Hyphal development in Candida albicans requires two temporally linked changes in promoter chromatin for initiation and maintenance

Conserved signaling modules regulate filamentous growth in fungi: a model for eukaryotic cell differentiation

Eukaryotic organisms are composed of different cell types with defined shapes and functions. Specific cell types are produced by the process of cell differentiation, which is regulated by signal transduction pathways. Signaling pathways regulate cell differentiation by sensing cues and controlling the expression of target genes whose products generate cell types with specific attributes. In studying how cells differentiate, fungi have proved valuable models because of their ease of genetic manipulation and striking cell morphologies. Many fungal species undergo filamentous growth-a specialized growth pattern where cells produce elongated tube-like projections. Filamentous growth promotes expansion into new environments, including invasion into plant and animal hosts by fungal pathogens. The same signaling pathways that regulate filamentous growth in fungi also control cell differentiation throughout eukaryotes and include highly conserved mitogen-activated protein kinase (MAPK) pathways, which is the focus of this review. In many fungal species, mucin-type sensors regulate MAPK pathways to control filamentous growth in response to diverse stimuli. Once activated, MAPK pathways reorganize cell polarity, induce changes in cell adhesion, and promote the secretion of degradative enzymes that mediate access to new environments. However, MAPK pathway regulation is complicated because related pathways can share components with each other yet induce unique responses (i.e. signal specificity). In addition, MAPK pathways function in highly integrated networks with other regulatory pathways (i.e. signal integration). Here, we discuss signal specificity and integration in several yeast models (mainly Saccharomyces cerevisiae and Candida albicans) by focusing on the filamentation MAPK pathway. Because of the strong evolutionary ties between species, a deeper understanding of the regulation of filamentous growth in established models and increasingly diverse fungal species can reveal fundamentally new mechanisms underlying eukaryotic cell differentiation.

The spliceosome impacts morphogenesis in the human fungal pathogen Candida albicans

At human body temperature, the fungal pathogen Candida albicans can transition from yeast to filamentous morphologies in response to host-relevant cues. Additionally, elevated temperatures encountered during febrile episodes can independently induce C. albicans filamentation. However, the underlying genetic pathways governing this developmental transition in response to elevated temperatures remain largely unexplored. Here, we conducted a functional genomic screen to unravel the genetic mechanisms orchestrating C. albicans filamentation specifically in response to elevated temperature, implicating 45% of genes associated with the spliceosome or pre-mRNA splicing in this process. Employing RNA-Seq to elucidate the relationship between mRNA splicing and filamentation, we identified greater levels of intron retention in filaments compared to yeast, which correlated with reduced expression of the affected genes. Intriguingly, homozygous deletion of a gene encoding a spliceosome component important for filamentation (PRP19) caused even greater levels of intron retention compared with wild type and displayed globally dysregulated gene expression. This suggests that intron retention is a mechanism for fine-tuning gene expression during filamentation, with perturbations of the spliceosome exacerbating this process and blocking filamentation. Overall, this study unveils a novel biological process governing C. albicans filamentation, providing new insights into the complex regulation of this key virulence trait.IMPORTANCEFungal pathogens such as Candida albicans can cause serious infections with high mortality rates in immunocompromised individuals. When C. albicans is grown at temperatures encountered during human febrile episodes, yeast cells undergo a transition to filamentous cells, and this process is key to its virulence. Here, we expanded our understanding of how C. albicans undergoes filamentation in response to elevated temperature and identified many genes involved in mRNA splicing that positively regulate filamentation. Through transcriptome analyses, we found that intron retention is a mechanism for fine-tuning gene expression in filaments, and perturbation of the spliceosome exacerbates intron retention and alters gene expression substantially, causing a block in filamentation. This work adds to the growing body of knowledge on the role of introns in fungi and provides new insights into the cellular processes that regulate a key virulence trait in C. albicans.

Hyphal Als proteins act as CR3 ligands to promote immune responses against Candida albicans

Patients with decreased levels of CD18 (β2 integrins) suffer from life-threatening bacterial and fungal infections. CD11b, the α subunit of integrin CR3 (CD11b/CD18, αMβ2), is essential for mice to fight against systemic Candida albicans infections. Live elongating C. albicans activates CR3 in immune cells. However, the hyphal ligands that activate CR3 are not well defined. Here, we discovered that the C. albicans Als family proteins are recognized by the I domain of CD11b in macrophages. This recognition synergizes with the β-glucan-bound lectin-like domain to activate CR3, thereby promoting Syk signaling and inflammasome activation. Dectin-2 activation serves as the "outside-in signaling" for CR3 activation at the entry site of incompletely sealed phagosomes, where a thick cuff of F-actin forms to strengthen the local interaction. In vitro, CD18 partially contributes to IL-1β release from dendritic cells induced by purified hyphal Als3. In vivo, Als3 is vital for C. albicans clearance in mouse kidneys. These findings uncover a novel family of ligands for the CR3 I domain that promotes fungal clearance.

The short-chain fatty acid crotonate reduces invasive growth and immune escape of Candida albicans by regulating hyphal gene expression

IMPORTANCE

Macrophages curtail the proliferation of the pathogen Candida albicans within human body niches. Within macrophages, C. albicans adapts its metabolism and switches to invasive hyphal morphology. These adaptations enable fungal growth and immune escape by triggering macrophage lysis. Transcriptional programs regulate these metabolic and morphogenetic adaptations. Here we studied the roles of chromatin in these processes and implicate lysine crotonylation, a histone mark regulated by metabolism, in hyphal morphogenesis and macrophage interactions by C. albicans. We show that the short-chain fatty acid crotonate increases histone crotonylation, reduces hyphal formation within macrophages, and slows macrophage lysis and immune escape of C. albicans. Crotonate represses hyphal gene expression, and we propose that C. albicans uses diverse acylation marks to regulate its cell morphology in host environments. Hyphal formation is a virulence property of C. albicans. Therefore, a further importance of our study stems from identifying crotonate as a hyphal inhibitor.

Anti-virulence strategy of diaryl chalcogenide compounds against Candida albicans infection

Candida albicans is an important opportunistic pathogenic fungus that frequently causes serious systemic infection in humans. Due to the vital roles of biofilm formation and the yeast-to-hypha transition in the infection process, we have selected a series of diaryl chalcogenides and tested their efficacy against C. albicans SC5314 pathogenicity by the inhibition of biofilm formation and the yeast-to-hypha transition. The compounds 5-sulfenylindole and 5-selenylindole were found to have excellent abilities to inhibit both biofilm formation and hyphal formation in C. albicans SC5314. Intriguingly, the two leading compounds also markedly attenuated C. albicans SC5314 virulence in human cell lines and mouse infection models at micromolar levels. Furthermore, our results showed that the presence of the compounds at 100 µM resulted in a marked decrease in the expression of genes involved in the cAMP-PKA and MAPK pathways in C. albicans SC5314. Intriguingly, the compounds 5-sulfenylindole and 5-selenylindole not only attenuated the cytotoxicity of Candida species strains but also showed excellent synergistic effects with antifungal agents against the clinical drug-resistant C. albicans strain HCH12. The compound 5-sulfenylindole showed an obvious advantage over fluconazole as it could also restore the composition and richness of the intestinal microbiota in mice infected by C. albicans. Together, these results suggest that diaryl chalcogenides can potentially be designed as novel clinical therapeutic agents against C. albicans infection. The diaryl chalcogenides of 5-sulfenylindole and 5-selenylindole discovered in this study can provide new direction for developing antifungal agents against C. albicans infection.

Recent Advances in Understanding the Human Fungal Pathogen Hypoxia Response in Disease Progression

Fungal-mediated disease progression and antifungal drug efficacy are significantly impacted by the dynamic infection microenvironment. At the site of infection, oxygen often becomes limiting and induces a hypoxia response in both the fungal pathogen and host cells. The fungal hypoxia response impacts several important aspects of fungal biology that contribute to pathogenesis, virulence, antifungal drug susceptibility, and ultimately infection outcomes. In this review, we summarize recent advances in understanding the molecular mechanisms of the hypoxia response in the most common human fungal pathogens, discuss potential therapeutic opportunities, and highlight important areas for future research.

