Progress and prospects for targeting Hsp90 to treat fungal infections

Targeting Borrelia burgdorferi HtpG with a berserker molecule, a strategy for anti-microbial development

Conventional antimicrobial discovery relies on targeting essential enzymes in pathogenic organisms, contributing to a paucity of new antibiotics to address resistant strains. Here, by targeting a non-essential enzyme, Borrelia burgdorferi HtpG, to deliver lethal payloads, we expand what can be considered druggable within any pathogen. We synthesized HS-291, an HtpG inhibitor tethered to the photoactive toxin verteporfin. Reactive oxygen species, generated by light, enables HS-291 to sterilize Borrelia cultures by causing oxidation of HtpG, and a discrete subset of proteins in proximity to the chaperone. This caused irreversible nucleoid collapse and membrane blebbing. Tethering verteporfin to the HtpG inhibitor was essential, since free verteporfin was not retained by Borrelia in contrast to HS-291. For this reason, we liken HS-291 to a berserker, wreaking havoc upon the pathogen's biology once selectively absorbed and activated. This strategy expands the druggable pathogenic genome and offsets antibiotic resistance by targeting non-essential proteins.

Development and research progress of anti-drug resistant fungal drugs

With the widespread use of immunosuppressive agents and the increase in patients with severe infections, the incidence of fungal infections worldwide has increased year by year. The fungal pathogens Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus cause a total of more than 1 million deaths each year. Long-term use of antifungal drugs can easily lead to fungal resistance, and the prevalence of drug-resistant fungi is a major global health challenge. In order to effectively control global fungal infections, there is an urgent need for new drugs that can exert effective antifungal activity and overcome drug resistance. We must promote the discovery of new antifungal targets and drugs, and find effective ways to control drug-resistant fungi through different ways, so as to reduce the threat of drug-resistant fungi to human life, health and safety. In the past few years, certain progress has been made in the research and development of antifungal drugs. In addition to summarizing some of the antifungal drugs currently approved by the FDA, this review also focuses on potential antifungal drugs, the repositioned drugs, and drugs that can treat drug-resistant bacteria and fungal infections, and provide new ideas for the development of antifungal drugs in the future.

Chitosan hydrogel loaded with recombinant protein containing epitope C from HSP90 of Candida albicans induces protective immune responses against systemic candidiasis

We reported previously a recombinant protein (rP-HSP90C) containing epitope C from heat shock protein 90 of Candida albicans mediates protective immune responses against systemic candidiasis. However, it exhibits weak immunogenicity. Therefore, we evaluated the potential and mechanisms of thermosensitive chitosan hydrogel (CH-HG) as an adjuvant in rP-HSP90C vaccine. CH-HG synthesized by ionic cross-linking showed buffering capacity and control-released rP-HSP90C in vitro. In comparison to naked rP-HSP90C, CH-HG-loaded rP-HSP90C (CH-HG/rP-HSP90C) not only evoked a long-lasting rP-HSP90C-specific IgG, but also enhanced Th1, Th2, Th17 responses and the ratio of Th1/Th2 in vivo; Meanwhile, CH-HG/rP-HSP90C provoked a stronger CTL response than rP-HSP90C. Notably, CH-HG increased the protective immune responses against systemic candidiasis in rP-HSP90C-immunized mice since CH-HG/rP-HSP90C enhanced the survival rate of infected mice, and diminished the CFUs in kidneys compared to rP-HSP90C, which were similar to that of QuilA. Further in vitro investigation displayed CH-HG upgraded the expressions of costimulators, MHCs and cytokines in BMDCs compared to rP-HSP90C；CH-HG also promoted cellular uptake, endosomal escape and "cross-presentation" of rP-HSP90C. In addition, it recruited immune cells at the injection site. Our study demonstrated that CH-HG can be an efficient adjuvant in fungal vaccines.

