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

Protein kinase A signaling regulates immune evasion by shaving and concealing fungal β-1,3-glucan

Fungal pathogens infect billions and kill millions of people each year. Many of these pathogens have evolved strategies to evade our antifungal immune defenses. Candida albicans, for example, masks the proinflammatory pathogen-associated molecular pattern (PAMP) β-1,3-glucan, in response to specific host signals such as lactate. In C. albicans, most β-1,3-glucan lies in the inner cell wall shielded, by the outer mannan layer, from recognition by certain immune cells such as macrophages. β-1,3-glucan that becomes exposed at the cell surface can be shaved off by secreted enzymes. By integrating mathematical modeling with experimentation, we show that the dynamics of this shaving, together with the dynamics of β-1,3-glucan exposure during growth, can account for a range of β-1,3-glucan masking phenotypes. The mathematical model accurately simulates the dynamics of β-1,3-glucan exposure during growth and predicts levels of β-1,3-glucan shaving under a variety of conditions, revealing how subtle differences in growth contribute to observed variabilities in lactate-induced β-1,3-glucan masking. For example, clinical isolates previously thought to display minimal lactate-induced masking are shown to mask robustly. Using a range of C. albicans mutants, we confirm the importance of Gpr1/Gpa2-protein kinase A signaling for lactate-induced β-1,3-glucan shaving and define the contributions of the Xog1 and Eng1 glucanases to this shaving. Furthermore, examination of a shielding-defective C. albicans mnn2x6 mutant confirms that both β-1,3-glucan shaving and shielding contribute to the dynamism of β-1,3-glucan masking at the fungal cell surface. Dynamism in PAMP masking is likely to be relevant to other fungal pathogens of humans.

Chemical composition of the traditional Chinese medicine compound (ICAM), its antifungal effects against Candida albicans, and the underlying Mechanisms: Therapeutic potential and safety evaluation for vulvovaginal candidiasis

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicine (TCM) compound preparations play a significant role in the clinical treatment of vulvovaginal candidiasis (VVC).

AIM OF THE STUDY

Candida albicans (CA) is an opportunistic fungal pathogen responsible for various human diseases, including vulvovaginal candidiasis (VVC). Hyphal growth and biofilm formation are critical virulence factors contributing to CA's pathogenicity and drug resistance. ICAM, a topical TCM compound preparation developed by our laboratory, was investigated for its chemical component, antifungal mechanisms against CA and therapeutic efficacy against VVC.

MATERIALS AND METHODS

The main components of ICAM were analyzed using the Gas Chromatography-Mass Spectrometry (GC-MS) method. To elucidate the mechanisms underlying ICAM's antifungal activity, we combined phenotypic assays, transcriptomic and proteomic analyses. The therapeutic potential of ICAM for VVC and its irritancy to vaginal tissue were evaluated using cavity model experiments.

RESULTS

ICAM contained a diverse range of phenolic compounds, such as phenol, 2-methoxyphenol, and 4-ethyl-2-methoxyphenol, among others. The minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of ICAM against CA were 2.50 % and 10.00 % for the standard strain, and 5.00 % and 20.00 % for the clinical strain, respectively. At 1.25 %, ICAM significantly inhibited CA adherence, hyphal growth, and biofilm formation, while also reducing surface hydrophobicity and exopolysaccharide production. Treatment with 10.00 % ICAM completely disrupted CA membrane integrity. Transcriptome analysis revealed that multiple genes associated with biofilm and hyphal formation, including five MAPK signaling pathway genes (Ras1, Cdc24, Ste11, Cek1, Hst7), four hyphae-specific genes (Hgc1, Hwp1, Ece1, Als3), and three additional genes (Tec1, Csh1, Pmt1), were significantly downregulated. Additionally, proteins associated with the MAPK signaling pathway, including the 14-3-3 domain-containing protein, cell wall protein RTB1, Msb2p, Ras family protein, and RhoGAP domain family protein, were significantly downregulated. These findings suggest that the MAPK signaling pathway plays a crucial role in mediating ICAM's inhibition of hyphal growth and biofilm formation in CA. In vivo, 10.00 % ICAM completely eliminated the symptoms of CA infection. The vaginal fungal burden in the 20.00 % and 40.00 % ICAM groups was reduced to zero after 12 days of treatment. Furthermore, 40.00 % ICAM significantly reduced lactate dehydrogenase and inflammatory cytokine levels, demonstrating efficacy comparable to the positive control. ICAM demonstrated excellent mucosal compatibility in the cavity experiment.

CONCLUSIONS

These findings highlight the potential of ICAM as a novel antifungal agent for the treatment of VVC.

Myricetin Exerts Antibiofilm Effects on Candida albicans by Targeting the RAS1/cAMP/EFG1 Pathway and Disruption of the Hyphal Network

Increasing antifungal resistance and side effects of existing drugs demand alternative approaches for treating Candida (C.) infections. This study aimed to comprehensively evaluate the antifungal efficacy of myricetin (MYR), a natural flavonoid, against both fluconazole (FLC)-resistant and susceptible clinical Candida strains, with a particular focus on its inhibitory effects on C. albicans biofilms. Antifungal susceptibility was evaluated on Candida spp. by the broth microdilution method, and the impact of myricetin on C. albicans biofilms was determined using the Cell Counting Kit-8 (CCK-8) assay. To understand the molecular mechanisms underlying the antibiofilm properties of myricetin, expression analysis of genes in the RAS1/cAMP/EFG1 pathway (ALS3, HWP1, ECE1, UME6, HGC1) and cAMP-dependent protein kinase regulation (RAS1, CYR1, EFG1) involved in the transition from yeast to hyphae was performed. Field emission scanning electron microscopy (FESEM) was used to study the ultrastructural changes and morphological dynamics of Candida biofilms after exposure to MYR and FLC. The in vivo toxicity of myricetin was evaluated by survival analysis using the Galleria mellonella model. Myricetin significantly suppressed key genes related to hyphae development (RAS1, CYR1, EFG1, UME6, and HGC1) and adhesion (ALS3 and HWP1) in both clinical and reference Candida strains at a concentration of 640 µg/mL. FESEM analysis revealed that myricetin inhibited hyphae growth and elongation in C. albicans. This study highlights the promising antibiofilm potential of myricetin through a significant inhibition of biofilm formation and hyphal morphogenesis.

Signaling pathways governing the pathobiological features and antifungal drug resistance of Candida auris

Candida auris is an emerging multidrug-resistant fungal pathogen that poses a significant global health threat. Since its discovery in 2009, C. auris has rapidly spread worldwide, causing severe infections with high mortality rates, particularly in healthcare settings. Its ability to persist in the environment, form biofilms, and resist multiple antifungal drugs underscores the urgent need to understand its pathogenicity mechanisms and associated signaling pathways. Such insights are crucial for elucidating its unique virulence traits and developing targeted therapeutic strategies. Current studies have identified several key pathways involved in its pathogenicity and antifungal drug resistance. The Ras/cAMP/PKA pathway regulates critical virulence factors, including thermotolerance, morphological plasticity, and biofilm formation. The mitogen-activated protein kinase (MAPK) and calcineurin pathways contribute to stress responses and antifungal drug resistance. The regulation of Ace2 and morphogenesis (RAM) pathway influences cell aggregation, while the target of rapamycin (TOR) pathway affects filamentous growth and biofilm development. However, the distinct characteristics of C. auris, such as its rapid environmental spread and clade-specific traits, warrant further investigation into additional signaling pathways. This review provides a comprehensive analysis of known signaling pathways associated with C. auris pathogenicity and antifungal drug resistance, integrating insights from other fungal pathogens. By synthesizing current knowledge and identifying research gaps, this review offers new perspectives on future research directions and potential therapeutic targets against this formidable pathogen.

Signaling Pathways Regulating Dimorphism in Medically Relevant Fungal Species

Pathogenic fungi that exhibit the ability to alternate between hyphal and yeast morphology in response to environmental stimuli are considered dimorphic. Under saprobic conditions, some fungi exist as filamentous hyphae, producing conidia. When conidia are inhaled by mammals or traumatically inoculated, body temperature (37 °C) triggers dimorphism into yeast cells. This shift promotes fungal dissemination and immune evasion. Some fungal pathogens undergo dimorphism in the contrary way, forming pseudohyphae and hyphae within the host. While temperature is a major driver of dimorphism, other factors, including CO2 concentration, pH, nitrogen sources, and quorum-sensing molecules, also contribute to morphological shifts. This morphological transition is associated with increased expression of virulence factors that aid in adhesion, colonization, and immune evasion. Candida albicans is a fungus that is commonly found as a commensal on human mucous membranes but has the potential to be an opportunistic fungal pathogen of immunocompromised patients. C. albicans exhibits a dimorphic change from the yeast form to the hyphal form when it becomes established as a pathogen. In contrast, Histoplasma capsulatum is an environmental dimorphic fungus where human infection begins when conidia or hyphal fragments of the fungus are inhaled into the alveoli, where the dimorphic change to yeast occurs, this being the morphology associated with its pathogenic phase. This review examines the main signaling pathways that have been mostly related to fungal dimorphism, using as a basis the information available in the literature on H. capsulatum and C. albicans because these fungi have been widely studied for the morphological transition from hypha to yeast and from yeast to hypha, respectively. In addition, we have included the reported findings of these signaling pathways associated with the dimorphism of other pathogenic fungi, such as Paracoccidioides brasiliensis, Sporothrix schenckii, Cryptococcus neoformans, and Blastomyces dermatitis. Understanding these pathways is essential for advancing therapeutic approaches against systemic fungal infections.

