In vitro activity of calcineurin and heat shock protein 90 Inhibitors against Aspergillus fumigatus azole- and echinocandin-resistant strains

Calcineurin activity in Fonsecaea pedrosoi: tacrolimus and cyclosporine A inhibited conidia growth, filamentation and showed synergism with itraconazole

Fonsecaea pedrosoi is a melanized fungus that causes chromoblastomycosis (CBM), a tropical neglected disease responsible for chronic and disability-related subcutaneous mycosis. Given the challenging nature of CBM treatment, the study of new targets and novel bioactive drugs capable of improving patient life quality is urgent. In the present work, we detected a calcineurin activity in F. pedrosoi conidial form, employing primarily colorimetric, immunoblotting and flow cytometry assays. Our findings reveal that the calcineurin activity of F. pedrosoi was stimulated by Ca2+/calmodulin, inhibited by EGTA and specific inhibitors, such as tacrolimus (FK506) and cyclosporine A (CsA), and proved to be insensitive to okadaic acid. In addition, FK506 and CsA were able to affect the cellular viability and the fungal proliferation. This effect was corroborated by transmission electron microscopy that showed both calcineurin inhibitors promoted profound changes in the ultrastructure of conidia, causing mainly cytoplasm condensation and intense vacuolization that are clear indication of cell death. Our data indicated that FK506 exhibited the highest effectiveness, with a minimum inhibitory concentration (MIC) of 3.12 mg/L, whereas CsA required 15.6 mg/L to inhibit 100% of conidial growth. Interestingly, when both were combined with itraconazole, they demonstrated anti-F. pedrosoi activity, exhibiting a synergistic effect. Moreover, the fungal filamentation was affected after treatment with both calcineurin inhibitors. These data corroborate with other calcineurin studies in fungal cells and open up further discussions aiming to establish the role of this enzyme as a potential target for antifungal therapy against CBM infections.

Old and new strategies in therapy and diagnosis against fungal infections

Fungal infections represent a serious global health threat. The new emerging pathogens and the spread of different forms of resistance are now hardly challenging the tools available in therapy and diagnostics. With the commonly used diagnoses, fungal identification is often slow and inaccurate, and, on the other hand, some drugs currently used as treatments are significantly affected by the decrease in susceptibility. Herein, the antifungal arsenal is critically summarized. Besides describing the old approaches and their mechanisms, advantages, and limitations, the focus is dedicated to innovative strategies which are designed, identified, and developed to take advantage of the discrepancies between fungal and host cells. Relevant pathways and their role in survival and virulence are discussed as their suitability as sources of antifungal targets. In a similar way, molecules with antifungal activity are reported as potential agents/precursors of the next generation of antimycotics. Particular attention was devoted to biotechnological entities, to their novelty and reliability, to drug repurposing and restoration, and to combinatorial applications yielding significant improvements in efficacy. KEY POINTS: • New antifungal agents and targets are needed to limit fungal morbidity and mortality. • Therapeutics and diagnostics suffer of delays in innovation and lack of targets. • Biologics, drug repurposing and combinations are the future of antifungal treatments.

Antifungal resistance, combinations and pipeline: oh my!

Invasive fungal infections are a strong contributor to healthcare costs, morbidity and mortality, especially amongst hospitalized patients. Historically, Candida was responsible for approximately 15% of all nosocomial bloodstream infections. In the past 10 years, the epidemiology of Candida species has altered, with increasing prevalence of resistant species. With rising fungal resistance, especially in Candida spp., the demand for novel antifungal therapies has exponentially increased over the last decade. Newer antifungal agents have become an attractive option for patients needing long-term therapy for infections or those requiring antifungal prophylaxis. Despite advances in coverage of non-Candida pathogens with newer agents, clinical scenarios involving multidrug-resistant fungal pathogens continue to arise in practice. Combination antifungal therapy can lead to a host of side-effects, some of which can be drug limiting. Additional antifungal therapies with enhanced fungal spectrum of activity and decreased rates of adverse effects are warranted. Fosmanogepix, ibrexafungerp, olorofim and rezafungin may help fill some of these gaps in the antifungal armamentarium. This article is part of the Challenges and strategies in the management of invasive fungal infections Special Issue: https://www.drugsincontext.com/special_issues/challenges-and-strategies-in-the-management-of-invasive-fungal-infections.

Hypoviral-regulated HSP90 co-chaperone p23 (CpCop23) determines the colony morphology, virulence, and viral response of chestnut blight fungus Cryphonectria parasitica

We previously identified a protein spot that showed down-regulation in the presence of Cryphonectria hypovirus 1 (CHV1) and tannic acid supplementation as a Hsp90 co-chaperone p23 gene (CpCop23). The CpCop23-null mutant strain showed retarded growth with less aerial mycelia and intense pigmentation. Conidia of the CpCop23-null mutant were significantly decreased and their viability was dramatically diminished. The CpCop23-null mutant showed hypersensitivity to Hsp90 inhibitors. However, no differences in responsiveness were observed after exposure to other stressors such as temperature, reactive oxygen species, and high osmosis, the exception being cell wall-disturbing agents. A severe reduction in virulence was observed in the CpCop23-null mutant. Interestingly, viral transfer to the CpCop23-null mutant from CHV1-infected strain via anastomosis was more inefficient than a comparable transfer with the wild type as a result of decreased hyphal branching of the CpCop23-null mutant around the peripheral region, which resulted in less fusion of the hyphae. The CHV1-infected CpCop23-null mutant exhibited recovered mycelial growth with less pigmentation and sporulation. The CHV1-transfected CpCop23-null mutant demonstrated almost no virulence, that is, even less than that of the CHV1-infected wild type (UEP1), a further indication that reduced virulence of the mutant is not attributable exclusively to the retarded growth but rather is a function of the CpCop23 gene. Thus, this study indicates that CpCop23 plays a role in ensuring appropriate mycelial growth and development, spore viability, responses to antifungal drugs, and fungal virulence. Moreover, the CpCop23 gene acts as a host factor that affects CHV1-infected fungal growth and maintains viral symptom development.

A host defense peptide mimetic, brilacidin, potentiates caspofungin antifungal activity against human pathogenic fungi

Fungal infections cause more than 1.5 million deaths a year. Due to emerging antifungal drug resistance, novel strategies are urgently needed to combat life-threatening fungal diseases. Here, we identify the host defense peptide mimetic, brilacidin (BRI) as a synergizer with caspofungin (CAS) against CAS-sensitive and CAS-resistant isolates of Aspergillus fumigatus, Candida albicans, C. auris, and CAS-intrinsically resistant Cryptococcus neoformans. BRI also potentiates azoles against A. fumigatus and several Mucorales fungi. BRI acts in A. fumigatus by affecting cell wall integrity pathway and cell membrane potential. BRI combined with CAS significantly clears A. fumigatus lung infection in an immunosuppressed murine model of invasive pulmonary aspergillosis. BRI alone also decreases A. fumigatus fungal burden and ablates disease development in a murine model of fungal keratitis. Our results indicate that combinations of BRI and antifungal drugs in clinical use are likely to improve the treatment outcome of aspergillosis and other fungal infections.

