Ras pathway signaling accelerates programmed cell death in the pathogenic fungus Candida albicans

PPI network identifies interacting pathogenic signaling pathways in Candida albicans

Candida albicans, an opportunistic and systemic infection causing fungus, causes skin, nail, and mucosal layer lesions in healthy individuals and hospital borne catheter-related and nosocomial infections. This particular fungus exists in two distinct stages in its life cycle: yeast and hyphae. In this study, 20 signaling pathways associated with 177 proteins from C. albicans were identified to construct a PPI network. The core part of the network consisted of 165 proteins. Network topology analyses revealed that the formed PPI network is biologically robust and scale-free, with significant interactions between proteins through 19 252 shortest pathways. In this network, the top 10 hub proteins (RAS1, CDC42, HOG1, CPH1, STE11, EFG1, CEK1, HSP90, TEC1 and CST20) were identified using network analysis, which seem to be the most important proteins involved in different pathways for the development of pathogenesis and virulence. Modular analysis of the network resulted in top six sub-networks, three of which shared eight hub proteins. Ontology and functional enrichment analyses revealed that the majority of the proteins were associated with regulation of transcription by RNA polymerase II, plasma membrane and nucleic acid binding in biological processes, and cellular components and molecular functions, respectively. Enrichment analysis indicated that the proteins were mostly involved in oxidative phosphorylation and purine metabolism signaling pathways. We determined the complex web of signaling pathway involving proteins via PPI network analysis to unravel and decipher protein interactions within C. albicans to understand the complex pathogenesis processes for targeted therapeutic interferences using novel bioinformatics strategies.

Intratumor fungi specific mechanisms to influence cell death pathways and trigger tumor cell apoptosis

Cancer, uncontrolled cell growth due to the loss of cell cycle regulation, is often found to be associated with viral infections and, as recent studies show, with bacterial infections as well. Emerging reports also suggest a strong link between fungi and cancer. The crucial virulence trait of fungi, the switch from yeast (Y) to hyphal (H) form, is found to be associated with carcinogenesis. The physicochemical properties and signal transduction pathways involved in the switch to the hyphal form overlap with those of tumor cell formation. Inhibiting differentiation causes apoptosis in fungi, whereas preventing apoptosis leads to cancer in multicellular organisms. Literature on the fungi-cancer linkage, though limited, is increasing rapidly. This review examines cancer-specific fungal communities, the impact of fungal microbiome on cancer cell progression, similarities between fungal differentiation and cells turning cancerous at biochemical and molecular levels, including the overlaps in signal transduction pathways between fungi and cancer. Based on the available evidence, we suggest that molecules inhibiting the yeast-hyphal transition in fungi can be combined with those targeting tumor cell apoptosis for effective cancer treatment. The review points out fertile research areas where mycologists and cancer researchers can collaborate to unravel common molecular mechanisms. Moreover, antibodies targeting fungal-specific chitin and glucan can be used for the selective neutralization of tumor cells. These new combinations of potential therapies are expected to facilitate the development of target-specific, less harmful and commercially feasible anticancer therapies. We bring together available evidence to argue that fungal infections could either trigger cancer or have a significant role in the development and progression of cancer. Hence, cancer-associated fungal populations could be utilized as a target for a combination therapy involving the integration of anticancer and antifungal drugs as well as inhibitors of fungal morphogenesis to develop more effective anticancer therapies.

Current and novel approaches in yeast cell death research

The study of cell death mechanisms in fungi, particularly yeasts, has gained substantial interest in recent decades driven by the potential for biotechnological advancements and therapeutic interventions. Examples include the development of robust yeast strains for industrial fermentations and high-value compound production, novel food preservation strategies against spoilage yeasts, and the identification of targets for treating fungal infections in the clinic. In this review, we discuss a wide range of methods to characterize cellular alterations associated with yeast cell death, noting the advantages and limitations. We describe assays to monitor reversible events versus those that mark a commitment to cell death (point-of-no-return), as these distinctions are important to decipher the underlying regulatory mechanisms. Several well-known challenges remain, including the varied susceptibilities to death within a cell population and the delineation of detailed cell death mechanisms. The identification and characterization of morphologically distinct subsets of dying yeast cells within dynamic yeast populations provides opportunities to reveal novel vulnerabilities and survival mechanisms. Elucidating the intricacies of yeast regulated cell death (yRCD) will contribute to the advancement of scientific knowledge and foster breakthrough discoveries with broad-ranging implications.

Developing novel antifungals: lessons from G protein-coupled receptors

Up to 1.5 million people die yearly from fungal disease, but the repertoire of antifungal drug classes is minimal and the incidence of drug resistance is rising rapidly. This dilemma was recently declared by the World Health Organization as a global health emergency, but the discovery of new antifungal drug classes remains excruciatingly slow. This process could be accelerated by focusing on novel targets, such as G protein-coupled receptor (GPCR)-like proteins, that have a high likelihood of being druggable and have well-defined biology and roles in disease. We discuss recent successes in understanding the biology of virulence and in structure determination of yeast GPCRs, and highlight new approaches that might pay significant dividends in the urgent search for novel antifungal drugs.

The Cdc25 and Ras1 Proteins of Candida albicans Influence Epithelial Toxicity in a Niche-Specific Way

The PKA pathway is a signaling pathway involved in virulence in Candida albicans. This mechanism can be activated via addition of glucose and activation involves at least two proteins, namely Cdc25 and Ras1. Both proteins are involved in specific virulence traits. However, it is not clear if Cdc25 and Ras1 also affect virulence independently of PKA. C. albicans holds a second, atypical, Ras protein, Ras2, but its function in PKA activation is still unclear. We investigated the role of Cdc25, Ras1, and Ras2 for different in vitro and ex vivo virulence characteristics. We show that deletion of CDC25 and RAS1 result in less toxicity towards oral epithelial cells, while deletion of RAS2 has no effect. However, toxicity towards cervical cells increases in both the ras2 and the cdc25 mutants while it decreases in a ras1 mutant compared to the WT. Toxicity assays using mutants of the transcription factors downstream of the PKA pathway (Efg1) or the MAPK pathway (Cph1) show that the ras1 mutant shows similar phenotypes as the efg1 mutant, whereas the ras2 mutant shows similar phenotypes as the cph1 mutant. These data show niche-specific roles for different upstream components in regulating virulence through both signal transduction pathways.

Candida albicans biofilms: antifungal resistance, immune evasion, and emerging therapeutic strategies

Candida albicans is a fungal pathogen that can form biofilms on medical devices and host tissue, resulting in serious, life-threatening infections. These fungal biofilms are inherently resistant to traditional antifungal therapies and the host immune system; therefore, biofilm-associated infections are a huge clinical challenge. This review summarizes the most important insights into C. albicans biofilm-associated antifungal drug resistance mechanisms and immune evasion strategies. In addtion, this review also discusses the strategies for antifungal drug use to combat these processes, providing further evidence for novel drugs research and clinical therapies.

