The MAP kinase signal transduction network in Candida albicans

A synthetic peptide mimic kills Candida albicans and synergistically prevents infection

More than two million people worldwide are affected by life-threatening, invasive fungal infections annually. Candida species are the most common cause of nosocomial, invasive fungal infections and are associated with mortality rates above 40%. Despite the increasing incidence of drug-resistance, the development of novel antifungal formulations has been limited. Here we investigate the antifungal mode of action and therapeutic potential of positively charged, synthetic peptide mimics to combat Candida albicans infections. Our data indicates that these synthetic polymers cause endoplasmic reticulum stress and affect protein glycosylation, a mode of action distinct from currently approved antifungal drugs. The most promising polymer composition damaged the mannan layer of the cell wall, with additional membrane-disrupting activity. The synergistic combination of the polymer with caspofungin prevented infection of human epithelial cells in vitro, improved fungal clearance by human macrophages, and significantly increased host survival in a Galleria mellonella model of systemic candidiasis. Additionally, prolonged exposure of C. albicans to the synergistic combination of polymer and caspofungin did not lead to the evolution of tolerant strains in vitro. Together, this work highlights the enormous potential of these synthetic peptide mimics to be used as novel antifungal formulations as well as adjunctive antifungal therapy.

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.

Regulation of symbiotic interactions and primitive lichen differentiation by UMP1 MAP kinase in Umbilicaria muhlenbergii

Lichens are of great ecological importance but mechanisms regulating lichen symbiosis are not clear. Umbilicaria muhlenbergii is a lichen-forming fungus amenable to molecular manipulations and dimorphic. Here, we established conditions conducive to symbiotic interactions and lichen differentiation and showed the importance of UMP1 MAP kinase in lichen development. In the initial biofilm-like symbiotic complexes, algal cells were interwoven with pseudohyphae covered with extracellular matrix. After longer incubation, fungal-algal complexes further differentiated into primitive lichen thalli with a melanized cortex-like and pseudoparenchyma-like tissues containing photoactive algal cells. Mutants deleted of UMP1 were blocked in pseudohyphal growth and development of biofilm-like complexes and primitive lichens. Invasion of dividing mother cells that contributes to algal layer organization in lichens was not observed in the ump1 mutant. Overall, these results showed regulatory roles of UMP1 in symbiotic interactions and lichen development and suitability of U. muhlenbergii as a model for studying lichen symbiosis.

The interactions of Candida albicans with gut bacteria: a new strategy to prevent and treat invasive intestinal candidiasis

BACKGROUND

The gut microbiota plays an important role in human health, as it can affect host immunity and susceptibility to infectious diseases. Invasive intestinal candidiasis is strongly associated with gut microbiota homeostasis. However, the nature of the interaction between Candida albicans and gut bacteria remains unclear.

OBJECTIVE

This review aimed to determine the nature of interaction and the effects of gut bacteria on C. albicans so as to comprehend an approach to reducing intestinal invasive infection by C. albicans.

METHODS

This review examined 11 common gut bacteria's interactions with C. albicans, including Escherichia coli, Pseudomonas aeruginosa, Acinetobacter baumannii, Enterococcus faecalis, Staphylococcus aureus, Salmonella spp., Helicobacter pylori, Lactobacillus spp., Bacteroides spp., Clostridium difficile, and Streptococcus spp.

RESULTS

Most of the studied bacteria demonstrated both synergistic and antagonistic effects with C. albicans, and just a few bacteria such as P. aeruginosa, Salmonella spp., and Lactobacillus spp. demonstrated only antagonism against C. albicans.

CONCLUSIONS

Based on the nature of interactions reported so far by the literature between gut bacteria and C. albicans, it is expected to provide new ideas for the prevention and treatment of invasive intestinal candidiasis.

A 4-Hydroxybenzoic Acid-Mediated Signaling System Controls the Physiology and Virulence of Shigella sonnei

Many bacteria use small molecules, such as quorum sensing (QS) signals, to perform intraspecies signaling and interspecies or interkingdom communication. Previous studies demonstrated that some bacteria regulate their physiology and pathogenicity by employing 4-hydroxybenzoic acid (4-HBA). Here, we report that 4-HBA controls biological functions, virulence, and anthranilic acid production in Shigella sonnei. The biosynthesis of 4-HBA is performed by UbiC (SSON_4219), which is a chorismate pyruvate-lyase that catalyzes the conversion of chorismate to 4-HBA. Deletion of ubiC caused S. sonnei to exhibit impaired phenotypes, including impaired biofilm formation, extracellular polysaccharide (EPS) production, and virulence. In addition, we found that 4-HBA controls the physiology and virulence of S. sonnei through the response regulator AaeR (SSON_3385), which contains a helix-turn-helix (HTH) domain and a LysR substrate-binding (LysR_substrate) domain. The same biological functions are controlled by AaeR and the 4-HBA signal, and 4-HBA-deficient mutant phenotypes were rescued by in trans expression of AaeR. We found that 4-HBA binds to AaeR and then enhances the binding of AaeR to the promoter DNA regions in target genes. Moreover, we revealed that 4-HBA from S. sonnei reduces the competitive fitness of Candida albicans by interfering with morphological transition. Together, our results suggested that the 4-HBA signaling system plays crucial roles in bacterial physiology and interkingdom communication. IMPORTANCE Shigella sonnei is an important pathogen in human intestines. Following previous findings that some bacteria employ 4-HBA as a QS signal to regulate biological functions, we demonstrate that 4-HBA controls the physiology and virulence of S. sonnei. This study is significant because it identifies both the signal synthase UbiC and receptor AaeR and unveils the signaling pathway of 4-HBA in S. sonnei. In addition, this study also supports the important role of 4-HBA in microbial cross talk, as 4-HBA strongly inhibits hyphal formation by Candida albicans. Together, our findings describe the dual roles of 4-HBA in both intraspecies signaling and interkingdom communication.

30th Annual GP2A Medicinal Chemistry Conference

The Group for the Promotion of Pharmaceutical Chemistry in Academia (GP2A) held their 30th annual conference in August 2022 in Trinity College Dublin, Ireland. There were 9 keynote presentations, 10 early career researcher presentations and 41 poster presentations.

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.

Antifungal Activity of Plant Secondary Metabolites on Candida albicans: An Updated Review

Fungal infections have been increasing continuously worldwide, especially in immunocompromised individuals. Fungi, regarded as eukaryotic pathogens, have many similarities to the host cells, which inhibit anti-fungal drug development progress. Various fungal model systems have been studied, and it was concluded that Candida spp. is the most common disease-causing fungus. Candida species are well known to cause infections not only in our mouth, skin, and vagina, but they are also a frequent cause of life-threatening hospital bloodstream infections. The morphological and developmental pathways of Candida have been studied extensively, providing insight into the fungus development. Candida albicans is known to be the most pathogenic species responsible for a variety of infections in humans. Conventional anti-fungal drugs, mainly azoles drugs available in the market, have been used for years developing resistance in C. albicans. Hence, the production of new anti-fungal drugs, which require detailed molecular knowledge of fungal pathogenesis, needs to be encouraged. Therefore, this review targets the new approach of "Green Medicines" or the phytochemicals and their secondary metabolites as a source of novel anti-fungal agents to overcome the drug resistance of C. albicans, their mechanism of action, and their combined effects with the available anti-fungal drugs.

