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

Reducing Functional Domain of Histatin 5 Improves Antifungal Activity and Prevents Proteolytic Degradation

Histatin 5 (Hst5) is an antifungal peptide (AFP) naturally produced by parotid glands with strong activity against Candida albicans. One of its mechanisms of action is the generation of reactive oxygen species (ROS) inside the C. albicans cells. Despite being an important peptide for the human innate immune response, its activity is reduced or inactivated by proteolytic degradation caused by salivary enzymes. To overcome this barrier, we used solid phase peptide synthesis (SPPS) to modify the Hst5 amino acid sequence improving its antifungal action and minimizing its degradation. We synthesized five peptides, three of which were based on the Hst5 functional domain. We determined that the smallest peptides (8WH5, 7WH5 and 6WH5) demonstrated the greatest antifungal action against C. albicans, including one fluconazole-resistant strain. Besides that, cationic-PAGE and HPLC assays showed that the degradation in saliva was slower for the smaller peptides than for 0WHst5 and WP113. Furthermore, 8WH5, 7WH5 and 6WH5 were found in the samples even after 8 h in whole saliva, while 0WHst5 and WP113 completely disappear after 1.5 h. Finally, we found that the smaller peptides were less fragmented than the 0WHst5 and WP113, so they were the smallest fragments of Hst5 to preserve its antifungal action with reduced degradation in whole saliva. Thus, they can be considered promising molecules for the treatment of C. albicans in the oral cavity.

The Contribution of Human Antimicrobial Peptides to Fungi

Various species of fungi can be detected in the environment and within the human body, many of which may become pathogenic under specific conditions, leading to various forms of fungal infections. Antimicrobial peptides (AMPs) are evolutionarily ancient components of the immune response that are quickly induced in response to infections with many pathogens in almost all tissues. There is a wide range of AMP classes in humans, many of which exhibit broad-spectrum antimicrobial function. This review provides a comprehensive overview of the mechanisms of action of AMPs, their distribution in the human body, and their antifungal activity against a range of both common and rare clinical fungal pathogens. It also discusses the current research status of promising novel antifungal strategies, highlighting the challenges that must be overcome in the development of these therapies. The hope is that antimicrobial peptides, as a class of antimicrobial agents, will soon progress through large-scale clinical trials and be implemented in clinical practice, offering new treatment options for patients suffering from infections.

Antifungal peptides: Therapeutic potential and challenges before their commercial success

Antifungal peptides (AFPs) are small cationic peptides that are found in a diverse range of taxa including bacteria, plants, mammals and insects. AFPs exhibit the strong antifungal activities against several pathogenic fungi, making them potential candidates for developing novel antifungal agents. AFP cause fungal cell death by rupturing the membranes of the fungal cell wall and inhibits the vital enzymes. Since AFPs are isolated from a range of natural sources, efforts are being made to create synthetic versions of these peptides with improved pharmacological properties. One of their key advantages is that they are less likely to develop resistance as compared to conventional antifungal medications. Although AFPs display immense potential as antifungal agents, challenges still exist in their stability, solubility, absorption, and time-consuming extraction process. Still, the possibility for AFPs to evolve into a novel class of antifungal medicine gives hope for improved treatments for fungal infections. This article offers the comprehensive information on AFPs origin, mode of action, prospective use in antifungal treatments. It also discusses about the application of antifungal peptides beyond the therapeutic field, such as in agriculture for crop protection, in food industry and in aquaculture field. It further elaborates on the challenges and potential paths associated with the progression of AFPs as advanced antifungal agents.