Roles of the pro-apoptotic factors CaNma111 and CaYbh3 in apoptosis and virulence of Candida albicans

Candida albicans, a commensal and opportunistic pathogen, undergoes apoptosis in response to various stimuli, including hydrogen peroxide, acetic acid, and antifungal agents. Apoptotic processes are highly conserved among mammals, plants, and fungi, but little is known about the apoptosis-regulating factors in C. albicans. In this study, C. albicans homologs of the putative apoptosis factors were identified by database screening followed by overexpression analysis. CaNma111, a homolog of the pro-apoptotic mammalian HtrA2/Omi, and CaYbh3, a homolog of BH3-only protein, yielded increased apoptotic phenotypes upon overexpression. We showed that CaNma111 and CaYbh3 functions as pro-apoptotic regulators by examining intracellular ROS accumulation, DNA end breaks (TUNEL assay), and cell survival in Canma111/Canma111 and Caybh3/Caybh3 deletion strains. We found that the protein level of CaBir1, an inhibitor-of-apoptosis (IAP) protein, was down-regulated by CaNma111. Interestingly, the Canma111/Canma111 and Caybh3/Caybh3 deletion strains showed hyperfilamentation phenotypes and increased virulence in a mouse infection model. Together, our results suggest that CaNma111 and CaYbh3 play key regulatory roles in the apoptosis and virulence of C. albicans.

Growth modulation and metabolic responses of Ganoderma boninense to salicylic acid stress

Various phenolic compounds have been screened against Ganoderma boninense, the fungal pathogen causing basal stem rot in oil palms. In this study, we focused on the effects of salicylic acid (SA) on the growth of three G. boninense isolates with different levels of aggressiveness. In addition, study on untargeted metabolite profiling was conducted to investigate the metabolomic responses of G. boninense towards salicylic acid. The inhibitory effects of salicylic acid were both concentration- (P < 0.001) and isolate-dependent (P < 0.001). Also, growth-promoting effect was observed in one of the isolates at low concentrations of salicylic acid where it could have been utilized by G. boninense as a source of carbon and energy. Besides, adaptation towards salicylic acid treatment was evident in this study for all isolates, particularly at high concentrations. In other words, inhibitory effect of salicylic acid treatment on the fungal growth declined over time. In terms of metabolomics response to salicylic acid treatment, G. boninense produced several metabolites such as coumarin and azatyrosine, which suggests that salicylic acid modulates the developmental switch in G. boninense towards the defense mode for its survival. Furthermore, the liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis showed that the growth of G. boninense on potato dextrose agar involved at least four metabolic pathways: amino acid metabolism, lipid pathway, tryptophan pathway and phenylalanine pathway. Overall, there were 17 metabolites that contributed to treatment separation, each with P<0.005. The release of several antimicrobial metabolites such as eudistomin I may enhance G. boninense's competitiveness against other microorganisms during colonisation. Our findings demonstrated the metabolic versatility of G. boninense towards changes in carbon sources and stress factors. G. boninense was shown to be capable of responding to salicylic acid treatment by switching its developmental stage.

N-acetylglucosamine transporter, Ngt1, undergoes sugar-responsive endosomal trafficking in Candida albicans

N-acetylglucosamine (GlcNAc), an important amino sugar at the infection sites of the fungal pathogen Candida albicans, triggers multiple cellular processes. GlcNAc import at the cell surface is mediated by GlcNAc transporter, Ngt1 which seems to play a critical role during GlcNAc signaling. We have investigated the Ngt1 dynamics that provide a platform for further studies aimed at understanding the mechanistic insights of regulating process(es) in C. albicans. The expression of this transporter is prolific and highly sensitive to even very low levels (˂2 µM) of GlcNAc. Under these conditions, Ngt1 undergoes phosphorylation-associated ubiquitylation as a code for internalization. This ubiquitylation process involves the triggering proteins like protein kinase Snf1, arrestin-related trafficking adaptors (ART) protein Rod1, and yeast ubiquitin ligase Rsp5. Interestingly, analysis of ∆snf1 and ∆rsp5 mutants revealed that while Rsp5 is promoting the endosomal trafficking of Ngt1-GFPɤ, Snf1 hinders the process. Furthermore, colocalization experiments of Ngt1 with Vps17 (an endosomal marker), Sec7 (a trans-Golgi marker), and a vacuolar marker revealed the fate of Ngt1 during sugar-responsive endosomal trafficking. ∆ras1 and ∆ubi4 mutants showed decreased ubiquitylation and delayed endocytosis of Ngt1. According to our knowledge, this is the first report which illustrates the mechanistic insights that are responsible for endosomal trafficking of a GlcNAc transporter in an eukaryotic organism.

Extending the Proteomic Characterization of Candida albicans Exposed to Stress and Apoptotic Inducers through Data-Independent Acquisition Mass Spectrometry

Candida albicans is a commensal fungus that causes systemic infections in immunosuppressed patients. In order to deal with the changing environment during commensalism or infection, C. albicans must reprogram its proteome. Characterizing the stress-induced changes in the proteome that C. albicans uses to survive should be very useful in the development of new antifungal drugs. We studied the C. albicans global proteome after exposure to hydrogen peroxide (H₂O₂) and acetic acid (AA), using a data-independent acquisition mass spectrometry (DIA-MS) strategy. More than 2,000 C. albicans proteins were quantified using an ion library previously constructed using data-dependent acquisition mass spectrometry (DDA-MS). C. albicans responded to treatment with H₂O₂ with an increase in the abundance of many proteins involved in the oxidative stress response, protein folding, and proteasome-dependent catabolism, which led to increased proteasome activity. The data revealed a previously unknown key role for Prn1, a protein similar to pirins, in the oxidative stress response. Treatment with AA resulted in a general decrease in the abundance of proteins involved in amino acid biosynthesis, protein folding, and rRNA processing. Almost all proteasome proteins declined, as did proteasome activity. Apoptosis was observed after treatment with H₂O₂ but not AA. A targeted proteomic study of 32 proteins related to apoptosis in yeast supported the results obtained by DIA-MS and allowed the creation of an efficient method to quantify relevant proteins after treatment with stressors (H₂O₂, AA, and amphotericin B). This approach also uncovered a main role for Oye32, an oxidoreductase, suggesting this protein as a possible apoptotic marker common to many stressors.

IMPORTANCE Fungal infections are a worldwide health problem, especially in immunocompromised patients and patients with chronic disorders. Invasive candidiasis, caused mainly by C. albicans, is among the most common fungal diseases. Despite the existence of treatments to combat candidiasis, the spectrum of drugs available is limited. For the discovery of new drug targets, it is essential to know the pathogen response to different stress conditions. Our study provides a global vision of proteomic remodeling in C. albicans after exposure to different agents, such as hydrogen peroxide, acetic acid, and amphotericin B, that can cause apoptotic cell death. These results revealed the significance of many proteins related to oxidative stress response and proteasome activity, among others. Of note, the discovery of Prn1 as a key protein in the defense against oxidative stress as well the increase in the abundance of Oye32 protein when apoptotic process occurred point them out as possible drug targets.

Regulation of Cell Death Induced by Acetic Acid in Yeasts

Acetic acid has long been considered a molecule of great interest in the yeast research field. It is mostly recognized as a by-product of alcoholic fermentation or as a product of the metabolism of acetic and lactic acid bacteria, as well as of lignocellulosic biomass pretreatment. High acetic acid levels are commonly associated with arrested fermentations or with utilization as vinegar in the food industry. Due to its obvious interest to industrial processes, research on the mechanisms underlying the impact of acetic acid in yeast cells has been increasing. In the past twenty years, a plethora of studies have addressed the intricate cascade of molecular events involved in cell death induced by acetic acid, which is now considered a model in the yeast regulated cell death field. As such, understanding how acetic acid modulates cellular functions brought about important knowledge on modulable targets not only in biotechnology but also in biomedicine. Here, we performed a comprehensive literature review to compile information from published studies performed with lethal concentrations of acetic acid, which shed light on regulated cell death mechanisms. We present an historical retrospective of research on this topic, first providing an overview of the cell death process induced by acetic acid, including functional and structural alterations, followed by an in-depth description of its pharmacological and genetic regulation. As the mechanistic understanding of regulated cell death is crucial both to design improved biomedical strategies and to develop more robust and resilient yeast strains for industrial applications, acetic acid-induced cell death remains a fruitful and open field of study.

