Histatin 3-mediated killing of Candida albicans: effect of extracellular salt concentration on binding and internalization

Zn(II) coordination influences the secondary structure, but not antimicrobial activity of the N-terminal histatin 3 hydrolysis product

The relationship between the coordination chemistry and antimicrobial activity of Zn(II) and Cu(II)-bound histatins, salivary antimicrobial peptides, remains enigmatic. We focus on metal complexes of histatin 3 and its two products of hydrolysis: histatin 4 and its N-terminal fragment (histatin 3-4). The thermodynamic stability of these complexes is quite expected - the binding of Cu(II) via the ATCUN motif results in the formation of very stable complexes. In histatin-Zn(II) complexes, the {2Nim} type of coordination dominates, with polymorphic binding sites observed for histatin 3-4 and 5-8, resulting in their low thermodynamic stability compared to the complexes of histatin 3, 4, 5 and 8 with Zn(II), in which we observe a {2Nim, O-} type of coordination. Histatin 3, 3-4 and 4 have greater activity against Gram-positive bacteria than against Gram-negative ones, and Cu(II) or Zn(II) binding can, in some cases, moderately increase the antimicrobial activity of the native histatin 3 and 4, but not the remaining 3-4 fragment. The most probable reason for the metal-enhanced antimicrobial activity is, in this case, a local change of charge, while the chemically fascinating metal binding induced structural changes do not result in a change of biological activity. Neither histatin 3-4, the N-terminal fragment of histatin 3, which remains in solution after cleavage, nor its metal complexes have any antimicrobial activity, but histatin 3-4 presents intriguing Zn(II)-induced structural behavior, changing its secondary structure, with a tendency to form an α-helix.

Bioinspired peptides induce different cell death mechanisms against opportunistic yeasts

The management of fungal diseases imposes an urgent need for the development of effective antifungal drugs. Among new drug candidates are the antimicrobial peptides, and especially their derivatives. Here, we investigated the molecular mechanism of action of three bioinspired peptides against the opportunistic yeasts Candida tropicalis and Candida albicans. We assessed morphological changes, mitochondrial functionality, chromatin condensation, ROS production, activation of metacaspases, and the occurrence of cell death. Our results indicated that the peptides induced sharply contrasting death kinetics, of 6 h for RR and 3 h for D-RR to C. tropicalis and 1 h for WR to C. albicans. Both peptide-treated yeasts exhibited increased ROS levels, mitochondrial hyperpolarization, cell size reduction, and chromatin condensation. RR and WR induced necrosis in C. tropicalis and C. albicans, but not D-RR in C. tropicalis. The antioxidant ascorbic acid reverted the toxic effect of RR and D-RR, but not WR, suggesting that instead of ROS there is a second signal triggered that leads to yeast death. Our data suggest that RR induced a regulated accidental cell death in C. tropicalis, D-RR induced a programmed cell death metacaspase-independent in C. tropicalis, while WR induced an accidental cell death in C. albicans. Our results were obtained with the LD100 and within the time that the peptides induce the yeast death. Within this temporal frame, our results allow us to gain clarity on the events triggered by the peptide-cell interaction and their temporal order, providing a better understanding of the death process induced by them.

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.

A Novel Mutant of rLj-RGD3 (rLj-112) Suppressed the Proliferation and Metastasis of B16 Cells through the EGFR Signaling Pathway

Lj-RGD3, which contains three Arg⁻Gly⁻Asp (RGD) motifs, was first identified from the buccal glands of Lampetra japonica and has been shown to suppress the tumor progression in the previous studies. Apart from the three RGD motifs, Lj-RGD3 is also characterized by its high content of histidine in its amino acid sequence. In order to clarify whether the histidine-rich characterization of Lj-RGD3 is also associated with its anti-tumor activity, mutants were designed in which the three RGD motifs (Lj-112), or all histidines (Lj-27) or both (Lj-26) were deleted. Furthermore, a mutant (Lj-42) in which all histidines and three RGD motifs were respectively substituted with alanines and three Ala⁻Gly⁻Asp (AGD) motifs, as well as a mutant (Lj-41) in which all histidines were substituted with alanines was synthesized to avoid alterations in structure which might further cause changes in the peptides' functions. After recombination and purification, recombinant Lj-112 (rLj-112), recombinant Lj-27 (rLj-27), recombinant Lj-41 (rLj-41), and recombinant Lj-RGD3 (rLj-RGD3) exhibited anti-proliferative activity in B16 cells, respectively; while recombinant Lj-26 (rLj-26) and recombinant Lj-42 (rLj-42) did not affect the proliferation of B16 cells significantly. In addition, the anti-proliferative activity of rLj-112 in B16 cells was due to apoptosis. Typical apoptosis features were observed, including chromatin condensation, fragmented DNA, and increased levels of cleaved caspase 3/caspase 7/nuclear enzyme poly (ADP-ribose) polymerase (PARP) in B16 cells. Similar to rLj-RGD3, rLj-112 was also capable of suppressing the migration and invasion of B16 cells by disturbing the F-actin arrangement. After labeling with FITC, rLj-112 was found localized in the cytoplasm of B16 cells, which induced the internalization of epidermal growth factor receptor (EGFR), suggesting that rLj-112 might block the EGFR mediated signaling pathway. Actually, the phosphorylation level of EGFR and its downstream signal molecules including Akt, PI3K, p38, and ERK1/2 was reduced in the rLj-112 treated B16 cells. In vivo, rLj-112 also inhibited the growth, weight, and volume of the tumors in B16 xenografted C57BL/6 mice without reducing their body weight, indicating that rLj-112 might be safe and might be used as an effective anti-tumor drug in the near future.

Manganese transport is essential for N2 -fixation by Rhizobium leguminosarum in bacteroids from galegoid but not phaseoloid nodules

Rhizobium leguminosarum has two high-affinity Mn2+ transport systems encoded by sitABCD and mntH. In symbiosis, sitABCD and mntH were expressed throughout nodules and also strongly induced in Mn2+ -limited cultures of free-living cells. Growth of a sitA mntH double mutant was severely reduced under Mn2+ limitation and sitA and mntH single mutants were more sensitive to oxidative stress. The double sitA mntH mutant of R. leguminosarum was unable to fix nitrogen (Fix- ) with legumes belonging to the galegoid clade (Pisum sativum, Vicia faba and Vicia hirsuta). The presence of infection thread-like structures and sparsely-packed plant cells in nodules suggest that bacteroid development was blocked, either at a late stage of infection thread progression or during bacteroid-release. In contrast, a double sitA mntH mutant was Fix+ on common bean (Phaseoli vulgaris), a member of the phaseoloid clade of legumes, indicating a host-specific symbiotic requirement for Mn2+ transport.