Molecular Docking Reveals Critical Residues in Candida albicans Cyr1 for Peptidoglycan Recognition and Hyphal Growth

The key virulent characteristic of Candida albicans, the major fungal pathogen in humans, lies in its ability to switch between the benign yeast state and the invasive hyphal form upon exposure to specific stimuli. Among the numerous hyphal-inducing signals, bacterial peptidoglycan fragments (PGNs) represent the most potent inducers of C. albicans hyphal growth. The sole adenylyl cyclase Cyr1 in C. albicans is a known sensor for PGNs and activates downstream signaling of hyphal growth, yet the molecular details of PGN-Cyr1 interactions have remained unclear. In this study, we performed in silico docking of a PGN motif to the modeled structure of the Cyr1 leucine-rich repeat (LRR) domain and uncovered four putative PGN-interacting residues in Cyr1_LRR. The critical roles of these residues in PGN binding and supporting C. albicans hyphal growth were demonstrated by in-gel fluorescence binding assay and hyphal induction assay, respectively. Remarkably, the C. albicans mutant harboring the cyr1 variant allele that is defective for PGN recognition exhibits significantly reduced cytotoxicity in macrophage infection assay. Overall, our work offered important insights into the molecular recognition of PGNs by C. albicans Cyr1 sensor protein, establishing that disruption of PGN recognition by Cyr1 results in defective hyphal growth and reduced virulence of C. albicans. Our findings provide an exciting starting point for the future development of Cyr1 antagonists as novel anti-virulence therapeutics to combat C. albicans invasive growth and infection.

Apoptotic Factors, CaNma111 and CaYbh3, Function in Candida albicans Filamentation by Regulating the Hyphal Suppressors, Nrg1 and Tup1

The morphological switch from the yeast to hyphal form is a key virulence attribute of the opportunistic fungal pathogen, Candida albicans. Our recent report showed that deletion of the newly identified apoptotic factor, CaNma111 or CaYbh3, leads to hyperfilamentation and increased virulence in a mouse infection model. CaNma111 and CaYbh3 are homologs of the pro-apoptotic protease, HtrA2/Omi, and BH3-only protein, respectively. In this study, we examined the effects of CaNMA111 and CaYBH3 deletion mutations on the expression levels of the hypha-specific transcription factors, Cph1 (a hyphal activator), Nrg1 (a hyphal repressor), and Tup1 (a hyphal repressor). The protein levels of Nrg1 were decreased in Caybh3/Caybh3 cells while those of Tup1 were decreased in both Canma111/Canma111 and Caybh3/Caybh3 cells. These effects on Nrg1 and Tup1 proteins were retained during serum-induced filamentation and appear to explain the hyperfilamentation phenotypes of the CaNMA111 and CaYBH3 deletion mutants. Treatment with the apoptosis-inducing dose of farnesol decreased the Nrg1 protein levels in the wild-type strain and more evidently in Canma111/Canma111 and Caybh3/Caybh3 mutant strains. Together, our results suggest that CaNma111 and CaYbh3 are key regulators of Nrg1 and Tup1 protein levels in C. albicans.

Negative regulation of biofilm development by the CUG-Ser1 clade-specific histone H3 variant is dependent on the canonical histone chaperone CAF-1 complex in Candida albicans

The CUG-Ser1 clade-specific histone H3 variant (H3V^CTG ) has been reported to be a negative regulator of planktonic to biofilm growth transition in Candida albicans. The preferential binding of H3V^CTG at the biofilm gene promoters makes chromatin repressive for the biofilm mode of growth. The two evolutionarily conserved chaperone complexes involved in incorporating histone H3 are CAF-1 and HIRA. In this study, we sought to identify the chaperone complex(es) involved in loading H3V^CTG . We demonstrate that C. albicans cells lacking either Cac1 or Cac2 subunit of the CAF-1 chaperone complex, exhibit a hyper-filamentation phenotype on solid surfaces and form more robust biofilms than wild-type cells, thereby mimicking the phenotype of the H3V^CTG null mutant. None of the subunits of the HIRA chaperone complex shows any significant difference in biofilm growth as compared to the wild type. The occupancy of H3V^CTG is found to be significantly reduced at the promoters of biofilm genes in the absence of CAF-1 subunits. Hence, we provide evidence that CAF-1, a chaperone known to load canonical histone H3 in mammalian cells, is involved in chaperoning of variant histone H3V^CTG at the biofilm gene promoters in C. albicans. Our findings also illustrate the acquisition of an unconventional role of the CAF-1 chaperone complex in morphogenesis in C. albicans.

Recognition of galactose by a scaffold protein recruits a transcriptional activator for the GAL regulon induction in Candida albicans

The GAL pathway of yeasts has long served as a model system for understanding of how regulatory mode of eukaryotic metabolic pathways evolves. While Gal4 mode has been well-characterized in Saccharomycetaceae clade, little is known about the regulation of the GAL pathway in other yeasts. Here, we find that Rep1, a Ndt80-like family transcription factor, serves as a galactose sensor in the commensal-pathogenic fungus Candida albicans. It is presented at the GAL gene promoters independent of the presence of galactose. Rep1 recognizes galactose via a direct physical interaction. The net result of this interaction is the recruitment of a transcriptional activator Cga1 (Candida galactose gene activator, orf19.4959) and transcription of the GAL genes proceeds. Rep1 and Cga1 are conserved across the CTG species. Rep1 itself does not possess transcriptional activity. Instead, it provides a scaffold to recruit different factors for transcriptional regulation. Rep1-Cga1 mode of regulation represents a new example of network rewiring in fungi, which provides insight into how C. albicans evolves transcriptional programs to colonize diverse host niches.

Candida albicans-enteric viral interactions-The prostaglandin E2 connection and host immune responses

The human microbiome comprises trillions of microorganisms residing within different mucosal cavities and across the body surface. The gut microbiota modulates host susceptibility to viral infections in several ways, and microbial interkingdom interactions increase viral infectivity within the gut. Candida albicans, a frequently encountered fungal species in the gut, produces highly structured biofilms and eicosanoids such as prostaglandin E₂ (PGE₂), which aid in viral protection and replication. These biofilms encompass viruses and provide a shield from antiviral drugs or the immune system. PGE₂ is a key modulator of active inflammation with the potential to regulate interferon signaling upon microbial invasion or viral infections. In this review, we raise the perspective of gut interkingdom interactions involving C. albicans and enteric viruses, with a special focus on biofilms, PGE₂, and viral replication. Ultimately, we discuss the possible implications of C. albicans-enteric virus associations on host immune responses, particularly the interferon signaling pathway.

Development and in vitro characterization of a humanized scFv against fungal infections

The resistance and the birth of new intrinsic and multidrug-resistant pathogenic species like C. auris is creating great concern in the antifungal world. Given the limited drug arsenal and the lack of effectiveness of the available compounds, there is an urgent need for innovative approaches. The murine mAb 2G8 was humanized and engineered in silico to develop a single-chain fragment variable (hscFv) antibody against β-1,3-glucans which was then expressed in E. coli. Among the recombinant proteins developed, a soluble candidate with high stability and affinity was obtained. This selected protein is VL-linker-VH oriented, and it is characterized by the presence of two ubiquitin monomers at the N-terminus and a His tag at the C-terminus. This construct, Ub2-hscFv-His, guaranteed stability, solubility, efficient purification and satisfactory recovery of the recombinant product. HscFv can bind β-1,3-glucans both as coated antigens and on C. auris and C. albicans cells similarly to its murine parental and showed long stability and retention of binding ability when stored at 4°, -20° and -80° C. Furthermore, it was efficient in enhancing the antifungal activity of drugs caspofungin and amphotericin B against C. auris. The use of biological drugs as antifungals is limited; here we present a promising hscFv which has the potential to be useful in combination with currently available antifungal drugs.