Underlying mechanisms of the effect of minocycline against Candida albicans biofilms

Minocycline (MH) is a broad-spectrum antimicrobial agent and semisynthetic tetracycline derivative, which has been widely used in the clinic due to its efficacy. Having the strongest anti-microbial effect, MH exceeded the traditional scope of antibiotics and its previously unknown antifungal activity is also gradually being discovered. To preliminarily investigate the inhibitory effect of MH on Candida albicans (C. albicans), changes of cell growth, hyphal formation and transition, biofilm production and signaling pathway gene expression of C. albicans in the presence of MH were assessed in the present study. An XTT reduction assay was performed to quantitatively detect the metabolic activity of biofilms and evaluate the inhibition of MH on this. The results suggested that biofilm formation was clearly inhibited by 67% (P<0.0001) in the presence of 250 µg/ml MH, while mature biofilms were not significantly affected. In addition, MH inhibited the transition from yeast to hypha in a dose-dependent manner. Furthermore, reverse transcription-quantitative PCR revealed that several hyphae- and adhesion-specific genes associated with the Ras/cyclic (c)AMP/protein kinase A (PKA) pathway were differentially expressed following MH treatment, including downregulation of ras family GTPase (RAS1), adenylyl cyclase-associated protein 1 (CAP1), thiamin pyrophosphokinase 1 (TPK1), adenylate cyclase (CDC35), transcription factor (TEC1), agglutinin-like protein 3 (ALS3) and hyphal wall protein 1 (HWP1) and upregulation of EFG1 (enhanced filamentous growth protein 1 gene) and PDE2 (high-affinity phosphodiesterase gene). The most obviously changed genes were TPK1, HWP1 and RAS1, downregulated by 0.33-, 0.48- and 0.55-fold, respectively. It was suggested that MH is associated with alterations in the morphology of C. albicans, such as the repression of hypha and biofilm formation of cells, and MH affected the Ras/cAMP pathway to regulate the expression of cAMP-associated genes.

Dynamic proteomic analysis of Phanerochaete chrysosporium under copper stress

The model white rot fungus Phanerochaete chrysosporium is frequently preferred for heavy metal accumulation studies due to its high resistance to heavy metals, including copper (Cu). Here, the response of P. chrysosporium under Cu stress at different time points was investigated for the first time by a detailed proteomic analysis using 2DE MALDI-TOF/MS and nanoLC-MS/MS techniques. A total of 123 Cu-responsive protein spots were determined using 2DE approach, and 104 of them were corresponded to 73 distinct open reading frames (ORFs). Of identified ones, 88 spots were over-, and 16 spots were underrepresented. The majority of these proteins showed to the strongest response at 8th h of Cu exposure. Using nanoLC-MS/MS analysis, a total of 167 differentially produced proteins were identified from Cu-exposed cultures after enrichment of the membrane proteins followed by SILAC. Seventy four, 66, and 69 overrepresented, and 56, 71, and 64 underrepresented proteins were identified at 2 h, 4 h, and 8 h of Cu exposure, respectively. The bioinformatic analysis of these proteins revealed that intracellular trafficking proteins such as Ran GTPase and a p24 family protein, and certain proteins involved in posttranslational modification, protein turnover and folding were Cu-responsive. Three important transcription factors (TFs), NAC, BTF3, and homeobox TFs, 40S and 60S ribosomal proteins, chaperones such as Hsp26/Hsp42 and mortalin, as well as 20S proteasome, 14-3-3 proteins and Hsp90 involve in Cu-stress response of P. chrysosporium. Moreover, certain elements of translation machinery, the proteins related with aspartate, methionine, and pyruvate metabolisms, transketolase, and trehalase related with carbohydrate metabolism, citrate synthase, fumarase, V-ATPase, and F0F1-type ATPase playing role in energy production and conversion, transport proteins such as multidrug resistance and p24 family proteins as well as actin-related proteins involved in cytoskeleton remodeling were determined to be Cu-responsive. The present proteome analysis revealed that P. chrysosporium mainly regulates translational and posttranslational processes, certain transport processes, many metabolic pathways and cytoskeleton to overcome the Cu-induced oxidative stress.

Candida and Candidiasis-Opportunism Versus Pathogenicity: A Review of the Virulence Traits

One of the most important questions in microbiology nowadays, is how apparently harmless, commensal yeasts like Candida spp. can cause a rising number of infections. The occurrence of the disease requires firstly the attachment to the host cells, followed by the invasion of the tissue. The adaptability translates into a rapid ability to respond to stress factors, to take up nutrients or to multiply under different conditions. By forming complex intracellular networks such as biofilms, Candida spp. become not only more refractive to antifungal therapies but also more prone to cause disease. The inter-microbial interactions can enhance the virulence of a strain. In vivo, the fungal cells face a multitude of challenges and, as a result, they develop complex strategies serving one ultimate goal: survival. This review presents the virulence factors of the most important Candida spp., contributing to a better understanding of the onset of candidiasis and raising awareness of the highly complex interspecies interactions that can change the outcome of the disease.

Pkh1p-Ypk1p and Pkh1p-Sch9p Pathways Are Activated by Acetic Acid to Induce a Mitochondrial-Dependent Regulated Cell Death

The yeast Saccharomyces cerevisiae undergoes a mitochondrial-dependent regulated cell death (RCD) exhibiting typical markers of mammalian apoptosis. We have previously shown that ceramide production contributes to RCD induced by acetic acid and is involved in mitochondrial outer membrane permeabilization and cytochrome c release, especially through hydrolysis of complex sphingolipids catalyzed by Isc1p. Recently, we also showed that Sch9p regulates the translocation of Isc1p from the endoplasmic reticulum into mitochondria, perturbing sphingolipid balance and determining cell fate. In this study, we addressed the role of other signaling proteins in acetic acid-induced RCD. We found that single deletion of PKH1 or YPK1, as shown for SCH9 and ISC1, leads to an increase in cell survival in response to acetic acid and that Pkh1/2p-dependent phosphorylation of Ypk1p and Sch9p increases under these conditions. These results indicate that Pkh1p regulates acetic acid-induced RCD through Ypk1p and Sch9p. In addition, our results suggest that Pkh1p-Ypk1p is necessary for isc1Δ resistance to acetic acid-induced RCD. Moreover, double deletion of ISC1 and PKH1 has a drastic effect on cell survival associated with increased ROS accumulation and release of cytochrome c, which is counteracted by overexpression of the PKA pathway negative regulator PDE2. Overall, our results suggest that Pkh1p-Ypk1p and Pkh1p-Sch9p pathways contribute to RCD induced by acetic acid.