Target- and prodrug-based design for fungal diseases and cancer-associated fungal infections

Invasive fungal infections (IFIs) are emerging as a serious threat to public health and are associated with high incidence and mortality. IFIs also represent a frequent complication in patients with cancer who are undergoing chemotherapy. However, effective and safe antifungal agents remain limited, and the development of severe drug resistance further undermines the efficacy of antifungal therapy. Therefore, there is an urgent need for novel antifungal agents to treat life-threatening fungal diseases, especially those with new mode of action, favorable pharmacokinetic profiles, and anti-resistance activity. In this review, we summarize new antifungal targets and target-based inhibitor design, with a focus on their antifungal activity, selectivity, and mechanism. We also illustrate the prodrug design strategy used to improve the physicochemical and pharmacokinetic profiles of antifungal agents. Dual-targeting antifungal agents offer a new strategy for the treatment of resistant infections and cancer-associated fungal infections.

Potential Strategies to Control the Risk of Antifungal Resistance in Humans: A Comprehensive Review

Fungal infections are becoming one of the main causes of morbidity and mortality in people with weakened immune systems. Mycoses are becoming more common, despite greater knowledge and better treatment methods, due to the regular emergence of resistance to the antifungal medications used in clinical settings. Antifungal therapy is the mainstay of patient management for acute and chronic mycoses. However, the limited availability of antifungal drug classes limits the range of available treatments. Additionally, several drawbacks to treating mycoses include unfavourable side effects, a limited activity spectrum, a paucity of targets, and fungal resistance, all of which continue to be significant issues in developing antifungal drugs. The emergence of antifungal drug resistance has eliminated accessible drug classes as treatment choices, which significantly compromises the clinical management of fungal illnesses. In some situations, the emergence of strains resistant to many antifungal medications is a major concern. Although new medications have been developed to address this issue, antifungal drug resistance has grown more pronounced, particularly in patients who need long-term care or are undergoing antifungal prophylaxis. Moreover, the mechanisms that cause resistance must be well understood, including modifications in drug target affinities and abundances, along with biofilms and efflux pumps that diminish intracellular drug levels, to find novel antifungal drugs and drug targets. In this review, different classes of antifungal agents, and their resistance mechanisms, have been discussed. The latter part of the review focuses on the strategies by which we can overcome this serious issue of antifungal resistance in humans.

Therapeutic Approaches for Combating Aspergillus Associated Infection

Now-a-days fungal infection emerges as a significant problem to healthcare management systems due to high frequency of associated morbidity, mortality toxicity, drug-drug interactions, and resistance of the antifungal agents. Aspergillus is the most common mold that cause infection in immunocompromised hosts. It's a hyaline mold that is cosmopolitan and ubiquitous in nature. Aspergillus infects around 10 million population each year with a mortality rate of 30-90%. Clinically available antifungal formulations are restricted to four classes (i.e., polyene, triazole, echinocandin, and allylamine), and each of them have their own limitations associated with the activity spectrum, the emergence of resistance, and toxicity. Consequently, novel antifungal agents with modified and altered chemical structures are required to combat these invasive fungal infections. To overcome these limitations, there is an urgent need for new antifungal agents that can act as potent drugs in near future. Currently, some compounds have shown effective antifungal activity. In this review article, we have discussed all potential antifungal therapies that contain old antifungal drugs, combination therapies, and recent novel antifungal formulations, with a focus on the Aspergillus associated infections.

Filamentous fungal biofilms: Conserved and unique aspects of extracellular matrix composition, mechanisms of drug resistance and regulatory networks in Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is an ubiquitous mold that can cause invasive pulmonary infections in immunocompromised patients. Within the lung, A. fumigatus forms biofilms that can enhance resistance to antifungals and immune defenses, highlighting the importance of defining the mechanisms underlying biofilm development and associated emergent properties. A. fumigatus biofilms display a morphology and architecture that is distinct from bacterial and yeast biofilms. Moreover, A. fumigatus biofilms display unique characteristics in the composition of their extracellular matrix (ECM) and the regulatory networks governing biofilm formation. This review will discuss our current understanding of the form and function of A. fumigatus biofilms, including the unique components of ECM matrix, potential drug resistance mechanisms, the regulatory networks governing A. fumigatus biofilm formation, and potential therapeutics targeting these structures.

Potential Antifungal Targets for Aspergillus sp. from the Calcineurin and Heat Shock Protein Pathways

Aspergillus species, especially A. fumigatus, and to a lesser extent others (A. flavus, A. niger, A. terreus), although rarely pathogenic to healthy humans, can be very aggressive to immunocompromised patients (they are opportunistic pathogens). Although survival rates for such infections have improved in recent decades following the introduction of azole derivatives, they remain a clinical challenge. The fact that current antifungals act as fungistatic rather than fungicide, that they have limited safety, and that resistance is becoming increasingly common make the need for new, more effective, and safer therapies to become more acute. Over the last decades, knowledge about the molecular biology of A. fumigatus and other Aspergillus species, and particularly of calcineurin, Hsp90, and their signaling pathway proteins, has progressed remarkably. Although calcineurin has attracted much interest, its adverse effects, particularly its immunosuppressive effects, make it less attractive than it might at first appear. The situation is not very different for Hsp90. Other proteins from their signaling pathways, such as protein kinases phosphorylating the four SPRR serine residues, CrzA, rcnA, pmcA-pmcC (particularly pmcC), rfeF, BAR adapter protein(s), the phkB histidine kinase, sskB MAP kinase kinase, zfpA, htfA, ctfA, SwoH (nucleoside diphosphate kinase), CchA, MidA, FKBP12, the K27 lysine position from Hsp90, PkcA, MpkA, RlmA, brlA, abaA, wetA, other heat shock proteins (Hsp70, Hsp40, Hsp12) currently appear promising and deserve further investigation as potential targets for antifungal drug development.

Current Promising Therapeutic Targets for Aspergillosis Treatment

Aspergillosis is a fungal disease caused by different species of Aspergillus. They live in soil,dust and decomposed material. Number of Aspergillus species found till now is about 300 and more are still to be identified. Only few Aspergillus species can cause human disease and the most common species for human infection is Aspergillus fumigatus, which is a ubiquitous airborne saprophytic fungus. Severity of the disease ranges from an allergic response to life-threatening generalized infection. They grow optimally at 37°C and can grow upto 50°C. The fungal conidia are being constantly inhaled by humans and animals everyday normally gets eliminated by innate immune mechanism. Due to increasing number of immunocompromised patients, severe and fatal Aspergillosis cases have augmented. Currently, available antifungal drug for the treatment of Aspergillosis act on these three molecular target are 14 alpha demethylase for Azoles, ergosterol for Polyene and β-1,3-glucan synthase for Echinocandin. These antifungal drug show high resistance problem and toxicity. So, it is high time to develop new drugs for treatment with reduced toxicity and drug resistant problem. Synthesis of essential amino acid is absent in human as they obtain it from their diet but fungi synthesis these amino acid. Thus, enzymes in this pathway acts as novel drug target. This article summarizes promising drug targets presents in different metabolic pathway of Aspergillus genome and discusses their molecular functions in detail. This review also list down the inhibitors of these novel target. We present a comprehensive review that will pave way for discovery and development of novel antifungals against these drug targets for Aspergillosis treatment.