Associations of Rap1 with Cell Wall Integrity, Biofilm Formation, and Virulence in Candida albicans

Rap1 (repressor activator protein 1) is a multifunctional protein, playing important roles in telomeric and nontelomeric functions in many eukaryotes. Candida albicans Rap1 has been previously shown to be involved in telomeric regulation, but its other functions are still mostly unknown. In this study, we found that the deletion of the RAP1 gene altered cell wall properties, composition, and gene expression. In addition, deletion of RAP1 affected C. albicans biofilm formation and modulated phagocytosis and cytokine release by host immune cells. Finally, the RAP1 gene deletion mutant showed attenuation of C. albicans virulence in a Galleria mellonella infection model. Therefore, these findings provide new insights into Rap1 functions that are particularly relevant to pathogenesis and virulence of C. albicans. IMPORTANCE C. albicans is an important fungal pathogen of humans. The cell wall is the outermost layer of C. albicans and is important for commensalism and infection by this pathogen. Moreover, the cell wall is also an important target for antifungals. Studies of how C. albicans maintains its cell wall integrity are critical for a better understanding of fungal pathogenesis and virulence. This work focuses on exploring unknown functions of C. albicans Rap1 and reveals its contribution to cell wall integrity, biofilm formation, and virulence. Notably, these findings will also improve our general understanding of complex machinery to control pathogenesis and virulence of fungal pathogens.

Inhibition of fungal pathogenicity by targeting the H2S-synthesizing enzyme cystathionine β-synthase

The global emergence of antifungal resistance threatens the limited arsenal of available treatments and emphasizes the urgent need for alternative antifungal agents. Targeting fungal pathogenic functions is an appealing alternative therapeutic strategy. Here, we show that cystathionine β-synthase (CBS), compared with cystathionine γ-lyase, is the major enzyme that synthesizes hydrogen sulfide in the pathogenic fungus Candida albicans. Deletion of CBS leads to deficiencies in resistance to oxidative stress, retarded cell growth, defective hyphal growth, and increased β-glucan exposure, which, together, reduce the pathogenicity of C. albicans. By high-throughput screening, we identified protolichesterinic acid, a natural molecule obtained from a lichen, as an inhibitor of CBS that neutralizes the virulence of C. albicans and exhibits therapeutic efficacy in a murine candidiasis model. These findings support the application of CBS as a potential therapeutic target to fight fungal infections.

The Role of Sfp1 in Candida albicans Cell Wall Maintenance

The cell wall is the first interface for Candida albicans interaction with the surrounding environment and the host cells. Therefore, maintenance of cell wall integrity (CWI) is crucial for C. albicans survival and host-pathogen interaction. In response to environmental stresses, C. albicans undergoes cell wall remodeling controlled by multiple signaling pathways and transcription regulators. Here, we explored the role of the transcription factor Sfp1 in CWI. A deletion of the SFP1 gene not only caused changes in cell wall properties, cell wall composition and structure but also modulated expression of cell wall biosynthesis and remodeling genes. In addition, Cas5 is a known transcription regulator for C. albicans CWI and cell wall stress response. Interestingly, our results indicated that Sfp1 negatively controls the CAS5 gene expression by binding to its promoter element. Together, this study provides new insights into the regulation of C. albicans CWI and stress response.

A Special Phenotype of Aconidial Aspergillus niger SH2 and Its Mechanism of Formation via CRISPRi

The complex morphological structure of Aspergillus niger influences its production of proteins, metabolites, etc., making the genetic manipulation and clonal purification of this species increasingly difficult, especially in aconidial Aspergillus niger. In this study, we found that N-acetyl-D-glucosamine (GlcNAc) could induce the formation of spore-like propagules in the aconidial Aspergillus niger SH2 strain. The spore-like propagules possessed life activities such as drug resistance, genetic transformation, and germination. Transcriptomic analysis indicated that the spore-like propagules were resting conidia entering dormancy and becoming more tolerant to environmental stresses. The Dac1 gene and the metabolic pathway of GlcNAc converted to glycolysis are related to the formation of the spore-like propagules, as evidenced by the CRISPRi system, qPCR, and semi-quantitative RT-PCR. Moreover, a method based on the CRISPR-Cas9 tool to rapidly recycle screening tags and recover genes was suitable for Aspergillus niger SH2. To sum up, this suggests that the spore-like propagules are resting conidia and the mechanism of their formation is the metabolic pathway of GlcNAc converted to glycolysis, particularly the Dac1 gene. This study can improve our understanding of the critical factors involved in mechanisms of phenotypic change and provides a good model for researching phenotypic change in filamentous fungi.

Candida albicans SR-Like Protein Kinases Regulate Different Cellular Processes: Sky1 Is Involved in Control of Ion Homeostasis, While Sky2 Is Important for Dipeptide Utilization

Protein kinases play a crucial role in regulating cellular processes such as growth, proliferation, environmental adaptation and stress responses. Serine-arginine (SR) protein kinases are highly conserved in eukaryotes and regulate fundamental processes such as constitutive and alternative splicing, mRNA processing and ion homeostasis. The Candida albicans genome encodes two (Sky1, Sky2) and the Candida glabrata genome has one homolog (Sky1) of the human SR protein kinase 1, but their functions have not yet been investigated. We used deletion strains of the corresponding genes in both fungi to study their cellular functions. C. glabrata and C. albicans strains lacking SKY1 exhibited higher resistance to osmotic stress and toxic polyamine concentrations, similar to Saccharomyces cerevisiae sky1Δ mutants. Deletion of SKY2 in C. albicans resulted in impaired utilization of various dipeptides as the sole nitrogen source. Subsequent phosphoproteomic analysis identified the di- and tripeptide transporter Ptr22 as a potential Sky2 substrate. Sky2 seems to be involved in Ptr22 regulation since overexpression of PTR22 in the sky2Δ mutant restored the ability to grow on dipeptides and made the cells more susceptible to the dipeptide antifungals Polyoxin D and Nikkomycin Z. Altogether, our results demonstrate that C. albicans and C. glabrata Sky1 protein kinases are functionally similar to Sky1 in S. cerevisiae, whereas C. albicans Sky2, a unique kinase of the CTG clade, likely regulates dipeptide uptake via Ptr22.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

The antifungal and antibiofilm activity of Cymbopogon nardus essential oil and citronellal on clinical strains of Candida albicans

OBJECTIVE

This study investigated the antifungal and antibiofilm activity of Cymbopogon nardus essential oil (EO) and its major compound, citronellal, in association with miconazole and chlorhexidine on clinical strains of Candida albicans. The likely mechanism(s) of action of C. nardus EO and citronellal was further determined.

MATERIALS AND METHODS

The EO was chemically characterized by gas chromatography coupled with mass spectrometry (GC-MS). The antifungal activity (MIC/MFC) and antibiofilm effects of C. nardus EO and citronellal were determined by the microdilution method, and their likely mechanism(s) of action was determined by the sorbitol and ergosterol assays. Then, the samples were tested for a potential association with standard drugs through the checkerboard technique. Miconazole and chlorhexidine were used as positive controls and the assays were performed in triplicate.

RESULTS

The GC-MS analysis tentatively identified citronellal as the major compound in C. nardus EO. Both samples showed antifungal activity, with MIC of 256 µg/mL, as compared to 128 µg/mL and 8 µg/mL of miconazole and chlorhexidine, respectively. C. nardus EO and citronellal effectively inhibited biofilm formation (p < 0.05) and disrupted preformed biofilms (p < 0.0001). They most likely interact with the cell membrane, but not the cell wall, and did not present any synergistic activity when associated with standard drugs.

CONCLUSION

C. nardus EO and citronellal showed strong in vitro antifungal and antibiofilm activity on C. albicans.

CLINICAL RELEVANCE

Natural products have been historically bioprospected for novel solutions to control fungal biofilms. Our data provide relevant insights into the potential of C. nardus EO and citronellal for further clinical testing. However, additional bioavailability and toxicity studies must be carried out before these products can be used for the chemical control of oral biofilms.