Salivary proteins modulate Candida albicans virulence and may prevent oropharingeal candidiasis

Oral candidiasis can be presented in different ways due to the virulence factors of its etiology such as Candida albicans that have developed an effective set of these factors that are able to improve its pathogenesis. The role of salivary immunological components in the development of candidiasis can provide insights for the development of new methodologies aiming to control this disease. The aim of this study was to evaluate the antifungal activity of two salivary components, histatin 5 and lactoferrin on C. albicans viability and virulence using a fluconazole resistant C. albicans clinical strain. Results showed that histatin 5 and lactoferrin decreased cell viability, and the cell surface hydrophobicity was increased by 18% in presence of 151 µg/mL of histatin 5 but was not altered by lactoferrin. It was observed the reduction of 69.3% in the expression of mannoproteins on C. albicans surface in the presence of 151 µg/mL of histatin, but proteolytic activity of serine proteinases was not inhibited by any of the proteins. Histatin 5 altered cell ultrastructure predominantly in the cytoplasmic compartment. However, this peptide does not interfere with mitochondrial function neither in membrane permeability of the yeasts. The association index between C. albicans and epithelial cells was increased by 51% in presence of 151 µg/mL of histatin. Results suggest that histatin 5 and lactoferrin affects viability and virulence of C. albicans at physiological levels, and the maintenance of these levels may be essential in the prevention of oropharyngeal candidiasis. Exogenous administration of these proteins may become a therapeutic alternative for resistant strains of C. albicans, circumventing toxicity issues, considering their constitutive features.

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.

Exploring the frontiers of therapeutic breadth of antifungal peptides: A new avenue in antifungal drugs

The growing prevalence of fungal infections alongside rising resistance to antifungal drugs poses a significant challenge to public health safety. At the close of the 2000s, major pharmaceutical firms began to scale back on antimicrobial research due to repeated setbacks and diminished economic gains, leaving only smaller companies and research labs to pursue new antifungal solutions. Among various natural sources explored for novel antifungal compounds, antifungal peptides (AFPs) emerge as particularly promising. Despite their potential, AFPs receive less focus than their antibacterial counterparts. These peptides have been sourced extensively from nature, including plants, animals, insects, and especially bacteria and fungi. Furthermore, with advancements in recombinant biotechnology and computational biology, AFPs can also be synthesized in lab settings, facilitating peptide production. AFPs are noted for their wide-ranging efficacy, in vitro and in vivo safety, and ability to combat biofilms. They are distinguished by their high specificity, minimal toxicity to cells, and reduced likelihood of resistance development. This review aims to comprehensively cover AFPs, including their sources-both natural and synthetic-their antifungal and biofilm-fighting capabilities in laboratory and real-world settings, their action mechanisms, and the current status of AFP research.

ONE-SENTENCE SUMMARY

This comprehensive review of AFPs will be helpful for further research in antifungal research.

Histatin-5 interacts with cellular copper to promote antifungal activity against Candida albicans

Histatin-5 (Hist-5) is an antimicrobial peptide found in human saliva that functions to defend the oral cavity from microbial infections, such as those caused by the fungal pathogen Candida albicans (C. albicans). Hist-5 can bind Cu in multiple oxidation states, Cu2+ and Cu+in vitro, and supplemental Cu2+ has been shown to improve the fungicidal activity of the peptide against C. albicans in culture. However, the exact role of Cu on the antifungal activity of Hist-5 and whether direct peptide-Cu interactions occur intracellularly has yet to be fully determined. Here, we used a combination of fluorescence spectroscopy and confocal microscopy experiments to show reversible Cu-dependent quenching of a fluorescent Hist-5 analogue, Hist-5*, indicating a direct interaction between Hist-5 and intracellular Cu. X-ray fluorescence microscopy images revealed peptide-induced changes to cellular Cu distribution and cell-associated Cu content. These data support a model in which Hist-5 can facilitate the hyperaccumulation of Cu in C. albicans and directly interact with Cu intracellularly to increase the fungicidal activity of Hist-5.