Hsp90 Inhibitor Attenuates the Development of Pathophysiological Cardiac Fibrosis in Mouse Hypertrophy via Suppression of the Calcineurin-NFAT and c-Raf-Erk Pathways

ABSTRACT

In the previous study, we showed that an Hsp90 inhibitor, 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), attenuates hypertrophic remodeling of cardiomyocytes during the development of heart failure. In this present study, we investigated the effects of 17-AAG on cardiac fibrosis during the development of heart failure. We used pressure-loaded cardiac hypertrophic mice prepared by constriction of the transverse aorta (TAC), which induces significant cardiac fibrosis without scar tissue. From the sixth week after the TAC operation, vehicle or 17-AAG was administered intraperitoneally twice a week. Eight weeks after the operation, the vehicle-treated animals showed chronic heart failure. On the other hand, cardiac deterioration of the 17-AAG-treated animals was attenuated. In 17-AAG-treated animals, when the degree of fibrosis was observed by histological staining, their volume of fibrosis was found to be reduced. The content of calcineurin, an Hsp90 client protein, and the level of dephosphorylated NFATc2, a transcription factor in the cardiac fibroblasts, in the TAC mice was reduced by treatment with 17-AAG. Furthermore, c-Raf and Erk signaling, indicators for cell proliferation and collagen synthesis, was also attenuated. In in vitro experiments, the proliferation and collagen synthesis of the cultured cardiac fibroblasts were attenuated by the presence of 17-AAG. When cardiac fibroblasts were incubated with angiotensin II, calcineurin-NFATc2 and c-Raf-Erk signaling in the cells were activated. These activations were attenuated by 17-AAG. Our findings suggest that suppression of the calcineurin-NFAT and c-Raf-Erk pathways may partially contribute to the attenuation of myocardial fibrosis caused by treatment with 17-AAG. Therefore, our data imply that the Hsp90 inhibitor may have potential for novel therapeutic strategy for the treatment of heart failure.

Azole Resistance in Clinical and Environmental Aspergillus Isolates from the French West Indies (Martinique)

The emergence of azole resistant Aspergillus spp., especially Aspergillus fumigatus, has been described in several countries around the world with varying prevalence depending on the country. To our knowledge, azole resistance in Aspergillus spp. has not been reported in the West Indies yet. In this study, we investigated the antifungal susceptibility of clinical and environmental isolates of Aspergillus spp. from Martinique, and the potential resistance mechanisms associated with mutations in cyp51A gene. Overall, 208 Aspergillus isolates were recovered from clinical samples (n = 45) and environmental soil samples (n = 163). They were screened for resistance to azole drugs using selective culture media. The Minimum Inhibitory Concentrations (MIC) towards voriconazole, itraconazole, posaconazole and isavuconazole, as shown by the resistant isolates, were determined using the European Committee on Antimicrobial Susceptibility Testing (EUCAST) microdilution broth method. Eight isolates (A. fumigatus, n = 6 and A. terreus, n = 2) had high MIC for at least one azole drug. The sequencing of cyp51A gene revealed the mutations G54R and TR34/L98H in two A. fumigatus clinical isolates. Our study showed for the first time the presence of azole resistance in A. fumigatus and A. terreus isolates in the French West Indies.

CgEnd3 Regulates Endocytosis, Appressorium Formation, and Virulence in the Poplar Anthracnose Fungus Colletotrichum gloeosporioides

The hemibiotrophic ascomycete fungus Colletotrichum gloeosporioides is the causal agent of anthracnose on numerous plants, and it causes considerable economic losses worldwide. Endocytosis is an essential cellular process in eukaryotic cells, but its roles in C. gloeosporioides remain unknown. In our study, we identified an endocytosis-related protein, CgEnd3, and knocked it out via polyethylene glycol (PEG)-mediated protoplast transformation. The lack of CgEnd3 resulted in severe defects in endocytosis. C. gloeosporioides infects its host through a specialized structure called appressorium, and ΔCgEnd3 showed deficient appressorium formation, melanization, turgor pressure accumulation, penetration ability of appressorium, cellophane membrane penetration, and pathogenicity. CgEnd3 also affected oxidant adaptation and the expression of core effectors during the early stage of infection. CgEnd3 contains one EF hand domain and four calcium ion-binding sites, and it is involved in calcium signaling. A lack of CgEnd3 changed the responses to cell-wall integrity agents and fungicide fludioxonil. However, CgEnd3 regulated appressorium formation and endocytosis in a calcium signaling-independent manner. Taken together, these results demonstrate that CgEnd3 plays pleiotropic roles in endocytosis, calcium signaling, cell-wall integrity, appressorium formation, penetration, and pathogenicity in C. gloeosporioides, and it suggests that CgEnd3 or endocytosis-related genes function as promising antifungal targets.

The Heat Shock Transcription Factor HsfA Is Essential for Thermotolerance and Regulates Cell Wall Integrity in Aspergillus fumigatus

The deleterious effects of human-induced climate change have long been predicted. However, the imminent emergence and spread of new diseases, including fungal infections through the rise of thermotolerant strains, is still neglected, despite being a potential consequence of global warming. Thermotolerance is a remarkable virulence attribute of the mold Aspergillus fumigatus. Under high-temperature stress, opportunistic fungal pathogens deploy an adaptive mechanism known as heat shock (HS) response controlled by heat shock transcription factors (HSFs). In eukaryotes, HSFs regulate the expression of several heat shock proteins (HSPs), such as the chaperone Hsp90, which is part of the cellular program for heat adaptation and a direct target of HSFs. We recently observed that the perturbation in cell wall integrity (CWI) causes concomitant susceptibility to elevated temperatures in A. fumigatus, although the mechanisms underpinning the HS response and CWI cross talking are not elucidated. Here, we aim at further deciphering the interplay between HS and CWI. Our results show that cell wall ultrastructure is severely modified when A. fumigatus is exposed to HS. We identify the transcription factor HsfA as essential for A. fumigatus viability, thermotolerance, and CWI. Indeed, HS and cell wall stress trigger the coordinated expression of both hsfA and hsp90. Furthermore, the CWI signaling pathway components PkcA and MpkA were shown to be important for HsfA and Hsp90 expression in the A. fumigatus biofilms. Lastly, RNA-sequencing confirmed that hsfA regulates the expression of genes related to the HS response, cell wall biosynthesis and remodeling, and lipid homeostasis. Our studies collectively demonstrate the connection between the HS and the CWI pathway, with HsfA playing a crucial role in this cross-pathway regulation, reinforcing the importance of the cell wall in A. fumigatus thermophily.