MaSln1, a Conserved Histidine Protein Kinase, Contributes to Conidiation Pattern Shift Independent of the MAPK Pathway in Metarhizium acridum

As a conserved sensor kinase in the HOG-MAPK pathway, Sln1 plays distinct functions in different fungi. In this study, the roles of MaSln1 in Metarhizium acridum were analyzed using gene knockout and rescue strategies. Deletion of MaSln1 did not affect conidial germination, conidial yield, or resistance to chemical agents. However, fungal tolerance to heat shock and UV-B were significantly reduced after deletion of MaSln1. Insect bioassays showed that fungal pathogenicity was significantly impaired when MaSln1 was deleted. Further studies showed that MaSln1 did not affect either germination or appressorium formation of M. acridum on locust wings, but it significantly increased appressorium turgor pressure. In addition, disruption of MaSln1 resulted in a conidiation pattern shift in M. acridum. Microscopic observation revealed, however, that some genes located in the MAPK signaling pathway, including MaSho1, MaHog1, MaMk1, and MaSlt2, were not involved in the conidiation pattern shift on SYA medium (microcycle medium). Meanwhile, of the 143 differently expressed genes (DEGs) identified by RNA-seq, no genes related to the MAPK pathway were found, suggesting that MaSln1 regulation of the conidiation pattern shift was probably independent of the conserved MAPK signaling pathway. It was found that 22 of the 98 known DEGs regulated by MaSln1 were involved in mycelial growth, cell division, and cytoskeleton formation, indicating that MaSln1 likely regulates the expression of genes related to cell division and morphogenesis, thus regulating the conidiation pattern shift in M. acridum.

IMPORTANCE The productivity and quality of conidia are both crucial for mycopesticides. In this study, we systematically analyzed the roles of MaSln1 in fungal pathogens. Most importantly, our results revealed that deletion of MaSln1 resulted in a conidiation pattern shift in M. acridum. However, some other genes, located in the MAPK signaling pathway, were not involved in the conidiation pattern shift. RNA-seq revealed no genes related to the MAPK pathway, suggesting that the regulation of the conidiation pattern shift by MaSln1 was probably independent of the conserved MAPK signaling pathway. This study provided a new insight into the functions of Sln1 and laid a foundation for exploring the mechanisms of conidiation pattern shifts in M. acridum.

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.

Adaptation to Endoplasmic Reticulum Stress in Candida albicans Relies on the Activity of the Hog1 Mitogen-Activated Protein Kinase

Adaptation to ER stress is linked to the pathogenicity of C. albicans. The fungus responds to ER stress primarily by activating the conserved Ire1-Hac1-dependent unfolded protein response (UPR) pathway. Subsequently, when ER homeostasis is re-established, the UPR is attenuated in a timely manner, a facet that is unexplored in C. albicans. Here, we show that C. albicans licenses the HOG (high-osmolarity glycerol) MAPK pathway for abating ER stress as evidenced by activation and translocation of Hog1 to the nucleus during tunicamycin-induced ER stress. We find that, once activated, Hog1 attenuates the activity of Ire1-dependent UPR, thus facilitating adaptation to ER stress. We use the previously established assay, where the disappearance of the UPR-induced spliced HAC1 mRNA correlates with the re-establishment of ER homeostasis, to investigate attenuation of the UPR in C. albicans. hog1Δ/Δ cells retain spliced HAC1 mRNA levels for longer duration reflecting the delay in attenuating Ire1-dependent UPR. Conversely, compromising the expression of Ire1 (ire1 DX mutant strain) results in diminished levels of phosphorylated Hog1, restating the cross-talk between Ire1 and HOG pathways. Phosphorylation signal to Hog1 MAP kinase is relayed through Ssk1 in response to ER stress as inactivation of Ssk1 abrogates Hog1 phosphorylation in C. albicans. Additionally, Hog1 depends on its cytosolic as well as nuclear activity for mediating ER stress-specific responses in the fungus. Our results show that HOG pathway serves as a point of cross-talk with the UPR pathway, thus extending the role of this signaling pathway in promoting adaptation to ER stress in C. albicans. Additionally, this study integrates this MAPK pathway into the little known frame of ER stress adaptation pathways in C. albicans.

Adaptation of the emerging pathogenic yeast Candida auris to high caspofungin concentrations correlates with cell wall changes

Candida auris has emerged as a fungal pathogen that causes nosocomial outbreaks worldwide. Diseases caused by this fungus are of concern, due to its reduced susceptibility to several antifungals. C. auris exhibits paradoxical growth (PG; defined as growth at high, but not intermediate antifungal concentrations) in the presence of caspofungin (CPF). We have characterized the cellular changes associated with adaptation to CPF. Using EUCAST AFST protocols, all C. auris isolates tested showed PG to CPF, although in some isolates it was more prominent. Most isolates also showed a trailing effect (TE) to micafungin and anidulafungin. We identified two FKS genes in C. auris that encode the echinocandins target, namely β-1,3-glucan synthase. FKS1 contained the consensus hot-spot (HS) 1 and HS2 sequences. FKS2 only contained the HS1 region which had a change (F635Y), that has been shown to confer resistance to echinocandins in C. glabrata. PG has been characterized in other species, mainly C. albicans, where high CPF concentrations induced an increase in chitin, cell volume and aggregation. In C. auris CPF only induced a slight accumulation of chitin, and none of the other phenomena. RNAseq experiments demonstrated that CPF induced the expression of genes encoding several GPI-anchored cell wall proteins, membrane proteins required for the stability of the cell wall, chitin synthase and mitogen-activated protein kinases (MAPKs) involved in cell integrity, such as BCK2, HOG1 and MKC1 (SLT2). Our work highlights some of the processes induced in C. auris to adapt to echinocandins.

Effect of diglyceryl dicaprylate on Candida albicans growth and pathogenicity

The antifungal effect of diglyceryl dicaprylate, one of the emulsifiers used as a food additive, on Candida albicans that is a pathogenic fungus that is predominant in the oral cavity was investigated. This component did not affect C. albicans growth; however, it suppressed some virulence factors in a concentration-dependent manner. Furthermore, the suppression of pathogenic factors, such as biofilm formation, adhesion, highly pathogenic dimorphism, and ability to produce proteolytic enzymes, was due to reduction in mRNA expression levels of genes involved in fungal pathogenicities. From these results, this emulsifier could potentially prevent the development of intraoral and extraoral diseases involving C. albicans and could potentially use in oral care and improvement of quality of life.

The SAGA and NuA4 component Tra1 regulates Candida albicans drug resistance and pathogenesis

Candida albicans is the most common cause of death from fungal infections. The emergence of resistant strains reducing the efficacy of first-line therapy with echinocandins, such as caspofungin calls for the identification of alternative therapeutic strategies. Tra1 is an essential component of the SAGA and NuA4 transcriptional co-activator complexes. As a PIKK family member, Tra1 is characterized by a C-terminal phosphoinositide 3-kinase domain. In Saccharomyces cerevisiae, the assembly and function of SAGA and NuA4 are compromised by a Tra1 variant (Tra1Q3) with three arginine residues in the putative ATP-binding cleft changed to glutamine. Whole transcriptome analysis of the S. cerevisiae tra1Q3 strain highlights Tra1's role in global transcription, stress response, and cell wall integrity. As a result, tra1Q3 increases susceptibility to multiple stressors, including caspofungin. Moreover, the same tra1Q3 allele in the pathogenic yeast C. albicans causes similar phenotypes, suggesting that Tra1 broadly mediates the antifungal response across yeast species. Transcriptional profiling in C. albicans identified 68 genes that were differentially expressed when the tra1Q3 strain was treated with caspofungin, as compared to gene expression changes induced by either tra1Q3 or caspofungin alone. Included in this set were genes involved in cell wall maintenance, adhesion, and filamentous growth. Indeed, the tra1Q3 allele reduces filamentation and other pathogenesis traits in C. albicans. Thus, Tra1 emerges as a promising therapeutic target for fungal infections.