Immunocytochemical Localization of Histatins in Human Salivary Glands

Histatins are a family of salivary proteins with bactericidal and fungicidal activities that contribute to the innate defense of the oral cavity. Histatins are present in the serous granules of the parotid and submandibular glands. The important role of histatins in saliva, and the limited information on their cellular and subcellular distribution, prompted us to further define the localization of histatins in the major salivary glands. Immunogold-silver staining of 1-μm sections of plastic-embedded tissue with anti-histatin antibody revealed histatin immunoreactivity in the serous acinar cells of the parotid and submandibular glands, the serous demilune cells of the submandibular and sublingual glands, and in occasional intercalated duct cells. No reactivity was seen in mucous cells or in striated or excretory duct cells. Electron microscopic observations of thin sections labeled with anti-histatin and gold-labeled secondary antibodies revealed immunoreactivity associated with the rough endoplasmic reticulum and Golgi complex and in secretory granules of serous acinar and demilune cells. The granules of parotid acinar cells exhibited relatively uniform labeling of their content, whereas the granules of serous cells in the submandibular and sublingual glands showed variable labeling of the dense and light regions of their content. A few intercalated duct cells adjacent to the acinar cells also exhibited labeled granules. These results suggest that the serous cells of the major glands are the main source of histatins in human saliva. They are also consistent with several previous studies demonstrating the variable distribution of different proteins within the granule content.

Diagnostic model of saliva peptide finger print analysis of oral squamous cell carcinoma patients using weak cation exchange magnetic beads

Whole saliva (WS) was used for diagnosis of oral squamous cell carcinoma (OSCC); two polypeptides may be used for OSCC diagnosis.

Saliva diagnostics utilizing nanotechnology and molecular technologies to detect oral squamous cell carcinoma (OSCC) has become an attractive field of study. However, no specific methods have been established. To refine the diagnostic power of saliva peptide fingerprints for the early detection of OSCC, we screened the expression spectrum of salivary peptides in 40 T1 stage OSCC patients (and healthy controls) using MALDI-TOF-MS combined with magnetic beads. Fifty proteins showed significantly different expression levels in the OSCC samples (P<0.05). Potential biomarkers were also predicted. The novel diagnostic proteomic model with m/z peaks of 1285.6 Da and 1432.2 Da are of certain value for early diagnosis of OSCC.

Innate Humoral Defense Factors

Although innate immunity came into the research spotlight in the late 1990s when its instructive role in the adaptive immune response was recognized, innate humoral defense factors have a much older history. The exocrine secretions of the body contain a plethora of distinct soluble factors (lysozyme, lactoferrin, peroxidases, proline-rich proteins, histatins, etc.) that protect the body from mucosal microbial pathogens. More recent studies have established that the humoral arm of innate immunity contains a heterogeneous group of pattern-recognition molecules (e.g., pentraxins, collectins, and ficolins), which perform diverse host-defense functions, such as agglutination and neutralization, opsonization, control of inflammation, and complement activation and regulation. These pattern-recognition molecules, which act as functional predecessors of antibodies (“ante-antibodies”), and the classic soluble innate defense factors form an integrated system with complementary specificity, action, and tissue distribution, and they are the subject of this chapter.

A pathogenesis related protein, VpPR-10.1, from Vitis pseudoreticulata: an insight of its mode of antifungal activity

Previously, VpPR-10.1 was isolated and characterized from a cDNA library of a fungus-resistant accession of Chinese wild grape (Vitis pseudoreticulata). We found that expression of VpPR-10.1 is affected by the fungal pathogen Erysiphe necator. To investigate the biochemical basis of the nuclease activity of VpPR-10.1 and its role in antifungal resistance, we generated recombinant VpPR-10.1 as well as site-directed mutations targeting three conserved amino acid residues among plant PR-10 s: Lys55, Glu149, and Tyr151. We showed that wild-type recombinant VpPR-10.1 exhibits both RNase and DNase activities. Mutant VpPR10.1-Y151H essentially retained all these activities. In contrast, VpPR10.1-K55N, where Lys55 in the P-loop region is mutated to Asn, and VpPR10.1-E149G, where Glu149 is mutated to Gly, lost their nuclease activity, indicating that both residues play a critical role in catalyzing RNA and DNA degradation. Furthermore, VpPR10.1 and VpPR10.1-Y151H inhibited the growth of the cultured fungal pathogen Alternaria alternate. Through transient expression in grapevine, we also demonstrated that VpPR10.1-K55N and VpPR10.1-E149G compromised resistance to E. necator. Finally, we further found that VpPR-10.1 can lead to programmed cell death and DNA degradation when incubated with tobacco BY-2 suspension cells. We show here that Lys55 and Glu149, but not Tyr151, are required for the RNase, DNase and antifungal activities of VpPR-10.1. The strong correlation between the level of VpPR-10.1 nuclease activity and its antifungal property indicates that the former is the biochemical basis for the latter. Taken together, our experiments revealed that VpPR-10.1 is critical in mediating fungal resistance in grape, potentially playing a dual role by degrading pathogen RNA and inducing programmed death of host cells.

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.

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.

Uptake of the antifungal cationic peptide Histatin 5 by Candida albicans Ssa2p requires binding to non-conventional sites within the ATPase domain

Candida albicans Hsp70 Ssa1/2 proteins have been identified as cell wall binding partners for the antifungal cationic peptide Histatin 5 (Hst 5) in vivo. C. albicans Ssa2p plays a major role in binding and translocation of Hst 5 into fungal cells, as demonstrated by defective peptide uptake and killing in C. albicans SSA2 null mutants. Candidal Hsp70 proteins are classical chaperone proteins with two discrete functional domains consisting of peptide binding and ATP binding regions. Pull-down assays with full-length and truncated Ssa2 proteins found that the ATPase domain was required for Hst 5 binding. Further mapping of Ssa2p by limited digestion and peptide array analyses identified two discrete Hst 5-binding epitopes within the ATPase region. Expression of Ssa2p in C. albicans cells carrying mutations in the first epitope identified by thermolysin digestion (Ssa2_128−132A3) significantly reduced intracellular transport and fungicidal activity of Hst 5, confirming its importance as a binding site for Hst 5 function in vivo. Since this Hst 5 binding site lies within the Ssa2p ATPase domain near the ATP-binding cleft, it is possible that ATP modulates Hst 5 binding to Ssa2p. Indeed, gel filtration assays demonstrated that although nucleotides are not required for Hst 5 binding, their presence improved binding affinity by 10-fold. Thus, C. albicans Ssa2p binds Hst 5 at a surface-localized epitope in a subunit of the ATPase domain; and this region is required for intracellular translocation and killing functions of Hst 5.