Iron Chelator Deferasirox Reduces Candida albicans Invasion of Oral Epithelial Cells and Infection Levels in Murine Oropharyngeal Candidiasis

Candida albicans, the causative agent of mucosal infections, including oropharyngeal candidiasis (OPC), as well as bloodstream infections, is becoming increasingly resistant to existing treatment options. In the absence of novel drug candidates, drug repurposing aimed at using existing drugs to treat off-label diseases is a promising strategy. C. albicans requires environmental iron for survival and virulence, while host nutritional immunity deploys iron-binding proteins to sequester iron and reduce fungal growth. Here we evaluated the role of iron limitation using deferasirox (an FDA-approved iron chelator for the treatment of patients with iron overload) during murine OPC and assessed deferasirox-treated C. albicans for its interaction with human oral epithelial (OE) cells, neutrophils, and antimicrobial peptides. Therapeutic deferasirox treatment significantly reduced salivary iron levels, while a nonsignificant reduction in the fungal burden was observed. Preventive treatment that allowed for two additional days of drug administration in our murine model resulted in a significant reduction in the number of C. albicans CFU per gram of tongue tissue, a significant reduction in salivary iron levels, and significantly reduced neutrophil-mediated inflammation. C. albicans cells harvested from the tongues of animals undergoing preventive treatment had the differential expression of 106 genes, including those involved in iron metabolism, adhesion, and the response to host innate immunity. Moreover, deferasirox-treated C. albicans cells had a 2-fold reduction in survival in neutrophil phagosomes (with greater susceptibility to oxidative stress) and reduced adhesion to and invasion of OE cells in vitro Thus, deferasirox treatment has the potential to alleviate OPC by affecting C. albicans gene expression and reducing virulence.

Non-Toxic and Ultra-Small Biosilver Nanoclusters Trigger Apoptotic Cell Death in Fluconazole-Resistant Candida albicans via Ras Signaling

Silver-based nanostructures are suitable for many biomedical applications, but to be useful therapeutic agents, the high toxicity of these nanomaterials must be eliminated. Here, we biosynthesize nontoxic and ultra-small silver nanoclusters (rsAg@NCs) using metabolites of usnioid lichen (a symbiotic association of algae and fungi) that exhibit excellent antimicrobial activity against fluconazole (FCZ)-resistant Candida albicans that is many times higher than chemically synthesized silver nanoparticles (AgNPs) and FCZ. The rsAg@NCs trigger apoptosis via reactive oxygen species accumulation that leads to the loss of mitochondrial membrane potential, DNA fragmentation, chromosomal condensation, and the activation of metacaspases. The proteomic analysis clearly demonstrates that rsAg@NCs exposure significantly alters protein expression. Most remarkable among the down-regulated proteins are those related to glycolysis, metabolism, free radical scavenging, anti-apoptosis, and mitochondrial function. In contrast, proteins involved in plasma membrane function, oxidative stress, cell death, and apoptosis were upregulated. Eventually, we also established that the apoptosis-inducing potential of rsAg@NCs is due to the activation of Ras signaling, which confirms their application in combating FCZ-resistant C. albicans infections.

Synthesis of tetrazole derivatives bearing pyrrolidine scaffold and evaluation of their antifungal activity against Candida albicans

The increase of opportunistic fungal infections raises the need for design and synthesis of new antifungal agents. Taking into account that tetrazole derivatives exhibit antifungal activity, and some of them are in the phase of clinical trials, new tetrazole derivatives bearing pyrrolidine moiety were synthesized in order to present their action mode against C. albicans. The target compounds were obtained by N-alkylation of various 2-arylpyrrolidines with several 1-(3-chloropropyl)-5-aryl-2H-tetrazoles. Regardless of the substituents at tetrazole or pyrrolidine rings reactions took place in 48 h and with satisfactory yields ranging from 53 to 70%. We performed screen of the synthesized compounds to identify these nontoxic inhibiting the C. albicans planktonic and sessile cells, and conducted a series of follow up studies to examine the in vitro and in vivo activity of the most potent antifungals. The leading antifungal inhibitor: 2-{3-[2-(3-Methylphenyl)pyrrolidin-1-yl]propyl}-5-phenyl-2H-tetrazole (3aC) and the randomly selected ones: 5-phenyl-2-[3-(2-phenylpyrrolidin-1-yl)propyl]-2H-tetrazole (3aA), 5-(4-chlorophenyl)-2-{3-[2-(4-fluorophenyl)pyrrolidin-1-yl]propyl}-2H-tetrazole (3cD), and 5-(4-chlorophenyl)-2-{3-[2-(4-chlorophenyl)pyrrolidin-1-yl]propyl}-2H-tetrazole (3cE) showed little to no toxicity against the Vero cell line and Galleria mellonella. 3aC and 3aD, the most active against biofilm in vitro, demonstrated in vivo activity in the invertebrate model of disseminated candidiasis. Flow cytometry analysis showed that necrotic cell death was generated under 3aC due to its interactions with the fungal membrane; this confirmed by the mitochondrial damage (XTT assay) and reduced adhesion to the TR-146 cell line at 46.05 μM. Flow cytometry was used to directly measure the redox state of the treated cells with the fluorescent DCFH probe. Pro-necrotic tetrazole derivatives (3aA, 3aC, 3cD) are unable to induce ROS production in the C. albicans cells. Moreover, CLSM analyses revealed that the tetrazole derivatives (principally 3aC, 3aD, and 3aE) inhibit C. albicans' ability to neutralize macrophages; a more effective phagosomes organisation was observed. 3aC's and 3aD's activity reflected in an attenuation of virulence in disseminated candidiasis in vivo.

Candida albicans Ras1 Inactivation Increases Resistance to Phagosomal Killing by Human Neutrophils

Host phagocytic cells are crucial players in initial defense against Candida albicans infection. C. albicans utilizes MAP kinases and Ras1 stress response signaling pathways to protect itself from killing by immune cells. In this study, we tested the importance of these pathways in C. albicans phagocytosis by neutrophils and subsequent phagosomal survival. Phagocytosis was influenced by C. albicans morphology, so hyphal length of >10 μm reduced the phagocytic index (PI) 2- to 3-fold in human neutrophils. Primary human neutrophils killed 81% of phagocytosed C. albicans, while primary mouse neutrophils killed 63% of yeasts. We found that both the C. albicans Cek1 and Hog1 pathways were required for survival of phagocytosed yeast, whereas deletion of C. albicans RAS1 resulted in an 84% increase in survival within neutrophils compared to that of the wild type (WT). The absence of Ras1 did not alter reactive oxygen species (ROS) production by C. albicans; however, phagocytosed C. albicans Δ/Δras1 cells reduced ROS release by neutrophils by 86%. Moreover, C. albicans Δ/Δras1 cells had increased resistance to hydrogen peroxide as a result of high levels of catalase activity. This phenotype was specific to Ras1, since these effects were not observed in the absence of its partner Cyr1 or with its downstream target Efg1. In addition, C. albicans Δ/Δras1 cells had a significantly increased resistance to nonoxidative killing by human neutrophil peptide 1 (HNP-1) that was reversed by restoring cellular cAMP levels. These data show that C. albicans Ras1 inactivation leads to fungal resistance to both oxidative and nonoxidative mechanisms of neutrophil phagosomal killing.

Multiple roles and diverse regulation of the Ras/cAMP/protein kinase A pathway in Candida albicans

Candida albicans is a major fungal pathogen of humans, causing both superficial and life-threatening systemic infections in immunocompromised people. The conserved Ras/cAMP/PKA pathway plays a key role in regulating multiple traits important for the virulence of C. albicans such as cell growth, yeast-hyphal transition, white-opaque switching, sexual reproduction and biofilm development. Diverse external signals influence cell physiology by activating this signaling pathway. The key components of the Ras/cAMP/PKA pathway include two Ras GTPases (Ras1 and Ras2), an adenylyl cyclase (Cyr1, also known as Cdc35), two cyclic nucleotide phosphodiesterases (Pde1 and Pde2) and the catalytic (Tpk1 and Tpk2) and regulatory (Bcy1) subunits of PKA kinase. Activation of this pathway dramatically alters the gene expression profile via several transcription factors, leading to the activation of specific biological processes. Here, we review the progress made in the past two decades to elucidate the molecular mechanisms by which the Ras/cAMP/PKA pathway senses diverse environmental cues and controls specific cellular responses and its connection with other signaling pathways in C. albicans.