Antifungal Drug Repurposing

Control of fungal pathogens is increasingly problematic due to the limited number of effective drugs available for antifungal therapy. Conventional antifungal drugs could also trigger human cytotoxicity associated with the kidneys and liver, including the generation of reactive oxygen species. Moreover, increased incidences of fungal resistance to the classes of azoles, such as fluconazole, itraconazole, voriconazole, or posaconazole, or echinocandins, including caspofungin, anidulafungin, or micafungin, have been documented. Of note, certain azole fungicides such as propiconazole or tebuconazole that are applied to agricultural fields have the same mechanism of antifungal action as clinical azole drugs. Such long-term application of azole fungicides to crop fields provides environmental selection pressure for the emergence of pan-azole-resistant fungal strains such as Aspergillus fumigatus having TR34/L98H mutations, specifically, a 34 bp insertion into the cytochrome P450 51A (CYP51A) gene promoter region and a leucine-to-histidine substitution at codon 98 of CYP51A. Altogether, the emerging resistance of pathogens to currently available antifungal drugs and insufficiency in the discovery of new therapeutics engender the urgent need for the development of new antifungals and/or alternative therapies for effective control of fungal pathogens. We discuss the current needs for the discovery of new clinical antifungal drugs and the recent drug repurposing endeavors as alternative methods for fungal pathogen control.

In vitro and in vivo Study of Antifungal Effect of Pyrvinium Pamoate Alone and in Combination With Azoles Against Exophiala dermatitidis

Infections of Exophiala dermatitidis are often chronic and recalcitrant. Combination therapies with novel compounds and azoles could be an effective solution. Previously, we have demonstrated that pyrvinium pamoate exerted antifungal activity alone and favorable synergy with azoles against planktonic E. dermatitidis. Herein, the underlying antifungal mode of action were investigated. Pyrvinium alone showed sessile MIC50 (SMIC50) of 8->16 μg/ml against E. dermatitidis biofilms. However, synergism of PP with itraconazole, voriconazole, and posaconazole were observed against 16 (88.9%), 9 (50%), and 13 (72.2%) strains of E. dermatitidis biofilms. In accordance with in vitro susceptibilities, pyrvinium alone at concentration of 2 μg/ml resulted in significant growth restriction of planktonic E. dermatitidis. Pyrvinium alone resulted in reduction of biofilm formation. Higher concentration of pyrvinium was associate with more progressive reduction of biofilm mass. The in vivo activity of pyrvinium alone and combined with azoles was evaluated using Galleria mellonella model. Pyrvinium alone significantly improved the survival rate of larvae (P < 0.0001). The combination of pyrvinium and voriconazole or posaconazole acted synergistically in vivo (P < 0.05). Fungal burden determination revealed significant reduction of numbers of colony forming unit (CFU) in larvae treated with pyrvinium-itraconazole and pyrvinium-posaconazole compared to itraconazole or posaconazole alone group, respectively. The effect of pyrvinium on apoptosis, expression of TOR and HSP90, and drug efflux reversal were evaluated by PI/Annexin V staining, Real-Time Quantitative PCR and Rhodamine 6G assay, respectively. Pyrvinium alone or combined with azoles significantly (P < 0.05) increased late apoptosis or necrosis of E. dermatitidis cells. Pyrvinium combined with posaconazole significantly decreased the expression of TOR and Hsp90 compared to posaconazole alone group (P < 0.05). Pyrvinium resulted in significant (P < 0.05) decrease of the efflux of Rhodamine 6G. These findings suggested pyrvinium could be a promising synergist with azoles. The underlying mechanisms could be explained by inducing apoptosis/necrosis, inhibition of drug efflux pumps, and signaling pathways related with stress response and growth control.

Deciphering the Inhibition Mechanism of under Trial Hsp90 Inhibitors and Their Analogues: A Comparative Molecular Dynamics Simulation

