst 5 provides new insights into the uptake mechanism for C. albicans toxicity, and further suggests design for targeted fungal therapeutic agents.

Discovery and development of a synthetic peptide derived from lactoferrin for clinical use

There is an urgent need to develop new antimicrobial drugs especially for combating the rise of infections caused by multi-resistant pathogens such as MRSA and VRSA. The problem of antibiotic resistant micro-organisms is expected to increase disproportionally and controlling of infections is becoming difficult because of the rapid spread of those micro-organisms. Primary therapy with classical antibiotics is becoming more ineffective. Combinational therapy of antibiotics with antimicrobial peptides (AMP's) has been suggested as an alternative approach to improve treatment outcome. Their unique mechanism of action and safety profile makes AMP's appealing candidates for simultaneous or sequential use in different cases of infections. In this review, for antimicrobial treatment the application of synthetic antimicrobial peptide hLF(1-11), derived from the first 11 amino acids of human lactoferrin is evaluated in both pre-clinical and clinical settings. Present information indicates that this derivate from lactoferrin is well tolerated in pre-clinical tests and clinical trials and thus hLF(1-11) is an interesting candidate for further exploration in various clinical indications of obscure infections, including meningitis. Another approach of using AMP's is their use in prevention of infections e.g. as coating for dental or bone implants or in biosensing applications or useful as infection specific radiopharmaceutical.

Membrane interaction and antibacterial properties of two mildly cationic peptide diastereomers, bombinins H2 and H4, isolated from Bombina skin

Bombinins H are mildly cationic antimicrobial peptides isolated from the skin of the anuran genus Bombina, the fire-bellied toad. Some members of this peptide family coexist in skin secretions as diastereomers in which a single D: -amino acid (alloisoleucine or leucine) is incorporated as a result of the post-translational modification of the respective gene-encoded L-amino acid. Here we report on the antimicrobial properties and membrane interactions of bombinins H2 and H4. The latter differs from H2 by the presence of a D-alloisoleucine at the second N-terminal position. Specifically, we have evaluated the antimicrobial activity of H2 and H4 against a large panel of reference and clinical isolates of Gram-negative and Gram-positive bacteria; performed membrane permeation assays on both intact cells and model membranes (lipid monolayers and liposomes) mimicking the composition of the plasma membrane of Gram-negative/positive bacteria; used biochemical tools, such as trypsin-encapsulated liposomes and capillary electrophoresis, to monitor the peptides' ability to translocate through the membrane of liposomes mimicking Escherichia coli inner membrane. The results revealed interesting relationships between the presence of a single D: -amino acid in the sequence of an antimicrobial peptide and its target microbial cell selectivity/membrane-perturbing activity.

Escherichia coli cell surface perturbation and disruption induced by antimicrobial peptides BP100 and pepR

The potential of antimicrobial peptides (AMPs) as an alternative to conventional therapies is well recognized. Insights into the biological and biophysical properties of AMPs are thus key to understanding their mode of action. In this study, the mechanisms adopted by two AMPs in disrupting the gram-negative Escherichia coli bacterial envelope were explored. BP100 is a short cecropin A-melittin hybrid peptide known to inhibit the growth of phytopathogenic gram-negative bacteria. pepR, on the other hand, is a novel AMP derived from the dengue virus capsid protein. Both BP100 and pepR were found to inhibit the growth of E. coli at micromolar concentrations. Zeta potential measurements of E. coli incubated with increasing peptide concentrations allowed for the establishment of a correlation between the minimal inhibitory concentration (MIC) of each AMP and membrane surface charge neutralization. While a neutralization-mediated killing mechanism adopted by either AMP is not necessarily implied, the hypothesis that surface neutralization occurs close to MIC values was confirmed. Atomic force microscopy (AFM) was then employed to visualize the structural effect of the interaction of each AMP with the E. coli cell envelope. At their MICs, BP100 and pepR progressively destroyed the bacterial envelope, with extensive damage already occurring 2 h after peptide addition to the bacteria. A similar effect was observed for each AMP in the concentration-dependent studies. At peptide concentrations below MIC values, only minor disruptions of the bacterial surface occurred.