Identification and functional characterization of ORF19.5274, a novel gene involved in both azoles susceptibility and hypha development in Candida albicans

Azole resistance is becoming increasingly serious due to the frequent recurrence of fungal infections and the need for long-term clinical prevention. In our previous study, we discovered ORF19.5274 with an unknown function by TMT^™ quantitative proteomics technology after fluconazole (FLC) treatment of Candida albicans. In this study, we created the target gene deletion strain using CRISPR-Cas9 editing technology to see if ORF19.5274 regulates azole sensitivity. The data showed that ORF19.5274 was involved in hyphal development and susceptibility to antifungal azoles. Deleting this gene resulted in defective hyphal growth in solid medium, while only a weak lag in the initiation of hyphal development and restoring hyphal growth during the hyphal maintenance phase under liquid conditions. Moreover, intracellular reactive oxygen species (ROS) assay and propidium iodide staining assays showed increased endogenous ROS levels and membrane permeability, but decreased metabolic activity of biofilm in orf19.5274Δ/Δ after treatment with FLC in comparison with either SC5314 or orf19.5274Δ/Δ::ORF19.5274 strains. More importantly, orf19.5274Δ/Δ significantly enhanced the FLC efficacy against C. albicans in infected Galleria mellonella larvae. The above characteristics were fully or partially restored in the complemented strain indicating that the changes caused by ORF19.5274 deletion were specific. In summary, the ORF19.5274 gene is required for hyphal development of C. albicans, and is correlated with the response to antifungal azoles in vitro and in vivo. The identification of ORF19.5275 is promising to expand the potential candidate targets for azoles.

EFG1, Everyone’s Favorite Gene in Candida albicans: A Comprehensive Literature Review

Candida sp. are among the most common fungal commensals found in the human microbiome. Although Candida can be found residing harmlessly on the surface of the skin and mucosal membranes, these opportunistic fungi have the potential to cause superficial skin, nail, and mucus membrane infections as well as life threatening systemic infections. Severity of infection is dependent on both fungal and host factors including the immune status of the host. Virulence factors associated with Candida sp. pathogenicity include adhesin proteins, degradative enzymes, phenotypic switching, and morphogenesis. A central transcriptional regulator of morphogenesis, the transcription factor Efg1 was first characterized in Candida albicans in 1997. Since then, EFG1 has been referenced in the Candida literature over three thousand times, with the number of citations growing daily. Arguably one of the most well studied genes in Candida albicans, EFG1 has been referenced in nearly all contexts of Candida biology from the development of novel therapeutics to white opaque switching, hyphae morphology to immunology. In the review that follows we will synthesize the research that has been performed on this extensively studied transcription factor and highlight several important unanswered questions.

The zinc cluster transcription factor Rha1 is a positive filamentation regulator in Candida albicans

Zinc cluster transcription factors (TFs) are essential fungal regulators of gene expression. In the pathogen Candida albicans, the gene orf19.1604 encodes a zinc cluster TF regulating filament development. Hyperactivation of orf19.1604, which we have named RHA1 for Regulator of Hyphal Activity, generates wrinkled colony morphology under nonhyphal growth conditions, triggers filament formation, invasiveness, and enhanced biofilm formation and causes reduced virulence in the mouse model of systemic infection. The strain expressing activated Rha1 shows up-regulation of genes required for filamentation and cell-wall-adhesion-related proteins. Increased expression is also seen for the hyphal-inducing TFs Brg1 and Ume6, while the hyphal repressor Nrg1 is downregulated. Inactivation of RHA1 reduces filamentation under a variety of filament-inducing conditions. In contrast to the partial effect of either single mutant, the double rha1 ume6 mutant strain is highly defective in both serum- and Spider-medium-stimulated hyphal development. While the loss of Brg1 function blocks serum-stimulated hyphal development, this block can be significantly bypassed by Rha1 hyperactivity, and the combination of Rha1 hyperactivity and serum addition can generate significant polarization even in brg1 ume6 double mutants. Thus, in response to external signals, Rha1 functions with other morphogenesis regulators including Brg1 and Ume6, to mediate filamentation.

The Candida albicans Cdk8-dependent phosphoproteome reveals repression of hyphal growth through a Flo8-dependent pathway

Ssn3, also known as Cdk8, is a member of the four protein Cdk8 submodule within the multi-subunit Mediator complex involved in the co-regulation of transcription. In Candida albicans, the loss of Ssn3 kinase activity affects multiple phenotypes including cellular morphology, metabolism, nutrient acquisition, immune cell interactions, and drug resistance. In these studies, we generated a strain in which Ssn3 was replaced with a functional variant of Ssn3 that can be rapidly and selectively inhibited by the ATP analog 3-MB-PP1. Consistent with ssn3 null mutant and kinase dead phenotypes, inhibition of Ssn3 kinase activity promoted hypha formation. Furthermore, the increased expression of hypha-specific genes was the strongest transcriptional signal upon inhibition of Ssn3 in transcriptomics analyses. Rapid inactivation of Ssn3 was used for phosphoproteomic studies performed to identify Ssn3 kinase substrates associated with filamentation potential. Both previously validated and novel Ssn3 targets were identified. Protein phosphorylation sites that were reduced specifically upon Ssn3 inhibition included two sites in Flo8 which is a transcription factor known to positively regulate C. albicans morphology. Mutation of the two Flo8 phosphosites (threonine 589 and serine 620) was sufficient to increase Flo8-HA levels and Flo8 dependent transcriptional and morphological changes, suggesting that Ssn3 kinase activity negatively regulates Flo8.Under embedded conditions, when ssn3Δ/Δ and efg1Δ/Δ mutants were hyperfilamentous, FLO8 was essential for hypha formation. Previous work has also shown that loss of Ssn3 activity leads to increased alkalinization of medium with amino acids. Here, we show that the ssn3Δ/Δ medium alkalinization phenotype, which is dependent on STP2, a transcription factor involved in amino acid utilization, also requires FLO8 and EFG1. Together, these data show that Ssn3 activity can modulate Flo8 and its direct and indirect interactions in different ways, and underscores the potential importance of considering Ssn3 function in the control of transcription factor activities.

In Candida albicans, Ssn3 kinase activity co-regulates the transcription of numerous genes involved in hyphal growth, metabolism and nutrient acquisition, immune cell interactions, and drug resistance. Using a strain in which Ssn3 could be rapidly and selectively inhibited, we identified both known and novel Ssn3 targets. We identified two phosphosites in Flo8, a regulator of morphology and virulence, that were shown to negatively regulate Flo8 levels and activity. The data and tools presented here will enable a better understanding of how Ssn3 impacts transcriptional and post-transcriptional regulation in order to coordinate processes during physiological and morphological transitions as well as during steady state growth.

Inhibition of morphological transition and hyphae extension in Candida spp. by occidiofungin

AIMS

To assess the efficacy of the antifungal, occidiofungin, against Candida albicans and Candida tropicalis morphological transformation.

METHODS AND RESULTS

Susceptibility assays and morphological data were used to demonstrate that occidiofungin effectively targets C. albicans and C. tropicalis undergoing morphological transformation. Susceptibility assays found that cell sensitivity to occidiofungin varied with the media conditions used for morphological switching. Microscopy data showed that occidiofungin inhibited hyphae formation when added at the time of morphological induction and hyphal extension when added within the first hour following hyphae induction. Immunoblot analysis demonstrated that occidiofungin addition prevented activation of Cek1p MAPK signalling.

CONCLUSIONS

The data indicated that the antimicrobial compound, occidiofungin, effectively targets hyphae elongation in Candida spp. and suggests the biological target of occidiofungin is necessary for the morphological changes associated with yeast-to-hyphae switching.

SIGNIFICANCE AND IMPACT OF THE STUDY

Findings from this study demonstrated that occidiofungin effectively targets the invasive growth of dimorphic Candida which suggests this compound may also inhibit the heterogenous population of cells present in a clinical setting. This presents occidiofungin as a promising candidate for the treatment of Candida associated infections.