Programmed cell death in human pathogenic fungi - a possible therapeutic target

INTRODUCTION

Diseases caused by pathogenic fungi are increasing because of antibiotic overuse, the rise of immunosuppressive therapies, and climate change. The limited variety of antimycotics and the rapid adaptation of pathogenic fungi to antifungal agents serve to exacerbate this issue. Unfortunately, about 1.6 million people are killed by fungal infections annually. Areas covered: The discovery of the small antimicrobial proteins produced by microorganisms, animals, humans, and plants will hopefully overcome challenges in the treatment of fungal infections. These small proteins are highly stable and any resistance to them rarely evolves; therefore, they are potentially good candidates for the treatment and prevention of infections caused by pathogenic fungi. Some of these proteins target the programmed cell death machinery of pathogenic fungi; this is potentially a novel approach in antimycotic therapies. In this review, we highlight the elements of apoptosis in human pathogenic fungi and related model organisms and discuss the possible therapeutic potential of the apoptosis-inducing, small, antifungal proteins. Expert opinion: Small antimicrobial proteins may establish a new class of antimycotics in the future. The rarity of resistance and their synergistic effects with other frequently used antifungal agents may help pave the way for their use in the clinic.

Fungal persister cells: The basis for recalcitrant infections?

Persister cells are a small subpopulation within fungal biofilms that are highly resistant to high concentrations of antifungals and therefore most likely contribute to the resistance and recalcitrance of biofilm infections. Moreover, this subpopulation is defined as a nongrowing, phenotypic variant of wild-type cells that can survive high doses of antifungals. There are high degrees of heterogeneity and plasticity associated with biofilm formation, resulting in a strong variation in the amount of persister cells. The fraction of these cells in fungal biofilms also appear to be dependent on the type of substrate. The cells can be observed immediately after their adhesion to that substrate, which makes up the initial step of biofilm formation. Thus far, persister cells have primarily been studied in Candida spp. These fungi are the fourth most common cause of nosocomial systemic infections in the United States, with C. albicans being the most prevalent species. Remarkably, persisters exhibit characteristics of a dormant state similar to what is observed in cells deprived of glucose. This dormant state, together with attachment to a substrate, appears to provide the cells with characteristics that help them overcome the challenges with fungicidal drugs such as amphotericin B (AmB). AmB is known to induce apoptosis, and persister cells are able to cope with the increase in reactive oxygen species (ROS) by activating stress response pathways and the accumulation of high amounts of glycogen and trehalose-two known stress-protecting molecules. In this review, we discuss the molecular pathways that are involved in persister cell formation in fungal species and highlight that the eradication of persister cells could lead to a strong reduction of treatment failure in a clinical setting.

G1 and S phase arrest in Candida albicans induces filamentous growth via distinct mechanisms

Candida albicans is an opportunistic fungal pathogen. In immunocompromised individuals, it can cause bloodstream infections with high mortality rates. The ability to switch between yeast and hyphal morphologies is a critical virulence factor of C. albicans. In response to diverse environmental cues, several signaling pathways are activated resulting in filamentous growth. Interestingly, cell cycle arrest can also trigger filamentous growth although the pathways involved are not well-understood. Here, we demonstrate that the cAMP-PKA pathway is involved in the filamentous growth caused by G1 arrest due to the depletion of the G1 cyclin Cln3 and S phase arrest due to hydroxyurea treatment. The downstream mechanisms involved in filamentation are different between the two cell cycle arrest phenomena. Cln3-depleted cells require HGC1 and UME6 for filamentous growth, but hydroxyurea-induced filamentation does not. Also, the hyphal repressor Nrg1 is not involved in the suppression of Cln3-depletion and hydroxyurea-induced filamentous growth. The findings highlight the complexity of the signaling networks that control filamentous growth in which different mechanisms downstream of the cAMP-PKA pathway are activated based on the nature of the inducing signals.

Regulated Cell Death as a Therapeutic Target for Novel Antifungal Peptides and Biologics

The rise of microbial pathogens refractory to conventional antibiotics represents one of the most urgent and global public health concerns for the 21st century. Emergence of Candida auris isolates and the persistence of invasive mold infections that resist existing treatment and cause severe illness has underscored the threat of drug-resistant fungal infections. To meet these growing challenges, mechanistically novel agents and strategies are needed that surpass the conventional fungistatic or fungicidal drug actions. Host defense peptides have long been misunderstood as indiscriminant membrane detergents. However, evidence gathered over the past decade clearly points to their sophisticated and selective mechanisms of action, including exploiting regulated cell death pathways of their target pathogens. Such peptides perturb transmembrane potential and mitochondrial energetics, inducing phosphatidylserine accessibility and metacaspase activation in fungi. These mechanisms are often multimodal, affording target pathogens fewer resistance options as compared to traditional small molecule drugs. Here, recent advances in the field are examined regarding regulated cell death subroutines as potential therapeutic targets for innovative anti-infective peptides against pathogenic fungi. Furthering knowledge of protective host defense peptide interactions with target pathogens is key to advancing and applying novel prophylactic and therapeutic countermeasures to fungal resistance and pathogenesis.

Difunctional bacteriophage conjugated with photosensitizers for Candida albicans-targeting photodynamic inactivation

Background

Candida albicans is the most prevalent fungal pathogen of the human microbiota, causing infections ranging from superficial infections of the skin to life-threatening systemic infections. Due to the increasing occurrence of antibiotic-resistant C. albicans strains, new approaches to control this pathogen are needed. Photodynamic inactivation is an emerging alternative to treat infections based on the interactions between visible light and photosensitisers, in which pheophorbide a (PPA) is a chlorophyll-based photosensitizer that could induce cell death after light irradiation. Due to PPA’s phototoxicity and low efficiency, the main challenge is to implement photosensitizer cell targeting and attacking.

Methods

In this study, PPA was conjugated with JM-phage by EDC/NHS crosslinking. UV-Vis spectra was used to determine the optimum conjugation percentages of PPA and JM-phage complex for photodynamic inactivation. After photodynamic inactivation, the efficacy of PPA-JM-phage was assessed by performing in vitro experiments, such as MTS assay, scanning electron microscopy, measurement of dysfunctional mitochondria, ROS accumulation, S cell arrest and apoptotic pathway.

Results

A single-chain variable-fragment phage (JM) with high affinity to MP65 was screened from human single-fold single-chain variable-fragment libraries and designed as a binding target for C. albicans cells. Subsequently, PPa was integrated into JM phage to generate a combined nanoscale material, which was called PPA-JM-phage. After photodynamic inactivation, the growth of C. albicans was inhibited by PPA-JM-phage and apoptosis was observed. Scanning electron microscopy analysis revealed shrinking and rupturing of C. albicans. We also found that depolarization of mitochondrial membrane potential was decreased and intracellular reactive oxygen species levels were elevated significantly in C. albicans inhibited by PPA-JM-phage. Additionally, PPA-JM-phage also lead to S-phase arrest, and metacaspase activation resulting from mitochondrial dysfunction was also found to be involved in C. albicans apoptosis.

Conclusion

PPa-JM-phage may induce C. albicans apoptosis through a caspase-dependent pathway and the results herein shed light on the potential application of phtototherapeutic nanostructures in fungal inactivation.