Heat shock protein 90 (Hsp90) performs functions in cellular activities together with other signaling pathways. Hsp90 is evolutionarily conserved and universally articulated as a human cancer-causing agent involved in lung cancer and breast cancer followed by colon and rectum cancers. It has emerged as an effective drug candidate, and inhibition may affect several signaling pathways associated with cancer spread. Therefore, in-silico approaches, molecular docking, molecular dynamics simulation, and binding free energy calculations were applied to create insights into the inhibition mechanism against Hsp90 to identify new cancer therapeutic drugs. Top-docked Hsp90-inhibitor complexes with their analogues were selected as the best complexes based on the GOLD fitness score and orientation. The significant interaction of Hsp90 inhibitors and their analogues were observed to be bound with active site residues as well as residing within the same cavity region. System stability factors RMSD, RMSF, beta-factor, and radius of gyration were analyzed for top-docked complexes and ensure strong binding interaction between inhibitors and the Hsp90 cavity. Cavity bound inhibitors were found to retain consistent hydrogen bonding during the simulation. The radial distribution function (RDF) illustrated that interacting active site residues drive the binding and stability of the inhibitors. Similarly, the axial frequency distribution, which is an indigenously developed analytical tool, produced noteworthy knowledge of the hydrogen-bonding pattern. Results yielded new insights into the design of cancer therapeutic drugs against Hsp90. This finding suggests that under trial Hsp90 inhibitors MPC-3100 could be a potential starting point into the development of potential anticancer agents with the possibility of future directions for the improvement of early existing Hsp90 inhibitors CNF-2024 and SNX-5422 as an anticancer agent.

Mechanism of Candida pathogenesis: revisiting the vital drivers

Candida is the most implicated fungal pathogen in the clinical setting. Several factors play important roles in the pathogenesis of Candida spp. Multiple transcriptional circuits, morphological and phenotypic switching, biofilm formation, tissue damaging extracellular hydrolytic enzymes, metabolic flexibility, genome plasticity, adaptation to environmental pH fluctuation, robust nutrient acquisition system, adherence and invasions (mediated by adhesins and invasins), heat shock proteins (HSPs), cytolytic proteins, escape from phagocytosis, evasion from host immune system, synergistic coaggregation with resident microbiota, resistance to antifungal agents, and the ability to efficiently respond to multiple stresses are some of the major pathogenic determinants of Candida species. The existence of multiple connections, in addition to the interactions and associations among all of these factors, are distinctive features that play important roles in the establishment of Candida infections. This review describes all the underlying factors and mechanisms involved in Candida pathogenesis by evaluating pathogenic determinants of Candida species. It reinforces the already available pool of data on the pathogenesis of Candida species by providing a clear and simplified understanding of the most important factors implicated in the pathogenesis of Candida species. The Candida pathogenesis network, an illustration linking all the major determinants of Candida pathogenesis, is also presented. Taken together, they will further improve our current understanding of how these factors modulate virulence and consequent infection(s). Development of new antifungal drugs and better therapeutic approaches to candidiasis can be achieved in the near future with continuing progress in the understanding of the mechanisms of Candida pathogenesis.

A Long Non-coding RNA Lnc712 Regulates Breast Cancer Cell Proliferation

Great quantity of intergenic noncoding RNAs (lncRNAs) have been identified in the mammalian genome and involved in various biological processes, especially in the development and metastasis of cancer. In this study, we identified one lncRNA, lncRNA NONHSAT028712 (Lnc712), was highly expressed in breast cancer cell lines and tissues based on microarray screening. Knockdown of Lnc712 largely inhibited breast cancer cell proliferation. Mechanistically, Lnc712 bound specifically to heat-shock protein 90 (HSP90). Interaction between Lnc712 and HSP90 is required for HSP90 binding to cell division cycle 37 (Cdc37). The Lnc712/HSP90/Cdc37 complex regulated cyclin-dependent kinase 2 (CDK2) activation and then triggered breast cancer cell proliferation. In summary, our results identified a new lncRNA regulate breast cancer proliferation though interaction with HSP90.