A screen to identify antifungal antagonists reveals a variety of pharmacotherapies induce echinocandin tolerance in Candida albicans

Through screening a comprehensive collection of drugs approved for human use, we identified over 20 that oppose the antifungal activity of the echinocandins upon the infectious yeast, Candida albicans . More detailed evaluation of five such drugs, including the atypical antipsychotic aripiprazole and the tyrosine kinase inhibitor ponatinib, indicated they promote C. albicans survival following exposure to the echinocandin antifungals. The activity of the five selected antagonists was dependent upon the Mkc1p MAPK pathway, however, ponatinib was paradoxically shown to suppress phosphorylation and therefore activation of Mkc1p itself. Components of several other signaling pathways are also required, including those of calcineurin and casein kinase-2, suggesting the observed antagonism required much of the cell wall stress responses previously described for C. albicans . Transcriptome analysis revealed that the antagonists stimulated the expression of genes involved in xenobiotic and antifungal resistance, and suppressed the expression of genes associated with hyphal growth. Thus, the echinocandin antagonistic drugs modulate C. albicans physiology in ways that could impact its pathogenicity and/or response to therapeutic intervention. Finally, a mutant lacking the Efg1p transcription factor, which has a central role in the activation of C. albicans hyphal growth was found to have intrinsically high levels of echinocandin tolerance, suggesting a link between modulation of morphogenesis related signaling and echinocandin tolerance.

Importance

We report a substantial number of previously unknown drug interactions that modulate the echinocandin sensitivity of one of the most prevalent human fungal pathogens, Candida albicans . The echinocandins are the first line therapy for treating disseminated and often lethal Candida infections, that account for >75% of invasive fungal infections in the U.S.. For largely unknown reasons, a substantial number of patients with invasive candidiasis fail to respond to treatment with these drugs. The finding of this study suggest that co-administered medications have the potential to influence the therapeutic outcomes of invasive fungal infections through modulating antifungal drug tolerance and/or fungal pathogenicity. The potential for echinocandin antagonistic medications to influence therapeutic outcomes is discussed.

Shape-Shifting Mechanisms: Integrative Multi-Omics Insights Into Candida albicans Morphogenesis

The ability of Candida albicans to switch among yeast, hyphal, and pseudohyphal forms underlies its adaptability and pathogenicity. While cAMP-dependent signaling has long been considered central to hyphal growth, recent multi-omics studies show that cAMP-independent mechanisms also drive morphological changes. Basal PKA activity, cyclin-dependent kinases (e.g., Cdc28), and other regulators can promote shape-shifting even without classical cAMP pathways. In addition, N-acetylglucosamine (GlcNAc) acts as a potent signal that induces hyphal growth independently of its metabolic role, directly connecting environmental cues to morphological states. By integrating transcriptomic, proteomic, and phosphoproteomic data, this review exposes the intricate networks controlling C. albicans morphogenesis. A clearer understanding of these complex regulatory circuits lays the groundwork for future studies that employ advanced multi-omics analyses. Such approaches will help elucidate how these pathways converge, how they respond to changing environments, and how they might be harnessed or disrupted to influence fungal behavior.

Carbon dioxide suppresses filamentous growth in the human fungal pathogen Candida tropicalis

A striking characteristic of the human fungal pathogen Candida albicans is its ability to switch between budding yeast morphology and the filamentous form, facilitating its adaptation to changing host environments. The filamentous growth of C. albicans is mediated by various environmental factors, such as carbon dioxide (CO2), N-acetylglucosamine (GlcNAc), serum, and high temperature. Despite extensive studies in C. albicans, the regulatory mechanism of filamentation in Candida tropicalis, a fungal species that is closely related to C. albicans, has not been well characterized. In this study, we reveal opposite roles of CO2 in regulating filamentation among Candida species: CO2 promotes filamentous growth in C. albicans and Candida dubliniensis, whereas it inhibits filamentation in C. tropicalis. Despite the critical role of the canonical cAMP pathway in filamentation, it is dispensable in CO2-regulated filamentation in C. tropicalis. A CO2-specific signaling is involved in the regulation of filamentous growth in C. tropicalis. Additionally, we identify two key elements involved in CO2 sensing in C. tropicalis: a single carbonic anhydrase (CA) Nce103 and the bZIP transcription factor Rca1. Both Nce103 and Rca1 are important for cellular growth in ambient air and negatively regulate filamentous development in response to CO2 in C. tropicalis. These findings reveal a distinct mechanism underlying CO2-regulated filamentation in C. tropicalis, contributing to a deeper understanding of its unique survival strategies in diverse environmental niches and providing new insights into the adaptive evolution of CO2 sensing mechanisms among various fungal pathogens.

Antifungal activities of Equol against Candida albicans in vitro and in vivo

Candida albicans is an opportunistic fungal pathogen that can cause systemic infections in immunocompromised individuals. Morphological transition and biofilm formation are major virulence factors of C. albicans. Moreover, biofilm enhances resistance to antifungal agents. Therefore, it is urgent to identify new and effective compounds to target the biofilm of C. albicans. In the present study, the antifungal activities of equol against C. albicans were investigated. In vitro, the microdilution analysis and spot assay result showed that equol exhibited potent inhibitory activities against C. albicans. Further investigations confirmed that the antifungal effects of equol involved interference with the transition from yeast to hypha and biofilm formation of C. albicans. In addition, transcriptome sequencing and reverse transcription-quantitative PCR (qRT-PCR) analysis showed that equol significantly downregulated the expression of several genes in the Ras1-cAMP-PKA pathway related to hyphae and biofilm formation and significantly upregulated the expression of the negative transcriptional repressors RFG1 and TUP1. Moreover, equol effectively reduced the production of cAMP, a key messenger in the Ras1-cAMP-PKA pathway, while supplementation with cAMP partly rescued the equol-induced defects in hyphal development. Furthermore, in a mouse model of systemic candidiasis (SC), equol treatment significantly decreased the fungal burden (liver, kidneys, and lung) in mice and local tissue damage, while enhancing the production of interleukin-10 (IL-10). Together, these findings confirm that equol is a potentially effective agent for treatment of SC.

Screening and verification of differentially expressed serum proteins in children with severe adenovirus pneumonia

Background

Human adenoviruses are prevalent pathogens that cause severe acute respiratory infections. The clinical presentation of the adenoviral pneumonia is varied; in severe cases, they may cause systemic multi-system damages. Currently, early clinical differential diagnosis is difficult under the existing testing methods, the study identified potential biomarkers by screening and validating differentially expressed proteins (DEPs), and aimed at distinguishing between severe and non-severe adenovirus pneumonia in children aged <14 years.

Methods

DEPs were identified using data-independent acquisition (DIA) quantitative proteomics technology, and potential biomarkers were further validated using an enzyme-linked immunosorbent assay (ELISA).

Results

Twenty-seven identical DEPs were found in patients with severe adenovirus pneumonia. Among these, 10 were downregulated, and 17 were upregulated. In the protein-protein interaction network, five proteins were located at the center of the functional network. Among these, E-selectin showed significantly higher serum expression levels in the severe adenoviral pneumonia group than in adenoviral pneumonia and control groups (p < 0.001). ELISA results were consistent with the proteomic analyses. The receiver operating characteristic (ROC) curve for E-selectin revealed a sensitivity of 79.31% and a specificity of 96.55%, with an area under the curve (AUC) of 0.92.

Conclusion

E-selectin has potential as a novel biomarker for severe adenoviral pneumonia, and offers insights for improved diagnosis and clinical management.

Pullulan nanoparticles inhibit the pathogenicity of Candida albicans by regulating hypha-related gene expression

Candida albicans is a prevalent opportunistic pathogenic fungus that resides in the skin and gastrointestinal (GI) tract of humans. Under specific conditions, C. albicans cells transition from a commensal to a pathogenic state, leading to both superficial and invasive infections. Although systemic candidiasis poses a life-threatening risk, a limited number of antifungal drugs are employed for its treatment. Moreover, the emergence of resistant strains to antifungal agents underscores the pressing need for new treatment options. In this study, we propose the use of polysaccharide nanoparticles as a strategy for treating candidiasis. We synthesized phthalic pullulan nanoparticles (PPNPs) and examined their ability to inhibit the pathogenicity of C. albicans. We observed that PPNPs inhibit hyphal growth, adhesion to abiotic surfaces, and biofilm formation of C. albicans in a dose-dependent manner. This inhibitory effect is mediated by transcriptional modulation, particularly the downregulation of hypha-related genes and the upregulation of stress-responsive genes, involving the Ras/cAMP/PKA signaling pathway. Furthermore, we observed that PPNPs inhibit the adhesion of C. albicans to human epithelial cells without inducing toxicity in human cells. In addition, PPNPs inhibited the in vivo pathogenicity of C. albicans in Caenorhabditis elegans, suggesting an antagonistic effect on candidiasis. Our findings suggest that PPNPs exhibit inhibitory effects on C. albicans biofilm formation and in vivo pathogenicity, indicating their potential as a novel therapeutic agent for candidiasis.