Salivary histatin 5 internalization by translocation, but not endocytosis, is required for fungicidal activity in Candida albicans

Salivary histatin 5 (Hst 5) is a cationic salivary protein with high fungicidal activity against Candida albicans. Binding to the cell wall followed by intracellular translocation is required for killing; however, specific binding components and critical toxic events are not understood. In this study, laminarin (β-1,3-glucan) but not sialic acid, mannan or pustulan mediated Hst 5 binding to C. albicans, and was disassociated by 100 mM NaCl. Time-lapse confocal microscopy revealed a dose-dependent rate of cytosolic uptake of Hst 5 that invariably preceded propidium iodide (PI) entry, demonstrating that translocation itself does not disrupt membrane integrity. Cell toxicity was manifest by vacuolar expansion followed by PI entrance; however, loss of endocytotic vacuolar trafficking of Hst 5 did not reduce killing. Extracellular NaCl (100 mM), but not sorbitol, prevented vacuolar expansion and PI entry in cells already containing cytosolic Hst 5, thus showing a critical role for ionic balance in Hst 5 toxicity. Hst 5 uptake, but not cell wall binding, was blocked by pretreatment with azide or carbonyl cyanide m-chlorophenylhydrazone; however, 10% of de-energized cells had membrane disruption. Thus, Hst 5 is capable of heterogeneous intracellular entry routes, but only direct cytosolic translocation causes cell death as a result of ionic efflux.

Candida albicans biofilm formation on peptide functionalized polydimethylsiloxane

In order to prevent biofilm formation by Candida albicans, several cationic peptides were covalently bound to polydimethylsiloxane (PDMS). The salivary peptide histatin 5 and two synthetic variants (Dhvar 4 and Dhvar 5) were used to prepare peptide functionalized PDMS using 4-azido-2,3,5,6-tetrafluoro-benzoic acid (AFB) as an interlinkage molecule. In addition, polylysine-, polyarginine-, and polyhistidine-PDMS surfaces were prepared. Dhvar 4 functionalized PDMS yielded the highest reduction of the number of C. albicans biofilm cells in the Modified Robbins Device. Amino acid analysis demonstrated that the amount of peptide immobilized on the modified disks was in the nanomole range. Poly-d-lysine PDMS, in particular the homopeptides with low molecular weight (2500 and 9600) showed the highest activity against C. albicans biofilms, with reductions of 93% and 91%, respectively. The results indicate that the reductions are peptide dependent.

The pH sensitivity of histidine-containing lytic peptides

Many bioactive peptides are featured by their unique amino acid compositions such as argine/lysine-rich peptides. However, histidine-rich bioactive peptides are hardly found. In this study, histidine-containing peptides were constructed by selectively replacing the corresponded lysine residues in a lytic peptide LL-1 with histidines. Interestingly, all resulting peptides demonstrated pH-dependent activities. The cell lysis activities of these peptides could be increased up to four times as the solution pHs dropped from pH = 7.4 to pH = 5.5. The pH sensitivity of a histidine-containing peptide was determined by histidine substitution numbers. Peptide derivatives with more histidines were associated with increased pH sensitivity. Results showed that not the secondary structures but pH-affected cell affinity changes were responsible for the pH-dependent activities of histidine-containing peptides. The histidine substitution approach demonstrated here may present a general strategy to construct bioactive peptides with desired pH sensitivity for various applications.

Constructing bioactive peptides with pH-dependent activities

Many bioactive peptides are featured by their arginine and lysine rich contents. In this study, lysine and arginine residues in lytic peptides were selectively replaced by histidines. Although resulting histidine-containing lytic peptides had decreased activity, they did show pH-dependent cytotoxicity. The activity of the constructed histidine-containing lytic peptides increased 2-8 times as the solution pH changed from 7.4 to 5.5. More importantly, these histidine-containing peptides maintain the same cell killing mechanism as their parent peptides by causing cell lysis. Both the activity and pH-sensitivity of histidine-containing peptides are tunable by adjusting histidine substitution numbers and positions. This study has presented a general strategy to create bioactive peptides with desired pH-sensitivity to meet the needs of various applications such as cancer treatments.