On and Off: Epigenetic Regulation of C. albicans Morphological Switches

The human fungal pathogen Candida albicans is a dimorphic opportunistic pathogen that colonises most of the human population without creating any harm. However, this fungus can also cause life-threatening infections in immunocompromised individuals. The ability to successfully colonise different host niches is critical for establishing infections and pathogenesis. C. albicans can live and divide in various morphological forms critical for its survival in the host. Indeed, C. albicans can grow as both yeast and hyphae and can form biofilms containing hyphae. The transcriptional regulatory network governing the switching between these different forms is complex but well understood. In contrast, non-DNA based epigenetic modulation is emerging as a crucial but still poorly studied regulatory mechanism of morphological transition. This review explores our current understanding of chromatin-mediated epigenetic regulation of the yeast to hyphae switch and biofilm formation. We highlight how modification of chromatin structure and non-coding RNAs contribute to these morphological transitions.

The transcription factor Cas5 suppresses hyphal morphogenesis during yeast-form growth in Candida albicans

Candida albicans is an opportunistic human pathogen that exists as yeast, hyphal or pseudohyphal forms depending on pH, nutrients, and temperature. The morphological transition from yeast to hyphae, which is required for the complete virulence of C. albicans, is controlled by many transcription factors that activate or repress hypha-specific genes. The C. albicans transcriptional factor Cas5, a key regulator of genes involved in cell wall integrity, affects the susceptibility of C. albicans to fluconazole, an inhibitor of ergosterol synthesis. In this study, we found that deletion of CAS5 in C. albicans decreased the expression levels of a set of ergosterol biosynthesis genes, such as ERG2, ERG3, ERG5, ERG6, ERG11, and ERG24, resulting in the accumulation of lanosterol and zymosterol, which are intermediate metabolites in the ergosterol biosynthesis pathway. Interestingly, it was observed that the cas5Δ/Δ mutant could not maintain the yeast form under non-hypha-inducing conditions, while the CAS5-overexpressing cells could not form hyphae under hypha-inducing conditions. Consistent with these observations, the cas5Δ/Δ mutant highly expressed hypha-specific genes, ALS3, ECE1, and HWP1, under non-hypha-inducing conditions. In addition, CAS5 transcription was significantly downregulated immediately after hyphal initiation in the wild-type strain. Furthermore, the cas5Δ/Δ mutant reduced the transcription of NRG1, which encodes a major repressor of hyphal morphogenesis, while Cas5 overexpression increased the transcription of NRG1 under hypha-inducing conditions. Collectively, this study suggests the potential role of Cas5 as a repressor of hypha-specific genes during yeast-form growth of C. albicans.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Metabolic flexibility and extensive adaptability governing multiple drug resistance and enhanced virulence in Candida albicans

Commensal fungus-Candida albicans turn pathogenic during the compromised immunity of the host, causing infections ranging from superficial mucosal to dreadful systemic ones. C. albicans has evolved various adaptive measures which collectively contribute towards its enhanced virulence. Among fitness attributes, metabolic flexibility and vigorous stress response are essential for its pathogenicity and virulence. Metabolic flexibility provides a means for nutrient assimilation and growth in diverse host microenvironments and reduces the vulnerability of the pathogen to various antifungals besides evading host immune response(s). Inside the host micro-environments, C. albicans efficiently utilizes the multiple fermentable and non-fermentable carbon sources to sustain and proliferate in glucose deficit conditions. The utilization of alternative carbon sources further highlights the importance of understanding these pathways as the attractive and potential therapeutic target. A thorough understanding of metabolic flexibility and adaptation to environmental stresses is warranted to decipher in-depth insights into virulence and molecular mechanisms of fungal pathogenicity. In this review, we have attempted to provide a detailed and recent understanding of some key aspects of fungal biology. Particular focus will be placed on processes like nutrient assimilation and utilization, metabolic adaptability, virulence factors, and host immune response in C. albicans leading to its enhanced pathogenicity.

Genetic Analysis of Sirtuin Deacetylases in Hyphal Growth of Candida albicans

Candida albicans is a major human fungal pathogen that encounters varied host environments during infection. In response to environmental cues, C. albicans switches between ovoid yeast and elongated hyphal growth forms, and this morphological plasticity contributes to virulence. Environmental changes that alter the cell's metabolic state could be sensed by sirtuins, which are NAD⁺-dependent deacetylases. Here, we studied the roles of three sirtuin deacetylases-Sir2, Hst1, and Hst2-in the hyphal growth of C. albicans We made single, double, and triple sirtuin knockout strains and tested their ability to switch from yeast to hyphae. We found that true hypha formation was significantly reduced by the deletion of SIR2 but not HST1 or HST2 Moreover, the expression of hypha-specific genes HWP1, ALS3, and ECE1 decreased in the sir2Δ/Δ mutant compared to the wild type. This regulation of hypha formation was likely dependent on the deacetylase activity of Sir2, as a similar defect in hypha formation was observed when an asparagine known to be required for deacetylation was mutated. Finally, we found that Sir2 and Hst1 were localized to the nucleus, with Sir2 specifically focused in the nucleolus. This nuclear localization suggests a role for Sir2 and Hst1 in regulating gene expression. In contrast, Hst2 was localized to the cytoplasm. In conclusion, our results suggest that Sir2 plays a critical and nonredundant role in hyphal growth of C. albicans IMPORTANCE Candida albicans is one of the most common causes of hospital-acquired systemic fungal infections in the United States. It can switch between ovoid yeast and elongated hyphal growth forms in response to environmental cues. This morphological transition is essential for its survival in the host. Thus, identifying regulators involved in this process can lead to new therapies. In this study, we examined the contribution of three regulators called sirtuins (Sir2, Hst1, and Hst2) to the yeast-to-hypha transition of C. albicans We found that loss of Sir2 but not Hst1 or Hst2 hampered hypha formation. Moreover, the defect was caused by the loss of the catalytic activity of Sir2. Our study may lay the groundwork for discovering novel targets for antifungal therapies.

Transcriptional control of hyphal morphogenesis in Candida albicans

Candida albicans is a multimorphic commensal organism and opportunistic fungal pathogen in humans. A morphological switch between unicellular budding yeast and multicellular filamentous hyphal growth forms plays a vital role in the virulence of C. albicans, and this transition is regulated in response to a range of environmental cues that are encountered in distinct host niches. Many unique transcription factors contribute to the transcriptional regulatory network that integrates these distinct environmental cues and determines which phenotypic state will be expressed. These hyphal morphogenesis regulators have been extensively investigated, and represent an increasingly important focus of study, due to their central role in controlling a key C. albicans virulence attribute. This review provides a succinct summary of the transcriptional regulatory factors and environmental signals that control hyphal morphogenesis in C. albicans.

Role of the high-affinity reductive iron acquisition pathway of Candida albicans in prostaglandin E2 production, virulence, and interaction with Pseudomonas aeruginosa

Components of the iron reductive pathway of Candida albicans have been implicated in the production of prostaglandin E2 (PGE2) and virulence. However, it is unknown whether other components of this pathway influence PGE2. We investigated the role of the iron reductive pathway of C. albicans in biofilm formation, PGE2 production, and virulence in Caenorhabditis elegans. Additionally, as the co-occurrence of C. albicans and Pseudomonas aeruginosa in host tissues is frequent and involves competition for host-associated iron, we examined the effects of this interaction. Deletion of multicopper oxidase gene, FET99, and iron permease genes, FTH1 and FTH2, affected biofilm metabolic activity, and for the FTH2 mutant, also biofilm morphology. Deletion of CCC1 (vacuolar iron transporter) and CCC2 (P-type ATPase copper importer) also influenced biofilm morphology. For PGE2 production, deletion of FET99, FTH1, FTH2, CCC1, and CCC2 caused a significant reduction by monomicrobial biofilms, while FTH2deletion caused the highest reduction in polymicrobial biofilms. URA3 positive mutants of FET99 and FTH2 demonstrated attenuated virulence in C. elegans, potentially due to the inability of mutants to form hyphae in vivo. Deductively, the role of the iron reductive pathway in PGE2 synthesis is indirect, possibly due to their role in iron homeostasis.

LAY SUMMARY

Iron uptake is vital for disease-causing microbes like Candida albicans. Using strains deficient in some iron-uptake genes, we show that iron-uptake genes, especially FET99 and FTH2, play a role in biofilm formation, prostaglandin production, and virulence in the nematode infection model.