IMPORTANCE

The pathogenic process of Candida albicans, the primary causative species of candidiasis, involves hyphal growth, biofilm formation, and secretion of virulence factors. Of these factors, the biofilm, created by the secretion of extracellular matrix from adherent cells, shields cells from external threats, enabling them to withstand high concentrations of antifungal agents. Therefore, suppressing biofilm formation is a crucial aspect of combating candidiasis. This study developed phthalic pullulan nanoparticles (PPNPs) as a novel material for inhibiting C. albicans' pathogenicity. PPNPs were internalized within Candida cells and reduced pathogenicity at the gene expression level, resulting in reduced in vitro biofilm formation, adhesion to human cells, and mortality of infected Caenorhabditis elegans. Moreover, PPNPs exhibited these effects without toxicity to human cells and host animals. These findings not only indicate that PPNPs can be employed to hinder in vitro biofilm formation but also suggest their potential as a novel treatment for candidiasis.

The ER-Resident Ras Inhibitor 1 (Eri1) of Candida albicans Inhibits Hyphal Morphogenesis via the Ras-Independent cAMP-PKA Pathway

Ras signaling and glycosylphosphatidylinositol (GPI) biosynthesis are mutually inhibitory in S. cerevisiae (Sc). The inhibition is mediated via an interaction of yeast Ras2 with the Eri1 subunit of its GPI-N-acetylglucosaminyl transferase (GPI-GnT), the enzyme catalyzing the very first GPI biosynthetic step. In contrast, Ras signaling and GPI biosynthesis in C. albicans (Ca) are mutually activated and together control the virulence traits of the human fungal pathogen. What might be the role of Eri1 in this pathogen? The present manuscript addresses this question while simultaneously characterizing the cellular role of CaEri1. It is either nonessential or required at very low levels for cell viability in C. albicans. Severe depletion of CaEri1 results in reduced GPI biosynthesis and cell wall defects. It also produces hyperfilamentation phenotypes in Spider medium as well as in bicarbonate medium containing 5% CO2, suggesting that both the Ras-dependent and Ras-independent cAMP-PKA pathways for hyphal morphogenesis are activated in these cells. Pull-down and acceptor-photobleaching FRET experiments suggest that CaEri1 does not directly interact with CaRas1 but does so through CaGpi2, another GPI-GnT subunit. We showed previously that CaGpi2 is downstream of CaEri1 in cross talk with CaRas1 and for Ras-dependent hyphal morphogenesis. Here we show that CaEri1 is downstream of all GPI-GnT subunits in inhibiting Ras-independent filamentation. CaERI1 also participates in intersubunit transcriptional cross talk within the GPI-GnT, a feature unique to C. albicans. Virulence studies using G. mellonella larvae show that a heterozygous strain of CaERI1 is better cleared by the host and is attenuated in virulence.

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.

Systematic analysis of the Candida albicans kinome reveals environmentally contingent protein kinase-mediated regulation of filamentation and biofilm formation in vitro and in vivo

Protein kinases are critical regulatory proteins in both prokaryotes and eukaryotes. Accordingly, protein kinases represent a common drug target for a wide range of human diseases. Therefore, understanding protein kinase function in human pathogens such as the fungus Candida albicans is likely to extend our knowledge of its pathobiology and identify new potential therapies. To facilitate the study of C. albicans protein kinases, we constructed a library of 99 non-essential protein kinase homozygous deletion mutants marked with barcodes in the widely used SN genetic background. Here, we describe the construction of this library and the characterization of the competitive fitness of the protein kinase mutants under 11 different growth and stress conditions. We also screened the library for protein kinase mutants with altered filamentation and biofilm formation, two critical virulence traits of C. albicans. An extensive network of protein kinases governs these virulence traits in a manner highly dependent on the specific environmental conditions. Studies on specific protein kinases revealed that (i) the cell wall integrity MAPK pathway plays a condition-dependent role in filament initiation and elongation; (ii) the hyper-osmolar glycerol MAPK pathway is required for both filamentation and biofilm formation, particularly in the setting of in vivo catheter infection; and (iii) Sok1 is dispensable for filamentation in hypoxic environments at the basal level of a biofilm but is required for filamentation in normoxia. In addition to providing a new genetic resource for the community, these observations emphasize the environmentally contingent function of C. albicans protein kinases.IMPORTANCECandida albicans is one of the most common causes of fungal disease in humans for which new therapies are needed. Protein kinases are key regulatory proteins and are increasingly targeted by drugs for the treatment of a wide range of diseases. Understanding protein kinase function in C. albicans pathogenesis may facilitate the development of new antifungal drugs. Here, we describe a new library of 99 protein kinase deletion mutants to facilitate the study of protein kinases. Furthermore, we show that the function of protein kinases in two virulence-related processes, filamentation and biofilm formation, is dependent on the specific environmental conditions.

Morphological responses of filamentous fungi to stressful environmental conditions

The filamentous growth mode of fungi, with its modular design, facilitates fungal adaptation to stresses they encounter in diverse terrestrial and anthropogenic environments. Surface growth conditions elicit diverse morphological responses in filamentous fungi, particularly demonstrating the remarkable adaptability of mycelial systems to metal- and mineral-rich environments. These responses are coupled with fungal biogeochemical activity and can ameliorate hostile conditions. A tessellated agar tile system, mimicking natural environmental heterogeneity, revealed negative chemotropism to toxic metals, distinct extreme growth strategies, such as phalanx and guerrilla movements and transitions between them, and the formation of aggregated re-allocation structures (strands, cords, synnemata). Other systems showed intrahyphal growth, intense biomineralization, and extracellular hair-like structures. Studies on submerged mycelial growth, using the thermophilic fungus Thielavia terrestris as an example, provided mechanistic insights into the morphogenesis of two extreme forms of fungal submerged culture-pelleted and dispersed growth. It was found that the development of fungal pellets was related to fungal adaptation to unfavorable stressful conditions. The two key elements affecting morphogenesis leading to the formation of either pelleted or dispersed growth were found to be (1) a lag phase (or conidia swelling stage) as a specific period of fungal morphogenesis when a certain growth form is programmed in response to morphogenic stressors, and (2) cAMP as a secondary messenger of cell signaling, defining the implementation of the particular growth strategy. These findings can contribute to knowledge of fungal-based biotechnologies, providing a means for controllable industrial processes at both morphological and physiological levels.

Anticandidal activity of a wild Bacillus subtilis NAM against clinical isolates of pathogenic Candida albicans

Background

Resistance to antifungal medications poses a significant obstacle in combating fungal infections. The development of novel therapeutics for Candida albicans is necessary due to the increasing resistance of candidiasis to the existing medications. The utilization of biological control is seen as a more advantageous and less hazardous strategy therefore the objective of this study is to identify the antifungal properties of Bacillus subtilis against pathogenic C. albicans.

Results

We conducted a study to evaluate the antifungal properties of three bacterial isolates against the human pathogen Candida albicans. One of the bacterial isolates exhibited a potent antifungal activity against this fungal pathogen. This bacterium was identified as Bacillus subtilis based on the 16Sr RNA gene sequence. It exhibited inhibitory efficacy ranging from 33.5 to 44.4% against 15 Candida isolates. The optimal incubation duration for achieving the maximum antifungal activity was determined to be 48 h, resulting in a mean inhibition zone diameter of 29 ± 0.39 mm. The Potato Dextrose agar (PDA) medium was the best medium for the most effective antifungal activity. Incubation temperature of 25oC and medium pH value of 8.0 were the most favorable conditions for maximum antagonistic activity that resulted fungal growth inhibition of 40 ± 0.16 and 36 ± 0.94 mm respectively. Furthermore, the addition of 10.5 mg/ml of bacterial filtrate to C. albicans colonies resulted in 86.51%. decrease in the number of germinated cells. The fungal cell ultrastructural responses due to exposure to B. subtilis filtrate after 48 h were investigated using transmission electron microscopy (TEM). It revealed primary a drastic abnormality that lead to cellular disintegration including folding and lysis of the cell wall, total collapse of the yeast cells, and malformed germ tube following the exposure to the filtrate. However, the control culture treatment had a characteristic morphology of the normal fungal cells featuring a consistently dense central region, a well-organized nucleus, and a cytoplasm containing several components of the endomembrane system. The cells were surrounded by a uniform and intact cell wall.

Conclusion

The current study demonstrates a notable antifungal properties of B. subtilis against C. albicans as a result of production of bioactive components of the bacterial exudate. This finding could be a promising natural antifungal agent that could be utilized to combat C. albicans.