In vitro susceptibility of Burkholderia pseudomallei to antimicrobial peptides

Burkholderia pseudomallei, the causative agent of melioidosis, is intrinsically resistant to many antibiotics, resulting in high mortality rates of 19% in Australia and even 50% in Thailand. Antimicrobial peptides (AMPs) possess potent broad-spectrum bactericidal activities and are regarded as promising therapeutic alternatives in the fight against resistant microorganisms. Moreover, these peptides may also affect inflammation, immune activation and wound healing. In this study, the in vitro activities of 10 AMPs, including histatin 5 and histatin variants, human cathelicidin peptide LL-37 and lactoferrin peptides, against 24 isolates of B. pseudomallei were investigated. The results showed that the antibacterial activities of the individual peptides depended on peptide dose and bacterial isolate. Among the 10 peptides tested, LL-37 exhibited the most effective killing activity. The smooth type A lipopolysaccharide (LPS) phenotype B. pseudomallei appeared to be more susceptible than those expressing the smooth type B LPS and the rough type LPS. Four isolates of B. pseudomallei shown to be resistant to ceftazidime and trimethoprim/sulfamethoxazole were also highly susceptible to LL-37. These data indicate that LL-37 possesses antimicrobial activity against all isolates independent of the LPS phenotype and is therefore a promising peptide to combat B. pseudomallei infections.

Comparative in vivo and in vitro analyses of putative virulence factors of Burkholderia pseudomallei using lipopolysaccharide, capsule and flagellin mutants

Burkholderia pseudomallei is a gram-negative bacillus that is the causative agent of melioidosis. We evaluated host-pathogen interaction at different levels using three separate B. pseudomallei mutants generated by insertional inactivation. One of these mutants is defective in the production of the polysaccharide side chains associated with lipopolysaccharide; one does not produce the capsular polysaccharide with the structure -3)-2-O-acetyl-6-deoxy-beta-d-manno-heptopyranose-(1-; and the third mutant does not produce flagellin. We compared the in vivo virulence in BALB/c mice, the in vitro fate of intracellular survival inside human polymorphonuclear cells (PMNs) and macrophages (Mphis) and the susceptibility to killing by 30% normal human serum, reactive nitrogen and oxygen intermediates and antimicrobial peptides with that of their wild-type counterpart. The lipopolysaccharide and capsule mutants demonstrated a marked reduction in virulence for BALB/c mice, but the flagellin mutant was only slightly less virulent than the parent strain. The results from the BALB/c mice experiments correlated with survival in Mphis. The lipopolysaccharide and capsule mutants were also more susceptible to killing by antimicrobial agents. All bacteria were equally susceptible to killing by PMNs. Altogether, the data suggest that lipopolysaccharide and capsule and, to a much lesser extent, flagella, are most likely associated with the virulence of this bacterium and highlight the importance of intracellular killing by PMNs and Mphis in disease pathogenesis.

The antimicrobial peptide histatin-5 causes a spatially restricted disruption on the Candida albicans surface, allowing rapid entry of the peptide into the cytoplasm

Antimicrobial peptides play an important role in host defense against microbial pathogens. Their high cationic charge and strong amphipathic structure allow them to bind to the anionic microbial cell membrane and disrupt the membrane bilayer by forming pores or channels. In contrast to the classical pore-forming peptides, studies on histatin-5 (Hst-5) have suggested that the peptide is transported into the cytoplasm of Candida albicans in a non-lytic manner, and cytoplasmic Hst-5 exerts its candicidal activities on various intracellular targets, consistent with its weak amphipathic structure. To understand how Hst-5 is internalized, we investigated the localization of FITC-conjugated Hst-5. We find that Hst-5 is internalized into the vacuole through receptor-mediated endocytosis at low extracellular Hst-5 concentrations, whereas under higher physiological concentrations, Hst-5 is translocated into the cytoplasm through a mechanism that requires a high cationic charge on Hst-5. At intermediate concentrations, two cell populations with distinct Hst-5 localizations were observed. By cell sorting, we show that cells with vacuolar localization of Hst-5 survived, while none of the cells with cytoplasmic Hst-5 formed colonies. Surprisingly, extracellular Hst-5, upon cell surface binding, induces a perturbation on the cell surface, as visualized by an immediate and rapid internalization of Hst-5 and propidium iodide or rhodamine B into the cytoplasm from the site using time-lapse microscopy, and a concurrent rapid expansion of the vacuole. Thus, the formation of a spatially restricted site in the plasma membrane causes the initial injury to C. albicans and offers a mechanism for its internalization into the cytoplasm. Our study suggests that, unlike classical channel-forming antimicrobial peptides, action of Hst-5 requires an energized membrane and causes localized disruptions on the plasma membrane of the yeast. This mechanism of cell membrane disruption may provide species-specific killing with minimal damage to microflora and the host and may be used by many other antimicrobial peptides.