Transcriptional regulation of the salivary histatin gene: finding of a strong positive regulatory element and its binding protein

Histatins are salivary proteins found and expressed in human salivary glands. They play a role in the non-immune system of antimicrobial defense, for instance, against Candida albicans. The transcriptional regulatory sequences of the histatin gene, HIS1, have remained obscure for a long time. Here, we cloned the putative promoter from human genomic DNA and tested it in a luciferase reporter system. This promoter is much more active in salivary gland cells than in other cell types. Analysis of deletion mutants revealed that the region encompassing -2254 to -1748 is a strong positive transcriptional element, and its functional core sequence (termed HTN27 box) works in correct and reverse orientations in synergy with downstream sequences, the region spanning -680 to +28 and a proximal promoter. The plus single-stranded HTN27 box is specifically bound by a 100 kDa protein that is present in HSG cells, but not in HeLa cells. These findings indicate that the regulation of the histatin gene expression may be intricate, and it seems to have a cell-type preference in the salivary gland cells.

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.

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.

Factors affecting antimicrobial activity of MUC7 12-mer, a human salivary mucin-derived peptide

Background

MUC7 12-mer (RKSYKCLHKRCR), a cationic antimicrobial peptide derived from the human low-molecular-weight salivary mucin MUC7, possesses potent antimicrobial activity in vitro. In order to evaluate the potential therapeutic application of the MUC7 12-mer, we examined the effects of mono- and divalent cations, EDTA, pH, and temperature on its antimicrobial activity.

Methods

Minimal Inhibitory Concentrations (MICs) were determined using a liquid growth inhibition assay in 96-well microtiter plates. MUC7 12-mer was added at concentrations of 1.56–50 μM. MICs were determined at three endpoints: MIC-0, MIC-1, and MIC-2 (the lowest drug concentration showing 10%, 25% and 50% of growth, respectively). To examine the effect of salts or EDTA, a checkerboard microdilution technique was used. Fractional inhibitory concentration index (FICi) was calculated on the basis of MIC-0. The viability of microbial cells treated with MUC7 12-mer in the presence of sodium or potassium was also determined by killing assay or flow cytometry.

Results

The MICs of MUC7 12-mer against organisms tested ranged from 6.25–50 μM. For C. albicans, antagonism (FICi 4.5) was observed for the combination of MUC7 12-mer and calcium; however, there was synergism (FICi 0.22) between MUC7 12-mer and EDTA, and the synergism was retained in the presence of calcium at its physiological concentration (1–2 mM). No antagonism but additivity or indifference (FICi 0.55–2.5) was observed for the combination of MUC7 12-mer and each K⁺, Na⁺, Mg²⁺, or Zn²⁺. MUC7 12-mer peptide (at 25 μM) also exerted killing activity in the presence of NaCl, (up to 25 mM for C. albicans and up to 150 mM for E. coli, a physiological concentration of sodium in the oral cavity and serum, respectively) and retained candidacidal activity in the presence of KCl (up to 40 mM). The peptide exhibited higher inhibitory activity against C. albicans at pH 7, 8, and 9 than at pH 5 and 6, and temperature up to 60°C did not affect the activity.

Conclusion

MUC7 12-mer peptide is effective anticandidal agent at physiological concentrations of variety of ions in the oral cavity. These results suggest that, especially in combination with EDTA, it could potentially be applied as an alternative therapeutic agent for the treatment of human oral candidiasis.

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.

The role of released ATP in killing Candida albicans and other extracellular microbial pathogens by cationic peptides

A unifying theme common to the action of many cationic peptides that display lethal activities against microbial pathogens is their specific action at microbial membranes that results in selective loss of ions and small nucleotides chiefly ATP. One model cationic peptide that induces non-lytic release of ATP from the fungal pathogen Candida albicans is salivary histatin 5 (Hst 5). The major characteristic of Hst 5-induced ATP release is that it occurs rapidly while cells are still metabolically active and have polarized membranes, thus precluding cell lysis as the means of release of ATP. Other cationic peptides that induce selective release of ATP from target microbes are lactoferricin, human neutrophil defensins, bactenecin, and cathelicidin peptides. The role of released extracellular ATP induced by cationic peptides is not known, but localized increases in extracellular ATP concentration may serve to potentiate cell killing, facilitate further peptide uptake, or function as an additional signal to activate the host innate immune system at the site of infection.

A pathogenesis related protein, AhPR10 from peanut: an insight of its mode of antifungal activity

A pathogenesis related protein (AhPR10) is identified from a clone of 6-day old Arachis hypogaea L. (peanut) cDNA library. The clone expressed as a approximately 20 kDa protein in E. coli. Nucleotide sequence derived amino acid sequence of the coding region shows its homology with PR10 proteins having Betv1 domain and P loop motif. Recombinant AhPR10 has ribonuclease activity, and antifungal activity against the peanut pathogens Fusarium oxysporum and Rhizoctonia solani. Mutant protein AhPR10-K54N where lys54 is mutated to asn54 loses its ribonuclease and antifungal activities. FITC labeled AhPR10 and AhPR10-K54N are internalized by hyphae of F. oxysporum and R. solani but the later protein does not inhibit the fungal growth. This suggests that the ribonuclease function of AhPR10 is essential for its antifungal activity. Energy and temperature dependent internalization of AhPR10 into sensitive fungal hyphae indicate that internalization of the protein occurs through active uptake.

