Histatin 5 resistance of Candida glabrata can be reversed by insertion of Candida albicans polyamine transporter-encoding genes DUR3 and DUR31

Strategies and opportunities for engineering antifungal peptides for therapeutic applications

Antifungal peptides (AFPs) are widely described as promising prospects to treat and prevent fungal infections, though they are far less studied than their antibacterial counterparts. Although promising, AFPs have practical limitations that have hindered their use as therapeutics. Rational design and combinatorial engineering are powerful protein engineering strategies with much potential to address the limitations of AFPs by designing peptides with improved physiochemical and biological characteristics. We examine how rational design and combinatorial engineering approaches have already been used to improve the properties of AFPs and propose key opportunities for applying these strategies to push the design and application of AFPs forward.

Candida-associated denture stomatitis: clinical, epidemiological, and microbiological features

OBJECTIVE

The identification of Candida spp. in denture stomatitis, the clinical manifestations, and the antifungal susceptibility profile lead to a correct and individualized therapeutic management of the patients. This study is aimed at investigating the clinical manifestations and epidemiological and microbiological characteristics of Candida-associated denture stomatitis.

DESIGN

The samples were obtained by swabbing the oral mucosa of the subjects and then seeded onto Sabouraud Dextrose Agar and onto CHROMagar® Candida plates. The identification at the species level was confirmed by Matrix Assisted Laser Desorption Time of Flight Mass Spectrometry. Clinical classification was performed according to the criteria proposed by Newton (1962): (i) pinpoint hyperemia, (ii) diffuse hyperemia, and (iii) granular hyperemia. For carrying out the antifungal susceptibility testing, we adopted the CLSI M27-S4 protocol.

RESULTS

C. albicans was the most prevalent species in our study. Regarding non-albicans Candida species, C. glabrata was the most common species isolated from the oral mucosa (n = 4, 14.8%), while in the prosthesis, it was C. tropicalis (n = 4, 14.8%). The most prevalent clinical manifestation was pinpoint hyperemia and diffuse hyperemia. Candida albicans, C. glabrata, and C. parapsilosis were susceptible to all the tested antifungals. Concerning fluconazole and micafungin, only two strains showed dose-dependent sensitivity (minimum inhibitory concentration (MIC), 1 μg/mL) and intermediate sensitivity (MIC, 0.25 μg/mL). One C. tropicalis strain was resistant to voriconazole (MIC, 8 μg/mL).

CONCLUSIONS

C. albicans was the most common species found in oral mucosa and prosthesis. The tested antifungal drugs showed great activity against most isolates. The most prevalent clinical manifestations were Newton's type I and type II.

Antifungal activity of [K3]temporin-SHa against medically relevant yeasts and moulds

Few antifungal agents are currently available for the treatment of fungal infections. Antimicrobial peptides (AMPs), which are natural molecules involved in the innate immune response of many organisms, represent a promising research method because of their broad killing activity. The aim of this study was to assess the activity of a frog AMP, [K³]temporin-SHa, against some species of yeasts and moulds, and to further explore its activity against Candida albicans. MIC determinations were performed according to EUCAST guidelines. Next, the activity of [K³]temporin-SHa against C. albicans was explored using time-killing curve experiments, membrane permeabilization assays, and electron microscopy. Finally, chequerboard assays were performed to evaluate the synergy between [K³]temporin-SHa and amphotericin B or fluconazole. [K³]temporin-SHa was found to be active in vitro against several yeasts with MIC between 5.5 and 45 µM. [K³]temporin-SHa displayed rapid fungicidal activity against C. albicans (inoculum was divided into two in less than an hour and no viable colonies were recovered after 5 h) with a mechanism that could be due to membrane permeabilization. [K³]temporin-SHa was synergistic with amphotericin B against C. albicans (FICI = 0.303). [K³]temporin-SHa could represent an additional tool to treat several Candida species and C. neoformans.

tpo3 and dur3, Aspergillus fumigatus Plasma Membrane Regulators of Polyamines, Regulate Polyamine Homeostasis and Susceptibility to Itraconazole