Global proteomic analyses define an environmentally contingent Hsp90 interactome and reveal chaperone-dependent regulation of stress granule proteins and the R2TP complex in a fungal pathogen

Hsp90 is a conserved molecular chaperone that assists in the folding and function of diverse cellular regulators, with a profound impact on biology, disease, and evolution. As a central hub of protein interaction networks, Hsp90 engages with hundreds of protein-protein interactions within eukaryotic cells. These interactions include client proteins, which physically interact with Hsp90 and depend on the chaperone for stability or function, as well as co-chaperones and partner proteins that modulate chaperone function. Currently, there are no methods to accurately predict Hsp90 interactors and there has been considerable network rewiring over evolutionary time, necessitating experimental approaches to define the Hsp90 network in the species of interest. This is a pressing challenge for fungal pathogens, for which Hsp90 is a key regulator of stress tolerance, drug resistance, and virulence traits. To address this challenge, we applied a novel biochemical fractionation and quantitative proteomic approach to examine alterations to the proteome upon perturbation of Hsp90 in a leading human fungal pathogen, Candida albicans. In parallel, we performed affinity purification coupled to mass spectrometry to define physical interacting partners for Hsp90 and the Hsp90 co-chaperones and identified 164 Hsp90-interacting proteins, including 111 that are specific to the pathogen. We performed the first analysis of the Hsp90 interactome upon antifungal drug stress and demonstrated that Hsp90 stabilizes processing body (P-body) and stress granule proteins that contribute to drug tolerance. We also describe novel roles for Hsp90 in regulating posttranslational modification of the Rvb1-Rvb2-Tah1-Pih1 (R2TP) complex and the formation of protein aggregates in response to thermal stress. This study provides a global view of the Hsp90 interactome in a fungal pathogen, demonstrates the dynamic role of Hsp90 in response to environmental perturbations, and highlights a novel connection between Hsp90 and the regulation of mRNA-associated protein granules.

Regulatory mechanisms controlling morphology and pathogenesis in Candida albicans

Candida albicans, a major human fungal pathogen, can cause a wide variety of both mucosal and systemic infections, particularly in immunocompromised individuals. Multiple lines of evidence suggest a strong association between virulence and the ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous cells in response to host environmental cues. Most previous studies on mechanisms important for controlling the C. albicans morphological transition have focused on signaling pathways and sequence-specific transcription factors. However, in recent years a variety of novel mechanisms have been reported, including those involving global transcriptional regulation and translational control. A large-scale functional genomics screen has also revealed new roles in filamentation for certain key biosynthesis pathways. This review article will highlight several of these exciting recent discoveries and discuss how they are relevant to the development of novel antifungal strategies. Ultimately, components of mechanisms that control C. albicans morphogenesis and pathogenicity could potentially serve as viable antifungal targets.

Chaperone-mediated autophagy is involved in the execution of ferroptosis

Necroptosis and ferroptosis are two distinct necrotic cell death modalities with no known common molecular mechanisms. Necroptosis is activated by ligands of death receptors such as tumor necrosis factor-α (TNF-α) under caspase-deficient conditions, whereas ferroptosis is mediated by the accumulation of lipid peroxides upon the depletion/or inhibition of glutathione peroxidase 4 (GPX4). The molecular mechanism that mediates the execution of ferroptosis remains unclear. In this study, we identified 2-amino-5-chloro-N,3-dimethylbenzamide (CDDO), a compound known to inhibit heat shock protein 90 (HSP90), as an inhibitor of necroptosis that could also inhibit ferroptosis. We found that HSP90 defined a common regulatory nodal between necroptosis and ferroptosis. We showed that inhibition of HSP90 by CDDO blocked necroptosis by inhibiting the activation of RIPK1 kinase. Furthermore, we showed that the activation of ferroptosis by erastin increased the levels of lysosome-associated membrane protein 2a to promote chaperone-mediated autophagy (CMA), which, in turn, promoted the degradation of GPX4. Importantly, inhibition of CMA stabilized GPX4 and reduced ferroptosis. Our results suggest that activation of CMA is involved in the execution of ferroptosis.

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.