To each its own: Mechanisms of cross-talk between GPI biosynthesis and cAMP-PKA signaling in Candida albicans versus Saccharomyces cerevisiae

Candida albicans is an opportunistic fungal pathogen that can switch between yeast and hyphal morphologies depending on the environmental cues it receives. The switch to hyphal form is crucial for the establishment of invasive infections. The hyphal form is also characterized by the cell surface expression of hyphae-specific proteins, many of which are GPI-anchored and important determinants of its virulence. The coordination between hyphal morphogenesis and the expression of GPI-anchored proteins is made possible by an interesting cross-talk between GPI biosynthesis and the cAMP-PKA signaling cascade in the fungus; a parallel interaction is not found in its human host. On the other hand, in the nonpathogenic yeast, Saccharomyces cerevisiae, GPI biosynthesis is shut down when filamentation is activated and vice versa. This too is achieved by a cross-talk between GPI biosynthesis and cAMP-PKA signaling. How are diametrically opposite effects obtained from the cross-talk between two reasonably well-conserved pathways present ubiquitously across eukarya? This Review attempts to provide a model to explain these differences. In order to do so, it first provides an overview of the two pathways for the interested reader, highlighting the similarities and differences that are observed in C. albicans versus the well-studied S. cerevisiae model, before going on to explain how the different mechanisms of regulation are effected. While commonalities enable the development of generalized theories, it is hoped that a more nuanced approach, that takes into consideration species-specific differences, will enable organism-specific understanding of these processes and contribute to the development of targeted therapies.

Structure-Based Optimization of Novel Sterol 24-C-Methyltransferase Inhibitors for the Treatment of Candida albicans Infections

Interfering with sterol biosynthesis is an important strategy for developing safe and effective antifungal drugs. We previously identified compound H55 as an allosteric inhibitor of the fungal-specific C-24 sterol methyltransferase Erg6 for treating Candida albicans infections. Herein, 62 derivatives of H55 were designed and synthesized based on target-ligand interactions to identify more active candidates. Among them, d28 displayed the most potent antivirulence ability (MHIC50 = 0.25 μg/mL) by targeting Erg6, exhibiting an 8-fold increase in potency compared with H55. Moreover, d28 significantly outperformed H55 in inhibiting cell adhesion and biofilm formation, and exhibited minimal cytotoxicity and negligible potential to induce drug resistance. Of note, the coadministration of d28 and other sterol biosynthesis inhibitors, such as tridemorph or terbinafine, demonstrated a strong synergistic antifungal action in vitro and in vivo in a murine skin infection model. These results support the potential application of d28 in the treatment of C. albicans infections.

Mitochondria complex I deficiency in Candida albicans arrests the cell cycle at S phase through suppressive TOR and PKA pathways

How mutations in mitochondrial electron transport chain (ETC) proteins impact the cell cycle of Candida albicans was investigated in this study. Using genetic null mutants targeting ETC complexes I (CI), III (CIII), and IV (CIV), the cell cycle stages (G0/G1, S phase, and G2/M) were analyzed via fluorescence-activated cell sorting (FACS). Four CI null mutants exhibited distinct alterations, including extended S phase, shortened G2/M population, and a reduction in cells size exceeding 10 µM. Conversely, CIII mutants showed an increased population in G1/G0 phase. Among four CI mutants, ndh51Δ/Δ and goa1Δ/Δ displayed aberrant cell cycle patterns correlated with previously reported cAMP/PKA downregulation. Specifically, nuo1Δ/Δ and nuo2Δ/Δ mutants exhibited increased transcription of RIM15, a central hub linking cell cycle with nutrient-dependent TOR1 and cAMP/PKA pathways and Snf1 aging pathway. These findings suggest that suppression of TOR1 and cAMP/PKA pathways or enhanced Snf1 disrupts cell cycle progression, influencing cell longevity and growth among CI mutants. Overall, our study highlights the intricate interplay between mitochondrial ETC, cell cycle, and signaling pathways.

Green synthesis of biogenic selenium nanoparticles functionalized with ginger dietary extract targeting virulence factor and biofilm formation in Candida albicans

To treat the systemic infections caused by Candida albicans (C. albicans), various drugs have been used, however, infections still persisted due to virulence factors and increasing antifungal resistance. As a solution to this problem, we synthesized selenium nanoparticles (SeNPs) by using Bacillus cereus bacteria. This is the first study to report a higher (70 %) reduction of selenite ions into SeNPs in under 6 h. The as-synthesized, biogenic SeNPs were used to deliver bioactive constituents of aqueous extract of ginger for inhibiting the growth and biofilm (virulence factors) in C. albicans. UV-visible spectroscopy revealed a characteristic absorption at 280 nm, and Raman spectroscopy showed a characteristic peak shift at 253 cm-1 for the biogenic SeNPs. The synthesized SeNPs are spherical with 240-250 nm in size as determined by electron microscopy. Fourier transform infrared spectroscopy confirmed the functionalization of antifungal constituents of ginger over the SeNPs (formation of Ginger@SeNPs nanoconjugates). In contrast to biogenic SeNPs, nanoconjugates were active against C. albicans for inhibiting growth and biofilm formation. In order to reveal antifungal mechanism of nanoconjugates', real-time polymerase chain reaction (RT-PCR) analysis was performed, according to RT-PCR analysis, the nanoconjugates target virulence genes involved in C. albicans hyphae and biofilm formation. Nanoconjugates inhibited 25 % growth of human embryonic kidney (HEK) 293 cell line, indicating moderate cytotoxicity of active nanoconjugates in an in-vitro cytotoxicity study. Therefore, biogenic SeNPs conjugated with ginger dietary extract may be a potential antifungal agent and drug carrier for inhibiting C. albicans growth and biofilm formation.

Cytochrome c regulates hyphal morphogenesis by interfering with cAMP-PKA signaling in Candida albicans

In the human fungal pathogen Candida albicans, invasive hyphal growth is a well-recognized virulence trait. We employed transposon-mediated genome-wide mutagenesis, revealing that inactivating CTM1 blocks hyphal growth. CTM1 encodes a lysine (K) methyltransferase, which trimethylates cytochrome c (Cyc1) at K79. Mutants lacking CTM1 or expressing cyc1K79A grow as yeast under hyphae-inducing conditions, indicating that unmethylated Cyc1 suppresses hyphal growth. Transcriptomic analyses detected increased levels of the hyphal repressor NRG1 and decreased levels of hyphae-specific genes in ctm1Δ/Δ and cyc1K79A mutants, suggesting cyclic AMP (cAMP)-protein kinase A (PKA) signaling suppression. Co-immunoprecipitation and in vitro kinase assays demonstrated that unmethylated Cyc1 inhibits PKA kinase activity. Surprisingly, hyphae-defective ctm1Δ/Δ and cyc1K79A mutants remain virulent in mice due to accelerated proliferation. Our results unveil a critical role for cytochrome c in maintaining the virulence of C. albicans by orchestrating proliferation, growth mode, and metabolism. Importantly, this study identifies a biological function for lysine methylation on cytochrome c.

DYRK-family kinases regulate Candida albicans morphogenesis and virulence through the Ras1/PKA pathway

IMPORTANCE

Candida albicans is an opportunistic human fungal pathogen that frequently causes life-threatening infections in immunocompromised individuals. To cause disease, the fungus employs several virulence traits, including its ability to transition between yeast and filamentous states. Previous work identified a role for the kinase Yak1 in regulating C. albicans filamentation. Here, we demonstrate that Yak1 regulates morphogenesis through the canonical cAMP/PKA pathway and that this regulation is environmentally contingent, as host-relevant concentrations of CO2 bypass the requirement of Yak1 for C. albicans morphogenesis. We show a related kinase, Pom1, is important for filamentation in the absence of Yak1 under these host-relevant conditions, as deletion of both genes blocked filamentous growth under all conditions tested. Finally, we demonstrate that Yak1 is required for filamentation in a mouse model of C. albicans dermatitis using genetic and pharmacological approaches. Overall, our results expand our understanding of how Yak1 regulates an important virulence trait in C. albicans.

Artemisinins inhibit oral candidiasis caused by Candida albicans through the repression on its hyphal development

Candida albicans is the most abundant fungal species in oral cavity. As a smart opportunistic pathogen, it increases the virulence by switching its forms from yeasts to hyphae and becomes the major pathogenic agent for oral candidiasis. However, the overuse of current clinical antifungals and lack of new types of drugs highlight the challenges in the antifungal treatments because of the drug resistance and side effects. Anti-virulence strategy is proved as a practical way to develop new types of anti-infective drugs. Here, seven artemisinins, including artemisinin, dihydroartemisinin, artemisinic acid, dihydroartemisinic acid, artesunate, artemether and arteether, were employed to target at the hyphal development, the most important virulence factor of C. albicans. Artemisinins failed to affect the growth, but significantly inhibited the hyphal development of C. albicans, including the clinical azole resistant isolates, and reduced their damage to oral epithelial cells, while arteether showed the strongest activities. The transcriptome suggested that arteether could affect the energy metabolism of C. albicans. Seven artemisinins were then proved to significantly inhibit the productions of ATP and cAMP, while reduced the hyphal inhibition on RAS1 overexpression strain indicating that artemisinins regulated the Ras1-cAMP-Efg1 pathway to inhibit the hyphal development. Importantly, arteether significantly inhibited the fungal burden and infections with no systemic toxicity in the murine oropharyngeal candidiasis models in vivo caused by both fluconazole sensitive and resistant strains. Our results for the first time indicated that artemisinins can be potential antifungal compounds against C. albicans infections by targeting at its hyphal development.

