Floricolin C elicits intracellular reactive oxygen species accumulation and disrupts mitochondria to exert fungicidal action

Recent Progress in Research on Mitochondrion-Targeted Antifungal Drugs: a Review

Fungal infections, which commonly occur in immunocompromised patients, can cause high morbidity and mortality. Antifungal agents act by disrupting the cell membrane, inhibiting nucleic acid synthesis and function, or inhibiting β-1,3-glucan synthase. Because the incidences of life-threatening fungal infections and antifungal drug resistance are continuously increasing, there is an urgent need for the development of new antifungal agents with novel mechanisms of action. Recent studies have focused on mitochondrial components as potential therapeutic drug targets, owing to their important roles in fungal viability and pathogenesis. In this review, we discuss novel antifungal drugs targeting mitochondrial components and highlight the unique fungal proteins involved in the electron transport chain, which is useful for investigating selective antifungal targets. Finally, we comprehensively summarize the efficacy and safety of lead compounds in clinical and preclinical development. Although fungus-specific proteins in the mitochondrion are involved in various processes, the majority of the antifungal agents target dysfunction of mitochondria, including mitochondrial respiration disturbance, increased intracellular ATP, reactive oxygen species generation, and others. Moreover, only a few drugs are under clinical trials, necessitating further exploration of possible targets and development of effective antifungal agents. The unique chemical structures and targets of these compounds will provide valuable hints for further exploiting new antifungals.

Liverwort-Derived Metabolites Retard Endophyte Growth and Inspire Antifungal Application

Liverwort secondary metabolites play an important role in the peaceful relationship between liverwort endophytic fungi and the host. This study identified potential antifungal agents based on interactions between host plants and endophytic fungi. Two endophytic fungi strains and 25 metabolites, including nine new compounds, were isolated from the Chinese liverwort Herbertus herpocladioides. Endophytic fungi were identified using internal transcribed spacer and whole-genome sequencing, and the compound structures were determined using comprehensive spectroscopic analysis coupled with electronic circular dichroism calculations. Among these compounds, compounds 10-13 exhibited potent antifungal activities. Compound 10, the most potent antifungal agent, disrupted fungal mitochondrial respiration by inhibiting the activity of mitochondrial complexes I and IV and resulted in the intracellular ATP content of endophytic fungi being significantly reduced. The in vivo results show that compound 10 protected fruits and animals from infection by phytopathogen Alternaria citriarbusti and human pathogen Candida albicans, respectively.

Binding of Vialinin A and p-Terphenyl Derivatives to Ubiquitin-Specific Protease 4 (USP4): A Molecular Docking Study

The para-terphenyl derivative vialinin A (Vi-A), isolated from Thelephora fungi, has been characterized as a potent inhibitor of the ubiquitin-specific protease 4 (USP4). Blockade of USP4 contributes to the anti-inflammatory and anticancer properties of the natural product. We have investigated the interaction of Vi-A with USP4 by molecular modeling, to locate the binding site (around residue V98 within the domain in USP segment) and to identify the binding process and interaction contacts. From this model, a series of 32 p-terphenyl compounds were tested as potential USP4 binders, mainly in the vialinin, terrestrin and telephantin series. We identified 11 compounds presenting a satisfactory USP4 binding capacity, including two fungal products, vialinin B and aurantiotinin A, with a more favorable empirical energy of USP4 interaction (ΔE) than the reference product Vi-A. The rare p-terphenyl aurantiotinin A, isolated from the basidiomycete T. aurantiotincta, emerged as a remarkable USP4 binder. Structure-binding relationships have been identified and discussed, to guide the future design of USP4 inhibitors based on the p-terphenyl skeleton. The docking study should help the identification of other protease inhibitors from fungus.