Synergistic and antagonistic effects of immunomodulatory drugs on the action of antifungals against Candida glabrata and Saccharomyces cerevisiae

Candidemia and other forms of invasive fungal infections caused by Candida glabrata and to a lesser extent Saccharomyces cerevisiae are a serious health problem, especially if their steadily rising resistance to the limited range of antifungal drugs is taken into consideration. Various drug combinations are an attractive solution to the resistance problem, and some drug combinations are already common in the clinical environment due to the nature of diseases or therapies. We tested a few of the common antifungal-immunomodulatory drug combinations and evaluated their effect on selected strains of C. glabrata and S. cerevisiae. The combinations were performed using the checkerboard microdilution assay and interpreted using the Loewe additivity model and a model based on the Bliss independence criterion. A synergistic interaction was confirmed between calcineurin inhibitors (Fk506 and cyclosporine A) and antifungals (fluconazole, itraconazole, and amphotericin B). A new antagonistic interaction between mycophenolic acid (MPA) and azole antifungals was discovered in non-resistant strains. A possible mechanism that explains this is induction of the Cdr1 efflux pump by MPA in C. glabrata ATCC 2001. The Pdr1 regulatory cascade plays a role in overall resistance to fluconazole, but it is not essential for the antagonistic interaction. This was confirmed by the Cgpdr1Δ mutant still displaying the antagonistic interaction between the drugs, although at lower concentrations of fluconazole. This antagonism calls into question the use of simultaneous therapy with MPA and azoles in the clinical environment.

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.

Modulation of the Fungal-Host Interaction by the Intra-Species Diversity of C. albicans

The incidence of human infections caused by the opportunistic fungal pathogen Candida albicans is on the rise due to increasing numbers of immunosuppressed patients. The importance of the immune system in preventing overgrowth of the colonizing fungus and thereby limiting infection is well recognized and host protective mechanisms widely investigated. Only recently, it was recognized that the natural diversity in the fungal species could also influence the outcome of the interaction between the fungus and the host. C. albicans strain-specific differences are complex and their regulation at the genomic, genetic, and epigenetic level and by environmental factors is only partially understood. In this review, we provide an overview of the natural diversity of C. albicans and discuss how it impacts host-fungal interactions and thereby affects the balance between commensalism versus disease.

Evidence for Hsp90 Co-chaperones in Regulating Hsp90 Function and Promoting Client Protein Folding

Molecular chaperones are a diverse group of highly conserved proteins that transiently interact with partially folded polypeptide chains during normal cellular processes such as protein translation, translocation, and disassembly of protein complexes. Prior to folding or after denaturation, hydrophobic residues that are normally sequestered within a folded protein are exposed to the aqueous environment and are prone to aggregation or misfolding. Multiple classes of molecular chaperones, such as Hsp70s and Hsp40s, recognize and transiently bind polypeptides with exposed hydrophobic stretches in order to prevent misfolding. Other types of chaperones, such as Hsp90, have more specialized functions in that they appear to interact with only a subset of cellular proteins. This chapter focuses on the role of Hsp90 and partner co-chaperones in promoting the folding and activation of a diverse group of proteins with critical roles in cellular signaling and function.

Candida albicans Heat Shock Proteins and Hsps-Associated Signaling Pathways as Potential Antifungal Targets

In recent decades, the incidence of invasive fungal infections has increased notably. Candida albicans (C. albicans), a common opportunistic fungal pathogen that dwells on human mucosal surfaces, can cause fungal infections, especially in immunocompromised and high-risk surgical patients. In addition, the wide use of antifungal agents has likely contributed to resistance of C. albicans to traditional antifungal drugs, increasing the difficulty of treatment. Thus, it is urgent to identify novel antifungal drugs to cope with C. albicans infections. Heat shock proteins (Hsps) exist in most organisms and are expressed in response to thermal stress. In C. albicans, Hsps control basic physiological activities or virulence via interaction with a variety of diverse regulators of cellular signaling pathways. Moreover, it has been demonstrated that Hsps confer drug resistance to C. albicans. Many studies have shown that disrupting the normal functions of C. albicans Hsps inhibits fungal growth or reverses the tolerance of C. albicans to traditional antifungal drugs. Here, we review known functions of the diverse Hsp family, Hsp-associated intracellular signaling pathways and potential antifungal targets based on these pathways in C. albicans. We hope this review will aid in revealing potential new roles of C. albicans Hsps in addition to canonical heat stress adaptions and provide more insight into identifying potential novel antifungal targets.

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.