Human beta-defensins kill Candida albicans in an energy-dependent and salt-sensitive manner without causing membrane disruption

Human beta-defensin 2 (hBD-2) and hBD-3 have potent fungicidal activity in the micromolar range. Although little is known about their mechanism of action against Candida species, some similarities to the antifungal mechanism of salivary peptide histatin 5 (Hst 5) seem to exist. Since hBD-2 and hBD-3 have been reported to cause direct disruption of target cell membranes, we compared the effects of hBD-2 and hBD-3 on Candida albicans membrane integrity. Incubation of calcein-loaded C. albicans cells with a dose of hBD-2 lethal for 90% of the strains tested (LD(90)) resulted in a maximal dye efflux of only 10.3% +/- 2.8% at 90 min, similar to that induced by Hst 5. In contrast, an LD(90) of hBD-3 more than doubled calcein release from cells yet did not result in more than 24% of total release, showing that neither peptide caused gross membrane damage. As for Hst 5, killing of C. albicans cells by hBD-2 and hBD-3 was salt sensitive; however, Ca(2+) and Mg(2+) inhibited hBD-2 but not hBD-3 fungicidal activity. Pretreatment of C. albicans cells with sodium azide resulted in significantly decreased ATP release and susceptibility of cells to hBD-2 and hBD-3. However, hBD-3 killing was partially restored at concentrations of > or =0.8 microM, showing energy-independent mechanisms at higher doses. C. glabrata resistance to Hst 5, hBD-2, and hBD-3 is not a result of loss of expression of cell wall Ssa proteins. The candidacidal effects of hBD-2-hBD-3 and Hst 5-hBD-2 were additive, while the index of interaction between Hst 5 and hBD-3 was 0.717 (P < 0.05). Thus, the candidacidal action of hBD-2 shows many similarities to that of Hst 5 in terms of salt sensitivity, ion selectivity, and energy requirements while hBD-3 exhibits biphasic concentration-dependent mechanisms of candidacidal action complementary to those of Hst 5.

Roles of cellular respiration, CgCDR1, and CgCDR2 in Candida glabrata resistance to histatin 5

Histatin 5, a human salivary protein with broad-spectrum antifungal activity, is remarkably ineffective against Candida glabrata. Fluconazole resistance in this fungus is due in most cases to upregulation of CgCDR efflux pumps. We investigated whether the distinct resistance of C. glabrata to histatin 5 is related to similar mechanisms.

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.

Antimicrobial peptides in the oral environment: expression and function in health and disease

The oral cavity is a unique environment in which antimicrobial peptides play a key role in maintaining health and may have future therapeutic applications. Present evidence suggests that alpha-defensins, beta-defensins, LL-37, histatin, and other antimicrobial peptides and proteins have distinct but overlapping roles in maintaining oral health and preventing bacterial, fungal, and viral adherence and infection. The expression of the inducible hBD-2 in normal oral epithelium, in contrast to other epithelia, and the apparent differential signaling in response to commensal and pathogenic organisms, provides new insights into innate immunity in this body site. Commensal bacteria are excellent inducers of hBD-2 in oral epithelial cells, suggesting that the commensal bacterial community acts in a manner to benefit the overall innate immune readiness of oral epithelia. This may have major significance for understanding host defense in the complex oral environment.

Saliva affects the antifungal activity of exogenously added histatin 3 towardsCandida albicans

Antifungal activity of histatin 3 against two Candida albicans clinical isolates was determined in assays containing rabbit submandibular gland saliva. Histatin 3 inhibited the cell growth and germination of both isolates dose-dependently (10-100 microg ml(-1)) with maximum inhibition occurring after 60 min incubation. Adding fresh histatin 3 after 60 min caused further reduction in the viable cell count. Higher histatin 3 concentrations (50-100 microg ml(-1)) and prolonged exposure to peptide were required to inhibit germination. Histatin 3 was rapidly degraded in rabbit submandibular gland saliva and this may explain why fresh addition of histatin 3 increases candidacidal activity.

Histatin-derived peptides: potential agents to treat localised infections

Histatins are a family of histidine-rich, cationic peptides composed of up to 38 amino acids. They are secreted by the salivary glands of humans and some subhuman primates and are thought to be part of the host defence system in the oral cavity. Histatins exhibit in vitro activity against both bacteria and yeast, common to other antimicrobial peptides. Because of these activities, histatin-based peptides could play an important role in the treatment and prevention of infectious diseases. A 12 amino acid amidated fragment of histatin 5, designated P-113, has been identified as the smallest fragment that retains antimicrobial activity comparable to the parent compound. Animal studies and human clinical trials showed that P-113 has potential in preventing the development of gingivitis, with no adverse side effects. Histatin peptides also could be used for other therapeutic applications in which the infection is localised and accessible via topical delivery, such as treatment of candidiasis (thrush) and mucositis in the oral cavity, skin infections and treatment of lung infections afflicting cystic fibrosis patients.

Effects of human lactoferrin on the cytoplasmic membrane of Candida albicans cells related with its candidacidal activity

Human lactoferrin is an innate host defence protein with antimicrobial activity that exerts a candidacidal effect in a cation concentration-dependent manner. We investigated the ability of this cationic protein (with an isoelectric point of 8.7) to permeabilize the cytoplasmic membrane of Candida albicans cells. Despite minor K(+)-release in lactoferrin-treated C. albicans cells, the killing effect was not related to an extensive membrane permeabilization, as indicated by: (a) the non-release of macromolecular cytosolic constituents; (b) the non-permeabilization for extracellular propidium iodide nor for intracellular accumulated calcein; and (c) the inability to disrupt the phospholipid bilayer of 8-aminonaphthalene-1,3,6, trisulfonic acid/p-xylene-bis-pyridiniumbromide-loaded liposomes. These results suggest that lactoferrin exerts its candidacidal effect through a mechanism different from membrane permeabilization described for other cationic peptides.

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.

The TRK1 potassium transporter is the critical effector for killing of Candida albicans by the cationic protein, Histatin 5

The principal feature of killing of Candida albicans and other pathogenic fungi by the catonic protein Histatin 5 (Hst 5) is loss of cytoplasmic small molecules and ions, including ATP and K(+), which can be blocked by the anion channel inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. We constructed C. albicans strains expressing one, two, or three copies of the TRK1 gene in order to investigate possible roles of Trk1p (the organism's principal K(+) transporter) in the actions of Hst 5. All measured parameters (Hst 5 killing, Hst 5-stimulated ATP efflux, normal Trk1p-mediated K(+) ((86)Rb(+)) influx, and Trk1p-mediated chloride conductance) were similarly reduced (5-7-fold) by removal of a single copy of the TRK1 gene from this diploid organism and were fully restored by complementation of the missing allele. A TRK1 overexpression strain of C. albicans, constructed by integrating an additional TRK1 gene into wild-type cells, demonstrated cytoplasmic sequestration of Trk1 protein, along with somewhat diminished toxicity of Hst 5. These results could be produced either by depletion of intracellular free Hst 5 due to sequestered binding, or to cooperativity in Hst 5-protein interactions at the plasma membrane. Furthermore, Trk1p-mediated chloride conductance was blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid in all of the tested strains, strongly suggesting that the TRK1 protein provides the essential pathway for ATP loss and is the critical effector for Hst 5 toxicity in C. albicans.