Histidine-rich glycoprotein protects from systemic Candida infection

Fungi, such as Candida spp., are commonly found on the skin and at mucosal surfaces. Yet, they rarely cause invasive infections in immunocompetent individuals, an observation reflecting the ability of our innate immune system to control potentially invasive microbes found at biological boundaries. Antimicrobial proteins and peptides are becoming increasingly recognized as important effectors of innate immunity. This is illustrated further by the present investigation, demonstrating a novel antifungal role of histidine-rich glycoprotein (HRG), an abundant and multimodular plasma protein. HRG bound to Candida cells, and induced breaks in the cell walls of the organisms. Correspondingly, HRG preferentially lysed ergosterol-containing liposomes but not cholesterol-containing ones, indicating a specificity for fungal versus other types of eukaryotic membranes. Both antifungal and membrane-rupturing activities of HRG were enhanced at low pH, and mapped to the histidine-rich region of the protein. Ex vivo, HRG-containing plasma as well as fibrin clots exerted antifungal effects. In vivo, Hrg^−/− mice were susceptible to infection by C. albicans, in contrast to wild-type mice, which were highly resistant to infection. The results demonstrate a key and previously unknown antifungal role of HRG in innate immunity.

It has been estimated that humans contain about 1 kg of microbes, an observation that reflects our coexistence with colonizing microbes such as bacteria and fungi. The fungal species Candida is present as a commensal at mucosal surfaces and on skin. Although it may cause life-threatening infections, such as sepsis, particularly in immunocompromised individuals, it seldom causes disease in normal individuals. In order to control our microbial flora, humans as well as virtually all life forms are armoured with various proteins and peptides that comprise integral parts of our innate immune system. Here we describe a new component in this system; histidine-rich glycoprotein (HRG), an abundant plasma protein. We show, using a combination of microbiological, biochemical, and biophysical methods, that HRG exerts a potent antifungal activity, which is mediated via a histidine-rich region of the protein, and targets ergosterol-rich membrane structures such as those of Candida. HRG killed Candida both in plasma as well as when incorporated into fibrin clots. In mouse infection models, HRG was protective against systemic infection by Candida, indicating a novel antifungal role of HRG in innate immunity.

Human antimicrobial peptide histatin 5 is a cell-penetrating peptide targeting mitochondrial ATP synthesis in Leishmania

Histatin 5 (Hst5) is a human salivary antimicrobial peptide that targets fungal mitochondria. In the human parasitic protozoa Leishmania, the mitochondrial ATP production is essential, as it lacks the bioenergetic switch between glycolysis and oxidative phosphorylation described in some yeasts. On these premises, Hst5 activity was assayed on both stages of its life cycle, promastigotes and amastigotes (LC(50)=7.3 and 14.4 microM, respectively). In a further step, its lethal mechanism was studied. The main conclusions drawn were as follows: 1) Hst5 causes limited and temporary damage to the plasma membrane of the parasites, as assessed by electron microscopy, depolarization, and entrance of the vital dye SYTOX Green; 2) Hst5 translocates into the cytoplasm of Leishmania in an achiral receptor-independent manner with accumulation into the mitochondrion, as shown by confocal microscopy; and 3) Hst5 produces a bioenergetic collapse of the parasite, caused essentially by the decrease of mitochondrial ATP synthesis through inhibition of F(1)F(0)-ATPase, with subsequent fast ATP exhaustion. By using the Hst5 enantiomer, it was found that the key steps of its lethal mechanism involved no chiral recognition. Hst5 thus constitutes the first leishmanicidal peptide with a defined nonstereospecific intracellular target. The prospects of its development, by its own or as a carrier molecule for other leishmanicidal molecules, into a novel anti-Leishmania drug with a preferential subcellular accumulation are discussed.