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

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

Envisaging Antifungal Potential of Histatin 5: A Physiological Salivary Peptide

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

A Novel Role for Histatin 5 in Combination with Zinc to Promote Commensalism in C. albicans Survivor Cells

Candida albicans is maintained as a commensal by immune mechanisms at the oral epithelia. Oral antifungal peptide Histatin 5 (Hst 5) may function in innate immunity, but the specific role Hst 5 plays in C. albicans commensalism is unclear. Since Zn-binding potentiates the candidacidal activity of Hst 5, we hypothesized that Hst 5+Zn would elicit a unique fungal stress response to shape interactions between C. albicans and oral epithelial cells (OECs). We found that Hst 5+Zn but not Hst 5 alone resulted in the activation of cell wall integrity (CWI) signaling, and deletion mutants were then used to determine that CWI-mediated chitin synthesis was protective against killing. Using flow cytometry, we confirmed that Hst 5+Zn-treated cells had significantly elevated levels of cell-wall chitin, mannan and β-1,3 glucan compared to Hst 5-treated cells. We then tested the activation of host signaling components involved in C. albicans cell-wall recognition. The immunoblot assay of C. albicans-exposed oral epithelial cells showed increased activation of EphA2 and NF-κB but not EGFR. Interestingly, C. albicans treated with Hst 5+Zn induced the global suppression of pro-inflammatory cytokine release from OECs, but an increase in negative regulator IL-10. Hst 5+Zn-treated cells were more adherent but ultimately less invasive to OECs than control cells, thus indicating lowered virulence. Therefore, Hst 5+Zn-treated C. albicans cells are discerned by epithelial monolayers, but are less virulent and promote anti-inflammatory signaling, suggesting that Hst 5+Zn in combination could play a role in regulating commensalism of oral C. albicans through cell wall reorganization.

Membrane-Interacting Antifungal Peptides

The incidence of invasive fungal infections is increasing worldwide, resulting in more than 1.6 million deaths every year. Due to growing antifungal drug resistance and the limited number of currently used antimycotics, there is a clear need for novel antifungal strategies. In this context, great potential is attributed to antimicrobial peptides (AMPs) that are part of the innate immune system of organisms. These peptides are known for their broad-spectrum activity that can be directed toward bacteria, fungi, viruses, and/or even cancer cells. Some AMPs act via rapid physical disruption of microbial cell membranes at high concentrations causing cell leakage and cell death. However, more complex mechanisms are also observed, such as interaction with specific lipids, production of reactive oxygen species, programmed cell death, and autophagy. This review summarizes the structure and mode of action of antifungal AMPs, thereby focusing on their interaction with fungal membranes.

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

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

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

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

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.

Controlling biofilms using synthetic biology approaches

Bacterial biofilms are formed by the complex but ordered regulation of intra- or inter-cellular communication, environmentally responsive gene expression, and secretion of extracellular polymeric substances. Given the robust nature of biofilms due to the non-growing nature of biofilm bacteria and the physical barrier provided by the extracellular matrix, eradicating biofilms is a very difficult task to accomplish with conventional antibiotic or disinfectant treatments. Synthetic biology holds substantial promise for controlling biofilms by improving and expanding existing biological tools, introducing novel functions to the system, and re-conceptualizing gene regulation. This review summarizes synthetic biology approaches used to eradicate biofilms via protein engineering of biofilm-related enzymes, utilization of synthetic genetic circuits, and the development of functional living agents. Synthetic biology also enables beneficial applications of biofilms through the production of biomaterials and patterning biofilms with specific temporal and spatial structures. Advances in synthetic biology will add novel biofilm functionalities for future therapeutic, biomanufacturing, and environmental applications.

Impediment to growth and yeast-to-hyphae transition in Candida albicans by copper oxide nanoparticles