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

A number of cationic antimicrobial peptides, among which are histatin 5 and the derived peptides dhvar4 and dhvar5, enter their target cells and interact with internal organelles. There still are questions about the mechanisms by which antimicrobial peptides translocate across the membrane. We used a liposome model to study membrane binding, translocation and membrane-perturbing capacities of histatin 5, dhvar4 and dhvar5. Despite the differences in amphipathic characters of these peptides, they bound equally well to liposomes, whereas their membrane activities differed remarkably: dhvar4 translocated at the fastest rate, followed by dhvar5, whereas the histatin 5 translocation rate was much lower. The same pattern was seen for the extent of calcein release: highest with dhvar4, less with dhvar5 and almost none with histatin 5. The translocation and disruptive actions of dhvar5 did not seem to be coupled, because translocation occurred on a much longer timescale than calcein release, which ended within a few minutes. We conclude that peptide translocation can occur through peptide-phospholipid interactions, and that this is a possible mechanism by which antimicrobial peptides enter cells. However, the translocation rate was much lower in this model membrane system than that seen in yeast cells. Thus it is likely that, at least for some peptides, additional features promoting the translocation across biological membranes are involved as well.

Human salivary histatin 5 fungicidal action does not induce programmed cell death pathways in Candida albicans

Salivary histatins (Hsts) are potent candidacidal proteins that induce a nonlytic form of cell death in Candida albicans accompanied by loss of mean cell volume, cell cycle arrest, and elevation of intracellular levels of reactive oxygen species (ROS). Since these phenotypes are often markers of programmed cell death and apoptosis, we investigated whether other classical markers of apoptosis, including generation of intracellular ROS and protein carbonyl groups, chromosomal fragmentation (laddering), and cytochrome c release, are found in Hst 5-mediated cell death. Increased intracellular levels of ROS in C. albicans were detected in cells both following exogenous application of Hst 5 and following intracellular expression of Hst 5. However, Western blot analysis failed to detect specifically increased protein carbonylation in Hst 5-treated cells. There was no evidence of chromosomal laddering and no cytochrome c release was observed following treatment of C. albicans mitochondria with Hst 5. Superoxide dismutase enzymes of C. albicans and Saccharomyces cerevisiae provide essential protection against oxidative stress; therefore, we tested whether SOD mutants have increased susceptibility to Hst 5, as expected if ROS mediate fungicidal effects. Cell survival of S. cerevisiae SOD1/SOD2 mutants and C. albicans SOD1 mutants following Hst 5 treatment (31 micro M) was indistinguishable from the survival of wild-type cells treated with Hst 5. We conclude that ROS may not play a direct role in fungicidal activity and that Hst 5 does not initiate apoptosis or programmed cell death pathways.

Respiratory inhibition of isolated mammalian mitochondria by salivary antifungal peptide histatin-5

Histatin-5 is a peptide secreted in the human saliva, which possesses powerful antifungal activity. Previous studies have shown that this peptide exerts its candidacidal activity, through the inhibition of both mitochondrial respiration and the formation of reactive oxygen species. The purpose of the present study was to investigate the biological consequences of histatin-5 action on mammalian mitochondria to verify if the toxic mechanism exerted on mitochondria from Candida albicans is an exclusive for fungal cells. Moreover, hypothesising that the damage exerted on mitochondria may induce programmed cellular death pathways, we evaluated two main markers of apoptosis: the mitochondrial membrane potential (DeltaPsi) and the release of cytochrome c. The results obtained show that exposure of isolated mammalian mitochondria to histatin-5 determines: (i) a large inhibition of the respiratory chain at the level of complex I, (ii) a slight decrease in the mitochondrial membrane potential, and (iii) no release of cytochrome c.

Calcium blocks fungicidal activity of human salivary histatin 5 through disruption of binding with Candida albicans

Salivary histatin 5 (Hst 5) kills the fungal pathogen C. albicans via a mechanism that involves binding and subsequent efflux of cellular ATP. Our aims were to identify inorganic ions found in saliva that influence Hst 5 fungicidal activity. Increasing ionic strength with relevant salivary anions (Cl(-) and CO(3)(-)) did not reduce Hst 5 binding or uptake by yeast cells, but reduced the Hst-induced efflux of ATP. Extracellular MgCl(2) (25 mM) maximally inhibited 30-40% of Hst 5 killing with 40% reduction in ATP efflux, while pre-treatment of cells with only 2 mM CaCl(2) inhibited 80-90% of killing, and prevented ATP efflux. Loss of fungicidal activity by the addition of CaCl(2) or MgCl(2) was a result of inhibition of binding of Hst 5 to C. albicans cells. Calcium is a potent inhibitor of Hst 5 candidacidal activity at physiological concentrations and may be the primary salivary ion responsible for the masking effect of saliva.

Candida albicans Ssa1/2p is the cell envelope binding protein for human salivary histatin 5

Salivary histatins are a family of small histidine-rich peptides with potent antifungal activity. We previously identified a 70-kDa cell envelope protein in Candida albicans and Saccharomyces cerevisiae that mediates binding of histatin (Hst) 5. Isolation of Hst 5-binding protein followed by matrix-assisted laser desorption ionization mass spectrometry analysis identified this protein as the heat shock protein Ssa1p. Ssa protein and Hst 5-binding protein were found to be co-localized on immunoblots of yeast beta-mercaptoethanol cell wall extracts and cytosolic fractions. Yeast two-hybrid analysis showed strong interactions between Ssa1p and both Hst 3 and Hst 5. To assess functional roles of Ssa proteins in the Hst 5 antifungal mechanism in vivo, both binding and fungicidal assays were carried out using S. cerevisiae isogenic SSA1/SSA2 mutants. 125I-Hst 5 binding assays showed saturable binding (Kd = 2.57 x 10(-6) m) with the wild-type SSA1/SSA2 strain; however, Hst 5 binding with the Deltassa1ssa2 double mutant was reduced (Kd = 1.25 x 10(-6) m). Cell wall HSP70 proteins were also diminished, but still detectable, in S. cerevisiae Deltassa1ssa2 cells and are likely to be Ssa3p or Ssa4p. Hst 5 (31 microm) killed 80% of the wild-type cells in fungicidal assays at room temperature. However, only 50-60% killing of the single mutants (Deltassa1 and Deltassa2) was observed, and fungicidal activity was further reduced to 20-30% in the Deltassa1ssa2 double mutant. Incubation of cells under heat shock conditions increased the sensitivity of cells to Hst 5, which correlated with increased Hst 5-binding activity in Deltassa1ssa2 cells, but not in wild-type cells. This study provides evidence for a novel function for yeast Ssa1/2 proteins as cell envelope binding receptors for Hst 5 that mediate fungicidal activity.