Mathematical modeling of the Candida albicans yeast to hyphal transition reveals novel control strategies

Candida albicans, an opportunistic fungal pathogen, is a significant cause of human infections, particularly in immunocompromised individuals. Phenotypic plasticity between two morphological phenotypes, yeast and hyphae, is a key mechanism by which C. albicans can thrive in many microenvironments and cause disease in the host. Understanding the decision points and key driver genes controlling this important transition and how these genes respond to different environmental signals is critical to understanding how C. albicans causes infections in the host. Here we build and analyze a Boolean dynamical model of the C. albicans yeast to hyphal transition, integrating multiple environmental factors and regulatory mechanisms. We validate the model by a systematic comparison to prior experiments, which led to agreement in 17 out of 22 cases. The discrepancies motivate alternative hypotheses that are testable by follow-up experiments. Analysis of this model revealed two time-constrained windows of opportunity that must be met for the complete transition from the yeast to hyphal phenotype, as well as control strategies that can robustly prevent this transition. We experimentally validate two of these control predictions in C. albicans strains lacking the transcription factor UME6 and the histone deacetylase HDA1, respectively. This model will serve as a strong base from which to develop a systems biology understanding of C. albicans morphogenesis.

Transcriptomic and Metabolomic Analysis Revealed Roles of Yck2 in Carbon Metabolism and Morphogenesis of Candida albicans

Candida albicans is a part of the normal microbiome of human mucosa and is able to thrive in a wide range of host environments. As an opportunistic pathogen, the virulence of C. albicans is tied to its ability to switch between yeast and hyphal morphologies in response to various environmental cues, one of which includes nutrient availability. Thus, metabolic flexibility plays an important role in the virulence of the pathogen. Our previous study has shown that C. albicans Yeast Casein Kinase 2 (CaYck2) regulates the yeast-to-hyphal switch, but its regulatory mechanisms remain unknown. This study further elucidated the role of Yck2 in governing morphology and carbon metabolism by analyzing the transcriptome and metabolome of the C. albicans YCK2 deletion mutant strain (yck2Δ strain) in comparison to the wild type strain. Our study revealed that loss of CaYck2 perturbs carbon metabolism, leading to a transcriptional response that resembles a transcriptional response to glucose starvation with coinciding intracellular accumulation of glucose and depletion of TCA cycle metabolites. This shift in the metabolome is likely mediated by derepression of glucose-repressed genes in the Mig1/2-mediated glucose sensing pathway and by downregulation of glycolytic genes, possibly through the Rgt1-mediated SRR pathway. In addition, genes involved in beta-oxidation, glyoxylate cycle, oxidative stress response, and arginine biosynthesis were upregulated in the yck2Δ strain, which is highly reminiscent of C. albicans engulfment by macrophages. This coincides with an increase in arginine degradation intermediates in the yck2Δ strain, suggesting arginine catabolism as a potential mechanism of CaYck2-mediated filamentation as seen during C. albicans escape from macrophages. Transcriptome analysis also shows differential expression of hyphal transcriptional regulators Nrg1 and Ume6. This suggests dysregulation of hyphal initiation and elongation in the yck2Δ strain which may lead to the constitutive pseudohyphal phenotype of this strain. Metabolome analysis also detected a high abundance of methyl citrate cycle intermediates in the yck2Δ strain, suggesting the importance of CaYck2 in this pathway. Taken together, we discovered that CaYck2 is an integral piece of carbon metabolism and morphogenesis of C. albicans.

Acid pH Strategy Adaptation through NRG1 in Ustilago maydis

The role of the Ustilago maydis putative homolog of the transcriptional repressor ScNRG1, previously described in Saccharomyces cerevisiae, Candida albicans and Cryptococcus neoformans, was analyzed by means of its mutation. In S. cerevisiae this gene regulates a set of stress-responsive genes, and in C. neoformans it is involved in pathogenesis. It was observed that the U. maydisNRG1 gene regulates several aspects of the cell response to acid pH, such as the production of mannosyl-erythritol lipids, inhibition of the expression of the siderophore cluster genes, filamentous growth, virulence and oxidative stress. A comparison of the gene expression pattern of the wild type strain versus the nrg1 mutant strain of the fungus, through RNA Seq analyses, showed that this transcriptional factor alters the expression of 368 genes when growing at acid pH (205 up-regulated, 163 down-regulated). The most relevant genes affected by NRG1 were those previously reported as the key ones for particular cellular stress responses, such as HOG1 for osmotic stress and RIM101 for alkaline pH. Four of the seven genes included WCO1 codifying PAS domain ( These has been shown as the key structural motif involved in protein-protein interactions of the circadian clock, and it is also a common motif found in signaling proteins, where it functions as a signaling sensor) domains sensors of blue light, two of the three previously reported to encode opsins, one vacuolar and non-pH-responsive, and another one whose role in the acid pH response was already known. It appears that all these light-reactive cell components are possibly involved in membrane potential equilibrium and as virulence sensors. Among previously described specific functions of this transcriptional regulator, it was found to be involved in glucose repression, metabolic adaptation to adverse conditions, cellular transport, cell rescue, defense and interaction with an acidic pH environment.

N-acetylglucosamine Signaling: Transcriptional Dynamics of a Novel Sugar Sensing Cascade in a Model Pathogenic Yeast, Candida albicans

The amino sugar, N-acetylglucosamine (GlcNAc), has emerged as an attractive messenger of signaling in the pathogenic yeast Candida albicans, given its multifaceted role in cellular processes, including GlcNAc scavenging, import and metabolism, morphogenesis (yeast to hyphae and white to opaque switch), virulence, GlcNAc induced cell death (GICD), etc. During signaling, the exogenous GlcNAc appears to adopt a simple mechanism of gene regulation by directly activating Ngs1, a novel GlcNAc sensor and transducer, at the chromatin level, to activate transcriptional response through the promoter acetylation. Ngs1 acts as a master regulator in GlcNAc signaling by regulating GlcNAc catabolic gene expression and filamentation. Ndt80-family transcriptional factor Rep1 appears to be involved in the recruitment of Ngs1 to GlcNAc catabolic gene promoters. For promoting filamentation, GlcNAc adopts a little modified strategy by utilizing a recently evolved transcriptional loop. Here, Biofilm regulator Brg1 takes up the key role, getting up-regulated by Ngs1, and simultaneously induces Hyphal Specific Genes (HSGs) expression by down-regulating NRG1 expression. GlcNAc kinase Hxk1 appears to play a prominent role in signaling. Recent developments in GlcNAc signaling have made C. albicans a model system to understand its role in other eukaryotes as well. The knowledge thus gained would assist in designing therapeutic interventions for the control of candidiasis and other fungal diseases.

The Roles of Chromatin Accessibility in Regulating the Candida albicans White-Opaque Phenotypic Switch

Candida albicans, a diploid polymorphic fungus, has evolved a unique heritable epigenetic program that enables reversible phenotypic switching between two cell types, referred to as "white" and "opaque". These cell types are established and maintained by distinct transcriptional programs that lead to differences in metabolic preferences, mating competencies, cellular morphologies, responses to environmental signals, interactions with the host innate immune system, and expression of approximately 20% of genes in the genome. Transcription factors (defined as sequence specific DNA-binding proteins) that regulate the establishment and heritable maintenance of the white and opaque cell types have been a primary focus of investigation in the field; however, other factors that impact chromatin accessibility, such as histone modifying enzymes, chromatin remodelers, and histone chaperone complexes, also modulate the dynamics of the white-opaque switch and have been much less studied to date. Overall, the white-opaque switch represents an attractive and relatively "simple" model system for understanding the logic and regulatory mechanisms by which heritable cell fate decisions are determined in higher eukaryotes. Here we review recent discoveries on the roles of chromatin accessibility in regulating the C. albicans white-opaque phenotypic switch.