Deciphering the regulatory mechanisms of the cAMP/protein kinase A pathway and their roles in the pathogenicity of Candida auris

The emergence of multidrug-resistant fungal pathogens is a significant concern for global public health. Candida auris poses a considerable threat as a multidrug-resistant fungal pathogen. Our recent study revealed that the adenylyl cyclase Cyr1 and protein kinase A (PKA) pathways play distinct and redundant roles in drug resistance and pathogenicity of C. auris. However, the upstream and negative feedback regulatory mechanisms of C. auris are not yet fully understood. In this study, we discovered that the small GTPase Ras1, along with its nucleotide exchange factor Cdc25 and GTPase-activating protein Ira2, plays a major role in regulating cAMP/PKA-dependent traits, while G-protein-coupled receptor Gpr1 and heterotrimeric G-protein α subunit Gpa2 play a minor role. Pde2 plays a major role in negative feedback regulation of the cAMP/PKA pathway, while Pde1 plays a minor role. Hyperactivation of the Ras/cAMP/PKA pathway by deleting PDE2 or BCY1 renders C. auris cells thermosensitive and susceptible to nutrient deficiency, which leads to attenuated virulence. Our study demonstrates the distinct contributions of hyperactivation of the Ras/cAMP/PKA signaling pathway to C. auris pathogenesis and suggests potential therapeutic targets for C. auris-mediated candidiasis. IMPORTANCE Candida auris is a major concern as a multidrug-resistant fungal pathogen. While our previous studies highlighted the crucial roles of the cAMP/protein kinase A (PKA) pathway in regulating drug resistance, stress responses, morphogenesis, ploidy change, biofilm formation, and pathogenicity in this pathogen, their regulatory mechanism remains unclear. In our study, we provided evidence that the cAMP/PKA signaling pathway in C. auris is primarily governed by the small GTPase RAS rather than a G-protein-coupled receptor. Additionally, we discovered that the negative feedback regulation of cAMP, controlled by phosphodiesterases, is vital for C. auris virulence by promoting resistance to high temperatures and nutrient deficiencies. These findings underscore the diverse pathobiological significance of the Ras/cAMP/PKA signaling pathway in C. auris, shedding light on potential therapeutic targets and strategies for combating this multidrug-resistant fungal pathogen.

The Adenylate Cyclase-Encoding Gene crac Is Involved in Clonostachys rosea Mycoparasitism

Clonostachys rosea is an excellent biocontrol fungus against numerous fungal plant pathogens. The cAMP signaling pathway is a crucial signal transduction pathway in fungi. To date, the role of the cAMP signaling pathway in C. rosea mycoparasitism remains unknown. An adenylate cyclase-encoding gene, crac (an important component of the cAMP signaling pathway), was previously screened from C. rosea 67-1, and its expression level was dramatically upregulated during the C. rosea mycoparasitization of the sclerotia of Sclerotinia sclerotiorum. In this study, the function of crac in C. rosea mycoparasitism was explored through gene knockout and complementation. The obtained results show that the deletion of crac influenced the growth rate and colony morphology of C. rosea, as well as the tolerance to NaCl and H2O2 stress. The mycoparasitic effects on the sclerotia of S. sclerotiorum and the biocontrol capacity on soybean Sclerotinia stem rot in ∆crac-6 and ∆crac-13 were both attenuated compared with that of the wild-type strain and complementation transformants. To understand the regulatory mechanism of crac during C. rosea mycoparasitism, transcriptomic analysis was conducted between the wild-type strain and knockout mutant. A number of biocontrol-related genes, including genes encoding cell wall-degrading enzymes and transporters, were significantly differentially expressed during C. rosea mycoparasitism, suggesting that crac may be involved in C. rosea mycoparasitism by regulating the expression of these DEGs. These findings provide insight for further exploring the molecular mechanism of C. rosea mycoparasitism.

Streptococcus mutans sigX-inducing peptide inhibits the virulence of Candida albicans and oral candidiasis through the Ras1-cAMP-Efg1 pathway

Oral candidiasis is the most common fungal infectious disease in the human oral cavity, and Candida albicans is the major pathogenic agent. Increasing drug resistance and the lack of new types of antifungals greatly increase the challenges for treating fungal infections. Targeting hyphal transition provides a promising strategy to inhibit the virulence of C. albicans and overcome drug resistance. This study aimed to investigate the effects and mechanisms of sigX-inducing peptide (XIP), a quorum-sensing signal peptide secreted by Streptococcus mutans, on C. albicans hyphal development and biofilm formation in vitro and oropharyngeal candidiasis in vivo. XIP significantly inhibited C. albicans yeast-to-hypha transition and biofilm formation in a dose-dependent manner from 0.01 to 0.1 µM. XIP significantly downregulated expression of genes from the Ras1-cAMP-Efg1 pathway (RAS1, CYR1, TPK2, EFG1 and UME6), a key pathway to regulate C. albicans hyphal development. Importantly, XIP reduced the levels of key molecules cAMP and ATP from this pathway, while the addition of exogenous cAMP and overexpression of RAS1 restored the hyphal development inhibited by XIP. XIP also lost its hyphal inhibitory effects on ras1Δ/Δ and efg1Δ/Δ strains. These results further confirmed that XIP inhibited hyphal development through downregulation of the Ras1-cAMP-Efg1 pathway. A murine oropharyngeal candidiasis model was employed to evaluate the therapeutic effects of XIP on oral candidiasis. XIP effectively reduced the infected epithelial area, fungal burden, hyphal invasion and inflammatory infiltrates. These results revealed the antifungal effects of XIP, and highlighted that XIP can be a potential antifungal peptide against C. albicans infection.

Inhibitory Effects of a Maleimide Compound on the Virulence Factors of Candida albicans

Candidiasis caused by Candida albicans infection has long been a serious human health problem. The pathogenicity of C. albicans is mainly due to its virulence factors, which are the novel targets of antifungal drugs for low risk of resistance development. In this study, we identified a maleimide compound [1-(4-methoxyphenyl)-1hydro-pyrrole-2,5-dione, MPD] that exerts effective anti-virulence activity. It could inhibit the process of adhesion, filamentation, and biofilm formation in C. albicans. In addition, it exhibited low cytotoxicity, hemolytic activity and drug resistance development. Moreover, in Galleria mellonella-C. albicans (in vivo) infection model, the survival time of infected larvae was significantly prolonged under the treatment of MPD. Further, mechanism research revealed that MPD increased farnesol secretion by upregulating the expression of Dpp3. The increased farnesol inhibited the activity of Cdc35, which then decreased the intracellular cAMP content resulting in the inhibition of virulence factors via the Ras1-cAMP-Efg1 pathway. In all, this study evaluated the inhibitory effect of MPD on various virulence factors of C. albicans and identified the underlying mechanisms. This suggests a potential application of MPD to overcome fungal infections in clinic.

A cAMP phosphodiesterase is essential for sclerotia formation and virulence in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a plant pathogenic fungus that causes white mold or stem rot diseases. It affects mostly dicotyledonous crops, resulting in significant economic losses worldwide. Sclerotia formation is a special feature of S. sclerotiorum, allowing its survival in soil for extended periods and facilitates the spread of the pathogen. However, the detailed molecular mechanisms of how sclerotia are formed and how virulence is achieved in S. sclerotiorum are not fully understood. Here, we report the identification of a mutant that cannot form sclerotia using a forward genetics approach. Next-generation sequencing of the mutant's whole genome revealed candidate genes. Through knockout experiments, the causal gene was found to encode a cAMP phosphodiesterase (SsPDE2). From mutant phenotypic examinations, we found that SsPDE2 plays essential roles not only in sclerotia formation, but also in the regulation of oxalic acid accumulation, infection cushion functionality and virulence. Downregulation of SsSMK1 transcripts in Sspde2 mutants revealed that these morphological defects are likely caused by cAMP-dependent inhibition of MAPK signaling. Moreover, when we introduced HIGS construct targeting SsPDE2 in Nicotiana benthamiana, largely compromised virulence was observed against S. sclerotiorum. Taken together, SsPDE2 is indispensable for key biological processes of S. sclerotiorum and can potentially serve as a HIGS target to control stem rot in the field.

The Ras small GTPase RSR1 regulates cellulase production in Trichoderma reesei

BACKGROUND

Lignocellulose is the most abundant renewable resource in the world and has attracted widespread attention. It can be hydrolyzed into sugars with the help of cellulases and hemicellulases that are secreted by filamentous fungi. Several studies have revealed that the Ras small GTPase superfamily regulates important cellular physiological processes, including synthesis of metabolites, sporulation, and cell growth and differentiation. However, it remains unknown how and to what extent Ras small GTPases participate in cellulase production.