Study on the Inhibitory Activity and Possible Mechanism of Myriocin on Clinically Relevant Drug-Resistant Candida albicans and Its Biofilms

In order to prevent and control the infection of Candida albicans, the antifungal activity, possible mechanism of myriocin against C. albicans and its biofilm were studied. The antifungal activity of myriocin was investigated by microdilution method. The effect of myriocin on fungal cell wall or membrane was evaluated by adding sorbitol, ergosterol or phytosphingosine (PHS). The damage to the cell membrane was investigated with propidium iodide (PI) staining and visualized by scanning electron microscope (SEM). The effects on biofilms and extracellular polysaccharides (EPS) were observed by crystal violet staining method and phenol-sulfuric acid method respectively. The adhesion of C. albicans cells to hydrocarbons was tested to evaluate cell surface hydrophobic (CSH). The combined effects of myriocin and antifungal drugs commonly used in clinical practice were investigated by using the checkerboard microdilution method. Minimal inhibitory concentrations (MICs) were found to be 0.125-4 µg/mL. Myriocin was found to affect both cell wall and cell membrane. After exposure to myriocin, biofilm and EPS were found to be inhibited and removed, and the CSH was decreased. The combined fungistasis of myriocin and voriconazole (VCZ) or amphotericin B (AMB) were additive. Myriocin had significant antifungal activity against C. albicans, and the antifungal mechanisms might be cell wall and membrane damage. Myriocin effectively inhibited and eliminated biofilms, and its mechanism may be related to the inhibition of EPS and CSH.

Combination Therapy to Treat Fungal Biofilm-Based Infections

An increasing number of people is affected by fungal biofilm-based infections, which are resistant to the majority of currently-used antifungal drugs. Such infections are often caused by species from the genera Candida, Aspergillus or Cryptococcus. Only a few antifungal drugs, including echinocandins and liposomal formulations of amphotericin B, are available to treat such biofilm-based fungal infections. This review discusses combination therapy as a novel antibiofilm strategy. More specifically, in vitro methods to discover new antibiofilm combinations will be discussed. Furthermore, an overview of the main modes of action of promising antibiofilm combination treatments will be provided as this knowledge may facilitate the optimization of existing antibiofilm combinations or the development of new ones with a similar mode of action.

Novel Antifungal Agents and Their Activity against Aspergillus Species

There is a need for new antifungal agents, mainly due to increased incidence of invasive fungal infections (IFI), high frequency of associated morbidity and mortality and limitations of the current antifungal agents (e.g., toxicity, drug-drug interactions, and resistance). The clinically available antifungals for IFI are restricted to four main classes: polyenes, flucytosine, triazoles, and echinocandins. Several antifungals are hampered by multiple resistance mechanisms being present in fungi. Consequently, novel antifungal agents with new targets and modified chemical structures are required to combat fungal infections. This review will describe novel antifungals, with a focus on the Aspergillus species.

Structure-guided approaches to targeting stress responses in human fungal pathogens

Fungi inhabit extraordinarily diverse ecological niches, including the human body. Invasive fungal infections have a devastating impact on human health worldwide, killing ∼1.5 million individuals annually. The majority of these deaths are attributable to species of Candida, Cryptococcus, and Aspergillus Treating fungal infections is challenging, in part due to the emergence of resistance to our limited arsenal of antifungal agents, necessitating the development of novel therapeutic options. Whereas conventional antifungal strategies target proteins or cellular components essential for fungal growth, an attractive alternative strategy involves targeting proteins that regulate fungal virulence or antifungal drug resistance, such as regulators of fungal stress responses. Stress response networks enable fungi to adapt, grow, and cause disease in humans and include regulators that are highly conserved across eukaryotes as well as those that are fungal-specific. This review highlights recent developments in elucidating crystal structures of fungal stress response regulators and emphasizes how this knowledge can guide the design of fungal-selective inhibitors. We focus on the progress that has been made with highly conserved regulators, including the molecular chaperone Hsp90, the protein phosphatase calcineurin, and the small GTPase Ras1, as well as with divergent stress response regulators, including the cell wall kinase Yck2 and trehalose synthases. Exploring structures of these important fungal stress regulators will accelerate the design of selective antifungals that can be deployed to combat life-threatening fungal diseases.

Calcium signaling is involved in diverse cellular processes in fungi

Calcium (Ca²⁺) is a universal signalling molecule of life. The Ca²⁺ signalling is an evolutionarily conserved process from prokaryotes to eukaryotes. Ca²⁺ at high concentration is deleterious to the cell; therefore, cell maintains a low resting level of intracellular free Ca²⁺ concentration ([Ca²⁺]_c). The resting [Ca²⁺]_c is tightly regulated, and a transient increase of the [Ca²⁺]_c initiates a signalling cascade in the cell. Ca²⁺ signalling plays an essential role in various processes, including growth, development, reproduction, tolerance to stress conditions, and virulence in fungi. In this review, we describe the evolutionary aspects of Ca²⁺ signalling and cell functions of major Ca²⁺ signalling proteins in different fungi.

Silva L, Gonçales RA, Neves BJ, Soares CM, Pereira M. Setting New Routes for Antifungal Drug Discovery Against Pathogenic Fungi

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Fungal diseases are life-threatening to human health and responsible for millions of deaths around the world. Fungal pathogens lead to a high number of morbidity and mortality. Current antifungal treatment comprises drugs, such as azoles, echinocandins, and polyenes and the cure is not guaranteed. In addition, such drugs are related to severe side effects and the treatment lasts for an extended period. Thus, setting new routes for the discovery of effective and safe antifungal drugs should be a priority within the health care system. The discovery of alternative and efficient antifungal drugs showing fewer side effects is time-consuming and remains a challenge. Natural products can be a source of antifungals and used in combinatorial therapy. The most important natural products are antifungal peptides, antifungal lectins, antifungal plants, and fungi secondary metabolites. Several proteins, enzymes, and metabolic pathways could be targets for the discovery of efficient inhibitor compounds and recently, heat shock proteins, calcineurin, salinomycin, the trehalose biosynthetic pathway, and the glyoxylate cycle have been investigated in several fungal species. HSP protein inhibitors and echinocandins have been shown to have a fungicidal effect against azole-resistant fungi strains. Transcriptomic and proteomic approaches have advanced antifungal drug discovery and pointed to new important specific-pathogen targets. Certain enzymes, such as those from the glyoxylate cycle, have been a target of antifungal compounds in several fungi species. Natural and synthetic compounds inhibited the activity of such enzymes and reduced the ability of fungal cells to transit from mycelium to yeast, proving to be promisor antifungal agents. Finally, computational biology has developed effective approaches, setting new routes for early antifungal drug discovery since normal approaches take several years from discovery to clinical use. Thus, the development of new antifungal strategies might reduce the therapeutic time and increase the quality of life of patients.

Study of Humoral Responses against Lomentospora/Scedosporium spp. and Aspergillus fumigatus to Identify L. prolificans Antigens of Interest for Diagnosis and Treatment

The high mortality rates of Lomentospora prolificans infections are due, above all, to the tendency of the fungus to infect weakened hosts, late diagnosis and a lack of effective therapeutic treatments. To identify proteins of significance for diagnosis, therapy or prophylaxis, immunoproteomics-based studies are especially important. Consequently, in this study murine disseminated infections were carried out using L. prolificans, Scedosporium aurantiacum, Scedosporium boydii and Aspergillus fumigatus, and their sera used to identify the most immunoreactive proteins of L. prolificans total extract and secreted proteins. The results showed that L. prolificans was the most virulent species and its infections were characterized by a high fungal load in several organs, including the brain. The proteomics study showed a high cross-reactivity between Scedosporium/Lomentospora species, but not with A. fumigatus. Among the antigens identified were, proteasomal ubiquitin receptor, carboxypeptidase, Vps28, HAD-like hydrolase, GH16, cerato-platanin and a protein of unknown function that showed no or low homology with humans. Finally, Hsp70 deserves a special mention as it was the main antigen recognized by Scedosporium/Lomentospora species in both secretome and total extract. In conclusion, this study identifies antigens of L. prolificans that can be considered as potential candidates for use in diagnosis and as therapeutic targets and the production of vaccines.