Inhibiting roles of farnesol and HOG in morphological switching of Candida albicans

Candida albicans is a major opportunistic fungal pathogen of humans, especially in the oral cavity it involves in precancerous lesions. Numerous transcriptional regulators and hypha-specific genes involved in the morphogenesis mechanisms have been identified. Its virulence is predominantly attributed to the potentiality of morphological switching from yeast and pseudohyphae to hyphal growth. Giving attention in farnesol for prevention or intervention of its virulence sense and possible etiologic role in some uncovered premalignant diseases, in addition, to be a quorum-sensing signal molecule and relationship with HOG pathway, although its morphological switching inhibiting function has attracted high attention and got great progress in being elucidated, their exact mode of action is not completely understood. This report provides a review of characteristic aspects of farnesol signaling and HOG pathway during hyphal development. It also includes other associated pathways, molecules, and novel drug development based on the latest researches over the last decade. Furthermore, farnesol as immunomodulatory to host is an important inferring.

Changes in transcriptomic and metabolomic profiles of morphotypes of Ophiocordyceps sinensis within the hemocoel of its host larvae, Thitarodes xiaojinensis

Background

Ophiocordyceps sinensis (Berk.) is a well-known entomopathogenic and medicinal fungus. It parasitizes and mummifies the underground ghost moth larvae to produce a fruiting body named Chinese cordyceps. Specific for the fungus, O. sinensis experiences a biotrophic vegetative growth period spanning over 5 months. During this vegetative growth, it appears successively in the host hemocoel in three/four morphotypes, namely, the yeast-like blastospores (subdivided into proliferative (BP) and stationary phase (BS)), prehyphae (PreHy) and the hyphae (Hy). This peculiar morphogenesis has been elucidated through morphological and ultrastructural observations, but its molecular basis remains cryptic. In this study, transcriptome and metabolome profiling of BP, BS, PreHy and Hy stages were performed to characterize the key genes, metabolites, and signaling pathways that regulated the vegetative development of O. sinensis in Thitarodes xiaojinensis larva.

Results

The molecular events and metabolic pathways that regulated different intracellular processes at various stages were examined. Cluster analyses of differentially expressed genes across the four stages revealed the stage specifically enriched pathways. Analysis of metabolome profiles showed that carbon metabolism and several amino acids biosynthesis were significantly perturbed during the tested development stages of O. sinensis in the host hemocoel. Genes homologous to Saccharomyces cerevisiae MAPK cascade were significantly up-regulated during the transition from blastospore to hypha. The up-regulation of Sho1, a regulator protein, suggested nutrient starvation act a role in activation of MAPK pathway and filamentous growth. In addition, up-regulation of several fatty acid synthesis genes and their corresponding products accumulation in the samples of BS might explain more lipid droplets were observed in BS than in BP. Coupled with the up-regulation of fatty acid degradation during PreHy and Hy stages, it is presumed that lipid accumulation and mobilization play important roles in filamentous development.

Conclusions

This is the first report comprehensively describing developmental transcriptomics and metabolomics of O. sinensis in vivo. Our findings provide new perspectives into the key pathways and hub genes involved in morphological changes of fungus developed in the hemocoel of its host, and are expected to guide future studies on morphogenesis and morphotype changes of entomopathogenic fungi in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07209-2.

Bre1 and Ubp8 regulate H2B mono-ubiquitination and the reversible yeast-hyphae transition in Candida albicans

The reversible yeast-hyphae transition of the human fungal pathogen Candida albicans is tightly linked to its pathogenicity. In this study, we show that histone H2B mono-ubiquitination (H2Bub) at lysine 123 was maintained at a low level in the yeast state, whereas it increased significantly during yeast-to-hyphae transition and decreased when hyphae converted to yeast. The increased H2Bub level is correlated with activation of the hyphal program. H2B ubiquitination and deubiquitination are dynamically regulated by the E3 ligase Bre1 and the deubiquitinase Ubp8 during the reversible yeast-hyphae transition. The functions of Bre1 and Ubp8 in hypha-specific gene (HSG) regulation appears to be direct because both are recruited to the coding regions of HSGs during hyphal induction. The sequential recruitment of Bre1 and Ubp8 to HSGs coding regions is important for the initiation and maintenance of HSG expression. Additionally, Ubp8 contributes to the pathogenicity of C. albicans during early infection in a mouse model. Our study is the first to link H2B ubiquitination to the morphological plasticity and pathogenicity of the human fungal pathogen C. albicans and shed light on potential antifungal treatments.

Anti-biofilm activity of chlorhexidine digluconate against Candida albicans vaginal isolates

OBJECTIVES

Recurrent vulvovaginal candidiasis (RVVC) causes significant morbidity. Candida albicans is the main pathogen associated with both sporadic and recurrent candidiasis. Due to unsatisfactory treatment effect, the impact of chlorhexidine digluconate and fluconazole alone or in combination on C. albicans and biofilm was investigated.

METHODS

Vaginal C. albicans isolates from 18 patients with recurrent candidiasis and commensals from 19 asymptomatic women were isolated by culture. Crystal violet, XTT and colony forming unit assay were used to analyze the effect of chlorhexidine digluconate and fluconazole on growth of C. albicans, formation of new and already established, mature, biofilm.

RESULTS

Fluconazole reduced the growth of planktonic C. albicans. However, in established biofilm, fluconazole had no effect on the candida cells and was not able to disperse and reduce the biofilm. By contrast, chlorhexidine digluconate had a direct killing effect on C. albicans grown both planktonically and in biofilm. Chlorhexidine digluconate also dispersed mature biofilm and inhibited formation of new biofilm. No major differences were observed between commensal isolates and candida causing recurrent vulvovaginitis with respect to biofilm or growth after chlorhexidine digluconate treatment.

CONCLUSION

Biofilm is a problem in patients with recurrent vulvovaginal candidiasis reducing the effect of antifungal treatment. Development of new treatment strategies are urgently needed to decrease the recurrences. In already established biofilm, chlorhexidine digluconate dispersed the biofilm and was more effective in eradicating candida compared to fluconazole. Future treatment strategy may thus be a combination of chlorhexidine digluconate and fluconazole and prophylactic use of chlorhexidine digluconate to prevent biofilm formation and restrict infections.

Chromatin Structure and Drug Resistance in Candida spp

Anti-microbial resistance (AMR) is currently one of the most serious threats to global human health and, appropriately, research to tackle AMR garnishes significant investment and extensive attention from the scientific community. However, most of this effort focuses on antibiotics, and research into anti-fungal resistance (AFR) is vastly under-represented in comparison. Given the growing number of vulnerable, immunocompromised individuals, as well as the positive impact global warming has on fungal growth, there is an immediate urgency to tackle fungal disease, and the disturbing rise in AFR. Chromatin structure and gene expression regulation play pivotal roles in the adaptation of fungal species to anti-fungal stress, suggesting a potential therapeutic avenue to tackle AFR. In this review we discuss both the genetic and epigenetic mechanisms by which chromatin structure can dictate AFR mechanisms and will present evidence of how pathogenic yeast, specifically from the Candida genus, modify chromatin structure to promote survival in the presence of anti-fungal drugs. We also discuss the mechanisms by which anti-chromatin therapy, specifically lysine deacetylase inhibitors, influence the acquisition and phenotypic expression of AFR in Candida spp. and their potential as effective adjuvants to mitigate against AFR.

Proteomic profiling of the monothiol glutaredoxin Grx3 reveals its global role in the regulation of iron dependent processes

Iron is an essential nutrient required as a cofactor for many biological processes. As a fungal commensal-pathogen of humans, Candida albicans encounters a range of bioavailable iron levels in the human host and maintains homeostasis with a conserved regulatory circuit. How C. albicans senses and responds to iron availability is unknown. In model yeasts, regulation of the iron homeostasis circuit requires monothiol glutaredoxins (Grxs), but their functions beyond the regulatory circuit are unclear. Here, we show Grx3 is required for virulence and growth on low iron for C. albicans. To explore the global roles of Grx3, we applied a proteomic approach and performed in vivo cross-linked tandem affinity purification coupled with mass spectrometry. We identified a large number of Grx3 interacting proteins that function in diverse biological processes. This included Fra1 and Bol2/Fra2, which function with Grxs in intracellular iron trafficking in other organisms. Grx3 interacts with and regulates the activity of Sfu1 and Hap43, components of the C. albicans iron regulatory circuit. Unlike the regulatory circuit, which determines expression or repression of target genes in response to iron availability, Grx3 amplifies levels of gene expression or repression. Consistent with the proteomic data, the grx3 mutant is sensitive to heat shock, oxidative, nitrosative, and genotoxic stresses, and shows growth dependence on histidine, leucine, and tryptophan. We suggest Grx3 is a conserved global regulator of iron-dependent processes occurring within the cell.