Heat Shock Protein (HSP) Drug Discovery and Development: Targeting Heat Shock Proteins in Disease

Heat shock proteins (HSPs) present as a double edged sword. While they play an important role in maintaining protein homeostasis in a normal cell, cancer cells have evolved to co-opt HSP function to promote their own survival. As a result, HSPs such as HSP90 have attracted a great deal of interest as a potential anticancer target. These efforts have resulted in over 20 distinct compounds entering clinical evaluation for the treatment of cancer. However, despite the potent anticancer activity demonstrated in preclinical models, to date no HSP90 inhibitor has obtained regulatory approval. In this review we discuss the unique challenges faced in targeting HSPs that have likely contributed to their lack of progress in the clinic and suggest ways to overcome these so that the enormous potential of these compounds to benefit patients can finally be realized. We also provide a guideline for the future development of HSP-targeted agents based on the many lessons learned during the last two decades in developing HSP90 inhibitors.

Extensive functional redundancy in the regulation of Candida albicans drug resistance and morphogenesis by lysine deacetylases Hos2, Hda1, Rpd3 and Rpd31

Current treatment efforts for fungal infections are hampered by the limited availability of antifungal drugs and by the emergence of drug resistance. A powerful strategy to enhance the efficacy of antifungal drugs is to inhibit the molecular chaperone Hsp90. Hsp90 governs drug resistance, morphogenesis and virulence in a leading fungal pathogen of humans, Candida albicans. Our previous work with Saccharomyces cerevisiae established acetylation as a novel mechanism of posttranslational control of Hsp90 function in fungi. We implicated lysine deacetylases (KDACs) as key regulators of resistance to the most widely deployed class of antifungals, the azoles, in both S. cerevisiae and C. albicans. Here, we demonstrate high levels of functional redundancy among the KDACs that are important for regulating Hsp90 function. We identify Hos2, Hda1, Rpd3 and Rpd31 as the KDACs mediating azole resistance and morphogenesis in C. albicans. Furthermore, we identify lysine 30 and 271 as critical acetylation sites on C. albicans Hsp90, and substitutions at these residues compromise Hsp90 function. Finally, we show that pharmacological inhibition of KDACs phenocopies pharmacological inhibition of Hsp90 and abrogates Hsp90-dependent azole resistance in numerous Candida species. This work illuminates new facets to the impact of KDACs on fungal drug resistance and morphogenesis, provides important insights into the divergence of the C. albicans Hsp90 regulatory network and reveals new targets for development of antifungal drugs.

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.

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.

Novel strategies against Candida biofilms: interest of synthetic compounds

A biofilm is a consortium of microbial cells that are attached to a substratum or an interface. It should be considered a reservoir that may induce serious infections. Indeed, Candidaspp. biofilms may be involved in the persistence or worsening of some chronic inflammatory diseases as well as in systemic infections, which may lead to high morbidity and mortality rates. New strategies are currently being explored, utilizing several synthetic compounds to prevent or fight these Candida biofilms. This article focuses on active synthetic compounds classified with regards to their modes of action: inhibition of early adherence phase, inhibition or control of biofilm maturation and finally elimination of already formed biofilms. Some of them show promise in fighting biofilm.

Heat Shock Protein 90 (Hsp90) as a Molecular Target for the Development of Novel Drugs Against the Dermatophyte Trichophyton rubrum

Treatment of fungal infections is difficult due to several reasons, such as side effects of drugs, emergence of resistant strains, and limited number of molecular targets for the drug compounds. In fungi, heat shock proteins (Hsps) have been implicated in several processes with the conserved molecular chaperone Hsp90 emerging as a potential target for antifungal therapy. It plays key cellular roles by eliciting molecular response to environmental changes, morphogenesis, antifungal resistance, and fungal pathogenicity. Here, we evaluated the transcription profiles of hsp genes of the most prevalent dermatophyte Trichophyton rubrum in response to different environmental challenges including nutrient availability, interaction with cells and molecules of the host tissue, and drug exposure. The results suggest that each Hsp responds to a specific stress condition and that the cohort of Hsps facilitates fungal survival under various environmental challenges. Chemical inhibition of Hsp90 resulted in increased susceptibility of the fungus to itraconazole and micafungin, and decreased its growth in human nails in vitro. Moreover, some hsp and related genes were modulated by Hsp90 at the transcriptional level. We are suggesting a role of Hsp90 in the pathogenicity and drug susceptibility of T. rubrum as well as the regulation of other Hsps. The synergism observed between the inhibition of Hsp90 and the effect of itraconazole or micafungin in reducing the fungal growth is of great interest as a novel and potential strategy to treat dermatophytoses.