The effects of two prominent copper oxide nanoparticles (CuO-NP and Cu₂O-NP), with the oxidation state of Cu⁺⁺ (cupric) and Cu⁺ (cuprous), on Candida albicans were evaluated. CuO-NP and Cu₂O-NP were synthesized and characterized by XRD, FESEM, HR-TEM and Zeta potential. At sub-MIC (50 µg ml^-1), both cupric and cuprous oxide NPs prevented yeast-to-hyphae switching and wrinkling behaviour in C. albicans. The mechanism for the antifungal action of the two NPs differed; CuO-NP significantly elicited reactive oxygen species, whereas membrane damage was more pronounced with Cu₂O-NP. Real time PCR analysis revealed that CuO-NP suppressed the morphological switching of yeast-to-hyphae by down-regulating cph1, hst7 and ras1 and by up-regulation of the negative regulator tup1. In comparison, Cu₂O-NP resulted in down-regulation of ras1 and up-regulation of the negative regulators nrg1 and tup1. Between the two NPs, CuO exhibited increased antifungal activity due to its stable oxidation state (Cu⁺⁺) and its smaller dimensions compared with Cu₂O-NP.

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

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

Systematical Analysis of the Protein Targets of Lactoferricin B and Histatin-5 Using Yeast Proteome Microarrays

Antimicrobial peptides (AMPs) have potential antifungal activities; however, their intracellular protein targets are poorly reported. Proteome microarray is an effective tool with high-throughput and rapid platform that systematically identifies the protein targets. In this study, we have used yeast proteome microarrays for systematical identification of the yeast protein targets of Lactoferricin B (Lfcin B) and Histatin-5. A total of 140 and 137 protein targets were identified from the triplicate yeast proteome microarray assays for Lfcin B and Histatin-5, respectively. The Gene Ontology (GO) enrichment analysis showed that Lfcin B targeted more enrichment categories than Histatin-5 did in all GO biological processes, molecular functions, and cellular components. This might be one of the reasons that Lfcin B has a lower minimum inhibitory concentration (MIC) than Histatin-5. Moreover, pairwise essential proteins that have lethal effects on yeast were analyzed through synthetic lethality. A total of 11 synthetic lethal pairs were identified within the protein targets of Lfcin B. However, only three synthetic lethal pairs were identified within the protein targets of Histatin-5. The higher number of synthetic lethal pairs identified within the protein targets of Lfcin B might also be the reason for Lfcin B to have lower MIC than Histatin-5. Furthermore, two synthetic lethal pairs were identified between the unique protein targets of Lfcin B and Histatin-5. Both the identified synthetic lethal pairs proteins are part of the Spt-Ada-Gcn5 acetyltransferase (SAGA) protein complex that regulates gene expression via histone modification. Identification of synthetic lethal pairs between Lfcin B and Histatin-5 and their involvement in the same protein complex indicated synergistic combination between Lfcin B and Histatin-5. This hypothesis was experimentally confirmed by growth inhibition assay.

A Novel Antifungal System With Potential for Prolonged Delivery of Histatin 5 to Limit Growth of Candida albicans

Currently 75-88% of fungal infections are caused by Candida species, and Candida albicans is the main microorganism that causes these infections, especially oral candidiasis. An option for treatment involves the use of the antifungal peptide Histatin 5 (Hst 5), which is naturally found in human saliva but undergoes rapid degradation when present in the oral cavity, its site of action. For this reason, it is important to develop a way of applying this peptide to the oral lesions, which promotes the gradual release of the peptide. In the present study, we have evaluated the development of liposomes of different lipid compositions, loaded with the peptide as a way to promote its release slowly and gradually, preserving its antifungal potential. For this, the peptide 0WHistatin 5, an analog of the peptide Hst 5, was synthesized, which contains the amino acid tryptophan in its sequence. The solid phase synthesis method was used, followed by cleavage and purification. The liposomes were produced by thin film hydration technique in three different lipid compositions, F1, F2, and F3 and were submitted to an extrusion and sonication process to standardize the size and study the best technique for their production. The liposomes were characterized by dynamic light scattering, and tests were performed to determine the encapsulation efficiency, release kinetics, stability, and evaluation of antifungal activity. The extruded liposomes presented average size in the range of 100 nm, while sonicated liposomes presented a smaller size in the range of 80 nm. The encapsulation efficiency was higher for the sonicated liposomes, being 34.5% for F1. The sonicated F3 presented better stability when stored for 60 days at 4°C. The liposomes showed the ability to release the peptide for the total time of 96 h, with the first peak after 5 h, and a further increase of the released after 30 h. Time-kill assay showed that the liposomes were able to control yeast growth for 72 h. The data suggest that the liposomes loaded with 0WHistatin 5 maintained the action of the peptide and were able to limit the growth of C. albicans, being a suitable system for use in the treatment of oral candidiasis.

