The putative transcription factor CaRtg3 is involved in tolerance to cations and antifungal drugs as well as serum-induced filamentation in Candida albicans

RetroGREAT signaling: The lessons we learn from yeast

The mitochondrial retrograde signaling (RTG) pathway of communication from mitochondria to the nucleus was first studied in yeast Saccharomyces cerevisiae. It rewires cellular metabolism according to the mitochondrial state by reprogramming nuclear gene expression in response to mitochondrial triggers. The main players involved in retrograde signaling are the Rtg1 and Rtg3 transcription factors, and a set of positive and negative regulators, including the Rtg2, Mks1, Lst8, and Bmh1/2 proteins. Retrograde regulation is integrated with other processes, including stress response, osmoregulation, and nutrient sensing through functional crosstalk with cellular pathways such as high osmolarity glycerol or target of rapamycin signaling. In this review, we summarize metabolic changes observed upon retrograde stimulation and analyze the progress made to uncover the mechanisms underlying the integration of regulatory circuits. Comparisons of the evolutionary adaptations of the retrograde pathway that have occurred in the different yeast groups can help to fully understand the process.

Transcription factor GlbHLH regulates hyphal growth, stress resistance, and polysaccharide biosynthesis in Ganoderma lucidum

Basic helix-loop-helix (bHLH) transcription factors (TFs) participate in many physiological and cellular processes in eukaryotes. However, their functions remain unclear in the macro basidiomycete Ganoderma lucidum (G. lucidum). In this study, a gene encoding bHLH TF, GlbHLH, was identified in G. lucidum. The knockdown of GlbHLH by RNA interference reduced hyphal growth, hyphal branching, and resistant to osmotic, oxidative, and cell wall stress. The content of cell wall components β-1,3 glucan and chitin and the expression of their synthesis genes were decreased in the GlbHLH knockdown strains. The knockdown of GlbHLH led to an increase of intracellular reactive oxygen species by decreasing the enzyme activity and gene expression of antioxidant enzymes. Furthermore, the production of intracellular polysaccharides and extracellular polysaccharides was greatly decreased in the GlbHLH mutants. These results suggested that GlbHLH is involved in hyphal growth, stress response, and polysaccharide biosynthesis in G. lucidum.

Mechanistic Insights into the Anticandidal Action of Vanillin Reveal Disruption of Cell Surface Integrity and Mitochondrial Functioning

BACKGROUND

Considering the emergence of multidrug resistance (MDR) in prevalent human fungal pathogen, Candida albicans, there is a parallel spurt in the development of novel strategies aimed to disrupt MDR. Compounds from natural resources could be exploited as efficient antifungal drugs owing to their structural diversity, cost effectiveness and negligible side effects.

OBJECTIVE

The present study elucidates the antifungal mechanisms of Vanillin (Van), a natural food flavoring agent against Candida albicans.

METHODS

Antifungal activities were assessed by broth microdilution and spot assays. Membrane and cell wall perturbations were studied by PI uptake, electron microscopy, plasma membrane H+ extrusion activity and estimation of ergosterol and chitin contents. Mitochondrial functioning was studied by growth on non-fermentable carbon sources, rhodamine B labeling and using retrograde signaling mutants. Gene expressions were validated by semi-quantitative RT-PCR.

RESULTS

We observed that the antifungal activity of Van was not only limited to clinical isolates of C. albicans but also against non-albicans species of Candida. Mechanistic insights revealed the effect of Van on cell surface integrity as evident from hypersensitivity against membrane perturbing agent SDS, depleted ergosterol levels, transmission electron micrographs and diminished plasma membrane H+ extrusion activity. In addition, spot assays with cell wall perturbing agents, scanning electron micrographs, delayed sedimentation rate and lower chitin content further substantiate cell wall damage by Van. Furthermore, Van treated cells underwent mitochondrial dysfunctioning via impaired retrograde signaling leading to abrogated iron homeostasis and DNA damage. All the perturbed phenotypes were also validated by RT-PCR depicting differential regulation of genes (NPC2, KRE62, FTR2 and CSM3) in response to Van.

CONCLUSION

Together, our results suggested that Van is promising antifungal agent that may be advocated for further investigation in therapeutic strategies to treat Candida infections.