In Vitro Interaction of Geldanamycin with Triazoles and Echinocandins Against Common and Emerging Candida Species

The aim of this study was to determine the in vitro interactions of geldanamycin (Hsp90-inhibitor) with triazoles and echinocandins against common and emerging Candida species. Twenty clinically important Candida strains comprising C. auris, C. albicans, C. parapsilosis, and C. glabrata (each five strains) were included. In vitro interactions of geldanamycin with fluconazole, itraconazole, caspofungin and anidulafungin were determined using a checkerboard method. The results were interpreted as synergistic, indifferent and antagonistic based on the fractional inhibitory concentration index (FICI). In vitro combination of fluconazole with geldanamycin resulted in synergistic effect against C. albicans (100%), C. glabrata (80%) and C. parapsilosis (80%) (FICI range 0.009-0.5), while indifferent interactions were obtained against C. auris (FICI range 1.5-2). The overall minimum inhibitory concentration (MIC) range of fluconazole against C. albicans, C. glabrata and C. parapsilosis reduced from 16-256 to 0.25-64 mg/L when combined with geldanamycin. Regarding the synergistic effect of geldanamycin with itraconazole against all strains of C. albicans, C. glabrata and C. parapsilosis (FICI range 0.009-0.375), the MIC range of this antifungal was reduced from 0.125-32 mg/L when tested alone, to 0.03-1 mg/L. Combinations of geldanamycin with fluconazole and itraconazole against C. auris, as well as combination of geldanamycin with caspofungin and anidulafungin against all studied Candida species, resulted in indifferent effects. No antagonism was observed. Simultaneous targeting of Hsp90 and lanosterol 14-α demethylase seems an effective approach against C. albicans, C. glabrata and C. parapsilosis. However, this combination is ineffective against the emerging pathogen C. auris.

Targeting HSP90 attenuates angiotensin II-induced adventitial remodelling via suppression of mitochondrial fission

Aims

Adventitial remodelling presenting with the phenotypic switch of adventitial fibroblasts (AFs) to myofibroblasts is reportedly involved in the evolution of several vascular diseases, including hypertension. In our previous study, we reported that heat shock protein 90 (HSP90) inhibition by 17-dime-thylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) markedly attenuates angiotensin II (AngII)-induced abdominal aortic aneurysm formation by simultaneously inhibiting several key signalling and transcriptional pathways in vascular smooth muscle cells; however, little is known about its role on AFs. Given that the AF phenotypic switch is likely to be associated with mitochondrial function and calcineurin (CN), a client protein of HSP90 that mediates mitochondrial fission and function, the aim of this study was to investigate whether mitochondrial fission contributes to phenotypic switch of AF, and if it does, we further aimed to determine whether HSP90 inhibition attenuates mitochondrial fission and subsequently suppresses AF transformation and adventitial remodelling in AngII-induced hypertensive mice.

Methods and results

In primary mouse AFs, we found that CN-dependent dephosphorylation of Drp1 induced mitochondrial fission and regulated mitochondrial reactive oxygen species production, which stimulated AF proliferation, migration, and phenotypic switching in AngII-treated AFs. Moreover, AngII was found to increase the binding of HSP90 and CN in AFs, while HSP90 inhibition significantly reversed AngII-induced mitochondrial fission and AF phenotypic switching by modulating the CN-dependent dephosphorylation of Drp1. Consistent with the effects in AFs, in an animal model of AngII-induced adventitial remodelling, 17-DMAG markedly reduced mitochondrial fission, AF differentiation, vessel wall thickening, and fibrosis in the aortic adventitia, which were mediated by CN/Drp1 signalling pathways.

Conclusions

Our study suggests that CN/Drp1-dependent mitochondrial fission may be essential for understanding adventitial remodelling in hypertension and that HSP90 inhibition may serve as a novel approach for the treatment of adventitial remodelling-related diseases.

Genetic Analysis of Candida auris Implicates Hsp90 in Morphogenesis and Azole Tolerance and Cdr1 in Azole Resistance

Candida auris is an emerging fungal pathogen and a serious global health threat as the majority of clinical isolates display elevated resistance to currently available antifungal drugs. Despite the increased prevalence of C. auris infections, the mechanisms governing drug resistance remain largely elusive. In diverse fungi, the evolution of drug resistance is enabled by the essential molecular chaperone Hsp90, which stabilizes key regulators of cellular responses to drug-induced stress. Hsp90 also orchestrates temperature-dependent morphogenesis in Candida albicans, a key virulence trait. However, the role of Hsp90 in the pathobiology of C. auris remains unknown. In order to study regulatory functions of Hsp90 in C. auris, we placed HSP90 under the control of a doxycycline-repressible promoter to enable transcriptional repression. We found that Hsp90 is essential for growth in C. auris and that it enables tolerance of clinical isolates with respect to the azoles, which inhibit biosynthesis of the membrane sterol ergosterol. High-level azole resistance was independent of Hsp90 but dependent on the ABC transporter CDR1, deletion of which resulted in abrogated resistance. Strikingly, we discovered that C. auris undergoes a morphogenetic transition from yeast to filamentous growth in response to HSP90 depletion or cell cycle arrest but not in response to other cues that induce C. albicans filamentation. Finally, we observed that this developmental transition is associated with global transcriptional changes, including the induction of cell wall-related genes. Overall, this report provides a novel insight into mechanisms of drug tolerance and resistance in C. auris and describes a developmental transition in response to perturbation of a core regulator of protein homeostasis.IMPORTANCE Fungal pathogens pose a serious threat to public health. Candida auris is an emerging fungal pathogen that is often resistant to commonly used antifungal drugs. However, the mechanisms governing drug resistance and virulence in this organism remain largely unexplored. In this study, we adapted a conditional expression system to modulate the transcription of an essential gene, HSP90, which regulates antifungal resistance and virulence in diverse fungal pathogens. We showed that Hsp90 is essential for growth in C. auris and is important for tolerance of the clinically important azole antifungals, which block ergosterol biosynthesis. Further, we established that the Cdr1 efflux transporter regulates azole resistance. Finally, we discovered that C. auris transitions from yeast to filamentous growth in response to Hsp90 inhibition, accompanied by global transcriptional remodeling. Overall, this work provides a novel insight into mechanisms regulating azole resistance in C. auris and uncovers a distinct developmental program regulated by Hsp90.

An insight into new strategies to combat antifungal drug resistance

Invasive fungal infections especially in immunocompromised patients represent a dominating cause of mortality. The most commonly used antifungal agents can be divided into three broad categories, including triazoles, echinocandins and polyenes. Antifungal resistance is on the increase, posing a growing threat to the stewardship of immunocompromised patients with fungal infections. The paucity of currently available antifungals leads to the rapid emergence of drug resistance and thus aggravates the refractoriness of invasive fungal infections. Therefore, deep exploration into mechanisms of drug resistance and search for new antifungal targets are required. This review highlights the therapeutic strategies targeting Hsp90, calcineurin, trehalose biosynthesis and sphingolipids biosynthesis, in an attempt to provide clinical evidence for overcoming drug resistance and to form the rationale for combination therapy of conventional antifungals and agents with novel mechanisms of action. What’s more, this review also gives a concise introduction of three new-fashioned antifungals, including carboxymethyl chitosan, silver nanoparticles and chromogranin A-N46.