Candida innate immunity at the mucosa

The tremendous diversity in microbial species that colonise the mucosal surfaces of the human body is only now beginning to be fully appreciated. Distinguishing between the behaviour of commensal microbes and harmful pathogens that reside at mucosal sites in the body is a complex, and exquisitely fine-tuned process central to mucosal health. The fungal pathobiont Candida albicans is frequently isolated from mucosal surfaces with an asymptomatic carriage rate of approximately 60% in the human population. While normally a benign member of the microbiota, overgrowth of C. albicans often results in localised mucosal infection causing morbidity in otherwise healthy individuals, and invasive infection that often causes death in the absence of effective immune defence. C. albicans triggers numerous innate immune responses at mucosal surfaces, and detection of C. albicans hyphae in particular, stimulates the production of antimicrobial peptides, danger-associated molecular patterns and cytokines that function to reduce fungal burdens during infection. This review will summarise our current understanding of innate immune responses to C. albicans at mucosal surfaces.

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

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

Fungicide Bac8c triggers attenuation of mitochondrial homeostasis and caspase-dependent apoptotic death

Bac8c (RIWVIWRR-NH₂), an 8-mer peptide modified from amino acids 4-11 of Bac2a, shows broad-spectrum activity against pathogenic bacteria and yeast, and it has been the focus of attention owing to its low cost of synthesis. Although Bac8c is effective against Candida albicans, its mode of action needs to be investigated further. Bac8c causes yeast cell death in a dose-dependent manner by eliciting the production of reactive oxygen species, thereby attenuating the antioxidant defense system. It is also involved in Ca²⁺ signaling, and produces apoptotic features, such as phosphatidylserine externalization and DNA fragmentation. Bac8c induces cell death by oxidative stress-dependent apoptotic death via disruption of mitochondrial homeostasis and metacaspase activation. This suggests that the concentration of Bac8c is important for the induction of apoptotic death, which is not necessarily accompanied by cell cycle arrest in C. albicans.

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.

Development and In Vivo Evaluation of a Novel Histatin-5 Bioadhesive Hydrogel Formulation against Oral Candidiasis

Oral candidiasis (OC), caused by the fungal pathogen Candida albicans, is the most common opportunistic infection in HIV(+) individuals and other immunocompromised populations. The dramatic increase in resistance to common antifungals has emphasized the importance of identifying unconventional therapeutic options. Antimicrobial peptides have emerged as promising candidates for therapeutic intervention due to their broad antimicrobial properties and lack of toxicity. Histatin-5 (Hst-5) specifically has exhibited potent anticandidal activity indicating its potential as an antifungal agent. To that end, the goal of this study was to design a biocompatible hydrogel delivery system for Hst-5 application. The bioadhesive hydroxypropyl methylcellulose (HPMC) hydrogel formulation was developed for topical oral application against OC. The new formulation was evaluated in vitro for gel viscosity, Hst-5 release rate from the gel, and killing potency and, more importantly, was tested in vivo in our mouse model of OC. The findings demonstrated a controlled sustained release of Hst-5 from the polymer and rapid killing ability. Based on viable C. albicans counts recovered from tongues of treated and untreated mice, three daily applications of the formulation beginning 1 day postinfection with C. albicans were effective in protection against development of OC. Interestingly, in some cases, Hst-5 was able to clear existing lesions as well as associated tissue inflammation. These findings were confirmed by histopathology analysis of tongue tissue. Coupled with the lack of toxicity as well as anti-inflammatory and wound-healing properties of Hst-5, the findings from this study support the progression and commercial feasibility of using this compound as a novel therapeutic agent.