Mechanistic Asymmetry in Hsp90 Dimers

Hsp90 is a molecular chaperone that facilitates the maturation of signaling proteins including many kinases and steroid hormone receptors. Through these client proteins, Hsp90 is a key mediator of many physiological processes and has emerged as a promising drug target in cancer. Additionally, Hsp90 can mask or potentiate the impact of mutations in clients with remarkable influence on evolutionary adaptations. The influential roles of Hsp90 in biology and disease have stimulated extensive research into the molecular mechanism of this chaperone. These studies have shown that Hsp90 is a homodimeric protein that requires ATP hydrolysis and a host of accessory proteins termed co-chaperones to facilitate the maturation of clients to their active states. Flexible hinge regions between its three structured domains enable Hsp90 to sample dramatically distinct conformations that are influenced by nucleotide, client, and co-chaperone binding. While it is clear that Hsp90 can exist in symmetrical conformations, recent studies have indicated that this homodimeric chaperone can also assume a variety of asymmetric conformations and complexes that are important for client maturation. The visualization of Hsp90-client complexes at high resolution together with tools to independently manipulate each subunit in the Hsp90 dimer are providing new insights into the asymmetric function of each subunit during client maturation.

Augmenting the antifungal activity of an oxidizing agent with kojic Acid: control of penicillium strains infecting crops

Oxidative treatment is one of the strategies for preventing Penicillium contamination in crops/foods. The antifungal efficacy of hydrogen peroxide (H₂O₂; oxidant) was investigated in Penicillium strains by using kojic acid (KA) as a chemosensitizing agent, which can enhance the susceptibility of pathogens to antifungal agents. Co-application of KA with H₂O₂ (chemosensitization) resulted in the enhancement of antifungal activity of either compound, when compared to the independent application of each agent alone. Of note, heat enhanced the activity of H₂O₂ to a greater extent during chemosensitization, whereby the minimum inhibitory or minimum fungicidal concentrations of H₂O₂ was decreased up to 4 or 13 fold, respectively, at 35–45 °C (heat), when compared to that at 28 °C (normal growth temperature). However, heat didn’t increase the antifungal activity of KA, indicating specificity exists between heat and types of antifungals applied. The effect of chemosensitization was also strain-specific, where P. expansum (both parental and fludioxonil-resistant mutants) or P. italicum 983 exhibited relatively higher susceptibility to the chemosensitization, comparing to other Penicillium strains tested. Collectively, chemosensitization can serve as a potent antifungal strategy to lower effective dosages of toxic antifungal substances, such as H₂O₂. This can lead to coincidental lowering of environmental and health risks.

Emerging novel and antimicrobial-resistant respiratory tract infections: new drug development and therapeutic options

The emergence and spread of antimicrobial-resistant bacterial, viral, and fungal pathogens for which diminishing treatment options are available is of major global concern. New viral respiratory tract infections with epidemic potential, such as severe acute respiratory syndrome, swine-origin influenza A H1N1, and Middle East respiratory syndrome coronavirus infection, require development of new antiviral agents. The substantial rise in the global numbers of patients with respiratory tract infections caused by pan-antibiotic-resistant Gram-positive and Gram-negative bacteria, multidrug-resistant Mycobacterium tuberculosis, and multiazole-resistant fungi has focused attention on investments into development of new drugs and treatment regimens. Successful treatment outcomes for patients with respiratory tract infections across all health-care settings will necessitate rapid, precise diagnosis and more effective and pathogen-specific therapies. This Series paper describes the development and use of new antimicrobial agents and immune-based and host-directed therapies for a range of conventional and emerging viral, bacterial, and fungal causes of respiratory tract infections.