Anti-inflammatory and anticancer p-terphenyl derivatives from fungi of the genus Thelephora

Fungi from the genus Thelephora have been exploited to identify bioactive compounds. The main natural products characterized are para-terphenyl derivatives, chiefly represented by the lead anti-inflammatory compound vialinin A isolated from species T. vialis and T. terrestris. Different series of p-terphenyls have been identified, including vialinins, ganbajunins, terrestrins, telephantins and other products. Their mechanism of action is not always clearly identified, and different potential molecule targets have been proposed. The lead vialinin A functions as a protease inhibitor, efficiently targeting ubiquitin-specific peptidases USP4/5 and sentrin-specific protease SENP1 which are prominent anti-inflammatory and anticancer targets. Protease inhibition is coupled with a powerful inhibition of the cellular production of tumor necrosis factor TNFα. Other mechanisms contributing to the anti-inflammatory or anti-proliferative action of these p-terphenyl compounds have been invoked, including the formation of cytotoxic copper complexes for derivatives bearing a catechol central unit such vialinin A, terrestrin B and telephantin O. These p-terphenyl compounds could be further exploited to design novel anticancer agents, as evidenced with the parent compound terphenyllin (essentially found in Aspergillus species) which has revealed marked antitumor and anti-metastatic effects in xenograft models of gastric and pancreatic cancer. This review shed light on the structural and functional diversity of p-terphenyls compounds isolated from Thelephora species, their molecular targets and pharmacological properties.

Hinokitiol chelates intracellular iron to retard fungal growth by disturbing mitochondrial respiration

Introduction

The increasing morbidity of fungal infections and the prevalence of drug resistance highlighted the discovery of novel antifungal agents and investigation of their modes of action. Iron chelators have been used to treat superficial fungal infections or potentiate the efficacy of certain antifungal drugs. Hinokitiol exhibits potent antifungal activity and iron-chelating ability. However, their relationships have not been established.

Objectives

This study aims to explore the selectivity of hinokitiol against fungal cells and mammalian cells and determine the role of iron-chelating for the antifungal activity of hinokitiol.

Methods

Iron probe FeRhonox-1 was used to determine intracellular Fe²⁺ content. 5-Cyano-2,3-ditolyl tetrazolium chloride probe and Cell Counting Kit-8 were used to detect the mitochondrial respiratory activities. Quantitative real-time PCR and rescue experiments were performed to determine the effect of iron on the antifungal activity of hinokitiol. The effects of hinokitiol on fungal mitochondria were further evaluated using reactive oxygen species probes and several commercial Assay Kits. The ability of hinokitiol to induce resistance in Candida species was carried out using a serial passage method. The in vivo therapeutic effect of hinokitiol was evaluated using Galleria mellonella as an infectious model.

Results

Hinokitiol was effective against a panel of Candida strains with multiple azole-resistant mechanisms and persistently inhibited Candida albicans growth. Mechanism investigations revealed that hinokitiol chelated fungal intracellular iron and inhibited the respiration of fungal cells but had minor effects on mammalian cells. Hinokitiol further inhibited the activities of mitochondrial respiratory chain complexes I and II and reduced mitochondrial membrane potential, thereby decreasing intracellular ATP synthesis and increasing detrimental intracellular reductive stress. Moreover, hinokitiol exhibited low potential for inducing resistance in several Candida species and greatly improved the survival of Candida-infected Galleria mellonella.

Conclusions

These findings suggested the potential application of hinokitiol as an iron chelator to treat fungal infections.

Caenorhabditis elegans as an Infection Model for Pathogenic Mold and Dimorphic Fungi: Applications and Challenges

The threat burden from pathogenic fungi is universal and increasing with alarming high mortality and morbidity rates from invasive fungal infections. Understanding the virulence factors of these fungi, screening effective antifungal agents and exploring appropriate treatment approaches in in vivo modeling organisms are vital research projects for controlling mycoses. Caenorhabditis elegans has been proven to be a valuable tool in studies of most clinically relevant dimorphic fungi, helping to identify a number of virulence factors and immune-regulators and screen effective antifungal agents without cytotoxic effects. However, little has been achieved and reported with regard to pathogenic filamentous fungi (molds) in the nematode model. In this review, we have summarized the enormous breakthrough of applying a C. elegans infection model for dimorphic fungi studies and the very few reports for filamentous fungi. We have also identified and discussed the challenges in C. elegans-mold modeling applications as well as the possible approaches to conquer these challenges from our practical knowledge in C. elegans-Aspergillus fumigatus model.