Mammalian pathogens occupy a diverse set of niches within the host organism. These niches vary in iron and oxygen availability. As a commensal and pathogen of humans, its ability to regulate iron uptake and utilization in response to bioavailable iron level is critical for its survival in different host environments encompassing a broad range of iron levels. This study aims to understand how C. albicans senses and responds to iron level to regulate multiple aspects of its biology. The cytosolic monothiol glutaredoxin Grx3 is a critical regulator of C. albicans iron homeostasis and virulence. Taking a proteomic approach, we identified a large list of Grx3 associated proteins of diverse functions, including iron-sulfur trafficking, iron homeostasis, metabolism redox homeostasis, protein translation, DNA maintenance and repair. In support of these protein associations, Grx3 is important for all these processes. Thus, Grx3 is a global regulator of iron homeostasis and other iron dependent cellular processes.

Antifungal activity of hypocrellin compounds and their synergistic effects with antimicrobial agents against Candida albicans

Candida albicans is a common human fungal pathogen. The previous study revealed that quinone compounds showed antimicrobial activity against C. albicans by inhibiting cell growth. However, it was unclear whether quinones have other antifungal effects against C. albicans in addition to fungicidal effects. In this study, we assessed the inhibitory activity of a total of 25 quinone compounds against C. albicans morphological transition, which is essential for the pathogenicity of C. albicans. Several quinones exhibited strong inhibition of mycelium formation by C. albicans SC5314. Three leading compounds, namely hypocrellins A, B and C, also exhibited marked attenuation of C. albicans SC5314 virulence in both human cell lines and mouse infection models. These three compounds significantly suppressed the proliferation of C. albicans SC5314 cells in a mouse mucosal infection model. Intriguingly, hypocrellins not only attenuated the cytotoxicity of a nystatin-resistant C. albicans strain but also showed excellent synergistic effects with antifungal agents against both wild-type C. albicans SC5314 and the drug-resistant mutant strains. In addition, hypocrellins A, B and C interfered with the biological functions and virulence of various clinical Candida species, suggesting the promising potential of these compounds for development as new therapeutic agents against infections caused by Candida pathogens.

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.

Linking Sfl1 Regulation of Hyphal Development to Stress Response Kinases in Candida albicans

Candida albicans is a commensal as well as a pathogen of humans. C. albicans is able to mount a cellular response to a diverse range of external stimuli in the host and switch reversibly between the yeast and hyphal growth forms. Hyphal development is a key virulence determinant. Here, we studied how C. albicans senses different environmental signals to control its growth forms. Our study results suggest that robust hyphal development requires downregulation of two transcriptional repressors, Nrg1 and Sfl1. Acidic pH or cationic stress inhibits hyphal formation via stress-responsive kinases and Sfl1.

Candida albicans is an important human pathogen responsible for causing both superficial and systemic infections. Its ability to switch from the yeast form to the hyphal growth form is required for its pathogenicity. Acidic pH inhibits hyphal initiation, but the nature of the mechanism for this inhibition is not completely clear. We show that acidic pH represses hyphal initiation independently of the temperature- and farnesol-mediated Nrg1 downregulation. Using a collection of transcription factor deletion mutants, we observed that the sfl1 mutant induced hyphae in acidic pH but not in farnesol at 37°C. Furthermore, transcription of hyphal regulators BRG1 and UME6 was not induced in wild-type (WT) cells but was induced in the sfl1 mutant during hyphal induction in acidic pH. Using the same screening conditions with the collection of kinase mutants, we found that deletions of the core stress response mitogen-activated protein (MAP) kinase HOG1 and its kinase PBS2, the cell wall stress MAP kinase MKC1, and the calcium/calmodulin-dependent kinase CMK1 allowed hyphal initiation in acidic pH. Furthermore, Hog1 phosphorylation induced by high osmotic stress also retarded hyphal initiation, and the effect was abolished in the sfl1 and three kinase mutants but was enhanced in the phosphatase mutant ptp2 ptp3. We also found functional associations among Cmk1, Hog1, and Sfl1 for cation stress. Our study results suggest that robust hyphal initiation requires downregulation of both Nrg1 and Sfl1 transcriptional repressors as well as timely BRG1 expression. Acidic pH and cationic stress retard hyphal initiation via the stress-responsive kinases and Sfl1.

IMPORTANCECandida albicans is a commensal as well as a pathogen of humans. C. albicans is able to mount a cellular response to a diverse range of external stimuli in the host and switch reversibly between the yeast and hyphal growth forms. Hyphal development is a key virulence determinant. Here, we studied how C. albicans senses different environmental signals to control its growth forms. Our study results suggest that robust hyphal development requires downregulation of two transcriptional repressors, Nrg1 and Sfl1. Acidic pH or cationic stress inhibits hyphal formation via stress-responsive kinases and Sfl1.

N-Acetylglucosamine Regulates Morphogenesis and Virulence Pathways in Fungi

N-acetylglucosamine (GlcNAc) is being increasingly recognized for its ability to stimulate cell signaling. This amino sugar is best known as a component of cell wall peptidoglycan in bacteria, cell wall chitin in fungi and parasites, exoskeletons of arthropods, and the extracellular matrix of animal cells. In addition to these structural roles, GlcNAc is now known to stimulate morphological and stress responses in a wide range of organisms. In fungi, the model organisms Saccharomyces cerevisiae and Schizosaccharomyces pombe lack the ability to respond to GlcNAc or catabolize it, so studies with the human pathogen Candida albicans have been providing new insights into the ability of GlcNAc to stimulate cellular responses. GlcNAc potently induces C. albicans to transition from budding to filamentous hyphal growth. It also promotes an epigenetic switch from White to Opaque cells, which differ in morphology, metabolism, and virulence properties. These studies have led to new discoveries, such as the identification of the first eukaryotic GlcNAc transporter. Other results have shown that GlcNAc can induce signaling in C. albicans in two ways. One is to act as a signaling molecule independent of its catabolism, and the other is that its catabolism can cause the alkalinization of the extracellular environment, which provides an additional stimulus to form hyphae. GlcNAc also induces the expression of virulence genes in the C. albicans, indicating it can influence pathogenesis. Therefore, this review will describe the recent advances in understanding the role of GlcNAc signaling pathways in regulating C. albicans morphogenesis and virulence.

A Metabolic Checkpoint for the Yeast-to-Hyphae Developmental Switch Regulated by Endogenous Nitric Oxide Signaling

The yeast Candida albicans colonizes several sites in the human body and responds to metabolic signals in commensal and pathogenic states. The yeast-to-hyphae transition correlates with virulence, but how metabolic status is integrated with this transition is incompletely understood. We used the putative mitochondrial fission inhibitor mdivi-1 to probe the crosstalk between hyphal signaling and metabolism. Mdivi-1 repressed C. albicans hyphal morphogenesis, but the mechanism was independent of its presumed target, the mitochondrial fission GTPase Dnm1. Instead, mdivi-1 triggered extensive metabolic reprogramming, consistent with metabolic stress, and reduced endogenous nitric oxide (NO) levels. Limiting endogenous NO stabilized the transcriptional repressor Nrg1 and inhibited the yeast-to-hyphae transition. We establish a role for endogenous NO signaling in C. albicans hyphal morphogenesis and suggest that NO regulates a metabolic checkpoint for hyphal growth. Furthermore, identifying NO signaling as an mdivi-1 target could inform its therapeutic applications in human diseases.

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.