The P-113 fragment of histatin 5 requires a specific peptide sequence for intracellular translocation in Candida albicans, which is independent of cell wall binding

The activity of histatin 5 (Hst 5) against Candida albicans is initiated through cell wall binding, followed by translocation and intracellular targeting. The C. albicans cell wall protein Ssa2 is involved in the transport of Hst 5 into cells as part of cell killing. P-113 (a 12-amino-acid candidacidal active fragment of Hst 5) and P-113Q2.10 (which is inactivated by a glutamine substitution of the Lys residues at positions 2 and 10) were compared for their levels of cell wall binding and intracellular translocation in Candida wild-type (wt) and ssa2Delta strains. Both P-113 and P-113Q2.10 bound to the walls of C. albicans wt and ssa2Delta cells, although the quantity of P-113Q2.10 in cell wall extracts was higher than that of P-113 in both strains. Increasing the extracellular NaCl concentration to 100 mM completely inhibited the cell wall association of both peptides, suggesting that these interactions are primarily ionic. The accumulation of P-113 in the cytosol of wt cells reached maximal levels within 15 min (0.26 microg/10(7) cells), while ssa2Delta mutant cells had maximal cytosolic levels of less than 0.2 microg/10(7) cells even after 30 min of incubation. Furthermore, P-113 but not P-113Q2.10 showed specific binding with a peptide array of C. albicans Ssa2p. P-113Q2.10 was not transported into the cytosol of either C. albicans wt or ssa2Delta cells, despite the high levels of cell wall binding, showing that the two cationic lysine residues at positions 2 and 10 in the P-113 peptide are important for transport into the cytosol and that binding and transport are independent functional events.

Characterization of a heat-stable protein with antimicrobial activity from Arabidopsis thaliana

A heat-stable protein with antimicrobial activity was isolated from Arabidopsis thaliana plants by buffer-soluble extraction and two chromatographic procedures. The results of MALDI-TOF analysis revealed that the isolated protein shares high sequence identity with aspen SP1. To determine the exact antimicrobial properties of this protein, a cDNA encoding the protein was isolated from an A. thaliana leaf cDNA library and named AtHS1. AtHS1 mRNA was induced by exposure to external stresses, such as salicylic acid and jasmonic acid. We also analyzed the antimicrobial activity of recombinant AtHS1 expressed in Escherichia coli. This protein inhibited pathogenic fungal strains, except for Phytophthora infestans and Phytophthora nicotianae, and it exhibited antibacterial activity against E. coli and Staphylococcus aureus. These results suggest that AtHS1 shows good potential for use as a natural material in the study of antimicrobial agents.

Histatin 5 initiates osmotic stress response in Candida albicans via activation of the Hog1 mitogen-activated protein kinase pathway

Histatin 5 (Hst 5) is a salivary cationic peptide that has toxicity for Candida albicans by inducing rapid cellular ion imbalance and cell volume loss. Microarray analyses of peptide-treated cells were used to evaluate global gene responses elicited by Hst 5. The major transcriptional response of C. albicans to Hst 5 was expression of genes involved in adaptation to osmotic stress, including production of glycerol (RHR2, SKO1, and PDC11) and the general stress response (CTA1 and HSP70). The oxidative-stress genes AHP1, TRX1, and GPX1 were mildly induced by Hst 5. Cell defense against Hst 5 was dependent on the Hog1 mitogen-activated protein kinase (MAPK) pathway, since C. albicans hog1/hog1 mutants were significantly hypersensitive to Hst 5 but not to Mkc1 MAPK or Cek1 MAPK mutants. Activation of the high-osmolarity glycerol (HOG) pathway was demonstrated by phosphorylation of Hog1 MAPK as well as by glycerol production following Hst 5 treatment in a dose-dependent manner. C. albicans cells prestressed with sorbitol were less sensitive to subsequent Hst 5 treatment; however, cells treated concurrently with osmotic stress and Hst 5 were hypersensitive to Hst 5. In contrast, cells subjected to oxidative stress had no difference in sensitivity to Hst 5. These results suggest a common underlying cellular response to osmotic stress and Hst 5. The HOG stress response pathway likely represents a significant and effective challenge to physiological levels of Hst 5 and other toxic peptides in fungal cells.

Energy depletion protects Candida albicans against antimicrobial peptides by rigidifying its cell membrane

Inhibitors of the energy metabolism, such as sodium azide and valinomycin, render yeast cells completely resistant against the killing action of a number of cationic antimicrobial peptides, including the salivary antimicrobial peptide Histatin 5. In this study the Histatin 5-mediated killing of the opportunistic yeast Candida albicans was used as a model system to comprehensively investigate the molecular basis underlying this phenomenon. Using confocal and electron microscopy it was demonstrated that the energy poison azide reversibly blocked the entry of Histatin 5 at the level of the yeast cell wall. Azide treatment hardly induced depolarization of the yeast cell membrane potential, excluding it as a cause of the lowered sensitivity. In contrast, the diminished sensitivity to Histatin 5 of energy-depleted C. albicans was restored by increasing the fluidity of the membrane using the membrane fluidizer benzyl alcohol. Furthermore, rigidification of the membrane by incubation at low temperature or in the presence of the membrane rigidifier Me(2)SO increased the resistance against Histatin 5, while not affecting the energy charge of the cell. In line, azide induced alterations in the physical state of the interior of the lipid bilayer. These data demonstrate that changes in the physical state of the membrane underlie the increased resistance to antimicrobial peptides.