Transcriptional control of hyphal morphogenesis in Candida albicans

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

RNA sequencing reveals an additional Crz1-binding motif in promoters of its target genes in the human fungal pathogen Candida albicans

Background

The calcium/calcineurin signaling pathway is mediated by the transcription factors NFAT (nuclear factor of activated T cells) in mammals and Crz1 (calcineurin-responsive zinc finger 1) in yeasts and other lower eukaryotes. A previous microarray analysis identified a putative Crz1-binding motif in promoters of its target genes in Candida albicans, but it has not been experimentally demonstrated.

Methods

An inactivation mutant for CaCRZ1 was generated through CRISPR/Cas9 approach. Transcript profiling was carried out by RNA sequencing of the wild type and the inactivation mutant for CaCRZ1 in response to 0.2 M CaCl₂. Gene promoters were scanned by the online MEME (Multiple Em for Motif Elicitation) software. Gel electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) analysis were used for in vitro and in vivo CaCrz1-binding experiments, respectively.

Results

RNA sequencing reveals that expression of 219 genes is positively, and expression of 59 genes is negatively, controlled by CaCrz1 in response to calcium stress. These genes function in metabolism, cell cycling, protein fate, cellular transport, signal transduction, transcription, and cell wall biogenesis. Forty of these positively regulated 219 genes have previously been identified by DNA microarray analysis. Promoter analysis of these common 40 genes reveals a consensus motif [5′-GGAGGC(G/A)C(T/A)G-3′], which is different from the putative CaCrz1-binding motif [5′-G(C/T)GGT-3′] identified in the previous study, but similar to Saccharomyces cerevisiae ScCrz1-binding motif [5′-GNGGC(G/T)CA-3′]. EMSA and ChIP assays indicate that CaCrz1 binds in vitro and in vivo to both motifs in the promoter of its target gene CaUTR2. Promoter mutagenesis demonstrates that these two CaCrz1-binding motifs play additive roles in the regulation of CaUTR2 expression. In addition, the CaCRZ1 gene is positively regulated by CaCrz1. CaCrz1 can bind in vitro and in vivo to its own promoter, suggesting an autoregulatory mechanism for CaCRZ1 expression.

Conclusions

CaCrz1 differentially binds to promoters of its target genes to regulate their expression in response to calcium stress. CaCrz1 also regulates its own expression through the 5′-TGAGGGACTG-3′ site in its promoter.

The lipid flippase subunit Cdc50 is required for antifungal drug resistance, endocytosis, hyphal development and virulence in Candida albicans

Cdc50 is the non-catalytic subunit of the flippase that establishes phospholipid asymmetry in membranes and functions in vesicle-mediated trafficking in Saccharomyces cerevisiae. Here, we have identified the homologous gene CaCDC50 that encodes a protein of 396 amino acids with two conserved transmembrane domains in Candidaalbicans. Deletion of CaCDC50 results in C. albicans cells becoming sensitive to the antifungal drugs azoles, terbinafine and caspofungin, as well as to the membrane-perturbing agent sodium dodecyl sulfate. We also show that CaCDC50 is involved in both endocytosis and vacuolar function. CaCDC50 confers tolerance to high concentrations of cations, although it is not required for osmolar response. Moreover, deletion of CaCDC50 leads to severe defects in hyphal development of C. albicans cells and highly attenuated virulence in the mouse model of systemic infection. Therefore, CaCDC50 regulates cellular responses to antifungal drugs, cell membrane stress, endocytosis, filamentation and virulence in the human fungal pathogen C. albicans.

The protein kinase Cmk2 negatively regulates the calcium/calcineurin signalling pathway and expression of calcium pump genes PMR1 and PMC1 in budding yeast

Through a genome-wide screen we have identified calcium-tolerant deletion mutants for five genes in the budding yeast Saccharomyces cerevisiae. In addition to CNB1 and RCN1 that are known to play a role in the calcium signalling pathway, the protein kinase gene CMK2, the sphingolipid homeostasis-related gene ORM2 and the gene SIF2 encoding the WD40 repeat-containing subunit of Set3C histone deacetylase complex are involved in the calcium sensitivity of yeast cells to extracellular calcium. Cmk2 and the transcription factor Crz1 have opposite functions in the response of yeast cells to calcium stress. Deletion of CMK2 elevates the level of calcium/calcineurin signalling and increases the expression level of PMR1 and PMC1, which is dependent on Crz1. Effects of Cmk2 on calcium sensitivity and calcium/calcineurin signalling are dependent on its kinase activity. Therefore, Cmk2 is a negative feedback controller of the calcium/calcineurin signalling pathway. Furthermore, the cmk2 crz1 double deletion mutant is more resistant than the crz1 deletion mutant, suggesting that Cmk2 has an additional Crz1-independent role in promoting calcium tolerance.