RESULTS

In this study, we found that the putative Ras small GTPase RSR1 negatively regulated the expression of cellulases and xylanases. Deletion of rsr1 (∆rsr1) significantly increased cellulase production and decreased the expression levels of ACY1-cAMP-protein kinase A (PKA) signaling pathway genes and the concentration of intracellular cyclic adenosine monophosphate (cAMP). Loss of acy1 based on ∆rsr1 (∆rsr1∆acy1) could further increase cellulase production and the expression levels of cellulase genes, while overexpression of acy1 based on ∆rsr1 (∆rsr1-OEacy1) significantly reduced cellulase production and transcriptional levels of cellulase genes. In addition, our results revealed that RSR1 negatively controlled cellulase production via the ACY1-cAMP-PKA pathway. Transcriptome analysis revealed significantly increased expression of three G-protein coupled receptors (GPCRs; tre62462, tre58767, and tre53238) and approximately two-fold higher expression of ACE3 and XYR1, which transcriptionally activated cellulases with the loss of rsr1. ∆rsr1∆ tre62462 exhibited a decrease in cellulase activity compared to ∆rsr1, while that of ∆rsr1∆tre58767 and ∆rsr1∆tre53238 showed a remarkable improvement compared to ∆rsr1. These findings revealed that GPCRs on the membrane may sense extracellular signals and transmit them to rsr1 and then to ACY1-cAMP-PKA, thereby negatively controlling the expression of the cellulase activators ACE3 and XYR1. These data indicate the crucial role of Ras small GTPases in regulating cellulase gene expression.

CONCLUSIONS

Here, we demonstrate that some GPCRs and Ras small GTPases play key roles in the regulation of cellulase genes in Trichoderma reesei. Understanding the roles of these components in the regulation of cellulase gene transcription and the signaling processes in T. reesei can lay the groundwork for understanding and transforming other filamentous fungi.

Characterization of an allosteric inhibitor of fungal-specific C-24 sterol methyltransferase to treat Candida albicans infections

Filamentation is an important virulence factor of the pathogenic fungus Candida albicans. The abolition of Candida albicans hyphal formation by disrupting sterol synthesis is an important concept for the development of antifungal drugs with high safety. Here, we conduct a high-throughput screen using a C. albicans strain expressing green fluorescent protein-labeled Dpp3 to identify anti-hypha agents by interfering with ergosterol synthesis. The antipyrine derivative H55 is characterized to have minimal cytotoxicity and potent inhibition of C. albicans hyphal formation in multiple cultural conditions. H55 monotherapy exhibits therapeutic efficacy in mouse models of azole-resistant candidiasis. H55 treatment increases the accumulation of zymosterol, the substrate of C-24 sterol methyltransferase (Erg6). The results of enzyme assays, photoaffinity labeling, molecular simulation, mutagenesis, and cellular thermal shift assays support H55 as an allosteric inhibitor of Erg6. Collectively, H55, an inhibitor of the fungal-specific enzyme Erg6, holds potential to treat C. albicans infections.

The effect of herbal medicine in innate immunity to Candida albicans

Candida albicans (C. albicans) is an opportunistic pathogenic fungus that often causes mucosal and systemic infections. Several pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs) and C-type lectin receptors (CLRs), have been implicated in the host recognition of C. albicans. These PRRs recognize the pathogen-associated molecular patterns (PAMPs) of C. albicans to activate innate immune cells, thereby rapidly inducing various inflammatory responses by activating intracellular signaling cascades. Herbal medicine and its active components deserve priority development due to their low toxicity and high antibacterial, antiviral and antifungal activities. This review discussed the activities of herbal compounds against C. albicans and their related mechanisms, especially their regulatory role on innate immune cells such as neutrophils, macrophages, and dendritic cells (DCs) implicated in C. albicans infections. Our work aims to find new therapeutic drugs and targets to prevent and treat diseases caused by C. albicans infection with the mechanisms by which this fungus interacts with the innate immune response.

Candida albicans antibiofilm molecules: analysis based on inhibition and eradication studies

Biofilms are communities of microbial cells surrounded by an extracellular polysaccharide matrix, recognized as a fungal source for local and systemic infections and less susceptible to antifungal drugs. Thus, treatment of biofilm-related Candida spp. infections with popular antifungals such as fluconazole is limited and species-dependent and alternatively demands the use of expensive and high toxic drugs. In this sense, molecules with antibiofilm activity have been studied but without care regarding the use of important criteria such as antibiofilm concentration lower than antifungal concentration when considering the process of inhibition of formation and concentrations equal to or lower than 300 µM. Therefore, this review tries to gather the most promising molecules regarding the activity against the C. albicans biofilm described in the last 10 years, considering the activity of inhibition and eradication. From January 2011 to July 2021, articles were searched on Scopus, PubMed, and Science Direct, combining the keywords "antibiofilm," "candida albicans," "compound," and "molecule" with AND and OR operators. After 3 phases of selection, 21 articles describing 42 molecules were discussed in the review. Most of them were more promising for the inhibition of biofilm formation, with SM21 (24) being an interesting molecule for presenting inhibitory and eradication activity in biofilms with 24 and 48 h, as well as alizarin (26) and chrysazine (27), with concentrations well below the antifungal concentration. Despite the detection of these molecules and the attempts to determine the mechanisms of action by microscopic analysis and gene expression, no specific target has been determined. Thus, a gap is signaled, requiring further studies such as proteomic analyses to clarify it.

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.

Nature of β-1,3-Glucan-Exposing Features on Candida albicans Cell Wall and Their Modulation

Candida albicans exists as a commensal of mucosal surfaces and the gastrointestinal tract without causing pathology. However, this fungus is also a common cause of mucosal and systemic infections when antifungal immune defenses become compromised. The activation of antifungal host defenses depends on the recognition of fungal pathogen-associated molecular patterns (PAMPs), such as β-1,3-glucan. In C. albicans, most β-1,3-glucan is present in the inner cell wall, concealed by the outer mannan layer, but some β-1,3-glucan becomes exposed at the cell surface. In response to host signals, such as lactate, C. albicans induces the Xog1 exoglucanase, which shaves exposed β-1,3-glucan from the cell surface, thereby reducing phagocytic recognition. We show here that β-1,3-glucan is exposed at bud scars and punctate foci on the lateral wall of yeast cells, that this exposed β-1,3-glucan is targeted during phagocytic attack, and that lactate-induced masking reduces β-1,3-glucan exposure at bud scars and at punctate foci. β-1,3-Glucan masking depends upon protein kinase A (PKA) signaling. We reveal that inactivating PKA, or its conserved downstream effectors, Sin3 and Mig1/Mig2, affects the amounts of the Xog1 and Eng1 glucanases in the C. albicans secretome and modulates β-1,3-glucan exposure. Furthermore, perturbing PKA, Sin3, or Mig1/Mig2 attenuates the virulence of lactate-exposed C. albicans cells in Galleria. Taken together, the data are consistent with the idea that β-1,3-glucan masking contributes to Candida pathogenicity. IMPORTANCE Microbes that coexist with humans have evolved ways of avoiding or evading our immunological defenses. These include the masking by these microbes of their "pathogen-associated molecular patterns" (PAMPs), which are recognized as "foreign" and used to activate protective immunity. The commensal fungus Candida albicans masks the proinflammatory PAMP β-1,3-glucan, which is an essential component of its cell wall. Most of this β-1,3-glucan is hidden beneath an outer layer of the cell wall on these microbes, but some can become exposed at the fungal cell surface. Using high-resolution confocal microscopy, we examine the nature of the exposed β-1,3-glucan at C. albicans bud scars and at punctate foci on the lateral cell wall, and we show that these features are targeted by innate immune cells. We also reveal that downstream effectors of protein kinase A (Mig1/Mig2, Sin3) regulate the secretion of major glucanases, modulate the levels of β-1,3-glucan exposure, and influence the virulence of C. albicans in an invertebrate model of systemic infection. Our data support the view that β-1,3-glucan masking contributes to immune evasion and the virulence of a major fungal pathogen of humans.

Chemoenzymatic Probes Reveal Peptidoglycan Recognition and Uptake Mechanisms in Candida albicans

Candida albicans, the major fungal pathogen in humans, is under the strong influence of bacterial peptidoglycan fragments to undergo the yeast-to-hyphae transition, a key virulent step in C. albicans pathogenesis and infections. However, due to the synthetic difficulties of obtaining peptidoglycan fragments for biological studies, mechanistic details of how C. albicans recognizes and uptakes these peptidoglycan fragments have not been well elucidated. Notably, previous works have solely focused on the synthetic peptidoglycan ligand, muramyl dipeptide (MDP), despite its poor hyphal-inducing activity in C. albicans. In this work, we isolated and purified natural peptidoglycan fragments via enzymatic degradation of bacteria cell wall sacculi and chemoenzymatically installed a series of functional d-amino acids into the natural muropeptide, creating peptidoglycan probes that bear photoaffinity, bio-orthogonal, or fluorescent functionality. Using these chemoenzymatic peptidoglycan probes, we established that natural peptidoglycan fragments, which are potent hyphal-inducers, interact with the C. albicans Cyr1 sensor protein in the in-gel fluorescence assay as well as in in vitro pulldown studies. Moreover, we established that bacterial peptidoglycan probes enter C. albicans cells via an energy-dependent endocytic process.