Fungal Cell Wall Proteins and Signaling Pathways Form a Cytoprotective Network to Combat Stresses

Candida species are part of the normal flora of humans, but once the immune system of the host is impaired and they escape from commensal niches, they shift from commensal to pathogen causing candidiasis. Candida albicans remains the primary cause of candidiasis, accounting for about 60% of the global candidiasis burden. The cell wall of C. albicans and related fungal pathogens forms the interface with the host, gives fungal cells their shape, and also provides protection against stresses. The cell wall is a dynamic organelle with great adaptive flexibility that allows remodeling, morphogenesis, and changes in its components in response to the environment. It is mainly composed of the inner polysaccharide rich layer (chitin, and β-glucan) and the outer protein coat (mannoproteins). The highly glycosylated protein coat mediates interactions between C. albicans cells and their environment, including reprograming of wall architecture in response to several conditions, such as carbon source, pH, high temperature, and morphogenesis. The mannoproteins are also associated with C. albicans adherence, drug resistance, and virulence. Vitally, the mannoproteins contribute to cell wall construction and especially cell wall remodeling when cells encounter physical and chemical stresses. This review describes the interconnected cell wall integrity (CWI) and stress-activated pathways (e.g., Hog1, Cek1, and Mkc1 mediated pathways) that regulates cell wall remodeling and the expression of some of the mannoproteins in C. albicans and other species. The mannoproteins of the surface coat is of great importance to pathogen survival, growth, and virulence, thus understanding their structure and function as well as regulatory mechanisms can pave the way for better management of candidiasis.

The NDR kinase-MOB complex FgCot1-Mob2 regulates polarity and lipid metabolism in Fusarium graminearum

Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential for cell differentiation and polarized morphogenesis. However, their functions in plant pathogenic fungi are not well understood. Here, we characterized the NDR kinase FgCot1 and its activator FgMob2 in Fusarium graminearum, a major pathogen causing Fusarium head blight (FHB) in wheat. FgCot1 and FgMob2 formed a NDR kinase-MOB protein complex. Localization assays using FgCot1-GFP or FgMob2-RFP constructs showed diverse subcellular localizations, including cytoplasm, septum, nucleus and hyphal tip. ΔFgcot1 and ΔFgmob2 exhibited serious defects in hyphal growth, polarity, fungal development and cell wall integrity as well as reduced virulence in planta. In contrast, lipid droplet accumulation was significantly increased in these two mutants. Phosphorylation of FgCot1 at two highly conserved residues (S462 and T630) as well as five new sites synergistically contributed its role in various cellular processes. In addition, non-synonymous mutations in two MAPK (mitogen-activated protein kinase) proteins, FgSte11 and FgGpmk1, partially rescued the growth defect of ΔFgmob2, indicating a functional link between the FgCot1-Mob2 complex and the FgGpmk1 signalling pathway in regulating filamentous fungal growth. These results indicated that the FgCot1-Mob2 complex is critical for polarity, fungal development, cell wall organization, lipid metabolism and virulence in F. graminearum.

Revisiting the Vital Drivers and Mechanisms of β-Glucan Masking in Human Fungal Pathogen, Candida albicans

Among the several human fungal pathogens, Candida genus represents one of the most implicated in the clinical scenario. There exist several distinctive features that govern the establishment of Candida infections in addition to their capacity to adapt to multiple stress conditions inside humans which also include evasion of host immune responses. The complex fungal cell wall of the prevalent pathogen, Candida albicans, is one of the main targets of antifungal drugs and recognized by host immune cells. The wall consists of tiered arrangement of an outer thin but dense covering of mannan and inner buried layers of β-glucan and chitin. However, the pathogenic fungi adopt strategies to evade immune recognition by masking these molecules. This capacity to camouflage the immunogenic polysaccharide β-glucan from the host is a key virulence factor of C. albicans. The present review is an attempt to collate various underlying factors and mechanisms involved in Candida β-glucan masking from the available pool of knowledge and provide a comprehensive understanding. This will further improve therapeutic approaches to candidiasis by identifying new antifungal targets that blocks fungal immune evasion.

Rondonin: antimicrobial properties and mechanism of action

Infectious diseases are among the major causes of death in the human population. A wide variety of organisms produce antimicrobial peptides (AMPs) as part of their first line of defense. A peptide from Acanthoscurria rondoniae plasma, rondonin—with antifungal activity, a molecular mass of 1236 Da and primary sequence IIIQYEGHKH—was previously studied (UniProt accession number B3EWP8). It showed identity with the C terminus of subunit ‘D’ of the hemocyanin of the Aphonopelma hentzi spider. This result led us to propose a new pathway of the immune system of arachnids that suggests a new function to hemocyanin: production of antimicrobial peptides. Rondonin does not interact with model membranes and was able to bind to yeast nucleic acids but not bacteria. It was not cytotoxic against mammalian cells. The antifungal activity of rondonin is pH‐dependent and peaks at pH ˜ 4–5. The peptide presents synergism with gomesin (spider hemocyte antimicrobial peptide—UniProtKB—P82358) against human yeast pathogens, suggesting a new potential alternative treatment option. Antiviral activity was detected against RNA viruses, measles, H1N1, and encephalomyocarditis. This is the first report of an arthropod hemocyanin fragment with activity against human viruses. Currently, it is vital to invest in the search for natural and synthetic antimicrobial compounds that, above all, present alternative mechanisms of action to first‐choice antimicrobials.

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.

Specific Functional Features of the Cell Integrity MAP Kinase Pathway in the Dimorphic Fission Yeast Schizosaccharomyces japonicus

Mitogen activated protein kinase (MAPK) signaling pathways execute essential functions in eukaryotic organisms by transducing extracellular stimuli into adaptive cellular responses. In the fission yeast model Schizosaccharomyces pombe the cell integrity pathway (CIP) and its core effector, MAPK Pmk1, play a key role during regulation of cell integrity, cytokinesis, and ionic homeostasis. Schizosaccharomyces japonicus, another fission yeast species, shows remarkable differences with respect to S. pombe, including a robust yeast to hyphae dimorphism in response to environmental changes. We show that the CIP MAPK module architecture and its upstream regulators, PKC orthologs Pck1 and Pck2, are conserved in both fission yeast species. However, some of S. pombe's CIP-related functions, such as cytokinetic control and response to glucose availability, are regulated differently in S. japonicus. Moreover, Pck1 and Pck2 antagonistically regulate S. japonicus hyphal differentiation through fine-tuning of Pmk1 activity. Chimeric MAPK-swapping experiments revealed that S. japonicus Pmk1 is fully functional in S. pombe, whereas S. pombe Pmk1 shows a limited ability to execute CIP functions and promote S. japonicus mycelial development. Our findings also suggest that a modified N-lobe domain secondary structure within S. japonicus Pmk1 has a major influence on the CIP signaling features of this evolutionarily diverged fission yeast.

Exploring Small Heat Shock Proteins (sHSPs) for Targeting Drug Resistance in Candida albicans and other Pathogenic Fungi

Fungal infections have predominantly increased worldwide that leads to morbidity and mortality in severe cases. Invasive candidiasis and other pathogenic fungal infections are a major problem in immunocompromised individuals and post-operative patients. Increasing resistance to existing antifungal drugs calls for the identification of novel antifungal drug targets for chemotherapeutic interventions. This demand for identification and characterization of novel drug targets leads to the development of effective antifungal therapy against drug resistant fungi. Heat shock proteins (HSPs) are important for various biological processes like protein folding, posttranslational modifications, transcription, translation, and protein aggregation. HSPs are involved in maintaining homeostasis of the cell. A subgroup of HSPs is small heat shock proteins (sHSPs), which functions as cellular chaperones. They are having a significant role in the many cellular functions like development, cytoskeletal organization, apoptosis, membrane lipid polymorphism, differentiation, autophagy, in infection recognition and are major players in various stresses like osmotic stress, pH stress, etc. Studies have shown that fungal cells express increased levels of sHSPs upon antifungal drug induced stress responses. Here we review the important role of small heat shock proteins (sHSPs) in fungal diseases and their potential as antifungal targets.