Dermaseptins from Phyllomedusa oreades and Phyllomedusa distincta: liposomes fusion and/or lysis investigated by fluorescence and atomic force microscopy

Three dermaseptins, DS 01, DD K, and DD L, were compared with respect to their structural features and interactions with liposomes. Circular dichroic spectra at alcohols of different chain lengths revealed that DS 01 has the higher helicogenic potential in hydrophobic media. Binding of DS 01, DD K, and DD L to liposomes induced significant blue shifts of the emission spectra of the single tryptophan located at position 3 of all sequences indicating association of the peptides with bilayers. Kinetics evaluation of atomic force microscopy images evidenced the strong fusogenic activity of DS 01 whereas DD K and DD L showed increased lytic activities.

Advances in antimicrobial peptide immunobiology

Antimicrobial peptides are ancient components of the innate immune system and have been isolated from organisms spanning the phylogenetic spectrum. Over an evolutionary time span, these peptides have retained potency, in the face of highly mutable target microorganisms. This fact suggests important coevolutionary influences in the host-pathogen relationship. Despite their diverse origins, the majority of antimicrobial peptides have common biophysical parameters that are likely essential for activity, including small size, cationicity, and amphipathicity. Although more than 900 different antimicrobial peptides have been characterized, most can be grouped as belonging to one of three structural classes: (1) linear, often of alpha-helical propensity; (2) cysteine stabilized, most commonly conforming to beta-sheet structure; and (3) those with one or more predominant amino acid residues, but variable in structure. Interestingly, these biophysical and structural features are retained in ribosomally as well as nonribosomally synthesized peptides. Therefore, it appears that a relatively limited set of physicochemical features is required for antimicrobial peptide efficacy against a broad spectrum of microbial pathogens. During the past several years, a number of themes have emerged within the field of antimicrobial peptide immunobiology. One developing area expands upon known microbicidal mechanisms of antimicrobial peptides to include targets beyond the plasma membrane. Examples include antimicrobial peptide activity involving structures such as extracellular polysaccharide and cell wall components, as well as the identification of an increasing number of intracellular targets. Additional areas of interest include an expanding recognition of antimicrobial peptide multifunctionality, and the identification of large antimicrobial proteins, and antimicrobial peptide or protein fragments derived thereof. The following discussion highlights such recent developments in antimicrobial peptide immunobiology, with an emphasis on the biophysical aspects of host-defense polypeptide action and mechanisms of microbial resistance.

Histidine-rich glycoprotein exerts antibacterial activity

Histidine-rich glycoprotein (HRGP), an abundant heparin-binding protein found in plasma and thrombocytes, exerts antibacterial effects against Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). Fluorescence studies and electron microscopy to assess membrane permeation showed that HRGP induces lysis of E. faecalisbacteria in the presence of Zn2+ or at low pH. Heparin blocked binding of the protein to E. faecalis and abolished antibacterial activity. Furthermore, truncated HRGP, devoid of the heparin-binding and histidine-rich domain, was not antibacterial. It has previously been shown that peptides containing consensus heparin-binding sequences (Cardin and Weintraub motifs) are antibacterial. Thus, the peptide (GHHPH)4, derived from the histidine-rich region of HRGP and containing such a heparin-binding motif, was antibacterial for E. faecalis in the presence of Zn2+ or at low pH. The results show a previously undisclosed antibacterial activity of HRGP and suggest that the histidine-rich and heparin-binding domain of HRGP mediates the antibacterial activity of the protein.