The nuclear transcription factor Rtg1p functions as a cytosolic, post-transcriptional regulator in the methylotrophic yeast Pichia pastoris

Rtg1p and Rtg3p are two basic helix-loop-helix, retrograde transcription factors in the budding yeast Saccharomyces cerevisiae Both factors heterodimerize to activate the transcription of nuclear genes in response to mitochondrial dysfunction and glutamate auxotrophy, but are not well characterized in other yeasts. Here, we demonstrate that the Rtg1p/Rtg3p-mediated retrograde signaling pathway is absent in the methylotrophic yeast Pichia pastoris We observed that P. pastoris Rtg1p (PpRtg1p) heterodimerizes with S. cerevisiae Rtg3p and functions as a nuclear, retrograde transcription factor in S. cerevisiae, but not in P. pastoris. We noted that P. pastoris Rtg3p lacks a functional leucine zipper and interacts with neither S. cerevisiae Rtg1p (ScRtg1p) nor PpRtg1p. In the absence of an interaction with Rtg3p, PpRtg1p has apparently acquired a novel function as a cytosolic regulator of multiple P. pastoris metabolic pathways, including biosynthesis of glutamate dehydrogenase 2 and phosphoenolpyruvate carboxykinase required for the utilization of glutamate as the sole carbon source. PpRtg1p also had an essential role in methanol metabolism and regulated alcohol oxidase synthesis and was required for the metabolism of ethanol, acetate, and oleic acid, but not of glucose and glycerol. Although PpRtg1p could functionally complement ScRtg1p, ScRtg1p could not complement PpRtg1p, indicating that ScRtg1p is not a functional PpRtg1p homolog. Thus, PpRtg1p functions as a nuclear, retrograde transcription factor in S. cerevisiae and as a cytosolic, post-transcriptional regulator in P. pastoris We conclude that PpRtg1p is a key component of a signaling pathway that regulates multiple metabolic processes in P. pastoris.

Fungicidal action of geraniol against Candida albicans is potentiated by abrogated CaCdr1p drug efflux and fluconazole synergism

Among the several mechanisms of multidrug resistance (MDR), overexpression of drug efflux pumps CaCdr1p and CaMdr1p belonging to ATP binding cassette (ABC) and major facilitator superfamily (MFS) respectively remain the predominant mechanisms of candidal infections. Therefore inhibiting or modulating the function of these transporters continues to draw attention as effective strategy to combat MDR. We have previously reported the antifungal potential of Geraniol (Ger), a natural monoterpenoid from Palmarosa oil, against Candida albicans. Herein, we explored the fungicidal nature of Ger. The Rhodamine 6G (R6G) and Nile red accumulation confirms the specific effect on CaCdr1p. Mechanistic insights with Candida cells overexpressing CaCdr1p and CaMdr1p revealed that Ger specifically modulates CaCdr1p activity. Kinetic studies further unraveled the competitive inhibition of Ger for R6G efflux as evident from increased apparent Km without affecting Vmax value. The effect of Ger on CaCdr1p was substantiated by molecular docking analyses, which depicted in-silico binding affinity of Ger with CaCdr1p and explored that Ger binds to the active site of CaCdr1p with higher binding energy. Although RT-PCR and western blot revealed no change in expressions of CDR1 and CaCdr1p, confocal microscopy images however depicted CaCdr1p mislocalization in presence of Ger. Interestingly, Ger was synergistic (FICI<0.5) with fluconazole (FLC) which is a well known antifungal drug. Furthermore, Ger sensitizes the FLC sensitive and resistant clinical matched pair of isolates Gu4/Gu5 and led to abrogated R6G efflux and depleted ergosterol. Furthermore, Rhodamine B labeling demonstrates altered mitochondrial potential with Ger which suggest possible linkage of dysfunctional mitochondria with CaCdr1p activity. We also estimated phenotypic virulence marker extracellular phospholipase activity which was considerably diminished along with inhibited cell adherence and biofilm biomass. Lastly, antifungal efficacy of Ger was demonstrated by enhanced survival of Caenorhabditis elegans model and negligible hemolytic activity (20%). Together, modulation of efflux pump activity by Ger and FLC synergism represent a promising approach for combinatorial treatment of candidiasis.