Cyclic Nucleotide Phosphodiesterases in Alcohol Use Disorders: Involving Gut Microbiota

Alcohol abuse is 1 of the most significant public health problems in the world. Chronic, excessive alcohol consumption not only causes alcohol use disorder (AUD) but also changes the gut and lung microbiota, including bacterial and nonbacterial types. Both types of microbiota can release toxins, further damaging the gastrointestinal and respiratory tracts; causing inflammation; and impairing the functions of the liver, lung, and brain, which in turn deteriorate AUD. Phosphodiesterases (PDEs) are critical in the control of intracellular cyclic nucleotides, including cyclic adenosine monophosphate and cyclic guanosine monophosphate. Inhibition of certain host PDEs reduces alcohol consumption and attenuates alcohol-related impairment. These PDEs are also expressed in the microbiota and may play a role in controlling microbiota-associated inflammation. Here, we summarize the influences of alcohol on gut/lung bacterial and nonbacterial microbiota as well as on the gut-liver/brain/lung axis. We then discuss the relationship between gut and lung microbiota-mediated PDE signaling and AUD consequences in addition to highlighting PDEs as potential targets for treatment of AUD.

Magnolol as a potent antifungal agent inhibits Candida albicans virulence factors via the PKC and Cek1 MAPK signaling pathways

Magnolol, a lignin compound extracted from Magnolia officinalis Cortex, has been found to have prominent antifungal effects against Candida albicans. However, the specific mechanism still remains unclear. Therefore, this study aimed to further explore the inhibition mechanism of magnolol against Candida albicans virulence factors and the related signaling pathways. By an XTT reduction assay, a hyphal formation assay, confocal laser scanning microscopy, transmission electron microscopy, a calcofluor white staining assay, and a cell wall β-glucan quantitative detection assay, we evaluated the inhibitory effects of magnolol against the adhesion, hyphal formation, biofilm viability, biofilm spatial structure, and cell wall ultrastructure of Candida albicans. Moreover, by RNA sequencing and qRT-PCR, we confirmed the effects of magnolol in inhibiting the gene expression of Candida albicans virulence factors and the related signaling pathways. The results revealed that the adhesion and hyphal formation of Candida albicans were inhibited significantly by magnolol. The viability and spatial structures of Candida albicans biofilms were further weakened. Candida albicans ultrastructure showed partial thinning of cell walls and even rupture, with cytoplasmic leakage. The cell wall intergrity and β-glucan content were also radically reduced. Moreover, magnolol caused significant inhibition of the expression of Candida albicans adhesion, invasion, hyphal formation, biofilm formation, β-1,3-glucan synthesis, and hydrolase secretion-related genes, including ALS1, ALS3, EFG1, EAP1, FKS1, FKS2, PLB2, and SAP2. Furthermore, the PKC pathway-related genes (RHO1, PKC1, BCK1, MKK2, MKC1) and Cek1 pathway-related genes (CDC42, CST20, STE11, HST7, CEK1) were also significantly downregulated, indicating that the inhibition of magnolol against Candida albicans virulence factors might be related to PKC and Cek1 MAPK signaling pathways. In conclusion, the findings of this study confirmed the inhibition mechanism of magnolol against Candida albicans virulence factors, which might be related to PKC and Cek1 MAPK pathways, thus laying the theoretical foundation for its clinical antifungal applications.

cAMP Signalling Pathway in Biocontrol Fungi

Biocontrol is a complex process, in which a variety of physiological and biochemical characteristics are altered. The cAMP signalling pathway is an important signal transduction pathway in biocontrol fungi and consists of several key components. The G-protein system contains G-protein coupled receptors (GPCRs), heterotrimeric G-proteins, adenylate cyclase (AC), cAMP-dependent protein kinase (PKA), and downstream transcription factors (TFs). The cAMP signalling pathway can regulate fungal growth, development, differentiation, sporulation, morphology, secondary metabolite production, environmental stress tolerance, and the biocontrol of pathogens. However, few reviews of the cAMP signalling pathway in comprehensive biocontrol processes have been reported. This work reviews and discusses the functions and applications of genes encoding each component in the cAMP signalling pathway from biocontrol fungi, including the G-protein system components, AC, PKA, and TFs, in biocontrol behaviour. Finally, future suggestions are provided for constructing a complete cAMP signalling pathway in biocontrol fungi containing all the components and downstream effectors involved in biocontrol behavior. This review provides useful information for the understanding the biocontrol mechanism of biocontrol fungi by utilising the cAMP signalling pathway.

Candida albicans Filamentation Does Not Require the cAMP-PKA Pathway In Vivo

Candida albicans is one of the most prevalent human fungal pathogens. Its ability to transition between budding yeast and filamentous morphological forms (pseudohyphae and hyphae) is tightly associated with its pathogenesis. Based on in vitro studies, the cAMP-protein kinase A (PKA) pathway is a key regulator of C. albicans morphogenesis. Using an intravital imaging approach, we investigated the role of the cAMP-PKA pathway during infection. Consistent with their roles in vitro, the downstream effectors of the cAMP-PKA pathway Efg1 and Nrg1 function, respectively, as an activator and a repressor of in vivo filamentation. Surprisingly, strains lacking the adenylyl cyclase, CYR1, showed only slightly reduced filamentation in vivo despite being completely unable to filament in RPMI + 10% serum at 37°C. Consistent with these findings, deletion of the catalytic subunits of PKA (Tpk1 and Tpk2), either singly or in combination, generated strains that also filamented in vivo but not in vitro. In vivo transcription profiling of C. albicans isolated from both ear and kidney tissue showed that the expression of a set of 184 environmentally responsive genes correlated well with in vitro filamentation (R², 0.62 to 0.68) genes. This concordance suggests that the in vivo and in vitro transcriptional responses are similar but that the upstream regulatory mechanisms are distinct. As such, these data emphatically emphasize that C. albicans filamentation is a complex phenotype that occurs in different environments through an intricate network of distinct regulatory mechanisms. IMPORTANCE The fungus Candida albicans causes a wide range of disease in humans from common diaper rash to life-threatening infections in patients with compromised immune systems. As such, the mechanisms for its ability to cause disease are of wide interest. An intensely studied virulence property of C. albicans is its ability to switch from a round yeast form to filament-like forms (hyphae and pseudohyphae). Surprisingly, we have found that a key signaling pathway that regulates this transition in vitro, the protein kinase A pathway, is not required for filamentation during infection of the host. Our work not only demonstrates that the regulation of filamentation depends upon the specific environment C. albicans inhabits but also underscores the importance of studying these mechanisms during infection.

Development and Evaluation of a Film Forming System Containing Myricetin and Miconazole Nitrate for Preventing Candida albicans Catheter-Related Infection

Background: Candida albicans catheter-related infection (CRI) is a great challenge in clinic now, mainly due to the difficulty in eradicating the biofilms. Purpose: In this study, the mechanism of the antibiofilm effect of myricetin (MY) on C. albicans was illustrated. A film forming system (FFS) containing MY and miconazole nitrate (MN) was developed, optimized, and evaluated. The anti-infection effect of MY+MN@FFS against C. albicans CRI was investigated in vivo. Study Design and Methods: To clarify the mechanism of the action of MY, the influence of MY on each key process of the formation of C. albicans biofilms was evaluated. To deliver MY and MN into the skin and form a drug reservoir on the surface of the skin, the FFS was used as a carrier and MY+MN@FFS was developed, optimized, and evaluated. After preliminary confirmation of drug safety, a percutaneously inserted C. albicans CRI mouse model was established to investigate the in vivo anti-infection effect of MY+MN@FFS by fluorescence microscopy and scanning electron microscopy on the outer surface of the catheters, hematoxylin/eosin staining, and periodic acid-Schiff staining of the mice skin tissues. Results: MY was found to inhibit the morphological transition of C. albicans and the secretion of exopolysaccharides, resulting in a reduction in biofilms. MY+MN@FFS exhibited excellent properties and no irritation to mice skin. In an in vivo anti-infection study, MY+MN@FFS exhibited an excellent preventive effect against percutaneously inserted C. albicans CRI. Conclusion: MY+MN@FFS might be a potential approach for effectively preventing percutaneously inserted C. albicans CRI in clinic.

Antifungal Mechanisms of a Chinese Herbal Medicine, Cao Huang Gui Xiang, Against Candida Species

Cao Huang Gui Xiang (CHGX) formula, a Chinese herbal medicine, has been empirically used for the treatment of Candida infections. In the present study, we discovered that the CHGX showed potent antifungal activities against the major human fungal pathogen Candida albicans and other clinical Candida species. Besides, we indicated that CHGX had in vivo efficacy on treating C. albicans infection in mice without noticeable toxicity at the clinical therapeutic concentration. We then set out to investigate the antifungal mechanisms of CHGX against C. albicans. We found that CHGX played an important role in inhibiting biofilm formation and filament development, two critical virulence factors of C. albicans. We further demonstrated that CHGX disrupted cell membrane integrity, triggered the accumulation of reactive oxygen species (ROS) and consumption of adenosine triphosphate (ATP), followed by a rapid fungal cell death in C. albicans. Multiple pathways, including the conserved Ras1-cAMP pathway and mitochondrial protein Mcu1 are involved in CHGX-induced cell death. Our finding expands the understanding of antifungal mechanism of CHGX against C. albicans, and provides new insights in treating patients with Candida infections in clinical practice.