Adaptive Response and Tolerance to Acetic Acid in Saccharomyces cerevisiae and Zygosaccharomyces bailii: A Physiological Genomics Perspective

Acetic acid is an important microbial growth inhibitor in the food industry; it is used as a preservative in foods and beverages and is produced during normal yeast metabolism in biotechnological processes. Acetic acid is also a major inhibitory compound present in lignocellulosic hydrolysates affecting the use of this promising carbon source for sustainable bioprocesses. Although the molecular mechanisms underlying Saccharomyces cerevisiae response and adaptation to acetic acid have been studied for years, only recently they have been examined in more detail in Zygosaccharomyces bailii. However, due to its remarkable tolerance to acetic acid and other weak acids this yeast species is a major threat in the spoilage of acidic foods and beverages and considered as an interesting alternative cell factory in Biotechnology. This review paper emphasizes genome-wide strategies that are providing global insights into the molecular targets, signaling pathways and mechanisms behind S. cerevisiae and Z. bailii tolerance to acetic acid, and extends this information to other weak acids whenever relevant. Such comprehensive perspective and the knowledge gathered in these two yeast species allowed the identification of candidate molecular targets, either for the design of effective strategies to overcome yeast spoilage in acidic foods and beverages, or for the rational genome engineering to construct more robust industrial strains. Examples of successful applications are provided.

The Yeast Saccharomyces cerevisiae as a Model for Understanding RAS Proteins and their Role in Human Tumorigenesis

The exploitation of the yeast Saccharomyces cerevisiae as a biological model for the investigation of complex molecular processes conserved in multicellular organisms, such as humans, has allowed fundamental biological discoveries. When comparing yeast and human proteins, it is clear that both amino acid sequences and protein functions are often very well conserved. One example of the high degree of conservation between human and yeast proteins is highlighted by the members of the RAS family. Indeed, the study of the signaling pathways regulated by RAS in yeast cells led to the discovery of properties that were often found interchangeable with RAS proto-oncogenes in human pathways, and vice versa. In this work, we performed an updated critical literature review on human and yeast RAS pathways, specifically highlighting the similarities and differences between them. Moreover, we emphasized the contribution of studying yeast RAS pathways for the understanding of human RAS and how this model organism can contribute to unveil the roles of RAS oncoproteins in the regulation of mechanisms important in the tumorigenic process, like autophagy.

Guidelines and recommendations on yeast cell death nomenclature

Elucidating the biology of yeast in its full complexity has major implications for science, medicine and industry. One of the most critical processes determining yeast life and physiology is cel-lular demise. However, the investigation of yeast cell death is a relatively young field, and a widely accepted set of concepts and terms is still missing. Here, we propose unified criteria for the defi-nition of accidental, regulated, and programmed forms of cell death in yeast based on a series of morphological and biochemical criteria. Specifically, we provide consensus guidelines on the differ-ential definition of terms including apoptosis, regulated necrosis, and autophagic cell death, as we refer to additional cell death rou-tines that are relevant for the biology of (at least some species of) yeast. As this area of investigation advances rapidly, changes and extensions to this set of recommendations will be implemented in the years to come. Nonetheless, we strongly encourage the au-thors, reviewers and editors of scientific articles to adopt these collective standards in order to establish an accurate framework for yeast cell death research and, ultimately, to accelerate the pro-gress of this vibrant field of research.

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.

Ras signalling in pathogenic yeasts

The small GTPase Ras acts as a master regulator of growth, stress response and cell death in eukaryotic cells. The control of Ras activity is fundamental, as highlighted by the oncogenic properties of constitutive forms of Ras proteins. Ras also plays a crucial role in the pathogenicity of fungal pathogens where it has been found to regulate a number of adaptions required for virulence. The importance of Ras in fungal disease raises the possibility that it may provide a useful target for the development of new treatments at a time when resistance to available antifungals is increasing. New findings suggest that important regulatory sequences found within fungal Ras proteins that are not conserved may prove useful in the development of new antifungals. Here we review the roles of Ras protein function and signalling in the major human yeast pathogens Candida albicans and Cryptococcus neoformans and discuss the potential for targeting Ras as a novel approach to anti-fungal therapy.

Induction of apoptosis and ganoderic acid biosynthesis by cAMP signaling in Ganoderma lucidum

Apoptosis is an essential physiological process that controls many important biological functions. However, apoptosis signaling in relation to secondary metabolite biosynthesis in plants and fungi remains a mystery. The fungus Ganoderma lucidum is a popular herbal medicine worldwide, but the biosynthetic regulation of its active ingredients (ganoderic acids, GAs) is poorly understood. We investigated the role of 3′,5′-cyclic adenosine monophosphate (cAMP) signaling in fungal apoptosis and GA biosynthesis in G. lucidum. Two phosphodiesterase inhibitors (caffeine and 3-isobutyl-1-methylxanthine, IBMX) and an adenylate cyclase activator (sodium fluoride, NaF) were used to increase intracellular cAMP levels. Fungal apoptosis was identified by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) assay and a condensed nuclear morphology. Our results showed that GA production and fungal apoptosis were induced when the mycelium was treated with NaF, caffeine, or cAMP/IBMX. Downregulation of squalene synthase and lanosterol synthase gene expression by cAMP was detected in the presence of these chemicals, which indicates that these two genes are not critical for GA induction. Transcriptome analysis indicated that mitochondria might play an important role in cAMP-induced apoptosis and GA biosynthesis. To the best of our knowledge, this is the first report to reveal that cAMP signaling induces apoptosis and secondary metabolite production in fungi.

The Regulatory Subunit of Protein Kinase A (Bcy1) in Candida albicans Plays Critical Roles in Filamentation and White-Opaque Switching but Is Not Essential for Cell Growth

The conserved cAMP-dependent protein kinase (PKA) is composed of the regulatory and catalytic subunits and acts as the central component of the cAMP signaling pathway. In the human fungal pathogen Candida albicans, the PKA regulatory subunit Bcy1 plays a critical role in the regulation of cell differentiation and death. It has long been considered that Bcy1 is essential for cell viability in C. albicans. In the current study, surprisingly, we found that Bcy1 is not required for cell growth, and we successfully generated a bcy1/bcy1 null mutant in C. albicans. Deletion of BCY1 leads to multiple cellular morphologies and promotes the development of filaments. Filamentous and smooth colonies are two typical morphological types of the bcy1/bcy1 mutant, which can undergo spontaneous switching between the two types. Cells of filamentous colonies grow better on a number of different culture media and have a higher survival rate than cells of smooth colonies. In addition, deletion of BCY1 significantly increased the frequency of white-to-opaque switching on N-acetylglucosamine (GlcNAc)-containing medium. The bcy1/bcy1 null mutant generated herein provides the field a new resource to study the biological functions of the cAMP signaling pathway in C. albicans.

Synergistic Effects and Mechanisms of Budesonide in Combination with Fluconazole against Resistant Candida albicans

Candida albicans is an important opportunistic pathogen, causing both superficial mucosal infections and life-threatening systemic diseases in the clinic. The emergence of drug resistance in Candida albicans has become a noteworthy phenomenon due to the extensive use of antifungal agents and the development of biofilms. This study showed that budesonide potentiates the antifungal effect of fluconazole against fluconazole-resistant Candida albicans strains both in vitro and in vivo. In addition, our results demonstrated, for the first time, that the combination of fluconazole and budesonide can reverse the resistance of Candida albicans by inhibiting the function of drug transporters, reducing the formation of biofilms, promoting apoptosis and inhibiting the activity of extracellular phospholipases. This is the first study implicating the effects and mechanisms of budesonide against Candida albicans alone or in combination with fluconazole, which may ultimately lead to the identification of new potential antifungal targets.