Candida albicans infection disturbs the redox homeostasis system and induces reactive oxygen species accumulation for epithelial cell death

Candida albicans is a common pathogenic fungus with high mortality in immunocompromised patients. However, the mechanism by which C. albicans invades host epithelial cells and causes serious tissue damage remains to be further investigated. In this study, we established the C. albicans-293T renal epithelial cell interaction model to investigate the mechanism of epithelial infection by this pathogen. It was found that C. albicans infection causes severe cell death and reactive oxygen species (ROS) accumulation in epithelial cells. Further investigations revealed that C. albicans infection might up-regulate expression of nicotinamide adenine dinucleotide phosphate (NAPDH) oxidase (NOX), inhibit the activity of the antioxidant enzymes superoxide dismutase (SOD) and catalase (CAT), and suppress the p38-Nrf2-heme oxygenase-1 (HO-1) pathway which plays an important role in the elimination of intracellular ROS. Furthermore, epithelial cell death caused by the fungal infection could be strikingly alleviated by addition of the antioxidant agent glutathione, indicating the critical role of ROS accumulation in cell death caused by the fungus. This study revealed that disturbance of the redox homeostasis system and ROS accumulation in epithelial cells is involved in cell death caused by C. albicans infection, which sheds light on the application of antioxidants in the suppression of tissue damage caused by fungal infection.

Rimonabant potentiates the antifungal activity of amphotericin B by increasing cellular oxidative stress and cell membrane permeability

Amphotericin B (AmB) is a very effective antifungal agent, and resistance in clinical isolates is rare. However, clinical treatment with AmB is often associated with severe side effects. Reducing the administration dose of AmB by combining it with other agents is a promising strategy to minimize this toxicity. In this study, we screened a small compound library and observed that the anti-obesity drug rimonabant exhibited synergistic antifungal action with AmB against Candida species and Cryptococcus neoformans. Moreover, the combination of AmB and rimonabant exhibited synergistic or additive effects against Candida albicans biofilm formation and cell viability in preformed biofilms. The effects of this combination were further confirmed in vivo using a murine systemic infection model. Exploration of the mechanism of synergy revealed that rimonabant enhances the fungicidal activity of AmB by increasing cellular oxidative stress and cell membrane permeability. These findings provide a foundation for the possible development of AmB-rimonabant polytherapies for fungal infections.

Antifungal natural products

Natural products are specialized small molecules produced in Nature and play pivotal roles in many cellular processes. These compounds possess exquisite chemical diversity and represent some of the most important pharmaceutical agents in human health care. With the rampant rise of fungal pathogens that are becoming resistant to nearly all clinically available antibiotics, there is an increased urgency to find new antifungal therapies with novel modes of action. To meet this need, we must be able to quickly identify new bioactive chemical scaffolds within complex natural extracts, determine their mechanisms of action, and generate appreciable yields for preclinical studies. In this review, we will highlight naturally derived antifungal agents of clinical importance as well as those with strong potential as leads in drug development.

Reactive oxygen mediated apoptosis as a therapeutic approach against opportunistic Candida albicans

Candida albicans are polymorphic fungal species commonly occurs in a symbiotic association with the host's usual microflora. Certain specific changes in its usual microenvironment can lead to diseases ranging from external mucosal to severally lethal systemic infections like invasive candidiasis hospital-acquired fatal infection caused by different species of Candida. The patient acquired with this infection has a high mortality and morbidity rate, ranging from 40% to 60%. This is an ill-posed problem by its very nature. Hence, early diagnosis and management is a crucial part. Antifungal drug resistance against the first and second generation of antifungal drugs has made it difficult to treat such fatal diseases. After a few dormant years, recently, there has been a rapid turnover of identifying novel drugs with low toxicity to limit the problem of drug resistance. After an initial overview of related work, we examine specific prior work on how a change in oxidative stress can facilitate apoptosis in C. albicans. Subsequently, it was investigated that Candida spp. suppresses the production of ROS mediated host defense system. Here, we have reviewed possibly all the small molecule inhibitors, natural products, antimicrobial peptide, and some naturally derived semi-synthetic compounds which are known to influence oxidative stress, to generate a proper apoptotic response in C. albicans and thus might be a novel therapeutic approach to augment the current treatment options.

A Decade of Antifungal Leads from Natural Products: 2010-2019

In this review, we discuss novel natural products discovered within the last decade that are reported to have antifungal activity against pathogenic species. Nearly a hundred natural products were identified that originate from bacteria, algae, fungi, sponges, and plants. Fungi were the most prolific source of antifungal compounds discovered during the period of review. The structural diversity of these antifungal leads encompasses all the major classes of natural products including polyketides, shikimate metabolites, terpenoids, alkaloids, and peptides.