Anti-adherence and Anti-fungal Abilities of Thymol and Carvacrol Against Candida Species Isolated From Patients with Oral Candidiasis in Comparison with Fluconazole and Voriconazole

Background: Natural isopropyl cresols, such as thymol and carvacrol, have been known to have antifungal activities. Objectives: The current study aimed to investigate the anti-adherence and antifungal activities of thymol, carvacrol, fluconazole, and voriconazole against oral isolates of Candida albicans (C. albicans), C. glabrata, and C. krusei. Methods: The susceptibility assay for the test compounds was performed using the disk diffusion method against all Candida isolates. Also, anti-adherence activity was examined using a rapid and highly reproducible 96 well microtiter-based method. Results: Both natural phenols and antifungal drugs revealed various efficacies against studied Candida species. The susceptibility to fluconazole and voriconazole were 100% for C. albicans, 50% and 90% for C. glabrata, and 0% and 100% for C. krusei isolates, respectively. The mean diameter of the inhibition zone was greater for thymol than carvacrol in C. albicans (19.89 ± 0.80 mm versus 17.05 ± 0.61 mm), C. glabrata (18.87 ± 0.71 mm versus 15.77 ± 0.57 mm), and C. krusei (15.11 ± 0.91 mm versus 13.91 ± 1.04 mm) isolates tested. Thymol showed more effective inhibition on adherence of all Candida species than other treatments. The mean relative adherence ratios for C. albicans, C. glabrata, and C. krusei were 0.50, 0.60, and 0.64, respectively. Conclusions: This study demonstrated significant inhibitory properties of thymol and carvacrol on the adherence and growth of azole susceptible- and -resistant Candida isolates. Also, thymol was more effective for preventing the adherence of yeast cells to polystyrene in comparison to carvacrol.

N-acetylglucosamine-mediated morphological transition in Candida albicans and Candida tropicalis

Morphological transitions in Candida species are key factors in facilitating invasion and adapting to environmental changes. N-acetylglucosamine (GlcNAc) is a monosaccharide signalling molecule that can regulate morphological transitions in Candida albicans and Candida tropicalis. Interestingly, although the uptake and metabolic pathways of GlcNAc and GlcNAc-mediated white-to-opaque cell switching are similar between the two Candida species, GlcNAc induces hyphal development in C. albicans, whereas it suppresses hyphal development in C. tropicalis. These findings indicate that the characteristics of C. albicans and C. tropicalis in response to GlcNAc are remarkably different. Here, we compare the conserved and divergent GlcNAc-mediated signalling pathways and catabolism between the two Candida species. Deletion of NGT1, a GlcNAc transportation gene, inhibited hyphal formation in C. albicans but promoted hyphal development in C. tropicalis. To further understand these opposite effects on filamentous growth in response to GlcNAc in the two Candida species, the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) signalling pathways in both C. albicans and C. tropicalis were compared. Interestingly, GlcNAc activated the cAMP/PKA signalling pathway of the two Candida species, suggesting that the hyphal development-regulated circuit is remarkably diverse between the two species. Indeed, the Ndt80-like gene REP1, which is critical for regulating GlcNAc catabolism, exhibits distinct roles in the hyphal development of C. albicans and C. tropicalis. These data suggest possible reasons for the divergent hyphal growth response in C. albicans and C. tropicalis upon GlcNAc induction.

Transcriptomic and proteomic profiling revealed reprogramming of carbon metabolism in acetate-grown human pathogen Candida glabrata

BACKGROUND

Emergence of Candida glabrata, which causes potential life-threatening invasive candidiasis, has been widely associated with high morbidity and mortality. In order to cause disease in vivo, a robust and highly efficient metabolic adaptation is crucial for the survival of this fungal pathogen in human host. In fact, reprogramming of the carbon metabolism is believed to be indispensable for phagocytosed C. glabrata within glucose deprivation condition during infection.

METHODS

In this study, the metabolic responses of C. glabrata under acetate growth condition was explored using high-throughput transcriptomic and proteomic approaches.

RESULTS

Collectively, a total of 1482 transcripts (26.96%) and 242 proteins (24.69%) were significantly up- or down-regulated. Both transcriptome and proteome data revealed that the regulation of alternative carbon metabolism in C. glabrata resembled other fungal pathogens such as Candida albicans and Cryptococcus neoformans, with up-regulation of many proteins and transcripts from the glyoxylate cycle and gluconeogenesis, namely isocitrate lyase (ICL1), malate synthase (MLS1), phosphoenolpyruvate carboxykinase (PCK1) and fructose 1,6-biphosphatase (FBP1). In the absence of glucose, C. glabrata shifted its metabolism from glucose catabolism to anabolism of glucose intermediates from the available carbon source. This observation essentially suggests that the glyoxylate cycle and gluconeogenesis are potentially critical for the survival of phagocytosed C. glabrata within the glucose-deficient macrophages.

CONCLUSION

Here, we presented the first global metabolic responses of C. glabrata to alternative carbon source using transcriptomic and proteomic approaches. These findings implicated that reprogramming of the alternative carbon metabolism during glucose deprivation could enhance the survival and persistence of C. glabrata within the host.

The Fungicidal Action of Micafungin is Independent on Both Oxidative Stress Generation and HOG Pathway Signaling in Candida albicans

In fungi, the Mitogen-Activated Protein kinase (MAPK) pathways sense a wide variety of environmental stimuli, leading to cell adaptation and survival. The HOG pathway plays an essential role in the pathobiology of Candida albicans, including the colonization of the gastrointestinal tract in a mouse model, virulence, and response to stress. Here, we examined the role of Hog1 in the C. albicans response to the clinically relevant antifungal Micafungin (MF), whose minimum inhibitory concentration (MIC) was identical in the parental strain (RM100) and in the isogenic homozygous mutant hog1 (0.016 mg/L). The cell viability was impaired without significant differences between the parental strain, the isogenic hog1 mutant, and the Hog1⁺ reintegrant. This phenotype was quite similar in a collection of hog1 mutants constructed in a different C. albicans background. MF-treated cells failed to induce a relevant increase of both reactive oxygen species (ROS) formation and activation of the mitochondrial membrane potential in parental and hog1 cells. MF was also unable to trigger any significant activation of the genes coding for the antioxidant activities catalase (CAT1) and superoxide dismutase (SOD2), as well as on the corresponding enzymatic activities, whereas a clear induction was observed in the presence of Amphotericin B (AMB), introduced as a positive control of Hog1 signaling. Furthermore, Hog1 was not phosphorylated by the addition of MF, but, notably, this echinocandin caused Mkc1 phosphorylation. Our results strongly suggest that the toxic effect of MF on C. albicans cells is not mediated by the Hog1 MAPK and is independent of the generation of an internal oxidative stress in C. albicans.

The Two-Component Response Regulator Ssk1 and the Mitogen-Activated Protein Kinase Hog1 Control Antifungal Drug Resistance and Cell Wall Architecture of Candida auris

Candida auris is an emerging multidrug-resistant (MDR) fungal pathogen that presents a serious global threat to human health. The Centers for Disease Control and Prevention (CDC) have classified C. auris as an urgent threat to public health for the next decade due to its major clinical and economic impact and the lack of effective antifungal drugs and because of future projections concerning new C. auris infections. Importantly, the Global Antimicrobial Resistance Surveillance System (GLASS) has highlighted the need for more robust and efficacious global surveillance schemes enabling the identification and monitoring of antifungal resistance in Candida infections. Despite the clinical relevance of C. auris infections, our overall understanding of its pathophysiology and virulence, its response to human immune surveillance, and the molecular basis of multiple antifungal resistance remains in its infancy. Here, we show a marked phenotypic plasticity of C. auris clinical isolates. Further, we demonstrate critical roles of stress response mechanisms in regulating multidrug resistance and show that cell wall architecture and composition are key elements that determine antifungal drug susceptibilities. Our data promise new therapeutic options to treat drug-refractory C. auris infections.