In vitro interactions between 17-AAG and azoles against Exophiala dermatitidis

BACKGROUND

Exophiala dermatitidis causes a variety of illnesses in humans which are always refractory to available treatment modalities. Hsp90 governs crucial stress responses, cell wall repair mechanisms and antifungal resistance in pathogenic fungi. Thus, targeting Hsp90 with specific inhibitors holds considerable promise as combination strategy.

OBJECTIVES

To investigate the antifungal effect of 17-AAG alone or combined with azoles against E. dermatitidis.

METHODS

In vitro interactions of 17-AAG, a Hsp90 inhibitor, and azoles including itraconazole, voriconazole and posaconazole against E. dermatitidis were evaluated via broth microdilution chequerboard technique, adapted from the CLSI M38-A2 method. A total of 18 clinical strains were studied. Candida parapsilosis (ATCC22019) was included to ensure quality control.

RESULTS AND CONCLUSIONS

17-AAG alone exhibited minimal antifungal activity against all tested isolates. However, synergistic effects between 17-AAG and posaconazole, itraconazole or voriconazole were observed against 15 (83.3%), 12 (66.7%) and 1 (5.6%) isolates of E. dermatitidis, respectively. The effective working ranges of 17-AAG in synergistic combinations were mostly within 2-8 μg/mL. No antagonism was observed. In conclusion, harnessing fungal Hsp90 with 17-AAG might prove a potential antifungal regimen for E. dermatitidis infections. However, due to the host toxicity of 17-AAG, more efforts are needed to develop fungal specific Hsp90 inhibitors.

Epigenetic and Posttranslational Modifications in Regulating the Biology of Aspergillus Species

Epigenetic and posttranslational modifications have been proved to participate in multiple cellular processes and suggested to be an important regulatory mechanism on transcription of genes in eukaryotes. However, our knowledge about epigenetic and posttranslational modifications mainly comes from the studies of yeasts, plants, and animals. Recently, epigenetic and posttranslational modifications have also raised concern for the relevance of regulating fungal biology in Aspergillus. Emerging evidence indicates that these modifications could be a connection between genetic elements and environmental factors, and their combined effects may finally lead to fungal phenotypical changes. This article describes the advances in typical DNA and protein modifications in the genus Aspergillus, focusing on methylation, acetylation, phosphorylation, ubiquitination, sumoylation, and neddylation.

Molecular Evolution of Antifungal Drug Resistance

The fungal pathogens Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have transitioned from a rare curiosity to a leading cause of human mortality. The management of infections caused by these organisms is intimately dependent on the efficacy of antifungal agents; however, fungi that are resistant to these treatments are regularly isolated in the clinic, impeding our ability to control infections. Given the significant impact fungal pathogens have on human health, it is imperative to understand the molecular mechanisms that govern antifungal drug resistance. This review describes our current knowledge of the mechanisms by which antifungal drug resistance evolves in experimental populations and clinical settings. We explore current antifungal treatment options and discuss promising strategies to impede the evolution of drug resistance. By tackling antifungal drug resistance as an evolutionary problem, there is potential to improve the utility of current treatments and accelerate the development of novel therapeutic strategies.

Scanning Quadrupole Data-Independent Acquisition, Part B: Application to the Analysis of the Calcineurin-Interacting Proteins during Treatment of Aspergillus fumigatus with Azole and Echinocandin Antifungal Drugs

Calcineurin is a critical cell-signaling protein that orchestrates growth, stress response, virulence, and antifungal drug resistance in several fungal pathogens. Blocking calcineurin signaling increases the efficacy of several currently available antifungals and suppresses drug resistance. We demonstrate the application of a novel scanning quadrupole DIA method for the analysis of changes in the proteins coimmunoprecipitated with calcineurin during therapeutic antifungal drug treatments of the deadly human fungal pathogen Aspergillus fumigatus. Our experimental design afforded an assessment of the precision of the method as demonstrated by peptide- and protein-centric analysis from eight replicates of the study pool QC samples. Two distinct classes of clinically relevant antifungal drugs that are guideline recommended for the treatment of invasive "aspergillosis" caused by Aspergillus fumigatus, the azoles (voriconazole) and the echinocandins (caspofungin and micafungin), which specifically target the fungal plasma membrane and the fungal cell wall, respectively, were chosen to distinguish variations occurring in the proteins coimmunoprecipitated with calcineurin. Novel potential interactors were identified in response to the different drug treatments that are indicative of the possible role for calcineurin in regulating these effectors. Notably, treatment with voriconazole showed increased immunoprecipitation of key proteins involved in membrane ergosterol biosynthesis with calcineurin. In contrast, echinocandin (caspofungin or micafungin) treatments caused increased immunoprecipitation of proteins involved in cell-wall biosynthesis and septation. Furthermore, abundant coimmunoprecipitation of ribosomal proteins with calcineurin occurred exclusively in echinocandins treatment, indicating reprogramming of cellular growth mechanisms during different antifungal drug treatments. While variations in the observed calcineurin immunoprecipitated proteins may also be due to changes in their expression levels under different drug treatments, this study suggests an important role for calcineurin-dependent cellular mechanisms in response to antifungal treatment of A. fumigatus that warrants future studies.

Caspofungin-Mediated Growth Inhibition and Paradoxical Growth in Aspergillus fumigatus Involve Fungicidal Hyphal Tip Lysis Coupled with Regenerative Intrahyphal Growth and Dynamic Changes in β-1,3-Glucan Synthase Localization

Caspofungin targets cell wall β-1,3-glucan synthesis and is the international consensus guideline-recommended salvage therapy for invasive aspergillosis. Although caspofungin is inhibitory at low concentrations, it exhibits a paradoxical effect (reversal of growth inhibition) at high concentrations by an undetermined mechanism. Treatment with caspofungin at either the growth-inhibitory concentration (0.5 μg/ml) or paradoxical growth-inducing concentration (4 μg/ml) for 24 h caused similar abnormalities, including wider, hyperbranched hyphae, increased septation, and repeated hyphal tip lysis, followed by regenerative intrahyphal growth. By 48 h, only hyphae at the colony periphery treated with the high caspofungin concentration displayed paradoxical growth. A similar high concentration of caspofungin also induced the paradoxical growth of Aspergillus fumigatus during human A549 alveolar cell invasion. Localization of the β-1,3-glucan synthase complex (Fks1 and Rho1) revealed significant differences between cells exposed to the growth-inhibitory and paradoxical growth-inducing concentrations of caspofungin. At both concentrations, Fks1 initially mislocalized from the hyphal tips to vacuoles. However, only continuous exposure to 4 μg/ml of caspofungin for 48 h led to recovery of the normal hyphal morphology with renewed localization of Fks1 to hyphal tips. Rho1 remained at the hyphal tip after treatment with both caspofungin concentrations but was required for paradoxical growth. Farnesol blocked paradoxical growth and relocalized Fks1 and Rho1 to vacuoles. Our results highlight the importance of regenerative intrahyphal growth as a rapid adaptation to the fungicidal lytic effects of caspofungin on hyphal tips and the dynamic localization of Fks1 as part of the mechanism for the caspofungin-mediated paradoxical response in A. fumigatus.