Mechanism of action of the tri-hybrid antimicrobial peptide LHP7 from lactoferricin, HP and plectasin on Staphylococcus aureus

The tri-hybrid peptide-LHP7 has the potent activity against Gram-positive and Gram-negative as well as fungi, but its mechanism of action has remained elusive. The effluences of LHP7 on the Staphylococcus aureus cell membrane and targets of intracellular action were investigated. LHP7 exhibited an inhibitory effect on the S. aureus growth, similar to those achieved by plectasin, vancomycin and gramicidin. The membrane integrity studies confirmed that LHP7 disrupted the cell membrane, indicating a membrane permeabilizing killing action. A marginal decline in the intensity fluorescence indicated no significant depolarization of the membrane potential following LHP7 treatment. Furthermore, electron microscopy showed that cell shrinkage, cell wall thickening, cellular content leakage, and cell disruption were observed in the cells treated with LHP7. A gel retardation assay showed that LHP7 bound to the genomic DNA of S. aureus or plasmid DNA at a mass ratio of 2.5–10 (peptide/DNA). Circular dichroism indicated that LHP7 inserted into the groove of DNA. The cell cycle analysis showed that after the treatment with LHP7 for 30 and 60 min, the proportion of cells in I-phase increased from 8.71 to 12.09 % and from 8.71 to 15.68 %, indicating that LHP7 induced arrest of cells in the I-phase. These results would conduce to elucidate its underlying antibacterial mechanism.

A dual mechanism involved in membrane and nucleic acid disruption of AvBD103b, a new avian defensin from the king penguin, against Salmonella enteritidis CVCC3377

The food-borne bacterial gastrointestinal infection is a serious public health threat. Defensins are evolutionarily conserved innate immune components with broad-spectrum antibacterial activity that do not easily induce resistance. AvBD103b, an avian defensin with potent activity against Salmonella enteritidis, was isolated from the stomach contents of the king penguin (Aptenodytes patagonicus). To elucidate further the antibacterial mechanism of AvBD103b, its effect on the S. enteritidis CVCC3377 cell membrane and intracellular DNA was researched. The cell surface hydrophobicity and a N-phenyl-1-naphthylamine uptake assay demonstrated that AvBD103b treatment increased the cell surface hydrophobicity and outer membrane permeability. Atomic absorption spectrometry, ultraviolet spectrophotometry, flow cytometry, and transmission electron microscopy (TEM) indicated that AvBD103b treatment can lead to the release of the cellular contents and cell death through damage of the membrane. DNA gel retardation and circular dichroism analysis demonstrated that AvBD103b interacted with DNA and intercalated into the DNA base pairs. A cell cycle assay demonstrated that AvBD103b affected cellular functions, such as DNA synthesis. Our results confirmed that AvBD103b exerts its antibacterial activity by damaging the cell membrane and interfering with intracellular DNA, ultimately causing cell death, and suggested that AvBD103b may be a promising candidate as an alternative to antibiotics against S. enteritidis.

How does it kill?: understanding the candidacidal mechanism of salivary histatin 5

Histatins are salivary cationic peptides that provide the first line of defense against oral candidiasis caused by Candida albicans. This minireview presents a critical evaluation of our knowledge of the candidacidal mechanism of histatin 5 (Hst 5). Hst 5 is the most potent among all histatin family members with regard to its antifungal activity. The mode of action of Hst 5 has been a subject of intense debate. Unlike other classical host innate immune proteins, pore formation or membrane lysis by Hst 5 has largely been disproven, and it is now known that all targets of Hst 5 are intracellular. Hst 5 binds C. albicans cell wall proteins (Ssa1/2) and glycans and is taken up by the cells through fungal polyamine transporters in an energy-dependent manner. Once inside the fungal cells, Hst 5 may affect mitochondrial functions and cause oxidative stress; however, the ultimate cause of cell death is by volume dysregulation and ion imbalance triggered by osmotic stress. Besides these diverse targets, a novel mechanism based on the metal binding abilities of Hst 5 is discussed. Finally, translational approaches for Hst 5, based on peptide design and synergy with other known drugs, are considered a step forward for bench-to-bed application of Hst 5.