Staphylococcus aureus susceptibility to innate antimicrobial peptides, beta-defensins and CAP18, expressed by human keratinocytes

The antimicrobial peptides human beta-defensin-1 (hBD1), hBD2, hBD3, and CAP18 expressed by keratinocytes have been implicated in mediation of the innate defense against bacterial infection. To gain insight into Staphylococcus aureus infection, the susceptibility of S. aureus, including methicillin-resistant S. aureus (MRSA), to these antimicrobial peptides was examined. Based on quantitative PCR, expression of hBD2 mRNA by human keratinocytes was significantly induced by contact with S. aureus, and expression of hBD3 and CAP18 mRNA was slightly induced, while hBD1 mRNA was constitutively expressed irrespective of the presence of S. aureus. Ten clinical S. aureus isolates, including five MRSA isolates, induced various levels of expression of hBD2, hBD3, and CAP18 mRNA by human kertinocytes. The activities of hBD3 and CAP18 against S. aureus were found to be greater than those of hBD1 and hBD2. A total of 44 S. aureus clinical isolates, including 22 MRSA strains, were tested for susceptibility to hBD3 and CAP18. Twelve (55%) and 13 (59%) of the MRSA strains exhibited more than 20% survival in the presence of hBD3 (1 microg/ml) and CAP18 (0.5 microg/ml), respectively. However, only three (13%) and two (9%) of the methicillin-sensitive S. aureus isolates exhibited more than 20% survival with hBD3 and CAP18, respectively, suggesting that MRSA is more resistant to these peptides. A synergistic antimicrobial effect between suboptimal doses of methicillin and either hBD3 or CAP18 was observed with 10 MRSA strains. Furthermore, of several genes associated with methicillin resistance, inactivation of the fmtC gene in MRSA strain COL increased susceptibility to the antimicrobial effect mediated by hBD3 or CAP18.

Killing of Candida albicans by human salivary histatin 5 is modulated, but not determined, by the potassium channel TOK1

Salivary histatin 5 (Hst 5), a potent toxin for the human fungal pathogen Candida albicans, induces noncytolytic efflux of cellular ATP, potassium, and magnesium in the absence of cytolysis, implicating these ion movements in the toxin's fungicidal activity. Hst 5 action on Candida resembles, in many respects, the action of the K1 killer toxin on Saccharomyces cerevisiae, and in that system the yeast plasma membrane potassium channel, Tok1p, has recently been reported to be a primary target of toxin action. The question of whether the Candida homologue of Saccharomyces Tok1p might be a primary target of Hst 5 action has now been investigated by disruption of the C. albicans TOK1 gene. The resultant strains (TOK1/tok1) and (tok1/tok1) were compared with wild-type Candida (TOK1/TOK1) for relative ATP leakage and killing in response to Hst 5. Patch-clamp measurements on Candida protoplasts were used to verify the functional deletion of Tok1p and to provide its first description in Candida. Tok1p is an outwardly rectifying, noisily gated, 40-pS channel, very similar to that described in Saccharomyces. Knockout of CaTOK1 (tok1/tok1) completely abolishes the currents and gating events characteristic of Tok1p. Also, knockout (tok1/tok1) increases residual viability of Candida after Hst 5 treatment to 27%, from 7% in the wild type, while the single allele deletion (TOK1/tok1) increases viability to 18%. Comparable results were obtained for Hst-induced ATP efflux, but quantitative features of ATP loss suggest that wild-type TOK1 genes function cooperatively. Overall, very substantial killing and ATP efflux are produced by Hst 5 treatment after complete knockout of wild-type TOK1, making clear that Tok1p channels are not the primary site of Hst 5 action, even though they do play a modulating role.

Binding, internalisation and degradation of histatin 3 in histatin-resistant derivatives of Candida albicans

The antifungal mechanism of salivary histatin has been studied in Candida albicans and involves binding to a specific receptor, translocation across the membrane and targeting intracellularly. Cell death correlates with non-lytic release of ATP that may function as a cytotoxic mediator extracellularly. By sequential exposure to increasing concentrations of histatin 3, we generated histatin-resistant derivatives of C. albicans strain CA132A that show five-fold less killing at physiological concentrations of histatin 3. Protection against histatin killing in histatin-resistant derivatives is not due to alterations in binding, internalisation or degradation of histatin or efflux of ATP. These results indicate that protective mechanisms activated by exposure to histatin 3 may involve unidentified pathways downstream of binding and internalisation events.

MUC7 20-Mer: investigation of antimicrobial activity, secondary structure, and possible mechanism of antifungal action

This study was aimed at examining the spectrum of antimicrobial activity of MUC7 20-mer (N-LAHQKPFIRKSYKCLHKRCR-C; residues 32 to 51 of MUC7, the low-molecular-weight human salivary mucin, comprised of 357 residues) and comparing its antifungal properties to those of salivary histatin 5 (Hsn-5). We also examined the secondary structure of the 20-mer and the possible mechanism of its antifungal action. Our results showed that MUC7 20-mer displays potent killing activity against a variety of fungi and both gram-positive and gram-negative bacteria at micromolar concentrations. Time-dependent killing of Candida albicans and Cryptococcus neoformans by MUC7 20-mer and Hsn-5 indicated differences in killing rates between MUC7 20-mer and Hsn-5. The secondary structure prediction showed that MUC7 20-mer adopts an amphiphilic helix with distinguishable hydrophilic and hydrophobic faces (a characteristic that is associated with antimicrobial activity). In comparison to that of Hsn-5, the fungicidal activity of MUC7 20-mer against C. albicans seems to be independent of fungal cellular metabolic activity, as evidenced by its killing potency at a low temperature (4 degrees C) and in the presence of inhibitors of oxidative phosphorylation in the mitochondrial system. Fluorescence microscopy showed the ability of MUC7 20-mer to cross the fungal cell membrane and to accumulate inside the cells. The internalization of MUC7 20-mer was inhibited by divalent cations. Confocal microscopy of cells doubly labeled with MUC7 20-mer and a mitochondrion-specific dye indicated that mitochondria are not the target of MUC7 20-mer for either C. albicans or C. neoformans.