Aspergillus fumigatus is a well-known opportunistic pathogen that causes invasive aspergillosis (IA) infections, which have high mortality rates in immunosuppressed individuals. Long-term antifungal drug azole use in clinical treatment and agriculture results in loss of efficacy or drug resistance. Drug resistance is related to cellular metabolites and the corresponding gene transcription. In this study, through untargeted metabolomics and transcriptomics under itraconazole (ITC) treatment, we identified two plasma membrane-localized polyamine regulators tpo3 and dur3, which were important for polyamine homeostasis and susceptibility to ITC in A. fumigatus. In the absence of tpo3 and/or dur3, the levels of cytoplasmic polyamines had a moderate increase, which enhanced the tolerance of A. fumigatus to ITC. In comparison, overexpression of tpo3 or dur3 induced a drastic increase in polyamines, which increased the sensitivity of A. fumigatus to ITC. Further analysis revealed that polyamines concentration-dependently affected the susceptibility of A. fumigatus to ITC by scavenging reactive oxygen species (ROS) at a moderate concentration and promoting the production of ROS at a high concentration rather than regulating drug transport. Moreover, inhibition of polyamine biosynthesis reduced the intracellular polyamine content, resulted in accumulation of ROS and enhanced the antifungal activity of ITC. Interestingly, A. fumigatus produces much lower levels of ROS under voriconazole (VOC) treatment than under ITC-treatment. Accordingly, our study established the link among the polyamine regulators tpo3 and dur3, polyamine homeostasis, ROS content, and ITC susceptibility in A. fumigatus.

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.

An Amyloid Core Sequence in the Major Candida albicans Adhesin Als1p Mediates Cell-Cell Adhesion

The human fungal commensal Candida albicans can become a serious opportunistic pathogen in immunocompromised hosts. The C. albicans cell adhesion protein Als1p is a highly expressed member of a large family of paralogous adhesins. Als1p can mediate binding to epithelial and endothelial cells, is upregulated in infections, and is important for biofilm formation. Als1p includes an amyloid-forming sequence at amino acids 325 to 331, identical to the sequence in the paralogs Als5p and Als3p. Therefore, we mutated Val326 to test whether this sequence is important for activity. Wild-type Als1p (Als1pWT) and Als1p with the V326N mutation (Als1pV326N) were expressed at similar levels in a Saccharomyces cerevisiae surface display model. Als1pV326N cells adhered to bovine serum albumin (BSA)-coated beads similarly to Als1pWT cells. However, cells displaying Als1pV326N showed visibly smaller aggregates and did not fluoresce in the presence of the amyloid-binding dye Thioflavin-T. A new analysis tool for single-molecule force spectroscopy-derived surface mapping showed that statistically significant force-dependent Als1p clustering occurred in Als1pWT cells but was absent in Als1pV326N cells. In single-cell force spectroscopy experiments, strong cell-cell adhesion was dependent on an intact amyloid core sequence on both interacting cells. Thus, the major adhesin Als1p interacts through amyloid-like β-aggregation to cluster adhesin molecules in cis on the cell surface as well as in trans to form cell-cell bonds.IMPORTANCE Microbial cell surface adhesins control essential processes such as adhesion, colonization, and biofilm formation. In the opportunistic fungal pathogen Candida albicans, the agglutinin-like sequence (ALS) gene family encodes eight cell surface glycoproteins that mediate adherence to biotic and abiotic surfaces and cell-cell aggregation. Als proteins are critical for commensalism and virulence. Their activities include attachment and invasion of endothelial and epithelial cells, morphogenesis, and formation of biofilms on host tissue and indwelling medical catheters. At the molecular level, Als5p-mediated cell-cell aggregation is dependent on the formation of amyloid-like nanodomains between Als5p-expressing cells. A single-site mutation to valine 326 abolishes cellular aggregation and amyloid formation. Our results show that the binding characteristics of Als1p follow a mechanistic model similar to Als5p, despite its differential expression and biological roles.