Candida auris is an emerging multidrug-resistant human fungal pathogen refractory to treatment by several classes of antifungal drugs. Unlike other Candida species, C. auris can adhere to human skin for prolonged periods of time, allowing for efficient skin-to-skin transmission in the hospital environments. However, molecular mechanisms underlying pronounced multidrug resistance and adhesion traits are poorly understood. Two-component signal transduction and mitogen-activated protein (MAP) kinase signaling are important regulators of adherence, antifungal drug resistance, and virulence. Here, we report that genetic removal of SSK1 encoding a response regulator and the mitogen-associated protein kinase HOG1 restores the susceptibility to both amphotericin B (AMB) and caspofungin (CAS) in C. auris clinical strains. The loss of SSK1 and HOG1 alters membrane lipid permeability, cell wall mannan content, and hyperresistance to cell wall-perturbing agents. Interestingly, our data reveal variable functions of SSK1 and HOG1 in different C. auris clinical isolates, suggesting a pronounced genetic plasticity affecting cell wall function, stress adaptation, and multidrug resistance. Taken together, our data suggest that targeting two-component signal transduction systems could be suitable for restoring C. auris susceptibility to antifungal drugs.

IMPORTANCECandida auris is an emerging multidrug-resistant (MDR) fungal pathogen that presents a serious global threat to human health. The Centers for Disease Control and Prevention (CDC) have classified C. auris as an urgent threat to public health for the next decade due to its major clinical and economic impact and the lack of effective antifungal drugs and because of future projections concerning new C. auris infections. Importantly, the Global Antimicrobial Resistance Surveillance System (GLASS) has highlighted the need for more robust and efficacious global surveillance schemes enabling the identification and monitoring of antifungal resistance in Candida infections. Despite the clinical relevance of C. auris infections, our overall understanding of its pathophysiology and virulence, its response to human immune surveillance, and the molecular basis of multiple antifungal resistance remains in its infancy. Here, we show a marked phenotypic plasticity of C. auris clinical isolates. Further, we demonstrate critical roles of stress response mechanisms in regulating multidrug resistance and show that cell wall architecture and composition are key elements that determine antifungal drug susceptibilities. Our data promise new therapeutic options to treat drug-refractory C. auris infections.

Signalling mechanisms involved in stress response to antifungal drugs

The emergence of antifungal resistance is a serious threat in the treatment of mycoses. The primary susceptible fungal cells may evolve a resistance after longer exposure to antifungal agents. The exposure itself causes stress condition, to which the fungus needs to adapt. This review provides detailed description of evolutionary conserved molecular mechanisms contributing to the adaptation response to stress caused by antifungal agents as well as their interconnection. The knowledge may help us to find new ways to delay the emergence of drug resistance as the same mechanisms are used regardless of what antifungal compound causes stress.

Transcriptional regulation of the caspofungin-induced cell wall damage response in Candida albicans

The human fungal pathogen Candida albicans maintains pathogenic and commensal states primarily through cell wall functions. The echinocandin antifungal drug caspofungin inhibits cell wall synthesis and is widely used in treating disseminated candidiasis. Signaling pathways are critical in coordinating the adaptive response to cell wall damage (CWD). C. albicans executes a robust transcriptional program following caspofungin-induced CWD. A comprehensive analysis of signaling pathways at the transcriptional level facilitates the identification of prospective genes for functional characterization and propels the development of novel antifungal interventions. This review article focuses on the molecular functions and signaling crosstalk of the C. albicans transcription factors Sko1, Rlm1, and Cas5 in caspofungin-induced CWD signaling.

Iron alters the cell wall composition and intracellular lactate to affect Candida albicans susceptibility to antifungals and host immune response

Fungal pathogen Candida albicans has a complex cell wall consisting of an outer layer of mannans and an inner layer of β-glucans and chitin. The fungal cell wall is the primary target for antifungals and is recognized by host immune cells. Environmental conditions such as carbon sources, pH, temperature, and oxygen tension can modulate the fungal cell wall architecture. Cellular signaling pathways, including the mitogen-activated protein kinase (MAPK) pathways, are responsible for sensing environmental cues and mediating cell wall alterations. Although iron has recently been shown to affect β-1,3-glucan exposure on the cell wall, we report here that iron changes the composition of all major C. albicans cell wall components. Specifically, high iron decreased the levels of mannans (including phosphomannans) and chitin; and increased β-1,3-glucan levels. These changes increased the resistance of C. albicans to cell wall-perturbing antifungals. Moreover, high iron cells exhibited adequate mitochondrial functioning; leading to a reduction in accumulation of lactate that signals through the transcription factor Crz1 to induce β-1,3-glucan masking in C. albicans We show here that iron-induced changes in β-1,3-glucan exposure are lactate-dependent; and high iron causes β-1,3-glucan exposure by preventing lactate-induced, Crz1-mediated inhibition of activation of the fungal MAPK Cek1. Furthermore, despite exhibiting enhanced antifungal resistance, high iron C. albicans cells had reduced survival upon phagocytosis by macrophages. Our results underscore the role of iron as an environmental signal in multiple signaling pathways that alter cell wall architecture in C. albicans, thereby affecting its survival upon exposure to antifungals and host immune response.

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.

SPT20 Regulates the Hog1-MAPK Pathway and Is Involved in Candida albicans Response to Hyperosmotic Stress

Candida albicans is the most common fungal pathogen and relies on the Hog1-MAPK pathway to resist osmotic stress posed by the environment or during host invasions. Here, we investigated the role of SPT20 in response to osmotic stress. Testing a C. albicans spt20Δ/Δ mutant, we found it was sensitive to osmotic stress. Using sequence alignment, we identified the conserved functional domains between CaSpt20 and ScSpt20. Reconstitution of the Spt20 function in a spt20Δ/CaSPT20 complemented strain found CaSPT20 can suppress the high sensitivity to hyperosmotic stressors, a cell wall stress agent, and antifungal drugs in the Saccharomyces cerevisiae spt20Δ/Δ mutant background. We measured the cellular glycerol accumulation and found it was significantly lower in the C. albicans spt20Δ/Δ mutant strain, compared to the wild type strain SC5314 (P < 0.001). This result was also supported by quantitative reverse transcription-PCR, which showed the expression levels of gene contributing to glycerol accumulation were reduced in Caspt20Δ/Δ compared to wild type (GPD2 and TGL1, P < 0.001), while ADH7 and AGP2, whose expression can lead to glycerol decrease, were induced when cells were exposed to high osmolarity (ADH7, P < 0.001; AGP2, P = 0.002). In addition, we tested the transcription levels of Hog1-dependent osmotic stress response genes, and found that they were significantly upregulated in wild type cells encountering hyperosmolarity, while the expression of HGT10, SKO1, CAT1, and SLP3 were not induced when SPT20 was deleted. Although the transcript of ORF19.3661 and ORF19.4370 in Caspt20Δ/Δ was induced in the presence of 1 M NaCl, the levels were less than what was observed in the wild type (ORF19.3661, P = 0.007; ORF19.4370, P = 0.011). Moreover, the deletion of CaSPT20 in C. albicans reduced phosphorylation levels of Hog1. These findings suggested that SPT20 is conserved between yeast and C. albicans and plays an important role in adapting to osmotic stress through regulating Hog1-MAPK pathway.

The Aspergillus fumigatus Phosphoproteome Reveals Roles of High-Osmolarity Glycerol Mitogen-Activated Protein Kinases in Promoting Cell Wall Damage and Caspofungin Tolerance

Aspergillus fumigatus is an opportunistic human pathogen causing allergic reactions or systemic infections, such as invasive pulmonary aspergillosis in immunocompromised patients. The mitogen-activated protein kinase (MAPK) signaling pathways are essential for fungal adaptation to the human host. Fungal cell survival, fungicide tolerance, and virulence are highly dependent on the organization, composition, and function of the cell wall. Upon cell wall stress, MAPKs phosphorylate multiple target proteins involved in the remodeling of the cell wall. Here, we investigate the global phosphoproteome of the ΔsakA and ΔmpkCA. fumigatus and high-osmolarity glycerol (HOG) pathway MAPK mutants upon cell wall damage. This showed the involvement of the HOG pathway and identified novel protein kinases and transcription factors, which were confirmed by fungal genetics to be involved in promoting tolerance of cell wall damage. Our results provide understanding of how fungal signal transduction networks modulate the cell wall. This may also lead to the discovery of new fungicide drug targets to impact fungal cell wall function, fungicide tolerance, and virulence.