Regulation of Candida albicans Hyphal Morphogenesis by Endogenous Signals

Candida albicans is a human commensal fungus that is able to assume several morphologies, including yeast, hyphal, and pseudohyphal. Under a range of conditions, C. albicans performs a regulated switch to the filamentous morphology, characterized by the emergence of a germ tube from the yeast cell, followed by a mold-like growth of branching hyphae. This transition from yeast to hyphal growth has attracted particular attention, as it has been linked to the virulence of C. albicans as an opportunistic human pathogen. Signal transduction pathways that mediate the induction of the hyphal transcription program upon the imposition of external stimuli have been extensively investigated. However, the hyphal morphogenesis transcription program can also be induced by internal cellular signals, such as inhibition of cell cycle progression, and conversely, the inhibition of hyphal extension can repress hyphal-specific gene expression, suggesting that endogenous cellular signals are able to modulate hyphal gene expression as well. Here we review recent developments in the regulation of the hyphal morphogenesis of C. albicans, with emphasis on endogenous morphogenetic signals.

CO2 Signaling through the Ptc2-Ssn3 Axis Governs Sustained Hyphal Development of Candida albicans by Reducing Ume6 Phosphorylation and Degradation

Candida albicans is the most common cause of invasive fungal infections in humans. Its ability to sense and adapt to changing carbon dioxide levels is crucial for its pathogenesis. Carbon dioxide promotes hyphal development. The hypha-specific transcription factor Ume6 is rapidly degraded in air, but is stable under physiological CO₂ and hypoxia to sustain hyphal elongation. Here, we show that Ume6 stability is regulated by two parallel E3 ubiquitin ligases, SCF^Grr1 and Ubr1, in response to CO₂ and O₂, respectively. To uncover the CO₂ signaling pathway that regulates Ume6 stability, we performed genetic screens for mutants unable to respond to CO₂ for sustained filamentation. We find that the type 2C protein phosphatase Ptc2 is specifically required for CO₂-induced stabilization of Ume6 and hyphal elongation. In contrast, the cyclin-dependent kinase Ssn3 is found to be required for Ume6 phosphorylation and degradation in atmospheric CO₂ Furthermore, we find that Ssn3 is dephosphorylated in 5% CO₂ in a Ptc2-dependent manner, whereas deletion of PTC2 has no effect on Ssn3 phosphorylation in air. Our study uncovers the Ptc2-Ssn3 axis as a new CO₂ signaling pathway that controls hyphal elongation by regulating Ume6 stability in C. albicans IMPORTANCE The capacity to sense and adapt to changing carbon dioxide levels is crucial for all organisms. In fungi, CO₂ is a key determinant involved in fundamental biological processes, including growth, morphology, and virulence. In the pathogenic fungus Candida albicans, high CO₂ is directly sensed by adenylyl cyclase to promote hyphal growth. However, little is known about the mechanism by which hyphal development is maintained in response to physiological levels of CO₂ Here we report that a signal transduction system mediated by a phosphatase-kinase pair controls CO₂-responsive Ume6 phosphorylation and stability that in turn dictate hyphal elongation. Our results unravel a new regulatory mechanism of CO₂ signaling in fungi.

Synergistic Effect of Quinic Acid Derived From Syzygium cumini and Undecanoic Acid Against Candida spp. Biofilm and Virulence

In recent decades, fungal infections have incredibly increased with Candida genus as the major cause of morbidity and mortality in hospitalized and immunocompromised patients. Most of the Candida species are proficient in biofilm formation on implanted medical devices as well as human tissues. Biofilm related Candida infections are very difficult to treat using common antifungal agents owing to their increased drug resistance. To address these issues, the present study investigated the antibiofilm and antivirulent properties of Syzygium cumini derived quinic acid in combination with known antifungal compound undecanoic acid. Initially, antibiofilm potential of S. cumini leaf extract was assessed and the active principles were identified through gas chromatography and mass spectrometry analysis. Among the compounds identified, quinic acid was one of the major compounds. The interaction between quinic acid and undecanoic acid was found to be synergistic in the Fractional inhibitory concentration index (≤0.5). Results of in vitro assays and gene expression analysis suggested that the synergistic combinations of quinic acid and undecanoic acid significantly inhibited virulence traits of Candida spp. such as the biofilm formation, yeast-to-hyphal transition, extracellular polymeric substances production, filamentation, secreted hydrolases production and ergosterol biosynthesis. In addition, result of in vivo studies using Caenorhabditis elegans demonstrated the non-toxic nature of QA-UDA combination and antivirulence effect against Candida spp. For the first time, synergistic antivirulence ability of quinic acid and undecanoic acid was explored against Candida spp. Thus, results obtained from the present study suggest that combination of phytochemicals might be used an alternate therapeutic strategy for the prevention and treatment of biofilm associated Candida infection.

Caenorhabditis elegans as a Model Host to Monitor the Candida Infection Processes

C. elegans has several advantages as an experimental host for the study of infectious diseases. Worms are easily maintained and propagated on bacterial lawns. The worms can be frozen for long term storage and still maintain viability years later. Their short generation time and large brood size of thousands of worms grown on a single petri dish, makes it relatively easy to maintain at a low cost. The typical wild type adult worm grows to approximately 1.5 mm in length and are transparent, allowing for the identification of several internal organs using an affordable dissecting microscope. A large collection of loss of function mutant strains are readily available from the C. elegans genetic stock center, making targeted genetic studies in the nematode possible. Here we describe ways in which this facile model host has been used to study Candida albicans, an opportunistic fungal pathogen that poses a serious public health threat.

Efficacy of 7-benzyloxyindole and other halogenated indoles to inhibit Candida albicans biofilm and hyphal formation

Certain pathogenic bacteria and yeast form biofilms on biotic and abiotic surfaces including medical devices and implants. Hence, the development of antibiofilm coating materials becomes relevant. The virulence of those colonizing pathogens can be reduced by inhibiting biofilm formation rather than killing pathogens using excessive amounts of antimicrobials, which is touted as one of the main reasons for the development of drug resistance. Candida albicans is an opportunistic fungal pathogen, and the transition of yeast cells to hyphal cells is believed to be a crucial virulence factor. Previous studies have shown that indole and its derivatives possess antivirulence properties against various bacterial pathogens. In this study, we used various indole derivatives to investigate biofilm-inhibiting activity against C. albicans. Our study revealed that 7-benzyloxyindole, 4-fluoroindole and 5-iodoindole effectively inhibited biofilm formation compared to the antifungal agent fluconazole. Particularly, 7-benzyloxyindole at 0.02 mM (4.5 μg ml-1 ) significantly reduced C. albicans biofilm formation, but had no effect on planktonic cells, and this finding was confirmed by a 2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assay and three-dimensional confocal laser scanning microscopy. Scanning electron microscopy analyses revealed that 7-benzyloxyindole effectively inhibited hyphal formation, which explains biofilm inhibition. Transcriptomic analysis showed that 7-benzyloxyindole downregulated the expressions of several hypha/biofilm-related genes (ALS3, ECE1, HWP1 and RBT1). A C. albicans-infected Caenorhabditis elegans model system was used to confirm the antivirulence efficacy of 7-benzyloxyindole.

(1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives suppress Candida albicans virulence by interfering with morphological transition

Clinical treatment of Candida albicans infections has become more difficult due to the limited development of antifungal agents and the rapid emergence of drug resistance. In this study, we demonstrate the synthesis of a series of piperazine derivatives and the evaluation of their inhibitory activity against C. albicans virulence. Thirty-four (1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives, including 25 new compounds, were synthesized and assessed for their efficacy against the physiology and pathogenesis of C. albicans. Several compounds strongly inhibited the morphological transition and virulence of C. albicans cells, although they did not influence the growth rate of the fungal pathogen. A leading novel compound, (1-(4-ethoxyphenyl)-4-(1-biphenylol-2-hydroxypropyl)-piperazine), significantly attenuated C. albicans virulence by interfering with the process of hyphal development, but it showed no cytotoxicity against human cells at a micromolar level. These findings suggest that (1-aryloxy-2-hydroxypropyl)-phenylpiperazine derivatives could potentially be developed as novel therapeutic agents for the clinical treatment of C. albicans infections by interfering with morphological transition and virulence.