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.

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.

Histatins: antimicrobial peptides with therapeutic potential

Histatins are a group of antimicrobial peptides, found in the saliva of man and some higher primates, which possess antifungal properties. Histatins bind to a receptor on the fungal cell membrane and enter the cytoplasm where they target the mitochondrion. They induce the non-lytic loss of ATP from actively respiring cells, which can induce cell death. In addition, they have been shown to disrupt the cell cycle and lead to the generation of reactive oxygen species. Their mode of action is distinct from those exhibited by the conventional azole and polyene drugs, hence histatins may have applications in controlling drug-resistant fungal infections. The possibility of utilising histatins for the control of fungal infections of the oral cavity is being actively pursued with the antifungal properties of topical histatin preparations and histatin-impregnated denture acrylic being evaluated. Initial clinical studies are encouraging, having demonstrated the safety and efficacy of histatin preparations in blocking the adherence of the yeast Candida albicans to denture acrylic, retarding plaque formation and reducing the severity of gingivitis. Histatins may represent a new generation of antimicrobial compounds for the treatment of oral fungal infections and have the advantage, compared with conventional antifungal agents, of being a normal component of human saliva with no apparent adverse effects on host tissues and having a mode of action distinct to azole and polyene antifungals.

Effect of phosphate group addition on the properties of denture base resins

STATEMENT OF PROBLEM

Acrylic resins are prone to microbial adherence, especially by Candida albicans. Surface-charged resins alter the ionic interaction between the denture resin and Candida hyphae, and these resins are being developed as a means to reduce microbial colonization on the denture surface.

PURPOSE

The purpose of this study was to investigate the physical and mechanical properties of phosphate-containing polymethyl methacrylate resins for their suitability as a denture material.

MATERIAL AND METHODS

Using PMMA with cross-linker (Lucitone 199) as a control, 4 experimental groups containing various levels of phosphate with and without cross-linker were generated. The properties examined were impact strength, fracture toughness, wettability (contact angle), and resin bonding ability to denture teeth. Impact strength was tested in the Izod configuration (n=16), and fracture toughness (n=13) was measured using the single-edge notched bend test. Wettability was determined by calculating the contact angle of water on the material surface (n=12), while ISO 1567 was used for bonding ability (n=12). The data were analyzed by 1- and 2-way ANOVA (alpha=.05).

RESULTS

A trend of increased hydrophilicity, as indicated by lower contact angle, was observed with increased concentrations of phosphate. With regard to the other properties, no significant differences were found when compared with the control acrylic resin.

CONCLUSIONS

No adverse physical effect due to the addition of a phosphate-containing monomer was found in the acrylic denture resins. Additional mechanical and physical properties, biocompatibility, and clinical efficacy studies are needed to confirm the in vivo anti-Candida activity of these novel resins.

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.

Candida albicans cell wall proteins

The Candida albicans cell wall maintains the structural integrity of the organism in addition to providing a physical contact interface with the environment. The major components of the cell wall are fibrillar polysaccharides and proteins. The proteins of the cell wall are the focus of this review. Three classes of proteins are present in the candidal cell wall. One group of proteins attach to the cell wall via a glycophosphatidylinositol remnant or by an alkali-labile linkage. A second group of proteins with N-terminal signal sequences but no covalent attachment sequences are secreted by the classical secretory pathway. These proteins may end up in the cell wall or in the extracellular space. The third group of proteins lack a secretory signal, and the pathway(s) by which they become associated with the surface is unknown. Potential constituents of the first two classes have been predicted from analysis of genome sequences. Experimental analyses have identified members of all three classes. Some members of each class selected for consideration of confirmed or proposed function, phenotypic analysis of a mutant, and regulation by growth conditions and transcription factors are discussed in more detail.