The filamentous fungus Aspergillus fumigatus can cause a distinct set of clinical disorders in humans. Invasive aspergillosis (IA) is the most common life-threatening fungal disease of immunocompromised humans. The mitogen-activated protein kinase (MAPK) signaling pathways are essential to the adaptation to the human host. Fungal cell survival is highly dependent on the organization, composition, and function of the cell wall. Here, an evaluation of the global A. fumigatus phosphoproteome under cell wall stress caused by the cell wall-damaging agent Congo red (CR) revealed 485 proteins potentially involved in the cell wall damage response. Comparative phosphoproteome analyses with the ΔsakA, ΔmpkC, and ΔsakA ΔmpkC mutant strains from the osmotic stress MAPK cascades identify their additional roles during the cell wall stress response. Our phosphoproteomics allowed the identification of novel kinases and transcription factors (TFs) involved in osmotic stress and in the cell wall integrity (CWI) pathway. Our global phosphoproteome network analysis showed an enrichment for protein kinases, RNA recognition motif domains, and the MAPK signaling pathway. In contrast to the wild-type strain, there is an overall decrease of differentially phosphorylated kinases and phosphatases in ΔsakA, ΔmpkC, and ΔsakA ΔmpkC mutants. We constructed phosphomutants for the phosphorylation sites of several proteins differentially phosphorylated in the wild-type and mutant strains. For all the phosphomutants, there is an increase in the sensitivity to cell wall-damaging agents and a reduction in the MpkA phosphorylation upon CR stress, suggesting these phosphosites could be important for the MpkA modulation and CWI pathway regulation.

Cooperative Role of MAPK Pathways in the Interaction of Candida albicans with the Host Epithelium

Candida albicans is an important human fungal pathogen responsible for tens of millions of infections as well as hundreds of thousands of severe life-threatening infections each year. MAP kinase (MAPK) signal transduction pathways facilitate the sensing and adaptation to external stimuli and control the expression of key virulence factors such as the yeast-to-hypha transition, the biogenesis of the cell wall, and the interaction with the host. In the present study, we have combined molecular approaches and infection biology to analyse the role of C. albicans MAPK pathways during an epithelial invasion. Hog1 was found to be important for adhesion to abiotic surfaces but was dispensable for damage to epithelial cells. The Mkc1 cell wall integrity (CWI) and Cek1 pathways, on the other hand, were both required for oral epithelial damage. Analysis of the ability to penetrate nutrient-rich semi-solid media revealed a cooperative role for Cek1 and Mkc1 in this process. Finally, cek2Δ (as well as cek1Δ) but not mkc1Δ or hog1Δ mutants, exhibited elevated β-glucan unmasking as revealed by immunofluorescence studies. Therefore, the four MAPK pathways play distinct roles in adhesion, epithelial damage, invasion and cell wall remodelling that may contribute to the pathogenicity of C. albicans.

CgHog1 controls the adaptation to both sorbitol and fludioxonil in Colletotrichum gloeosporioides

The HOG (high-osmolarity glycerol) pathway is critical for the appropriate adaptation to adverse conditions. Here, we demonstrated that the deletion of CgHog1 resulted in enhanced sensitivity to osmotic stress and increased resistance to fludioxonil in the poplar anthracnose fungus Colletotrichum gloeosporioides. The accumulation of chitin around hyphal tips was obviously decreased in the ΔCgHog1 strain under sorbitol, whereas it strongly was increased in the response to fludioxonil compared with the wild type. To investigate the underlying mechanism of CgHog1-mediated adaption to osmotic stress and fludioxonil, transcriptomic profiles were performed in both the ΔCgHog1 strain and the wild type under the treatment of sorbitol and fludioxonil, respectively. Under sorbitol, genes associated with glycolysis, lipid metabolism, and accumulation of soluble sugars and amino acids were differentially expressed; under fludioxonil, vesicle trafficking-related genes were highly downregulated in the ΔCgHog1 strain, which was consistent with abnormal vacuoles distribution and morphology of hyphae, indicating that the growth defect caused by fludioxonil may be associated with disruption of endocytosis. Taken together, we elucidated the adaptation mechanisms of how CgHog1 regulates appropriate response to sorbitol and fludioxonil via different metabolism pathways. These findings extend our insights into the HOG pathway in fungi.

Lrg1 Regulates β (1,3)-Glucan Masking in Candida albicans through the Cek1 MAP Kinase Pathway

Candida albicans is among the most prevalent opportunistic human fungal pathogens. The ability to mask the immunogenic polysaccharide β (1,3)-glucan from immune detection via a layer of mannosylated proteins is a key virulence factor of C. albicans We previously reported that hyperactivation of the Cek1 mitogen-activated protein (MAP) kinase pathway promotes β (1,3)-glucan exposure. In this communication, we report a novel upstream regulator of Cek1 activation and characterize the impact of Cek1 activity on fungal virulence. Lrg1 encodes a GTPase-activating protein (GAP) that has been suggested to inhibit the GTPase Rho1. We found that disruption of LRG1 causes Cek1 hyperactivation and β (1,3)-glucan unmasking. However, when GTPase activation was measured for a panel of GTPases, the lrg1ΔΔ mutant exhibited increased activation of Cdc42 and Ras1 but not Rho1 or Rac1. Unmasking and Cek1 activation in the lrg1ΔΔ mutant can be blocked by inhibition of the Ste11 MAP kinase kinase kinase (MAPKKK), indicating that the lrg1ΔΔ mutant acts through the canonical Cek1 MAP kinase cascade. In order to determine how Cek1 hyperactivation specifically impacts virulence, a doxycycline-repressible hyperactive STE11ΔN467 allele was expressed in C. albicans In the absence of doxycycline, this allele overexpressed STE11ΔN467 , which induced production of proinflammatory tumor necrosis factor alpha (TNF-α) from murine macrophages. This in vitro phenotype correlates with decreased colonization and virulence in a mouse model of systemic infection. The mechanism by which Ste11ΔN467 causes unmasking was explored with RNA sequencing (RNA-Seq) analysis. Overexpression of Ste11ΔN467 caused upregulation of the Cph1 transcription factor and of a group of cell wall-modifying proteins which are predicted to impact cell wall architecture.IMPORTANCECandida albicans is an important source of systemic infections in humans. The ability to mask the immunogenic cell wall polymer β (1,3)-glucan from host immune surveillance contributes to fungal virulence. We previously reported that the hyperactivation of the Cek1 MAP kinase cascade promotes cell wall unmasking, thus increasing strain immunogenicity. In this study, we identified a novel regulator of the Cek1 pathway called Lrg1. Lrg1 is a predicted GTPase-activating protein (GAP) that represses Cek1 activity by downregulating the GTPase Cdc42 and its downstream MAPKKK, Ste11. Upregulation of Cek1 activity diminished fungal virulence in the mouse model of infection, and this correlates with increased cytokine responses from macrophages. We also analyzed the transcriptional profile determined during β (1,3)-glucan exposure driven by Cek1 hyperactivation. Our report provides a model where Cek1 hyperactivation causes β (1,3)-glucan exposure by upregulation of cell wall proteins and leads to more robust immune detection in vivo, promoting more effective clearance.

The small GTPase Rhb1 is involved in the cell response to fluconazole in Candida albicans

Candida albicans is an important fungal pathogen in humans. Rhb1 is a small GTPase of the Ras superfamily and is conserved from yeasts to humans. In C. albicans, Rhb1 regulates the expression of secreted protease 2, low nitrogen-mediated morphogenesis, and biofilm formation. Moreover, our previous studies have indicated that Rhb1 is associated with the target of rapamycin (TOR) signaling pathway. In this study, we further explored the relationship between Rhb1 and drug susceptibility. The RHB1 deletion mutant exhibited reduced fluconazole susceptibility, and this phenotype occurred mainly through the increased gene expression and activity of efflux pumps. In addition, Mrr1 and Tac1 are transcription factors that can activate efflux pump gene expression. However, the RHB1 deletion, RHB1/MRR1 and RHB1/TAC1 double deletion mutants had no significant differences in efflux pump gene expression and fluconazole susceptibility, suggesting that Rhb1-regulated efflux pump genes do not act through Mrr1 and Tac1. We also showed that membrane localization is crucial for Rhb1 activity in response to fluconazole. Finally, Rhb1 was linked not only to the TOR but also to the Mkc1 mitogen-activated protein kinase signaling pathway in response to fluconazole. In sum, this study unveiled a new role of Rhb1 in the regulation of C. albicans drug susceptibility.

