Characterization of a hyperactive Cyr1 mutant reveals new regulatory mechanisms for cellular cAMP levels in Candida 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.

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.

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

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

Fluorescent toys 'n' tools lighting the way in fungal research

Although largely overlooked compared to bacterial infections, fungal infections pose a significant threat to the health of humans and other organisms. Many pathogenic fungi, especially Candida species, are extremely versatile and flexible in adapting to various host niches and stressful situations. This leads to high pathogenicity and increasing resistance to existing drugs. Due to the high level of conservation between fungi and mammalian cells, it is hard to find fungus-specific drug targets for novel therapy development. In this respect, it is vital to understand how these fungi function on a molecular, cellular as well as organismal level. Fluorescence imaging allows for detailed analysis of molecular mechanisms, cellular structures and interactions on different levels. In this manuscript, we provide researchers with an elaborate and contemporary overview of fluorescence techniques that can be used to study fungal pathogens. We focus on the available fluorescent labelling techniques and guide our readers through the different relevant applications of fluorescent imaging, from subcellular events to multispecies interactions and diagnostics. As well as cautioning researchers for potential challenges and obstacles, we offer hands-on tips and tricks for efficient experimentation and share our expert-view on future developments and possible improvements.

Molecular Elucidation of Riboflavin Production and Regulation in Candida albicans, toward a Novel Antifungal Drug Target

Candida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

Candida albicans is a major cause of fungal infections, both superficial and invasive. The economic costs as well as consequences for patient welfare are substantial. Only a few treatment options are available due to the high resemblance between fungal targets and host molecules, as both are eukaryotes. Riboflavin is a yellow pigment, also termed vitamin B₂. Unlike animals, fungi can synthesize this essential component themselves, thereby leading us to appreciate that targeting riboflavin production is a promising novel strategy against fungal infections. Here, we report that the GTP cyclohydrolase encoded by C. albicansRIB1 (CaRIB1) is essential and rate-limiting for production of riboflavin in the fungal pathogen. We confirm the high potential of CaRib1 as an antifungal drug target, as its deletion completely impairs in vivo infectibility by C. albicans in model systems. Furthermore, the stimulating effect of iron deprivation and PKA activation on riboflavin production seems to involve CaRib1 and the upstream transcription factor CaSef1. Gathering insights in the synthesis mechanism of riboflavin in pathogenic fungi, like C. albicans, will allow us to design a novel strategy and specifically target this process to combat fungal infections.

IMPORTANCECandida albicans is an important fungal pathogen causing common superficial infections as well as invasive diseases with an extremely high morbidity and mortality. Antifungal therapies are limited in efficiency and availability. In this research, we describe the regulation of riboflavin production in C. albicans. Since riboflavin biosynthesis is essential to this organism, we can appreciate that targeting it would be a promising new strategy to combat these fungal infections. We provide evidence that one particular enzyme in the production process, CaRib1, would be most promising as an antifungal drug target, as it plays a central role in regulation and proves to be essential in a mouse model of systemic infection.

N-Acetylglucosamine Regulates Morphogenesis and Virulence Pathways in Fungi

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

Protein-Protein Interactions in Candida albicans

Despite being one of the most important human fungal pathogens, Candida albicans has not been studied extensively at the level of protein-protein interactions (PPIs) and data on PPIs are not readily available in online databases. In January 2018, the database called "Biological General Repository for Interaction Datasets (BioGRID)" that contains the most PPIs for C. albicans, only documented 188 physical or direct PPIs (release 3.4.156) while several more can be found in the literature. Other databases such as the String database, the Molecular INTeraction Database (MINT), and the Database for Interacting Proteins (DIP) database contain even fewer interactions or do not even include C. albicans as a searchable term. Because of the non-canonical codon usage of C. albicans where CUG is translated as serine rather than leucine, it is often problematic to use the yeast two-hybrid system in Saccharomyces cerevisiae to study C. albicans PPIs. However, studying PPIs is crucial to gain a thorough understanding of the function of proteins, biological processes and pathways. PPIs can also be potential drug targets. To aid in creating PPI networks and updating the BioGRID, we performed an exhaustive literature search in order to provide, in an accessible format, a more extensive list of known PPIs in C. albicans.

The possible molecular mechanisms of farnesol on the antifungal resistance of C. albicans biofilms: the regulation of CYR1 and PDE2

Background

Farnesol has potential antifungal activity against Candida albicans biofilms, but the molecular mechanism of this activity is still unclear. Farnesol inhibits hyphal growth by regulating the cyclic AMP (cAMP) signalling pathway in C. albicans, and CYR1 and PDE2 regulate a pair of enzymes that are directly responsible for cAMP synthesis and degradation. Here, we hypothesize that farnesol enhances the antifungal susceptibility of C. albicans biofilms by regulating CYR1 and PDE2.

Results

The resistance of the CYR1- and PDE2-overexpressing strains to caspofungin, itraconazole and terbinafine was increased in planktonic cells, and that to amphotericin B was increased in biofilms. Meanwhile, the biofilms of the CYR1- and PDE2-overexpressing strains were thicker (all p < 0.05) and consisted of more hyphae than that of the wild strain. The intracellular cAMP levels were higher in the biofilms of the CYR1-overexpressing strain than that in the biofilms of the wild strain (all p < 0.01), while no changes were found in the PDE2-overexpressing strain. Exogenous farnesol decreased the resistance of the CYR1- and PDE2-overexpressing strains to these four antifungals, repressed the hyphal and biofilm formation of the strains, and decreased the intracellular cAMP level in the biofilms (all p < 0.05) compared to the untreated controls. In addition, farnesol decreased the expression of the gene CYR1 and the protein CYR1 in biofilms of the CYR1-overexpressing strain (all p < 0.05) but increased the expression of the gene PDE2 and the protein PDE2 in biofilms of the PDE2-overexpressing strain (all p < 0.01).