The Hsp90 Chaperone Network Modulates Candida Virulence Traits

Hsp90 is a conserved molecular chaperone that facilitates the folding and function of client proteins. Hsp90 function is dynamically regulated by interactions with co-chaperones and by post-translational modifications. In the fungal pathogen Candida albicans, Hsp90 enables drug resistance and virulence by stabilizing diverse signal transducers. Here, we review studies that have unveiled regulators of Hsp90 function, as well as downstream effectors that govern the key virulence traits of morphogenesis and drug resistance. We highlight recent work mapping the Hsp90 genetic network in C. albicans under diverse environmental conditions, and how these interactions provide insight into circuitry important for drug resistance, morphogenesis, and virulence. Ultimately, elucidating the Hsp90 chaperone network will aid in the development of therapeutics to treat fungal disease.

Calcineurin in fungal virulence and drug resistance: Prospects for harnessing targeted inhibition of calcineurin for an antifungal therapeutic approach

Increases in the incidence and mortality due to the major invasive fungal infections such as aspergillosis, candidiasis and cryptococcosis caused by the species of Aspergillus, Candida and Cryptococcus, are a growing threat to the immunosuppressed patient population. In addition to the limited armamentarium of the current classes of antifungal agents available (pyrimidine analogs, polyenes, azoles, and echinocandins), their toxicity, efficacy and the emergence of resistance are major bottlenecks limiting successful patient outcomes. Although these drugs target distinct fungal pathways, there is an urgent need to develop new antifungals that are more efficacious, fungal-specific, with reduced or no toxicity and simultaneously do not induce resistance. Here we review several lines of evidence which indicate that the calcineurin signaling pathway, a target of the immunosuppressive drugs FK506 and cyclosporine A, orchestrates growth, virulence and drug resistance in a variety of fungal pathogens and can be exploited for novel antifungal drug development.

The anti-Aspergillus drug pipeline: Is the glass half full or empty?

Aspergillosis has emerged as important human mycoses, in view of the ever expanding population at risk. The emergence of resistance to the most commonly used drugs for aspergillosis, the azoles, the mediocre activity, and frequent toxicity of the current antifungal armamentarium, support the need for development of novel antifungals for treatment of this disease. In this minireview, we describe recent efforts by small drug companies and University research labs to develop novel therapies for invasive aspergillus infections. We specifically discuss four small-molecule antifungals (T-2307, E1210/APX001, ASP2397, and F901318) with novel modes-of-action, which are currently entering phase I clinical trials. In addition, we provide a nonexhaustive discussion of some interesting, yet early developments in the quest for improved therapeutic strategies such as (i) novel formulations of amphotericin B including AMB nanoparticle suspensions and AMB-arabinogalactan or AMB-PEG conjugates that show low toxicity and high efficacy in preclinical animal models, (ii) repurposed drugs that synergize with existing antifungals (clozafimine, trichostatin A, MGCD290, geldanamycin, tacrolimus, cyclosporin), (iii) natural products (psoriasin, humidimycin), and (iv) immunotherapy using adoptive transfer of activated immune cells with antifungal activity. We argue that despite the plethora of candidates, the extremely low success rates of drug development leading to clinically useful drugs reinforces the need for continued clinical reliance on mainstream antifungals and their improved derivatives.

Uptake and efflux kinetics, and intracellular activity of voriconazole against Aspergillus fumigatus in human pulmonary epithelial cells: a new application for the prophylaxis and early treatment of invasive pulmonary aspergillosis

Invasive pulmonary aspergillosis (IPA), most caused by Aspergillus fumigatus, is a serious life-threatening infection in immunocompromised patients. Voriconazole is used to prevent and treat IPA. However, little is known about the pharmacological characteristics of voriconazole in pulmonary epithelial cells, which are the target site for the prophylaxis and early treatment of IPA. The aim of the study was to evaluate the kinetics and activity of voriconazole against A. fumigatus in A549 cells. High-performance liquid chromatography/tandem mass spectrometry and time-kill method were used to study the cellular pharmacokinetic and pharmacodynamics of voriconazole. Voriconazole exerted a concentration-dependent toxic effect on A549 cells and could penetrate into cells, reaching plateau concentrations of 1.14 ± 0.64, 3.72 ± 1.38 and 6.36 ± 0.95 ng/mg protein after A549 cells were exposed to voriconazole at extracellular concentrations of 2, 8 and 16 mg/L for 2 h, respectively. The efflux of voriconazole was rapid, with a half-life of 10.2 min. Voriconazole can decrease the A. fumigatus conidia invade cells, and the number of viable A. fumigatus conidia in cells can be decreased 2.1- to 20.6-fold when A549 cells were cultured in medium containing voriconazole. After 24-h incubation, 75.6% and 80.5% of intracellular A. fumigatus were killed when extracellular voriconazole concentration was 8 and 16 mg/L, respectively. This study illustrated a new application for the prophylaxis and early treatment of IPA from the cellular pharmacokinetics and pharmacodynamics and emphasized the importance of monitoring concentrations of voriconazole in epithelial lining fluid in immunocompromised patients receiving voriconazole therapy.

Antifungal Drugs: The Current Armamentarium and Development of New Agents

Invasive fungal infections are becoming an increasingly important cause of human mortality and morbidity, particularly for immunocompromised populations. The fungal pathogens Candida albicans , Cryptococcus neoformans , and Aspergillus fumigatus collectively contribute to over 1 million human deaths annually. Hence, the importance of safe and effective antifungal therapeutics for the practice of modern medicine has never been greater. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for drug development remains limited. Only three classes of molecules are currently approved for the treatment of invasive mycoses. The efficacy of these agents is compromised by host toxicity, fungistatic activity, or the emergence of drug resistance in pathogen populations. Here we describe our current arsenal of antifungals and highlight current strategies that are being employed to improve the therapeutic safety and efficacy of these drugs. We discuss state-of-the-art approaches to discover novel chemical matter with antifungal activity and highlight some of the most promising new targets for antifungal drug development. We feature the benefits of combination therapy as a strategy to expand our current repertoire of antifungals and discuss the antifungal combinations that have shown the greatest potential for clinical development. Despite the paucity of new classes of antifungals that have come to market in recent years, it is clear that by leveraging innovative approaches to drug discovery and cultivating collaborations between academia and industry, there is great potential to bolster the antifungal armamentarium.