The human cathelicidin peptide LL-37 and truncated variants induce segregation of lipids and proteins in the plasma membrane of Candida albicans

The human cathelicidin peptide LL-37 and several truncated variants differ in their capability to transmigrate over the plasma membrane of Candida albicans. We investigated whether retention at the cell perimeter or membrane transmigration affects their membrane-disrupting activities and candidacidal properties. Using fluorescein-labeled peptides, we demonstrate that LL-37 and its C-terminally truncated peptide LL-31 remain permanently associated with the perimeter of the cell. The N-terminally truncated peptide RK-31 initially accumulated at the cell boundary, but transmigrated into the cytoplasm within 30 min. The C-terminally truncated peptide LL-25 transmigrated instantaneously into the cytoplasm. The ultrastructural effects on the plasma membrane were studied by freeze-fracture electron microscopy combined with filipin cytochemistry. All peptides, whether they transmigrated over the plasma membrane or not, induced phase separation in the plasma membrane. All peptides induced leakage of cell components, including nucleotides and proteins. Proteins were identified by SDS-PAGE in combination with mass spectrometry, which revealed that predominantly proteins smaller than 50 kDa had leaked out of C. albicans.

Zinc potentiates the antibacterial effects of histidine-rich peptides against Enterococcus faecalis

Antimicrobial peptides are effector molecules of the innate immune system. We have recently shown that peptides containing multiples of the heparin-binding Cardin and Weintraub motifs AKKARA and ARKKAAKA exert antimicrobial activities. Here, we show that replacement of lysine and arginine in these motifs by histidine abrogates the antibacterial effects of these peptides. Antibacterial activity of the histidine-rich peptides against the Gram-positive bacterium Enterococcus faecalis was restored by the addition of Zn2+. Fluorescence microscopy experiments showed that Zn2+ enabled binding of the histidine-rich peptides to Enterococcus faecalis bacteria. Similar Zn2+-dependent antibacterial activities were shown for histatin 5 as well as histidine-containing peptides derived from the Zn2+- and heparin-binding domain 5 of human kininogen. Thus, the results demonstrate a previously undisclosed Zn2+-dependent antibacterial activity of kininogen-derived peptides and indicate an important role for Zn2+ in regulating the antimicrobial activities of histidine-rich peptides.

Distinct antifungal mechanisms: beta-defensins require Candida albicans Ssa1 protein, while Trk1p mediates activity of cysteine-free cationic peptides

Salivary histatin 5 (Hst 5) kills the fungal pathogen Candida albicans via a multistep process which includes binding to Ssa1/2 proteins on the cell surface and requires the TRK1 potassium transporter. Hst 5-induced membrane permeability to propidium iodide (PI) was nearly abolished in strain CaTK1 (TRK1/trk1), suggesting that Hst 5-induced influx of PI is via Trk1p. To explore the functional role of Trk1p in the mechanism of other antifungal peptides, we evaluated candidacidal activity and PI uptake in wild-type strain CaTK2 (TRK1/TRK1) and strain CaTK1 following treatment with lactoferricin 11 (LFcn 11), bactenecin 16 (BN 16), and virion-associated protein VPR 12. Strain CaTK1 was resistant to killing with these peptides (VPR 12 > LFcn 11 > BN 16), showing the requirement of Trk1p for fungicidal activity. In contrast, human neutrophil defensin 1 (HNP-1), human beta-defensin 2 (hBD-2), and hBD-3 effects on viability of and membrane permeability to PI were not different between mutant and wild-type strains, clearly showing that their candidacidal mechanism does not involve Trk1p as a functional effector. To test whether defensins require binding to Candida surface Ssa1/2 proteins for their activity, we measured the killing effectiveness in SSA1/2 mutant strains. Both hBD-2 and hBD-3, but not HNP-1, exhibited reduced killing of ssa1Delta and ssa2Delta strains compared to the wild type, showing that Ssa1 and Ssa2 proteins are required for their fungicidal activity. These results demonstrate that (i) Trk1p mediates candidacidal activities of cysteine-free peptides, but not of defensins, and (ii) hBD-2 and hBD-3, but not HNP-1, require Ssa1/2p for antifungal activity.

A membrane-destabilizing peptide in capsid protein L2 is required for egress of papillomavirus genomes from endosomes

Papillomaviruses are internalized via clathrin-dependent endocytosis. However, the mechanism by which viral genomes pass endosomal membranes has not been elucidated. In this report we show that the minor capsid protein L2 is required for egress of viral genomes from endosomes but not for initial uptake and uncoating and that a 23-amino-acid peptide at the C terminus of L2 is necessary for this function. Pseudogenomes encapsidated by L1 and L2 lacking this peptide accumulated in vesicular compartments similar to that observed with L1-only viral particles, and these mutant pseudoviruses were noninfectious. This L2 peptide displayed strong membrane-disrupting activity, induced cytolysis of bacteria and eukaryotic cells in a pH-dependent manner, and permeabilized cells after exogenous addition. Fusions between green fluorescent protein and the L2 peptide integrated into cellular membranes like the wild type but not like C-terminal mutants of L2. Our data indicate that the L2 C terminus facilitates escape of viral genomes from the endocytic compartment and that this feature is conserved among papillomaviruses. Furthermore, the characteristic of this peptide differs from the classical virus-encoded membrane-penetrating peptides.