Susceptibility of Candida dubliniensis to salivary histatin 3

Candida dubliniensis is a recently described Candida species associated with oral candidiasis in human immunodeficiency virus (HIV)-infected patients and patients with AIDS. The majority of C. dubliniensis clinical isolates tested to date are susceptible to the commonly used antifungal drugs, including fluconazole, ketoconazole, itraconazole, and amphotericin B. However, the appearance of fluconazole-resistant C. dubliniensis strains in this patient group is increasing. Histatins are a family of basic histidine-rich proteins present in human saliva which have therapeutic potential in the treatment of oral candidiasis. The mechanism of action of histatin is distinct from that of commonly used azole and polyene drugs. Characterization of the antifungal activity of histatin has mainly been carried out using C. albicans but it is also effective in killing C. glabrata and C. krusei. Here we report that C. dubliniensis is also susceptible to killing by histatin 3. The concentration of histatin 3 giving 50% killing (the IC(50) value) ranged from 0.043 to 0.196 mg/ml among different strains of C. dubliniensis. The least-susceptible C. dubliniensis strain, P9224, was found to internalize histatin at a lower rate than the C. albicans reference strain CA132A. The dissociation constant (K(d)) for the least-susceptible strain (C. dubliniensis 9224) was ninefold higher than that for the C. albicans reference strain. These results suggest that histatin 3 may have potential as an effective antifungal agent, particularly in the treatment of oral candidiasis in HIV-infected patients and patients with AIDS in which resistance to the commonly used antifungal drug fluconazole has emerged.

Internalisation and degradation of histatin 5 by Candida albicans

Histatins, salivary antimicrobial peptides, are susceptible to proteolytic degradation, often ascribed to host proteinases. In this study, we addressed the question whether proteolytic activity from microbial sources can contribute to this degradation. Candida albicans, an opportunistic yeast that is susceptible to the histatins, was used as target organism. The most potent histatin (histatin 5: sequence: DSHAKRHHGYKRKFHEKHHSHRGY), two histatin 5 fragments (dh-5: sequence: KRKFHEKHHSHRGY; P-113: sequence: AKRHHGYKRKFH) and an all-D isomer of the latter (P-113D) were used as model peptides. All L-peptides were susceptible to degradation by C. albicans. Cleavage was established at Lys5 and His19 of histatin 5, Lys11, Arg12, Phe14, Glu16, Lys17, His18 and Ser20 of dh-5 and Ala4 and Lys11 of P-113. In addition, it was found that secreted C. albicans enzymes are not involved in the degradation process and that blocking cell entry of the peptides greatly impedes degradation. Moreover, P-113D, which is biologically as active as P-113, was hardly susceptible to proteolysis. These data imply that proteolysis occurs mainly intracellularly and is not used as a protective mechanism against histatin activity. Together, our results suggest that, besides host proteinases, microbial enzymes play an important role in histatin degradation.

Human salivary histatin 5 causes disordered volume regulation and cell cycle arrest in Candida albicans

Human salivary histatin 5 (Hst 5) is a nonimmune salivary protein with antifungal activity against an important human pathogen, Candida albicans. The candidacidal activity of histatins appears to be a distinctive multistep mechanism involving depletion of the C. albicans intracellular ATP content as a result of nonlytic ATP efflux. Hst 5 caused a loss of cell viability concomitant with a decrease in cellular volume as determined both by a classical candidacidal assay with exogenous Hst 5 and by using a genetically engineered C. albicans strain expressing Hst 5. Preincubation of C. albicans cells with pharmacological inhibitors of anion transport provided complete or substantial protection from Hst 5-induced killing and volume reduction of cells. Moreover, intracellular expression of Hst 5 resulted in a reduction in the population mean cell volume that was accompanied by an increase in the percentage of unbudded cells and C. albicans cells in the G(1) phase. Following expression of Hst 5, the smallest cells sorted by fluorescence-activated cell sorting from the total population did not replicate and were exclusively in the G(1) phase. Cells with intracellularly expressed Hst 5 had greatly reduced G(1) cyclin transcript levels, indicating that they arrested in the G(1) phase before the onset of Start. Our data demonstrate that a key determinant in the mechanism of Hst 5 toxicity in C. albicans cells is the disruption of regulatory circuits for cell volume homeostasis that is closely coupled with loss of intracellular ATP. This novel process of fungicidal activity by a human salivary protein has highlighted potential interactions of Hst 5 with volume regulatory mechanisms and the process of yeast cell cycle control.

Histatin 5 and derivatives. Their localization and effects on the ultra-structural level

Histatins, a family of cationic peptides present in saliva, are active against the opportunistic yeast Candida albicans. The mechanism of action is still unclear. Histatin 5 and more potent synthetic variants, dhvar4 and dhvar5, were used to study localization and effects on morphology on the ultra-structural level. Although all peptides induced leakage, no association with the plasma membrane, indicative for permanent pores, was observed with immuno-gold-labeling. Freeze-fracturing showed severe changes of the plasma membrane. Together with, for the dhvars, the loss of intracellular integrity, this suggests that leakage may be a secondary effect rather than an effect of formation of permanent pores.

Antimicrobial peptides: therapeutic potential for the treatment of Candida infections

The increasing frequency of fungal infections in immunocompromised patients together with the emergence of strains resistant to currently used antifungal drugs point to an increased need for a new class of antimycotics. Antimicrobial peptides are promising candidates for the treatment of fungal infections since they have both mechanisms of action distinct from available antifungal agents and the ability to regulate the host immune defence systems as well. This review focuses on Candida albicans as a large amount of work on the mechanisms of action of classical antifungals as well as antimicrobial peptides, such as defensins, protegrins, histatins and lactoferrin (LF)-derived peptides, has been performed in this yeast. Analogues of these antimicrobial peptides and combinations of antimicrobial peptides with classical antimycotics are under investigation for treatment of candidiasis.

The human salivary peptide histatin 5 exerts its antifungal activity through the formation of reactive oxygen species

Previous studies have shown that the human salivary antifungal peptide histatin 5 is taken up by Candida albicans cells and associates intracellularly with mitochondria. The purpose of the present study was to investigate the biological consequence of this specific subcellular targeting. Histatin 5 inhibited respiration of isolated C. albicans mitochondria as well as the respiration of intact blastoconidia in a dose and time-dependent manner. A nearly perfect correlation was observed between histatin-induced inhibition of respiration and cell killing with either logarithmic- or stationary-phase cells, but stationary-phase cells were less sensitive. Because nonrespiring yeast cells are insensitive to histatin 5, the potential mechanistic relationship between histatin 5 interference with the respiratory apparatus and cell killing was explored by using an oxygen radical sensitive probe (dihydroethidium). Fluorimetric measurements showed that histatin 5 induced the formation of reactive oxygen species (ROS) in C. albicans cells as well as in isolated mitochondria and that ROS levels were highly correlated with cell death. In the presence of an oxygen scavenger (l-cysteine), cell killing and ROS formation were prevented. In addition, the membrane-permeant superoxide dismutase mimetic 2,2,6,6-tetramethylpiperidine-N-oxyl, abolished histatin-induced ROS formation in isolated mitochondria. In contrast to histatin 5, the conventional inhibitors of the respiratory chain, sodium cyanide or sodium azide, neither induced ROS nor killed yeast cells. These data provide strong evidence for a comprehensive mechanistic model of histatin-5-provoked yeast cell death in which oxygen radical formation is the ultimate and essential step.