The putative transient receptor potential channel protein encoded by the orf19.4805 gene is involved in cation sensitivity, antifungal tolerance, and filamentation in Candida albicans

Transient receptor potential (TRP) channels, an ancient family of cation channels, are highly conserved in eukaryotes and play various physiological functions, ranging from sensation of ion homeostasis to reception of pain and vision. Calcium-permeable TRP channels have been identified from the plant Arabidopsis thaliana (AtCsc1) and the budding yeast Saccharomyces cerevisiae (ScCsc1). In this study, we characterized the functions of the Csc1 homolog, orf19.4805, in Candida albicans. Orf19.4805 is a protein of 866 amino acids and 11 transmembrane domains, which shares 49% identity (69% similarity) in amino acid sequence with ScRsn1. Here, we demonstrate that deletion of the orf19.4805 gene causes C. albicans cells to be sensitive to SDS (sodium dodecyl sulfate) and antifungal drugs, and tolerance to zinc, manganese, and cadmium ions. Candida albicans cells lacking orf19.4805 show a defect in filamentation in vitro. Therefore, orf19.4805 is involved in the regulation of cation homeostasis and filamentation in C. albicans.

Retrograde Signaling as a Mechanism of Yeast Adaptation to Unfavorable Factors

Mitochondria perform many essential functions in eukaryotic cells. Being the main producers of ATP and the site of many catabolic and anabolic reactions, they participate in intracellular signaling, proliferation, aging, and formation of reactive oxygen species. Mitochondrial dysfunction is the cause of many diseases and even cell death. The functioning of mitochondria in vivo is impossible without interaction with other cellular compartments. Mitochondrial retrograde signaling is a signaling pathway connecting mitochondria and the nucleus. The major signal transducers in the yeast retrograde response are Rtg1p, Rtg2p, and Rtg3p proteins, as well as four additional negative regulatory factors - Mks1p, Lst8p, and two 14-3-3 proteins (Bmh1/2p). In this review, we analyze current information on the retrograde signaling in yeast that is regarded as a stress or homeostatic response mechanism to changes in various metabolic and biosynthetic activities that occur upon mitochondrial dysfunction. We also discuss relations between retrograde signaling and other signaling pathways in the cell.

The plasma membrane protein Rch1 and the Golgi/ER calcium pump Pmr1 have an additive effect on filamentation in Candida albicans

Pmr1 is the Golgi/ER calcium pump, while Rch1 is a newly identified negative regulator of calcium influx in the plasma membrane of yeast cells. We show here that CaRch1 plays a dominant role over CaPmr1 in response of Candida albicans to SDS and tunicamycin stresses, while CaPmr1 has a major role in cell wall stress. Deletion of CaRCH1 increases the calcium/calcineurin signaling level in cells lacking CaPMR1. Calcineurin function is required for the role of CaRch1 in SDS stresses, while it is required for the function of CaPmr1 under all conditions examined. Disruption of CaRCH1 alone does not reduce the cell wall chitin, mannan or β-glucan content, but lack of CaRCH1 slightly decreases the chitin content of cells lacking CaPMR1. Furthermore, CaRch1 and CaPmr1 have an additive effect on filamentation of C. albicans cells in vitro. Cells lacking both CaRCH1 and CaPMR1 and cells lacking CaPMR1 alone show a similar degree of virulence attenuation, being much more attenuated than cells lacking CaRCH1 alone. Therefore, CaRch1 genetically interacts with CaPmr1 in the regulation of in vitro filamentation in C. albicans.

Sfp1 and Rtg3 reciprocally modulate carbon source-conditional stress adaptation in the pathogenic yeast Candida albicans

The pathogenicity of the clinically important yeast, Candida albicans, is dependent on robust responses to host-imposed stresses. These stress responses have generally been dissected in vitro at 30°C on artificial growth media that do not mimic host niches. Yet host inputs, such as changes in carbon source or temperature, are known to affect C. albicans stress adaptation. Therefore, we performed screens to identify novel regulators that promote stress resistance during growth on a physiologically relevant carboxylic acid and at elevated temperatures. These screens revealed that, under these 'non-standard' growth conditions, numerous uncharacterised regulators are required for stress resistance in addition to the classical Hog1, Cap1 and Cta4 stress pathways. In particular, two transcription factors (Sfp1 and Rtg3) promote stress resistance in a reciprocal, carbon source-conditional manner. SFP1 is induced in stressed glucose-grown cells, whereas RTG3 is upregulated in stressed lactate-grown cells. Rtg3 and Sfp1 regulate the expression of key stress genes such as CTA4, CAP1 and HOG1 in a carbon source-dependent manner. These mechanisms underlie the stress sensitivity of C. albicans sfp1 cells during growth on glucose, and rtg3 cells on lactate. The data suggest that C. albicans exploits environmentally contingent regulatory mechanisms to retain stress resistance during host colonisation.