CRISPR-Cas9 approach confirms Calcineurin-responsive zinc finger 1 (Crz1) transcription factor as a promising therapeutic target in echinocandin-resistant Candida glabrata

Invasive fungal infections, which kill more than 1.6 million patients each year worldwide, are difficult to treat due to the limited number of antifungal drugs (azoles, echinocandins, and polyenes) and the emergence of antifungal resistance. The transcription factor Crz1, a key regulator of cellular stress responses and virulence, is an attractive therapeutic target because this protein is absent in human cells. Here, we used a CRISPR-Cas9 approach to generate isogenic crz1Δ strains in two clinical isolates of caspofungin-resistant C. glabrata to analyze the role of this transcription factor in susceptibility to echinocandins, stress tolerance, biofilm formation, and pathogenicity in both non-vertebrate (Galleria mellonella) and vertebrate (mice) models of candidiasis. In these clinical isolates, CRZ1 disruption restores the susceptibility to echinocandins in both in vitro and in vivo models, and affects their oxidative stress response, biofilm formation, cell size, and pathogenicity. These results strongly suggest that Crz1 inhibitors may play an important role in the development of novel therapeutic agents against fungal infections considering the emergence of antifungal resistance and the low number of available antifungal drugs.

When Is It Appropriate to Take Off the Mask? Signaling Pathways That Regulate ß(1,3)-Glucan Exposure in Candida albicans

Candida spp. are an important source of systemic and mucosal infections in immune compromised populations. However, drug resistance or toxicity has put limits on the efficacy of current antifungals. The C. albicans cell wall is considered a good therapeutic target due to its roles in viability and fungal pathogenicity. One potential method for improving antifungal strategies could be to enhance the detection of fungal cell wall antigens by host immune cells. ß(1,3)-glucan, which is an important component of fungal cell walls, is a highly immunogenic epitope. Consequently, multiple host pattern recognition receptors, such as dectin-1, complement receptor 3 (CR3), and the ephrin type A receptor A (EphA2) are capable of recognizing exposed (unmasked) ß(1,3)-glucan moieties on the cell surface to initiate an anti-fungal immune response. However, ß(1,3)-glucan is normally covered (masked) by a layer of glycosylated proteins on the outer surface of the cell wall, hiding it from immune detection. In order to better understand possible mechanisms of unmasking ß(1,3)-glucan, we must develop a deeper comprehension of the pathways driving this phenotype. In this review, we describe the medical importance of ß(1,3)-glucan exposure in anti-fungal immunity, and highlight environmental stimuli and stressors encountered within the host that are capable of inducing changes in the levels of surface exposed ß(1,3)-glucan. Furthermore, particular focus is placed on how signal transduction cascades regulate changes in ß(1,3)-glucan exposure, as understanding the role that these pathways have in mediating this phenotype will be critical for future therapeutic development.

Sodium houttuyfonate enhances the mono-therapy of fluconazole on oropharyngeal candidiasis (OPC) through HIF-1α/IL-17 axis by inhibiting cAMP mediated filamentation in Candida albicans-Candida glabrata dual biofilms

Candida albicans and Candida glabrata are two common opportunistic fungi that can be co-isolated in oropharyngeal candidiasis (OPC). Hypha is a hallmark of the biofilm formation of C. albicans, indispensable for the attachment of C. glabrata, which is seldom in mycelial morphology. Increasing evidence reveals a hypoxic microenvironment in interior fungal biofilms, reminding of a fact that inflammation is usually accompanied by oxygen deprivation. As a result, it is assumed that the disaggregation of hypha-mediated hypoxia of biofilms might be a solution to alleviate OPC. Based on this hypothesis, sodium houttuyfonate (SH), a well-identified traditional herbal compound with antifungal activity, is used in combination with fluconazole (FLU), a well-informed synthesized antimycotics, to investigate their impact on filamentation in C. albicans and C. glabrata dual biofilms and the underlying mechanism of their combined treatment on OPC. The results show that compared with the single therapy, SH plus FLU can inhibit the hyphal growth in the mixed biofilms in vitro, decrease the fungal burden of oral tissues and internal organs, restore mucosal epithelial integrity and function, and reduce hypoxic microenvironment and inflammation in a mice OPC model. The possible mechanism of the combined therapy of SH plus FLU can be attributed to the regulation of HIF-1α/IL-17A axis through direct abrogation of the dual Candida biofilm formation. This study highlights the role of HIF-1α/IL-17A axis and the promising application of SH as a sensitizer of conventional antifungals in the treatment of OPC.

The δ subunit of F1Fo-ATP synthase is required for pathogenicity of Candida albicans

Fungal infections, especially candidiasis and aspergillosis, claim a high fatality rate. Fungal cell growth and function requires ATP, which is synthesized mainly through oxidative phosphorylation, with the key enzyme being F₁F_o-ATP synthase. Here, we show that deletion of the Candida albicans gene encoding the δ subunit of the F₁F_o-ATP synthase (ATP16) abrogates lethal infection in a mouse model of systemic candidiasis. The deletion does not substantially affect in vitro fungal growth or intracellular ATP concentrations, because the decrease in oxidative phosphorylation-derived ATP synthesis is compensated by enhanced glycolysis. However, the ATP16-deleted mutant displays decreased phosphofructokinase activity, leading to low fructose 1,6-bisphosphate levels, reduced activity of Ras1-dependent and -independent cAMP-PKA pathways, downregulation of virulence factors, and reduced pathogenicity. A structure-based virtual screening of small molecules leads to identification of a compound potentially targeting the δ subunit of fungal F₁F_o-ATP synthases. The compound induces in vitro phenotypes similar to those observed in the ATP16-deleted mutant, and protects mice from succumbing to invasive candidiasis. Our findings indicate that F₁F_o-ATP synthase δ subunit is required for C. albicans lethal infection and represents a potential therapeutic target.

Antifungal Activity and Potential Mechanism of 6,7, 4'-O-Triacetylscutellarein Combined With Fluconazole Against Drug-Resistant C. albicans

The increased resistance of Candida albicans to conventional antifungal drugs poses a huge challenge to the clinical treatment of this infection. In recent years, combination therapy, a potential treatment method to overcome C. albicans resistance, has gained traction. This study assessed the effect of 6,7,4′-O-triacetylscutellarein (TA) combined with fluconazole (FLC) on C. albicans in vitro and in vivo. TA combined with FLC showed good synergistic antifungal activity against drug-resistant C. albicans in vitro, with a partial inhibitory concentration index (FICI) of 0.0188–0.1800. In addition, the time-kill curve confirmed the synergistic effect of TA and FLC. TA combined with FLC showed a strong synergistic inhibitory effect on the biofilm formation of resistant C. albicans. The combined antifungal efficacy of TA and FLC was evaluated in vivo in a mouse systemic fungal infection model. TA combined with FLC prolonged the survival rate of mice infected with drug-resistant C. albicans and reduced tissue invasion. TA combined with FLC also significantly inhibited the yeast-hypha conversion of C. albicans and significantly reduced the expression of RAS-cAMP-PKA signaling pathway-related genes (RAS1 and EFG1) and hyphal-related genes (HWP1 and ECE1). Furthermore, the mycelium growth on TA combined with the FLC group recovered after adding exogenous db-cAMP. Collectively, these results show that TA combined with FLC inhibits the formation of hyphae and biofilms through the RAS-cAMP-PKA signaling pathway, resulting in reduced infectivity and resistance of C. albicans. Therefore, this study provides a basis for the treatment of drug-resistant C. albicans infections.

Integrative multi-omics profiling reveals cAMP-independent mechanisms regulating hyphal morphogenesis in Candida albicans

Microbial pathogens grow in a wide range of different morphologies that provide distinct advantages for virulence. In the fungal pathogen Candida albicans, adenylyl cyclase (Cyr1) is thought to be a master regulator of the switch to invasive hyphal morphogenesis and biofilm formation. However, faster growing cyr1Δ/Δ pseudorevertant (PR) mutants were identified that form hyphae in the absence of cAMP. Isolation of additional PR mutants revealed that their improved growth was due to loss of one copy of BCY1, the negative regulatory subunit of protein kinase A (PKA) from the left arm of chromosome 2. Furthermore, hyphal morphogenesis was improved in some of PR mutants by multigenic haploinsufficiency resulting from loss of large regions of the left arm of chromosome 2, including global transcriptional regulators. Interestingly, hyphal-associated genes were also induced in a manner that was independent of cAMP. This indicates that basal protein kinase A activity is an important prerequisite to induce hyphae, but activation of adenylyl cyclase is not needed. Instead, phosphoproteomic analysis indicated that the Cdc28 cyclin-dependent kinase and the casein kinase 1 family member Yck2 play key roles in promoting polarized growth. In addition, integrating transcriptomic and proteomic data reveals hyphal stimuli induce increased production of key transcription factors that contribute to polarized morphogenesis.

Quorum Sensing in Fungal Species

Quorum sensing (QS) is one of the most studied cell-cell communication mechanisms in fungi. Research in the last 20 years has explored various fungal QS systems that are involved in a wide range of biological processes, especially eukaryote- or fungus-specific behaviors, mirroring the significant contribution of QS regulation to fungal biology and evolution. Based on recent progress, we summarize in this review fungal QS regulation, with an emphasis on its functional role in behaviors unique to fungi or eukaryotes. We suggest that using fungi as genetically amenable eukaryotic model systems to address why and how QS regulation is integrated into eukaryotic reproductive strategies and molecular or cellular processes could be an important direction for QS research. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.