Conclusions

The results indicate that CYR1 and PDE2 regulate the resistance of C. albicans biofilms to antifungals. Farnesol suppresses the resistance of C. albicans biofilms to antifungals by regulating the expression of the gene CYR1 and PDE2, while PDE2 regulation was subordinate to CYR1 regulation.

A Bimolecular Fluorescence Complementation Tool for Identification of Protein-Protein Interactions in Candida albicans

Investigation of protein-protein interactions (PPI) in Candida albicans is essential for understanding the regulation of the signal transduction network that triggers its pathogenic lifestyle. Unique features of C. albicans, such as its alternative codon usage and incomplete meiosis, have enforced the optimization of standard genetic methods as well as development of novel approaches. Since the existing methods for detection of PPI are limited for direct visualization of the interacting complex in vivo, we have established a bimolecular fluorescence complementation (BiFC) assay in C. albicans, a powerful technique for studying PPI. We have developed an optimized set of plasmids that allows for N- and C-terminal tagging of proteins with split yeast-enhanced monomeric Venus fragments, so that all eight combinations of fusion orientations can be analyzed. With the use of our BiFC assay we demonstrate three interaction complexes in vivo, which were also confirmed by two-hybrid analysis. Our Candida-optimized BiFC assay represents a useful molecular tool for PPI studies and shows great promise in expanding our knowledge of molecular mechanisms of protein functions.

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

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

cAMP-independent signal pathways stimulate hyphal morphogenesis in Candida albicans

The fungal pathogen Candida albicans can transition from budding to hyphal growth, which promotes biofilm formation and invasive growth into tissues. Stimulation of adenylyl cyclase to form cAMP induces hyphal morphogenesis. The failure of cells lacking adenylyl cyclase (cyr1Δ) to form hyphae has suggested that cAMP signaling is essential for hyphal growth. However, cyr1Δ mutants also grow slowly and have defects in morphogenesis, making it unclear whether hyphal inducers must stimulate cAMP, or if normal basal levels of cAMP are required to maintain cellular health needed for hyphal growth. Interestingly, supplementation of cyr1Δ cells with low levels of cAMP enabled them to form hyphae in response to the inducer N-acetylglucosamine (GlcNAc), suggesting that a basal level of cAMP is sufficient for stimulation. Furthermore, we isolated faster-growing cyr1Δ pseudorevertant strains that can be induced to form hyphae even though they lack cAMP. The pseudorevertant strains were not induced by CO₂ , consistent with reports that CO₂ directly stimulates adenylyl cyclase. Mutational analysis showed that induction of hyphae in a pseudorevertant strain was independent of RAS1, but was dependent on the EFG1 transcription factor that acts downstream of protein kinase A. Thus, cAMP-independent signals contribute to the induction of hyphal responses.

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

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

Looking into Candida albicans infection, host response, and antifungal strategies

Candida albicans, a commonly encountered fungal pathogen, causes diseases varying from superficial mucosal complaints to life-threatening systemic disorders. Among the virulence traits of C. albicans, yeast-to-hypha transition is most widely acknowledged. Host innate immunity to C. albicans critically requires pattern recognition receptors (PRRs), and defence against C. albicans infection is provided by an exquisite interplay between the innate and adaptive arms of the host immune system.

Farnesol and cyclic AMP signaling effects on the hypha-to-yeast transition in Candida albicans

Candida albicans, a fungal pathogen of humans, regulates its morphology in response to many environmental cues and this morphological plasticity contributes to virulence. Farnesol, an autoregulatory molecule produced by C. albicans, inhibits the induction of hyphal growth by inhibiting adenylate cyclase (Cyr1). The role of farnesol and Cyr1 in controlling the maintenance of hyphal growth has been less clear. Here, we demonstrate that preformed hyphae transition to growth as yeast in response to farnesol and that strains with increased cyclic AMP (cAMP) signaling exhibit more resistance to farnesol. Exogenous farnesol did not induce the hypha-to-yeast transition in mutants lacking the Tup1 or Nrg1 transcriptional repressors in embedded conditions. Although body temperature is not required for embedded hyphal growth, we found that the effect of farnesol on the hypha-to-yeast transition varies inversely with temperature. Our model of Cyr1 activity being required for filamentation is also supported by our liquid assay data, which show increased yeast formation when preformed filaments are treated with farnesol. Together, these data suggest that farnesol can modulate morphology in preformed hyphal cells and that the repression of hyphal growth maintenance likely occurs through the inhibition of cAMP signaling.

Molecular mechanisms of feedback inhibition of protein kinase A on intracellular cAMP accumulation

The cAMP-protein kinase A (PKA) pathway is a major signalling pathway in the yeast Saccharomyces cerevisiae, but also in many other eukaryotic cell types, including mammalian cells. Since cAMP plays a crucial role as second messenger in the regulation of this pathway, its levels are strictly controlled, both in the basal condition and after induction by agonists. A major factor in the down-regulation of the cAMP level after stimulation is PKA itself. Activation of PKA triggers feedback down-regulation of the increased cAMP level, stimulating its return to the basal concentration. This is accomplished at different levels. The best documented mechanisms are: inhibition of cAMP synthesis by down-regulation of adenylate cyclase and/or its regulatory proteins, stimulation of cAMP breakdown by phosphodiesterases and spatial regulation of cAMP levels in the cell by A-Kinase Anchoring Proteins (AKAPs). In this review we describe these processes in detail for S. cerevisiae, for cells of mammals and selected other organisms, and we hint at other possible targets for feedback regulation of intracellular cAMP levels.