Genetic interactions between Rch1 and the high-affinity calcium influx system Cch1/Mid1/Ecm7 in the regulation of calcium homeostasis, drug tolerance, hyphal development and virulence in Candida albicans

The high-affinity calcium influx system (HACS) consisted of CaCch1, CaMid1 and CaEcm7 controls calcium influx into the cell in response to environmental stimuli. The plasma membrane protein CaRch1 is a negative regulator of calcium influx in Candida albicans. In this study, we show that deletion of any of the HACS components suppresses the calcium hypersensitivity of, and the elevated activation level of calcium/calcineurin signaling in, C. albicans cells lacking CaRCH1. In contrast, CaRCH1 is epistatic to the HACS system in the tolerance of antifungal drugs. In addition, cells lacking CaRCH1 are sensitive to tunicamycin, show a delay in in vitro filamentation and an altered colony surface morphology, and are attenuated in virulence in a mouse systemic model. Cells lacking CaCCH1 and CaMID1, but not CaECM7, are sensitive to tunicamycin. Deletion of CaRCH1 increases the tunicamycin sensitivity of cells lacking CaECM7 or CaMID1, but not CaCCH1. Furthermore, deletion of CaRCH1 suppresses the defect in hyphal development due to the deletion of CaCCH1 or CaECM7, and increases the virulence of cells lacking any of the HACS components. Therefore, CaRch1 genetically interacts with the HACS components in different fashions for these functions.

Examining the virulence of Candida albicans transcription factor mutants using Galleria mellonella and mouse infection models

The aim of the present study was to identify Candida albicans transcription factors (TFs) involved in virulence. Although mice are considered the gold-standard model to study fungal virulence, mini-host infection models have been increasingly used. Here, barcoded TF mutants were first screened in mice by pools of strains and fungal burdens (FBs) quantified in kidneys. Mutants of unannotated genes which generated a kidney FB significantly different from that of wild-type were selected and individually examined in Galleria mellonella. In addition, mutants that could not be detected in mice were also tested in G. mellonella. Only 25% of these mutants displayed matching phenotypes in both hosts, highlighting a significant discrepancy between the two models. To address the basis of this difference (pool or host effects), a set of 19 mutants tested in G. mellonella were also injected individually into mice. Matching FB phenotypes were observed in 50% of the cases, highlighting the bias due to host effects. In contrast, 33.4% concordance was observed between pool and single strain infections in mice, thereby highlighting the bias introduced by the "pool effect." After filtering the results obtained from the two infection models, mutants for MBF1 and ZCF6 were selected. Independent marker-free mutants were subsequently tested in both hosts to validate previous results. The MBF1 mutant showed impaired infection in both models, while the ZCF6 mutant was only significant in mice infections. The two mutants showed no obvious in vitro phenotypes compared with the wild-type, indicating that these genes might be specifically involved in in vivo adapt.

Sesamol: a natural phenolic compound with promising anticandidal potential

We investigated the antifungal effects of sesamol (Ses), a natural phenolic compound, and exemplified that it could be mediated through disruption of calcineurin signaling pathway in C. albicans, a human fungal pathogen. The repertoire of antifungal activity not only was limited to C. albicans and its six clinical isolates tested but also was against non-albicans species of Candida. Interestingly, the antifungal effect of Ses affects neither the MDR efflux transporter activity nor passive diffusion of drug. We found that C. albicans treated with Ses copies the phenotype displayed by cells having defect in calcineurin signaling leading to sensitivity against alkaline pH, ionic, membrane, salinity, endoplasmic reticulum, and serum stresses but remained resistant to thermal stress. Furthermore, the ergosterol levels were significantly decreased by 63% confirming membrane perturbations in response to Ses as also visualized through transmission electron micrographs. Despite the fact that Ses treatment mimics the phenotype of compromised calcineurin signaling, it was independent of cell wall integrity pathway as revealed by spot assays and the scanning electron micrographs. Taken together, the data procured from this study clearly ascertains that Ses is an effectual antifungal agent that could be competently employed in treating Candida infections.