Not just the wall: the other ways to turn the yeast CWI pathway on

The Saccharomyces cerevisiae cell wall integrity (CWI) pathway took this name when its role in the cell response to cell wall aggressions was clearly established. The receptors involved in sensing the damage, the relevant components operating in signaling to the MAPK Slt2, the transcription factors activated by this MAPK, as well as some key regulatory mechanisms have been identified and characterized along almost 30 years. However, other stimuli that do not alter specifically the yeast cell wall, including protein unfolding, low or high pH, or plasma membrane, oxidative and genotoxic stresses, have been also found to trigger the activation of this pathway. In this review, we compile almost forty non-cell wall-specific compounds or conditions, such as tunicamycin, hypo-osmotic shock, diamide, hydroxyurea, arsenate, and rapamycin, which induce these stresses. Relevant aspects of the CWI-mediated signaling in the response to these non-conventional pathway activators are discussed. The data presented here highlight the central and key position of the CWI pathway in the safeguard of yeast cells to a wide variety of external aggressions.

The Fungal Histone Acetyl Transferase Gcn5 Controls Virulence of the Human Pathogen Candida albicans through Multiple Pathways

Fungal virulence is regulated by a tight interplay of transcriptional control and chromatin remodelling. Despite compelling evidence that lysine acetylation modulates virulence of pathogenic fungi such as Candida albicans, the underlying mechanisms have remained largely unexplored. We report here that Gcn5, a paradigm lysyl-acetyl transferase (KAT) modifying both histone and non-histone targets, controls fungal morphogenesis - a key virulence factor of C. albicans. Our data show that genetic removal of GCN5 abrogates fungal virulence in mice, suggesting strongly diminished fungal fitness in vivo. This may at least in part arise from increased susceptibility to killing by macrophages, as well as by other phagocytes such as neutrophils or monocytes. Loss of GCN5 also causes hypersensitivity to the fungicidal drug caspofungin. Caspofungin hypersusceptibility requires the master regulator Efg1, working in concert with Gcn5. Moreover, Gcn5 regulates multiple independent pathways, including adhesion, cell wall-mediated MAP kinase signaling, hypersensitivity to host-derived oxidative stress, and regulation of the Fks1 glucan synthase, all of which play critical roles in virulence and antifungal susceptibility. Hence, Gcn5 regulates fungal virulence through multiple mechanisms, suggesting that specific inhibition of Gcn5 could offer new therapeutic strategies to combat invasive fungal infections.

Deletion of FgHOG1 Is Suppressive to the mgv1 Mutant by Stimulating Gpmk1 Activation and Avoiding Intracellular Turgor Elevation in Fusarium graminearum

Fusarium head blight caused by Fusarium graminearum is an important disease of wheat and barley. Previous studies have showed that all three MAP kinase genes, MGV1, FgHOG1, and GPMK1, are involved in regulating hyphal growth, sexual reproduction, plant infection, and stress responses in this pathogen. To determine the relationship between the Mgv1 and FgHog1 pathways, in this study, we generated and characterized the mgv1 Fghog1 double mutant. Deletion of FgHOG1 partially rescued the defects of the mgv1 mutant in vegetative growth and cell wall integrity but had no effects on its defects in plant infection and DON production. The mgv1 Fghog1 mutant grew faster and was more tolerant to cell wall stressors than the mgv1 mutant. Swollen compartments and cell burst were observed frequently in the mgv1 mutant but rarely in the mgv1 Fghog1 mutant when treated with fungicide fludioxonil or cell wall stressor Congo red. Conversely, the deletion of MGV1 also alleviated the hyperosmotic sensitivity of the Fghog1 mutant in vegetative growth. TGY assays indicated increased phosphorylation of FgHog1 in the mgv1 mutant, and TEY assays further revealed elevated activation of Gpmk1 in the mgv1 Fghog1 double mutant, particularly under cell wall stress conditions. Overall, our data showed that deletion of FgHOG1 partially suppressed the defects of the mgv1 mutant, possibly by affecting genes related to cell wall integrity and osmoregulation via the over-activation of Gpmk1 MAP kinase and avoiding intracellular turgor elevation.

The Transcription Factor Sfp1 Regulates the Oxidative Stress Response in Candida albicans

Candida albicans is a commensal that inhabits the skin and mucous membranes of humans. Because of the increasing immunocompromised population and the limited classes of antifungal drugs available, C. albicans has emerged as an important opportunistic pathogen with high mortality rates. During infection and therapy, C. albicans frequently encounters immune cells and antifungal drugs, many of which exert their antimicrobial activity by inducing the production of reactive oxygen species (ROS). Therefore, antioxidative capacity is important for the survival and pathogenesis of C. albicans. In this study, we characterized the roles of the zinc finger transcription factor Sfp1 in the oxidative stress response against C. albicans. A sfp1-deleted mutant was more resistant to oxidants and macrophage killing than wild-type C. albicans and processed an active oxidative stress response with the phosphorylation of the mitogen-activated protein kinase (MAPK) Hog1 and high CAP1 expression. Moreover, the sfp1-deleted mutant exhibited high expression levels of antioxidant genes in response to oxidative stress, resulting in a higher total antioxidant capacity, glutathione content, and glutathione peroxidase and superoxide dismutase enzyme activity than the wild-type C. albicans. Finally, the sfp1-deleted mutant was resistant to macrophage killing and ROS-generating antifungal drugs. Together, our findings provide a new understanding of the complex regulatory machinery in the C. albicans oxidative stress response.

Aspergillus fumigatus High Osmolarity Glycerol Mitogen Activated Protein Kinases SakA and MpkC Physically Interact During Osmotic and Cell Wall Stresses

Aspergillusfumigatus, a saprophytic filamentous fungus, is a serious opportunistic pathogen of mammals and it is the primary causal agent of invasive aspergillosis (IA). Mitogen activated protein Kinases (MAPKs) are important components involved in diverse cellular processes in eukaryotes. A. fumigatus MpkC and SakA, the homologs of the Saccharomyces cerevisiae Hog1 are important to adaptations to oxidative and osmotic stresses, heat shock, cell wall damage, macrophage recognition, and full virulence. We performed protein pull-down experiments aiming to identify interaction partners of SakA and MpkC by mass spectrometry analysis. In presence of osmotic stress with sorbitol, 118, and 213 proteins were detected as possible protein interactors of SakA and MpkC, respectively. Under cell wall stress caused by congo red, 420 and 299 proteins were detected interacting with SakA and MpkC, respectively. Interestingly, a group of 78 and 256 proteins were common to both interactome analysis. Co-immunoprecipitation (Co-IP) experiments showed that SakA::GFP is physically associated with MpkC:3xHA upon osmotic and cell wall stresses. We also validated the association between SakA:GFP and the cell wall integrity MAPK MpkA:3xHA and the phosphatase PtcB:3xHA, under cell wall stress. We further characterized A. fumigatus PakA, the homolog of the S. cerevisiae sexual developmental serine/threonine kinase Ste20, as a component of the SakA/MpkC MAPK pathway. The ΔpakA strain is more sensitive to cell wall damaging agents as congo red, calcofluor white, and caspofungin. Together, our data supporting the hypothesis that SakA and MpkC are part of an osmotic and general signal pathways involved in regulation of the response to the cell wall damage, oxidative stress, drug resistance, and establishment of infection. This manuscript describes an important biological resource to understand SakA and MpkC protein interactions. Further investigation of the biological roles played by these protein interactors will provide more opportunities to understand and combat IA.