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.

Human lactoferrin induces apoptosis-like cell death in Candida albicans: critical role of K+-channel-mediated K+ efflux

Human lactoferrin (hLf) induced an apoptosis-like phenotype in Candida albicans cells, which includes phosphatidylserine externalization, nuclear chromatin condensation, DNA degradation, and increased reactive oxygen species (ROS) production. Intracellular ROS accumulation was seen to correlate with candidacidal activity in hLf-treated cells. Mitochondrial activity was involved as indicated by mitochondrial depolarization and increased hLf resistance of cells preincubated with sordarin or erythromycin, the latter of which inhibits protein synthesis in mitoribosomes. Interestingly, Cl(-)- and K(+)-channel blockers prevented the hLf antimicrobial activity, but only when cells were pretreated with the blocking agent (tetraethylammonium) prior to the hLf-induced K(+)-release period. These results indicate for the first time that K(+)-channel-mediated K(+) efflux is required for the progression of apoptosis-like process in yeast, suggesting that this essential apoptotic event of higher eukaryotes has been evolutionary conserved among species ranging from yeasts to humans.

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.

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.

Drug-induced apoptosis in yeast

In order to alter the impact of diseases on human society, drug development has been one of the most invested research fields. Nowadays, cancer and infectious diseases are leading targets for the design of effective drugs, in which the primary mechanism of action relies on the modulation of programmed cell death (PCD). Due to the high degree of conservation of basic cellular processes between yeast and higher eukaryotes, and to the existence of an ancestral PCD machinery in yeast, yeasts are an attractive tool for the study of affected pathways that give insights into the mode of action of both antitumour and antifungal drugs. Therefore, we covered some of the leading reports on drug-induced apoptosis in yeast, revealing that in common with mammalian cells, antitumour drugs induce apoptosis through reactive oxygen species (ROS) generation and altered mitochondrial functions. The evidence presented suggests that yeasts may be a powerful model for the screening/development of PCD-directed drugs, overcoming the problem of cellular specificity in the design of antitumour drugs, but also enabling the design of efficient antifungal drugs, targeted to fungal-specific apoptotic regulators that do not have major consequences for human cells.

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.

Histatin-induced alterations in Candida albicans: a microscopic and submicroscopic comparison

Despite the numerous studies performed in an attempt to clarify the issue, the mechanism of action of salivary histatins remains unclear. The aim of the present study was to correlate histatin-induced morphological changes in Candida albicans by fluorescence microscopy (FM), transmission electron microscopy (TEM), and high resolution scanning electron microscopy (HRSEM). Each of the fluorescent dyes used by FM (i.e., tetramethylrhodamine methyl ester perchlorate for mitochondrial potential, Lysotracker for lysosome acidic compartment, and 4',6-diamino-2-phenylindole dihydrochloride for DNA) exhibited a specific staining in control cells. Following histatin treatment, we observed a recurring staining pattern, corresponding to fluorescence concentration along the cell periphery, suggesting a loss of dye specificity. To assess histatin-induced cytoplasmic modifications, ultrastructural analysis was then carried out. After treatments with histatins, TEM revealed characteristic intracellular modifications including: vacuole overgrowth, nuclear disappearance, loss of organelle identity, as well as the appearance of electron-dense membranes, likely of mitochondrial origin. Additionally, structures resembling autophagosomes were occasionally observed. By HRSEM, mitochondrial swelling was invariably the first sign of a histatin-induced effect. Other modifications included intracellular membrane disarrangement, organelles in disarray, and a large central cavity with deformed bodies displaced to the cell periphery, similar to what was detected by TEM. In summary, our study illustrates the occurrence of ultrastructural modifications following administration of histatins. Observations made with FM, TEM, and HRSEM provided different views of the same signs, demonstrating a definite action of histatins on C. albicans morphology. The possible functional meanings of these morphological results is discussed in light of the most recent biochemical data on histatin fungicidal activity.

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.

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.