The Plant Defensin NaD1 Enters the Cytoplasm of Candida Albicans via Endocytosis

Antimicrobial peptides are widespread in nature and are produced by many organisms as a first line of defence against pathogens. These peptides have a broad range of biological activities, such as antibacterial or antifungal activities and act with varied mechanisms of action. A large number of the peptides are amphipathic α-helices which act by disrupting plasma membranes and allowing leakage of intracellular contents. However, some peptides have more complex mechanisms of action that require internalisation into the target organisms’ cytoplasm. The method by which these peptides enter the cytoplasm varies, with some requiring the energy dependent processes of endocytosis or polyamine transport and others entering via passive transport. Here we describe the mechanism that the antimicrobial peptide, the plant defensin NaD1, uses to transverse the fungal membrane and gain access to the fungal cytoplasm. By inhibiting ATP synthesis and using an inhibitor of actin polymerisation, we show that NaD1 is internalised into C. albicans yeast cells by the energy-dependent process of endocytosis.

Fluconazole-Resistant Candida auris Is Susceptible to Salivary Histatin 5 Killing and to Intrinsic Host Defenses

Candida auris is a newly identified species causing invasive candidemia and candidiasis. It has broad multidrug resistance (MDR) not observed for other pathogenic Candida species. Histatin 5 (Hst 5) is a well-studied salivary cationic peptide with significant antifungal activity against Candida albicans and is an attractive candidate for treating MDR fungi, since antimicrobial peptides induce minimal drug resistance. We investigated the susceptibility of C. auris to Hst 5 and neutrophils, two first-line innate defenses in the human host. The majority of C. auris clinical isolates, including fluconazole-resistant strains, were highly sensitive to Hst 5: 55 to 90% of cells were killed by use of 7.5 μM Hst 5. Hst 5 was translocated to the cytosol and vacuole in C. auris cells; such translocation is required for the killing of C. albicans by Hst 5. The inverse relationship between fluconazole resistance and Hst 5 killing suggests different cellular targets for Hst 5 than for fluconazole. C. auris showed higher tolerance to oxidative stress than C. albicans, and higher survival within neutrophils, which correlated with resistance to oxidative stress in vitro Thus, resistance to reactive oxygen species (ROS) is likely one, though not the only, important factor in the killing of C. auris by neutrophils. Hst 5 has broad and potent candidacidal activity, enabling it to combat MDR C. auris strains effectively.

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.

Activity of Scorpion Venom-Derived Antifungal Peptides against Planktonic Cells of Candida spp. and Cryptococcus neoformans and Candida albicans Biofilms

The incidence of fungal infections has been increasing in the last decades, while the number of available antifungal classes remains the same. The natural and acquired resistance of some fungal species to available therapies, associated with the high toxicity of these drugs on the present scenario and makes an imperative of the search for new, more efficient and less toxic therapeutic choices. Antimicrobial peptides (AMPs) are a potential class of antimicrobial drugs consisting of evolutionarily conserved multifunctional molecules with both microbicidal and immunomodulatory properties being part of the innate immune response of diverse organisms. In this study, we evaluated 11 scorpion-venom derived non-disulfide-bridged peptides against Cryptococcus neoformans and Candida spp., which are important human pathogens. Seven of them, including two novel molecules, showed activity against both genera with minimum inhibitory concentration values ranging from 3.12 to 200 μM and an analogous activity against Candida albicans biofilms. Most of the peptides presented low hemolytic and cytotoxic activity against mammalian cells. Modifications in the primary peptide sequence, as revealed by in silico and circular dichroism analyses of the most promising peptides, underscored the importance of cationicity for their antimicrobial activity as well as the amphipathicity of these molecules and their tendency to form alpha helices. This is the first report of scorpion-derived AMPs against C. neoformans and our results underline the potential of scorpion venom as a source of antimicrobials. Further characterization of their mechanism of action, followed by molecular optimization to decrease their cytotoxicity and increase antimicrobial activity, is needed to fully clarify their real potential as antifungals.

The Antimicrobial Peptides P-113Du and P-113Tri Function against Candida albicans

Two antimicrobial P-113 peptide derivatives, P-113Du and P-113Tri, were investigated in this study. Notably, P-113Du and P-113Tri contained significant fractions of α-helix conformation and were less sensitive to high salt and low pH than P-113. Moreover, compared to P-113, these peptides exhibited increased antifungal activity against planktonic cells, biofilm cells, and clinical isolates of Candida albicans and non-albicans Candida spp. These results suggest that P-113Du and P-113Tri are promising candidates for development as novel antifungal agents.