Plant phenolic acids induce programmed cell death of a fungal pathogen: MAPK signaling and survival of Cochliobolus heterostrophus

Plant aromatic compounds provide signals and a nutrient source to pathogens, and also act as stressors. Structure-activity relationships suggest two pathways sensing these compounds in the maize pathogen Cochliobolus heterostrophus, one triggering a stress response, and one inducing enzymes for their degradation. Focusing on the stress pathway, we found that ferulic acid causes rapid appearance of TUNEL-positive nuclei, dispersion of histone H1:GFP, hyphal shrinkage, and eventually membrane damage. These hallmarks of programmed cell death (PCD) were not seen upon exposure to caffeic acid, a very similar compound. Exposure to ferulic acid dephosphorylated two MAP kinases: Hog1 (stress activated) and Chk1 (pathogenicity related), while increasing phosphorylation of Mps1 (cell integrity related). Mutants lacking Hog1 or Chk1 are hypersensitive to ferulic acid while Mps1 mutants are not. These results implicate three MAPK pathways in the stress response. Ferulic acid and the antifungal fludioxonil have opposite additive effects on survival and on dephosphorylation of Hog1, which is thus implicated in survival. The results may explain why some fungal pathogens of plants undergo cell death early in host invasion, when phenolics are released from plant tissue.

A functional link between hyphal maintenance and quorum sensing in Candida albicans

Morphogenesis in Candida albicans requires hyphal initiation and maintenance, and both processes are regulated by the fungal quorum sensing molecule (QSM) farnesol. We show that deletion of C. albicans EED1, which is crucial for hyphal extension and maintenance, led to a dramatically increased sensitivity to farnesol, and thus identified the first mutant hypersensitive to farnesol. Furthermore, farnesol decreased the transient filamentation of an eed1Δ strain without inducing cell death, indicating that two separate mechanisms mediate quorum sensing and cell lysis by farnesol. To analyze the cause of farnesol hypersensitivity we constructed either hyperactive or deletion mutants of factors involved in farnesol signaling, by introducing the hyperactive RAS1^G13V or pADH1-CYR1^CAT allele, or deleting CZF1 or NRG1 respectively. Neither of the constructs nor the exogenous addition of dB-cAMP was able to rescue the farnesol hypersensitivity, highlighting that farnesol mediates its effects not only via the cAMP pathway. Interestingly, the eed1Δ strain also displayed increased farnesol production. When eed1Δ was grown under continuous medium flow conditions, to remove accumulating QSMs from the supernatant, maintenance of eed1Δ filamentation, although not restored, was significantly prolonged, indicating a link between farnesol sensitivity, production, and the hyphal maintenance-defect in the eed1Δ mutant strain.

The Role of Signaling via Aqueous Pore Formation in Resistance Responses to Amphotericin B

Drug resistance studies have played an important role in the validation of antibiotic targets. In the case of the polyene antibiotic amphotericin B (AmB), such studies have demonstrated the essential role that depletion of ergosterol plays in the development of AmB-resistant (AmB-R) organisms. However, AmB-R strains also occur in fungi and parasitic protozoa that maintain a normal level of ergosterol at the plasma membrane. Here, I review evidence that shows not only that there is increased protection against the deleterious consequences of AmB-induced ion leakage across the membrane in these resistant pathogens but also that a set of events are activated that block the cell signaling responses that trigger the oxidative damage produced by the antibiotic. Such signaling events appear to be the consequence of a membrane-thinning effect that is exerted upon lipid-anchored Ras proteins by the aqueous pores formed by AmB. A similar membrane disturbance effect may also explain the activity of AmB on mammalian cells containing Toll-like receptors. These resistance mechanisms expand our current understanding of the role that the formation of AmB aqueous pores plays in triggering signal transduction responses in both pathogens and host immune cells.

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.

Role of Aif1 in regulation of cell death under environmental stress in Candida albicans

Apoptosis-inducing factor (AIF) is a conserved flavoprotein localized in the mitochondria, inducing apoptosis after translocation into the nucleus. However, its role in the important fungal pathogen, Candida albicans, remains to be investigated. In this study, we find that the C. albicans AIF protein Aif1, similar to its homologues in other organisms, is localized at the mitochondria and translocated into the nucleus under apoptosis-inducing conditions. Moreover, deletion of AIF1 causes attenuated apoptosis in this pathogen under apoptosis-inducing conditions, such as the treatment of 2 mm H2 O2 , 10 mm acetic acid or 0.08 mg/l caspofungin, and its overexpression enhances this process. Interestingly, treatment with high levels of these agents leads to reversed sensitivity of aif1Δ/Δ and the overexpression strain AIF1ov. In addition, the virulence of C. albicans is not affected by deletion or overexpression of AIF1. Hence, C. albicans Aif1, as a mitochondria-localized protein, plays a dual role in the regulation of cell death under different concentrations of the stress-caused agents. Copyright © 2016 John Wiley & Sons, Ltd.

Apoptosis of Candida albicans during the Interaction with Murine Macrophages: Proteomics and Cell-Death Marker Monitoring

Macrophages may induce fungal apoptosis to fight against C. albicans, as previously hypothesized by our group. To confirm this hypothesis, we analyzed proteins from C. albicans cells after 3 h of interaction with macrophages using two quantitative proteomic approaches. A total of 51 and 97 proteins were identified as differentially expressed by DIGE and iTRAQ, respectively. The proteins identified and quantified were different, with only seven in common, but classified in the same functional categories. The analyses of their functions indicated that an increase in the metabolism of amino acids and purine nucleotides were taking place, while the glycolysis and translation levels dropped after 3 h of interaction. Also, the response to oxidative stress and protein translation were reduced. In addition, seven substrates of metacaspase (Mca1) were identified (Cdc48, Fba1, Gpm1, Pmm1, Rct1, Ssb1, and Tal1) as decreased in abundance, plus 12 proteins previously described as related to apoptosis. Besides, the monitoring of apoptotic markers along 24 h of interaction (caspase-like activity, TUNEL assay, and the measurement of ROS and cell examination by transmission electron microscopy) revealed that apoptotic processes took place for 30% of the fungal cells, thus supporting the proteomic results and the hypothesis of macrophages killing C. albicans by apoptosis.

A common mechanism involving the TORC1 pathway can lead to amphotericin B-persistence in biofilm and planktonic Saccharomyces cerevisiae populations

Fungal infections are an increasing clinical problem. Decreased treatment effectiveness is associated with biofilm formation and drug recalcitrance is thought to be biofilm specific. However, no systematic investigations have tested whether resistance mechanisms are shared between biofilm and planktonic populations. We performed multiplexed barcode sequencing (Bar-seq) screening of a pooled collection of gene-deletion mutants cultivated as biofilm and planktonic cells. Screening for resistance to the ergosterol-targeting fungicide amphotericin B (AmB) revealed that the two growth modes had significant overlap in AmB-persistent mutants. Mutants defective in sterol metabolism, ribosome biosynthesis, and the TORC1 and Ras pathways showed increased persistence when treated with AmB. The ras1, ras2 and tor1 mutants had a high-persister phenotype similar to wild-type biofilm and planktonic cells exposed to the TORC1 pathway inhibitor rapamycin. Inhibition of TORC1 with rapamycin also increased the proportion of persisters in Candida albicans and Candida glabrata. We propose that decreased TORC1-mediated induction of ribosome biosynthesis via Ras can lead to formation of AmB-persister cells regardless of whether the cells are in planktonic or biofilm growth mode. Identification of common pathways leading to growth mode-independent persister formation is important for developing novel strategies for treating fungal infections.