The Hsp90 Co-chaperones Sti1, Aha1, and P23 Regulate Adaptive Responses to Antifungal Azoles

Heat Shock Protein 90 (Hsp90) is essential for tumor progression in humans and drug resistance in fungi. However, the roles of its many co-chaperones in antifungal resistance are unknown. In this study, by susceptibility test of Neurospora crassa mutants lacking each of 18 Hsp90/Calcineurin system member genes (including 8 Hsp90 co-chaperone genes) to antifungal drugs and other stresses, we demonstrate that the Hsp90 co-chaperones Sti1 (Hop1 in yeast), Aha1, and P23 (Sba1 in yeast) were required for the basal resistance to antifungal azoles and heat stress. Deletion of any of them resulted in hypersensitivity to azoles and heat. Liquid chromatography-mass spectrometry (LC-MS) analysis showed that the toxic sterols eburicol and 14α-methyl-3,6-diol were significantly accumulated in the sti1 and p23 deletion mutants after ketoconazole treatment, which has been shown before to led to cell membrane stress. At the transcriptional level, Aha1, Sti1, and P23 positively regulate responses to ketoconazole stress by erg11 and erg6, key genes in the ergosterol biosynthetic pathway. Aha1, Sti1, and P23 are highly conserved in fungi, and sti1 and p23 deletion also increased the susceptibility to azoles in Fusarium verticillioides. These results indicate that Hsp90-cochaperones Aha1, Sti1, and P23 are critical for the basal azole resistance and could be potential targets for developing new antifungal agents.

New Horizons in Antifungal Therapy

Recent investigations have yielded both profound insights into the mechanisms required by pathogenic fungi for virulence within the human host, as well as novel potential targets for antifungal therapeutics. Some of these studies have resulted in the identification of novel compounds that act against these pathways and also demonstrate potent antifungal activity. However, considerable effort is required to move from pre-clinical compound testing to true clinical trials, a necessary step toward ultimately bringing new drugs to market. The rising incidence of invasive fungal infections mandates continued efforts to identify new strategies for antifungal therapy. Moreover, these life-threatening infections often occur in our most vulnerable patient populations. In addition to finding completely novel antifungal compounds, there is also a renewed effort to redirect existing drugs for use as antifungal agents. Several recent screens have identified potent antifungal activity in compounds previously indicated for other uses in humans. Together, the combined efforts of academic investigators and the pharmaceutical industry is resulting in exciting new possibilities for the treatment of invasive fungal infections.

Potential Microbiological Effects of Higher Dosing of Echinocandins

The antifungal "paradoxical effect" has been described as the reversal of growth inhibition at high doses of echinocandins, most usually caspofungin. This microbiological effect appears to be a cellular compensatory response to cell wall damage, resulting in alteration of cell wall content and structure as well as fungal morphology and growth. In vitro studies demonstrate this reproducible effect in a certain percentage of fungal isolates, but animal model and clinical studies are less consistent. The calcineurin and Hsp90 cell signaling pathways appear to play a major role in regulating these cellular and structural changes. Regardless of the clinical relevance of this paradoxical growth effect, understanding the specific actions of echinocandins is paramount to optimizing their use at either standard or higher dosing schemes, as well as developing future improvements in our antifungal arsenal.

The fungal resistome: a risk and an opportunity for the development of novel antifungal therapies

The risks for toxicity of novel antifungal compounds, together with the emergence of resistance, makes the use of inhibitors of resistance, in combination with antifungal compounds, a suitable strategy for developing novel antifungal formulations. Among them, inhibitors of efflux pumps are suitable candidates. Increasing drug influx or interfering with the stress response may also improve the efficacy of antifungals. Therapies as induction of fungal apoptosis or immunostimulation are also good strategies for reducing the risks for resistance and to improve antifungals' efficacy. Understanding the effect of the acquisition of resistance on the fungal physiology and determining the collateral sensitivity networks are useful for the development of novel strategies based on combination of antifungals for improving the efficacy of the therapy.

Combinatorial strategies for combating invasive fungal infections

Invasive fungal infections are an important cause of human mortality and morbidity, particularly for immunocompromised populations. However, there remains a paucity of antifungal drug treatments available to combat these fungal pathogens. Further, antifungal compounds are plagued with problems such as host toxicity, fungistatic activity, and the emergence of drug resistance in pathogen populations. A promising therapeutic strategy to increase drug effectiveness and mitigate the emergence of drug resistance is through the use of combination drug therapy. In this review we describe the current arsenal of antifungals in medicine and elaborate on the benefits of combination therapy to expand our current antifungal drug repertoire. We examine those antifungal combinations that have shown potential against fungal pathogens and discuss strategies being employed to discover novel combination therapeutics, in particular combining antifungal agents with non-antifungal bioactive compounds. The findings summarized in this review highlight the promise of combinatorial strategies in combatting invasive mycoses.

Immunoproteomics-Based Analysis of the Immunocompetent Serological Response to Lomentospora prolificans

The filamentous fungus Lomentospora prolificans is an emerging pathogen causing severe infections mainly among the immunocompromised population. These diseases course with high mortality rates due to great virulence of the fungus, its inherent resistance to available antifungals, and absence of specific diagnostic tools. Despite being widespread in humanized environments, L. prolificans rarely causes infections in immunocompetent individuals likely due to their developed protective immune response. In this study, conidial and hyphal immunomes against healthy human serum IgG were analyzed, identifying immunodominant antigens and establishing their prevalence among the immunocompetent population. Thirteen protein spots from each morph were detected as reactive against at least 70% of serum samples, and identified by liquid chromatography tandem mass spectrometry (LC-MS/MS). Hence, the most seroprevalent antigens were WD40 repeat 2 protein, malate dehydrogenase, and DHN1, in conidia, and heat shock protein (Hsp) 70, Hsp90, ATP synthase β subunit, and glyceraldehyde-3-phosphate dehydrogenase, in hyphae. More interestingly, the presence of some of these seroprevalent antigens was determined on the cell surface, as Hsp70, enolase, or Hsp90. Thus, we have identified a diverse set of antigenic proteins, both in the entire proteome and cell surface subproteome, which may be used as targets to develop innovative therapeutic or diagnostic tools.

Calcineurin Orchestrates Hyphal Growth, Septation, Drug Resistance and Pathogenesis of Aspergillus fumigatus: Where Do We Go from Here?

Studies on fungal pathogens belonging to the ascomycota phylum are critical given the ubiquity and frequency with which these fungi cause infections in humans. Among these species, Aspergillus fumigatus causes invasive aspergillosis, a leading cause of death in immunocompromised patients. Fundamental to A. fumigatus pathogenesis is hyphal growth. However, the precise mechanisms underlying hyphal growth and virulence are poorly understood. Over the past 10 years, our research towards the identification of molecular targets responsible for hyphal growth, drug resistance and virulence led to the elucidation of calcineurin as a key signaling molecule governing these processes. In this review, we summarize our salient findings on the significance of calcineurin for hyphal growth and septation in A. fumigatus and propose future perspectives on exploiting this pathway for designing new fungal-specific therapeutics.

Medically important fungi respond to azole drugs: an update

The increased numbers of patients with compromised immune systems in the last three decades have increased the chances of life-threatening fungal infections. Numerous antifungal drugs have been developed in the last 20 years to treat these infections. The largest group, the azoles, inhibits the synthesis of fungal sterols. The use of these fungistatic azoles has subsequently led to the emergence of acquired azole resistance. The most common mechanisms that result in azole resistance include the overexpression or mutation of the azole target enzyme, and overexpression of drug transporters that are responsible for azole efflux from cells. Additional, less-frequent mechanisms have also been identified. Understanding azole resistance mechanisms is crucial for current antifungal treatment and for the future development of new treatment strategies.