Candidacidal effects of two antimicrobial peptides: histatin 5 causes small membrane defects, but LL-37 causes massive disruption of the cell membrane

The effects of antimicrobial peptides on artificial membranes have been well-documented; however, reports on the ultrastructural effects on the membranes of micro-organisms are relatively scarce. We compared the effects of histatin 5 and LL-37, two antimicrobial peptides present in human saliva, on the functional and morphological properties of the Candida albicans cell membrane. Fluorescence microscopy and immunogold transmission electron microscopy revealed that LL-37 remained associated with the cell wall and cell membrane, whereas histatin 5 transmigrated over the membrane and accumulated intracellularly. Freeze-fracture electron microscopy revealed that LL-37 severely affected the membrane morphology, resulting in the disintegration of the membrane bilayer into discrete vesicles, and an instantaneous efflux of small molecules such as ATP as well as larger molecules such as proteins with molecular masses up to 40 kDa. The effects of histatin 5 on the membrane morphology were less pronounced, but still resulted in the efflux of nucleotides. As the morphological defects induced by histatin 5 are much smaller than those induced by LL-37, but the efflux of nucleotides is similar at comparable candidacidal concentrations, we suggest that the loss of nucleotides plays an important role in the killing process.

Reactive oxygen species play no role in the candidacidal activity of the salivary antimicrobial peptide histatin 5

The mechanism of action of antimicrobial peptides is still a matter of debate. The formation of ROS (reactive oxygen species) has been suggested to be the crucial step in the fungicidal mechanism of a number of antimicrobial peptides, including histatin 5 and lactoferrin-derived peptides. In the present study we have investigated the effects of histatin 5 and of a more amphipathic synthetic derivative, dhvar4, on the generation of ROS in the yeast Candida albicans, using dihydroethidium as an indicator for ROS. With both peptides, a substantial enhancement of fluorescence was observed. However, TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), a cell-permeant ROS scavenger, did not have an inhibitory effect on killing or on the enhancement of fluorescence. Furthermore, antimycin and azide, which have been reported to induce ROS in vitro, were not able to enhance the dihydroethidium fluorescence, while chlorhexidine, a non-specific antiseptic agent, enhanced dihydroethidium fluorescence to the same extent as did the peptides. Fluorescence microscopy showed the fluorescence enhancement to be a consequence of the release of unbound preformed ethidium from the mitochondrial matrix within the cell. It is concluded that ROS do not play a role in the histatin 5-mediated killing of C. albicans.

In vivo comparison of Dhvar-5 and gentamicin in an MRSA osteomyelitis prevention model

OBJECTIVES

The continued rise in drug-resistant pathogens has led to global research efforts into new antimicrobial agents. A promising class of new agents are the antimicrobial peptides. The aim of the study was to investigate the efficacy of the antimicrobial peptide Dhvar-5 in a prophylactic, methicillin-resistant Staphylococcus aureus (MRSA) osteomyelitis model.

METHODS

Dhvar-5 (12 mg or 24 mg/rabbit) was incorporated into polymethyl methacrylate (PMMA) beads as a local drug delivery system. For comparison, plain beads (control) and beads containing gentamicin as a sulphate (10 mg or 24 mg per rabbit) were also prepared. The beads were inserted into the inoculated femoral cavity of 36 rabbits, and 1 week later they were killed. The presence and severity of MRSA osteomyelitis was assessed by culture and histology.

RESULTS

Both the 24 mg Dhvar-5 beads and the 24 mg gentamicin sulphate beads significantly reduced the bacterial load of the inoculated femora compared with the control chain. Although a 24 mg Dhvar-5 dose inhibited MRSA growth, it did not completely sterilize the femora. Sterilization occurred only in some of the gentamicin-treated specimens.

CONCLUSION

We conclude that both the gentamicin beads and the Dhvar-5 beads were only partially effective at preventing MRSA infection in this model.