Characterization of the cell penetrating properties of a human salivary proline-rich peptide

Saliva contains hundreds of small proline-rich peptides most of which derive from the post-translational and post-secretory processing of the acidic and basic salivary proline-rich proteins. Among these peptides we found that a 20 residue proline-rich peptide (p1932), commonly present in human saliva and patented for its antiviral activity, was internalized within cells of the oral mucosa. The cell-penetrating properties of p1932 have been studied in a primary gingival fibroblast cell line and in a squamous cancer cell line, and compared to its retro-inverso form. We observed by mass-spectrometry, flow cytometry and confocal microscopy that both peptides were internalized in the two cell lines on a time scale of minutes, being the natural form more efficient than the retro-inverso one. The cytosolic localization was dependent on the cell type: both peptide forms were able to localize within nuclei of tumoral cells, but not in the nuclei of gingival fibroblasts. The uptake was shown to be dependent on the culture conditions used: peptide internalization was indeed effective in a complete medium than in a serum-free one allowing the hypothesis that the internalization could be dependent on the cell cycle. Both peptides were internalized likely by a lipid raft-mediated endocytosis mechanism as suggested by the reduced uptake in the presence of methyl-ß-cyclodextrin. These results suggest that the natural peptide may play a role within the cells of the oral mucosa after its secretion and subsequent internalization. Furthermore, lack of cytotoxicity of both peptide forms highlights their possible application as novel drug delivery agents.

Candida albicans Cek1 mitogen-activated protein kinase signaling enhances fungicidal activity of salivary histatin 5

Candida albicans is a major etiological organism for oropharyngeal candidiasis (OPC), while salivary histatin 5 (Hst 5) is a human fungicidal protein that protects the oral cavity from OPC. C. albicans senses its environment by mitogen-activated protein kinase (MAPK) activation that can also modulate the activity of some antifungal drugs, including Hst 5. We found that phosphorylation of the MAPK Cek1, induced either by N-acetylglucosamine (GlcNAc) or serum, or its constitutive activation by deletion of its phosphatase Cpp1 elevated the susceptibility of C. albicans cells to Hst 5. Cek1 phosphorylation but not hyphal formation was needed for increased Hst 5 sensitivity. Interference with the Cek1 pathway by deletion of its head sensor proteins, Msb2 and Sho1, or by addition of secreted aspartyl protease (SAP) cleavage inhibitors, such as pepstatin A, reduced Hst 5 susceptibility under Cek1-inducing conditions. Changes in fungal cell surface glycostructures also modulated Hst 5 sensitivity, and Cek1-inducing conditions resulted in a higher uptake rate of Hst 5. These results show that there is a consistent relationship between activation of Cek1 MAPK and increased Hst 5 susceptibility in C. albicans.

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.

Saliva-microbe interactions and salivary gland dysfunction

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

Agp2p, the plasma membrane transregulator of polyamine uptake, regulates the antifungal activities of the plant defensin NaD1 and other cationic peptides

Cationic antifungal peptides (AFPs) act through a variety of mechanisms but share the common feature of interacting with the fungal cell surface. NaD1, a defensin from Nicotiana alata, has potent antifungal activity against a variety of fungi of both hyphal and yeast morphologies. The mechanism of action of NaD1 occurs via three steps: binding to the fungal cell surface, permeabilization of the plasma membrane, and internalization and interaction with intracellular targets to induce fungal cell death. The targets at each of these three stages have yet to be defined. In this study, the screening of a Saccharomyces cerevisiae deletion collection led to the identification of Agp2p as a regulator of the potency of NaD1. Agp2p is a plasma membrane protein that regulates the transport of polyamines and other molecules, many of which carry a positive charge. Cells lacking the agp2 gene were more resistant to NaD1, and this resistance was accompanied by a decreased uptake of defensin. Agp2p senses and regulates the uptake of the polyamine spermidine, and competitive inhibition of the antifungal activity of NaD1 by spermidine was observed in both S. cerevisiae and the plant pathogen Fusarium oxysporum. The resistance of agp2Δ cells to other cationic antifungal peptides and decreased binding of the cationic protein cytochrome c to agp2Δ cells compared to that of wild-type cells have led to a proposed mechanism of resistance whereby the deletion of agp2 leads to an increase in positively charged molecules at the cell surface that repels cationic antifungal peptides.

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

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