Genetically engineered human salivary histatin genes are functional in Candida albicans: development of a new system for studying histatin candidacidal activity

Histatins are a structurally related family of salivary proteins known as histidine-rich proteins that are produced and secreted by the human major salivary glands. In vitro, histatins are potent cytotoxic proteins with selectivity for pathogenic yeasts including Candida albicans. Studies that investigate the mechanism of action of histatin proteins upon this important human pathogen have used a candidacidal assay in which the histatin is applied extracellularly. In order to develop a model system to study the mechanism of histatin action independently from binding and translocation events, the authors constructed C. albicans strains that contain chromosomally encoded human salivary histatin genes under the control of a regulated promoter. Intracellular expression of either histatin 5 or histatin 3 induced cell killing and ATP release in parallel. Since histatin killing can be initiated solely from intracellular sites, extracellular binding and internalization are preceding transport events. Thus the mechanism of histatin-induced ATP release does not require extracellular binding, and intracellular targets alone can activate ATP release. By employing a codon-optimization strategy it was shown that expression of heterologous sequences in C. albicans can be a useful tool for functional studies.

Internalization of tenecin 3 by a fungal cellular process is essential for its fungicidal effect on Candida albicans

Tenecin 3 is a glycine-rich, antifungal protein of 78 residues isolated from the insect Tenebrio molitor larva. As an initial step towards understanding the antifungal mechanism of tenecin 3, we examined how this protein interacts with the pathogenic fungus Candida albicans to exert its antifungal action. Tenecin 3 did not induce the release of a fluorescent dye trapped in the artificial membrane vesicles and it did not perturb the membrane potential of C. albicans by the initial interaction. Fluorescence confocal microscopy and flow cytometric analysis revealed that tenecin 3 is rapidly internalized into the cytoplasmic space in energy-dependent and temperature-dependent manners. This internalization is also dependent on the ionic environment and cellular metabolic states. These results suggest that the internalization of tenecin 3 into the cytoplasm of C. albicans is mediated by a fungal cellular process. The internalized tenecin 3 is dispersed in the cytoplasm, and the loss of cell viability occurs after this internalization.

Effects of histatin 5 and derived peptides on Candida albicans

Three anti-microbial peptides were compared with respect to their killing activity against Candida albicans and their ability to disturb its cellular and internal membranes. Histatin 5 is an anti-fungal peptide occurring naturally in human saliva, while dhvar4 and dhvar5 are variants of its active domain, with increased anti-microbial activity. dhvar4 has increased amphipathicity compared with histatin 5, whereas dhvar5 has amphipathicity comparable with that of histatin 5. All three peptides caused depolarization of the cytoplasmic and/or mitochondrial membrane, indicating membranolytic activity. For the variant peptides both depolarization and killing occurred at a faster rate. With FITC-labelled peptides, no association with the cytoplasmic membrane was observed, contradicting the formation of permanent transmembrane multimeric peptide pores. Instead, the peptides were internalized and act on internal membranes, as demonstrated with mitochondrion- and vacuole-specific markers. In comparison with histatin 5, the variant peptides showed a more destructive effect on mitochondria. Entry of the peptides and subsequent killing were dependent on the metabolic state of the cells. Blocking of the mitochondrial activity led to complete protection against histatin 5 activity, whereas that of dhvar4 was hardly affected and that of dhvar5 was affected only intermediately.

Antimicrobial peptides: properties and applicability

All organisms need protection against microorganisms, e. g. bacteria, viruses and fungi. For many years, attention has been focused on adaptive immunity as the main antimicrobial defense system. However, the adaptive immune system, with its network of humoral and cellular responses is only found in higher animals, while innate immunity is encountered in all living creatures. The turning point in the appreciation of the innate immunity was the discovery of antimicrobial peptides in the early eighties. In general these peptides act by disrupting the structural integrity of the microbial membranes. It has become clear that membrane-active peptides and proteins play a crucial role in both the innate and the adaptive immune system as antimicrobial agents. This review is focused on the functional and structural features of the naturally occurring antimicrobial peptides, and discusses their potential as therapeutics.

Characterization of histatin 5 with respect to amphipathicity, hydrophobicity, and effects on cell and mitochondrial membrane integrity excludes a candidacidal mechanism of pore formation

Histatin 5 is a 24-residue peptide from human saliva with antifungal properties. We recently demonstrated that histatin 5 translocates across the yeast membrane and targets to the mitochondria, suggesting an unusual antifungal mechanism (Helmerhorst, E. J., Breeuwer, P., van't Hof, W., Walgreen-Weterings, E., Oomen, L. C. J. M., Veerman, E. C. I., Nieuw Amerongen, A. V., and Abee, T. (1999) J. Biol. Chem. 274, 7286-7291). The present study used specifically designed synthetic analogs of histatin 5 to elucidate the role of peptide amphipathicity, hydrophobicity, and the propensity to adopt alpha-helical structures in relation to membrane permeabilization and fungicidal activity. Studies included circular dichroism measurements, evaluation of the effects on the cytoplasmic transmembrane potential and on the respiration of isolated mitochondria, and analysis of the peptide hydrophobicity/amphipathicity relationship (Eisenberg, D. (1984) Annu. Rev. Biochem. 53, 595-623). The 14-residue synthetic peptides used were dh-5, comprising the functional domain of histatin 5, and dhvar1 and dhvar4, both designed to maximize amphipathic characteristics. The results obtained show that the amphipathic analogs exhibited a high fungicidal activity, a high propensity to form an alpha-helix, dissipated the cytoplasmic transmembrane potential, and uncoupled the respiration of isolated mitochondria, similar to the pore-forming peptide PGLa (Peptide with N-terminal Glycine and C-terminal Leucine-amide). In contrast, histatin 5 and dh-5 showed fewer or none of these features. The difference in these functional characteristics between histatin 5 and dh-5 on the one hand and dhvar1, dhvar4, and PGLa on the other hand correlated well with their predicted affinity for membranes based on hydrophobicity/amphipathicity analysis. These data indicate that the salivary protein histatin 5 exerts its antifungal function through a mechanism other than pore formation.