Lysine enhances the effect of amphotericin B against Candida albicans in vitro

Amphotericin B (AmB) is a polyene antibiotic produced by Streptomyces nodosus and has been used for >50 years in the treatment of acute systemic fungal infections. In the present study, we demonstrated that lysine, an essential amino acid, could enhance the effect of AmB against Candida albicans in vitro, although lysine itself did not exert a fungicidal effect. In addition, the combination of AmB with lysine could provide an enhanced action against Candida parapsilosis and Cryptococcus neoformans compared with AmB alone. Lysine could also enhance the antifungal effect of caspofungin or nystatin. An enhanced effect of the combination of lysine with AmB was observed for the prevention of biofilm and hypha formation. Furthermore, our results demonstrated that lysine-mediated oxidative damage, such as the generation of endogenous reactive oxygen species, may be the mechanism underlying the enhancing effect of lysine on AmB. Our results also showed that CaMCA1 gene plays an important role in increasing the sensitivity of C. albicans cells upon AmB treatment. Using AmB together with lysine may be a promising strategy for the therapy of disseminated candidiasis.

Integrative Model of Oxidative Stress Adaptation in the Fungal Pathogen Candida albicans

The major fungal pathogen of humans, Candida albicans, mounts robust responses to oxidative stress that are critical for its virulence. These responses counteract the reactive oxygen species (ROS) that are generated by host immune cells in an attempt to kill the invading fungus. Knowledge of the dynamical processes that instigate C. albicans oxidative stress responses is required for a proper understanding of fungus-host interactions. Therefore, we have adopted an interdisciplinary approach to explore the dynamical responses of C. albicans to hydrogen peroxide (H₂O₂). Our deterministic mathematical model integrates two major oxidative stress signalling pathways (Cap1 and Hog1 pathways) with the three major antioxidant systems (catalase, glutathione and thioredoxin systems) and the pentose phosphate pathway, which provides reducing equivalents required for oxidative stress adaptation. The model encapsulates existing knowledge of these systems with new genomic, proteomic, transcriptomic, molecular and cellular datasets. Our integrative approach predicts the existence of alternative states for the key regulators Cap1 and Hog1, thereby suggesting novel regulatory behaviours during oxidative stress. The model reproduces both existing and new experimental observations under a variety of scenarios. Time- and dose-dependent predictions of the oxidative stress responses for both wild type and mutant cells have highlighted the different temporal contributions of the various antioxidant systems during oxidative stress adaptation, indicating that catalase plays a critical role immediately following stress imposition. This is the first model to encapsulate the dynamics of the transcriptional response alongside the redox kinetics of the major antioxidant systems during H₂O₂ stress in C. albicans.

N-Acetylglucosamine-Induced Cell Death in Candida albicans and Its Implications for Adaptive Mechanisms of Nutrient Sensing in Yeasts

Single-celled organisms have different strategies to sense and utilize nutrients in their ever-changing environments. The opportunistic fungal pathogen Candida albicans is a common member of the human microbiota, especially that of the gastrointestinal (GI) tract. An important question concerns how C. albicans gained a competitive advantage over other microbes to become a successful commensal and opportunistic pathogen. Here, we report that C. albicans uses N-acetylglucosamine (GlcNAc), an abundant carbon source present in the GI tract, as a signal for nutrient availability. When placed in water, C. albicans cells normally enter the G₀ phase and remain viable for weeks. However, they quickly lose viability when cultured in water containing only GlcNAc. We term this phenomenon GlcNAc-induced cell death (GICD). GlcNAc triggers the upregulation of ribosomal biogenesis genes, alterations of mitochondrial metabolism, and the accumulation of reactive oxygen species (ROS), followed by rapid cell death via both apoptotic and necrotic mechanisms. Multiple pathways, including the conserved cyclic AMP (cAMP) signaling and GlcNAc catabolic pathways, are involved in GICD. GlcNAc acts as a signaling molecule to regulate multiple cellular programs in a coordinated manner and therefore maximizes the efficiency of nutrient use. This adaptive behavior allows C. albicans’ more efficient colonization of the gut.

The ability to rapidly and appropriately respond to nutrients in the environment is crucial to free-living microorganisms. To maximize the use of available nutrients, microorganisms often use a limiting nutritional component as a signal to coordinate multiple biological processes. The human fungal pathogen Candida albicans uses N-acetylglucosamine (GlcNAc) as a signal for the availability of external nutrient resources. GlcNAc induces rapid cell death in C. albicans due to the constitutive activation of oxidative metabolism and accumulation of reactive oxygen species (ROS), and multiple pathways are involved in its regulation. This study sheds light on the mechanisms of niche specialization of pathogenic fungi and raises the possibility that this cell death pathway could be an unexplored therapeutic target.

In vitro antifungal activity of baicalin against Candida albicans biofilms via apoptotic induction

The aim of this study was to investigate the antifungal activity of baicalin and its potential mechanism of action against Candida albicans biofilms. The standard techniques including microdilution method and checkerboard assay were employed to evaluate the susceptibilities of baicalin alone and in combination with fluconazole against planktonic and biofilm cells of C. albicans. Transmission electron microscope (TEM), scanning electron microscope (SEM), fluorescent microscope and flow cytometry were used to assess the apoptotic incidences induced by baicalin in biofilm cells. The expressions of four genes (RAS1, CAP1, PDE2 and TPK1) related to Ras-cAMP-PKA pathway were also analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR). The results showed that minimum inhibitory concentration (MIC) and sessile minimum inhibitory concentration (SMIC50) of baicalin were 500 and 2000 μg/mL with fractional inhibitory concentration indexs (FICIs) ranging from 0.28 to 0.75. A series of events related to apoptosis were observed in baicalin-treated C. albicans biofilms, including extensive chromatin condensation along the nuclear envelope, ROS accumulation, MMP reduction, PS externalization, nuclear fragmentation, chromatin condensation, metacaspase activation and Cyt C release. Additionally, the expressions of RAS1 and TPK1 were up-regulated by 3.2 and 2.9 folds respectively, while those of CAP1 and PDE2 were down-regulated by 3.3 and 6.6 folds respectively after exposure to baicalin in biofilm cells. In conclusion, baicalin can suppress the development of C. albicans biofilms most likely due to inducing cell death via apoptosis.

Lignocellulose depolymerization occurs via an environmentally adapted metabolic cascades in the wood-rotting basidiomycete Phanerochaete chrysosporium

Plant biomass can be utilized by a lignocellulose-degrading fungus, Phanerochaete chrysosporium, but the metabolic and regulatory mechanisms involved are not well understood. A polyomics-based analysis (metabolomics, proteomics, and transcriptomics) of P. chrysosporium has been carried out using statistically optimized conditions for lignocellulolytic reaction. Thirty-nine metabolites and 123 genes (14 encoded proteins) that consistently exhibited altered regulation patterns were identified. These factors were then integrated into a comprehensive map that fully depicts all signaling cascades involved in P. chrysosporium. Despite the diversity of these cascades, they showed complementary interconnection among themselves, ensuring the efficiency of passive biosystem and thereby yielding energy expenditure for the cells. Particularly, many factors related to intracellular regulatory networks showed compensating activity in homeostatic lignocellulolysis. In the main platform of proactive biosystem, although several deconstruction-related targets (e.g., glycoside hydrolase, ureidoglycolate hydrolase, transporters, and peroxidases) were systematically utilized, well-known supporters (e.g., cellobiose dehydrogenase and ferroxidase) were rarely generated.