Deciphering fungal dimorphism: Farnesol's unanswered questions

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Deciphering the molecular components of the quorum sensing system in the fungus Ophiostoma piceae

IMPORTANCE

This manuscript presents a comprehensive study on the molecular mechanisms triggered by the quorum sensing (QS) molecule farnesol in the biotechnologically relevant fungus Ophiostoma piceae. We present for the first time, using a multiomics approach, an in-depth analysis of the QS response in a saprotroph fungus, detailing the molecular components involved in the response and their possible mechanisms of action. We think that these results are particularly relevant in the knowledge of the functioning of the QS in eukaryotes, as well as for the exploitation of these mechanisms.

Farnesol as an antifungal agent: comparisons among MTL and MTLα haploid and diploid Candida albicans and Saccharomyces cerevisiae

Aims: Farnesol was identified 20 years ago in a search for Candida albicans quorum sensing molecules (QSM), but there is still uncertainty regarding many aspects of its mode of action including whether it employs farnesol transport mechanisms other than diffusion. Based on the structural similarity between farnesol and the farnesylated portion of the MTL a pheromone, we explored the effects of ploidy and mating type locus (MTL) on the antifungal activity of exogenous farnesol. Methods and results: We approached this question by examining five MTL a and five MTLα haploid strains with regard to their farnesol sensitivity in comparison to six heterozygous MTL a/ α diploids. We examined the haploid and diploid strains for percent cell death after exposure of exponentially growing cells to 0-200 µM farnesol. The heterozygous (MTL a/α) diploids were tolerant of exogenous farnesol whereas the MTL a and MTLα haploids were on average 2- and 4-times more sensitive, respectively. In the critical range from 10-40 µM farnesol their cell death values were in the ratio of 1:2:4. Very similar results were obtained with two matched sets of MAT a, MATα, and MAT a/α Saccharomyces cerevisiae strains. Conclusion: We propose that the observed MTL dependence of farnesol is based on differentially regulated mechanisms of entry and efflux which determine the actual cellular concentration of farnesol. The mechanisms by which pathogens such as C. albicans tolerate the otherwise lethal effects of farnesol embrace a wide range of physiological functions, including MTL type, ubiquinone type (UQ6-UQ9), energy availability, and aerobic/anaerobic status.

Fungal quorum-sensing molecules and antiseptics: a promising strategy for biofilm modulation?

New strategies to control fungal biofilms are essential, especially those that interfere in the biofilm organization process and cellular communication, known as quorum sensing. The effect of antiseptics and quorum-sensing molecules (QSMs) have been considered with regard to this; however, little has been elucidated, particularly because studies are often restricted to the action of antiseptics and QSMs against a few fungal genera. In this review, we discuss progress reported in the literature thus far and analyze, through in silico methods, 13 fungal QSMs with regard to their physicochemical, pharmacological, and toxicity properties, including their mutagenicity, tumorigenicity, hepatotoxicity, and nephrotoxicity. From these in silico analyses, we highlight 4-hydroxyphenylacetic acid and tryptophol as having satisfactory properties and, thus, propose that these should be investigated further as antifungal agents. We also recommend future in vitro approaches to determine the association of QSMs with commonly used antiseptics as potential antibiofilm agents.

Quantitative assay for farnesol and the aromatic fusel alcohols from the fungus Candida albicans

Abstract

The dimorphic fungus Candida albicans is a commensal and opportunistic fungal pathogen of humans. It secretes at least four small lipophilic molecules, farnesol and three aromatic fusel alcohols. Farnesol has been identified as both a quorum sensing molecule (QSM) and a virulence factor. Our gas chromatography (GC)-based assay for these molecules exhibits high throughput, prevention of analyte loss by avoiding filtration and rotary evaporation, simultaneous cell lysis and analyte extraction by ethyl acetate, and the ability to compare whole cultures with their cell pellets and supernatants. Farnesol synthesis and secretion were separable phenomena and pellet:supernatant ratios for farnesol were high, up to 12:1. The assay was validated in terms of precision, specificity, ruggedness, accuracy, solution stability, detection limits (DL), quantitation limits (QL), and dynamic range. The DL for farnesol was 0.02 ng/µl (0.09 µM). Measurement quality was assessed by the relative error of the whole culture versus the sum of pellet and supernatant fractions (WPS). C. albicans strain SC5314 grown at 30 °C in complex and defined media (YPD and mRPMI) was assayed in biological triplicate 17 times over 3 days. Farnesol and the three aromatic fusel alcohols can be measured in the same assay. The levels of all four are greatly altered by the growth medium chosen. Significantly, the three fusel alcohols are synthesized during stationary phase, not during growth. They are secreted quickly without being retained in the cell pellet and may accumulate up to mM concentrations.

Key points

• Quantitative analysis of both intra- and extracellular farnesol, and aromatic fusel oils.

• High throughput, whole culture assay with simultaneous lysis and extraction.

• Farnesol secretion and synthesis are distinct and separate events.

Physiological and transcriptional profiling of surfactin exerted antifungal effect against Candida albicans

Given the risk of Candida albicans overgrowth in the gut, novel complementary therapies should be developed to reduce fungal dominancy. This study highlights the antifungal characteristics of a Bacillus subtilis-derived secondary metabolite, surfactin with high potential against C. albicans. Surfactin inhibited the growth of C. albicans following a 1-hour exposure, in addition to reduced adhesion and morphogenesis. Specifically, surfactin did not affect the level of reactive oxygen species but increased the level of reduced glutathione. Surprisingly, ethanol production was increased following 2 h of surfactin exposure. Surfactin treatment caused a significant reduction in intracellular iron, manganese and zinc content compared to control cells, whereas the level of copper was not affected. Alongside these physiological properties, surfactin also enhanced fluconazole efficacy. To gain detailed insights into the surfactin-related effects on C. albicans, genome-wide gene transcription analysis was performed. Surfactin treatment resulted in 1390 differentially expressed genes according to total transcriptome sequencing (RNA-Seq). Of these, 773 and 617 genes with at least a 1.5-fold increase or decrease in transcription, respectively, were selected for detailed investigation. Several genes involved in morphogenesis or related to metabolism (e.g., glycolysis, ethanol and fatty acid biosynthesis) were down-regulated. Moreover, surfactin decreased the expression of ERG1, ERG3, ERG9, ERG10 and ERG11 involved in ergosterol synthesis, whereas genes associated with ribosome biogenesis and iron metabolism and drug transport-related genes were up-regulated. Our data demonstrate that surfactin significantly influences the physiology and gene transcription of C. albicans, and could contribute to the development of a novel innovative complementary therapy.

Bacterial Quorum-Sensing Molecules as Promising Natural Inhibitors of Candida albicans Virulence Dimorphism: An In Silico and In Vitro Study

Farnesol, a self-secreted quorum-sensing molecule (QSM) of Candida albicans, has been known to limit yeast-to-hyphal transition by blocking the RAS1-cAMP-PKA pathway. In a similar fashion, certain bacterial QSMs have also been reported to be successful in attenuating C. albicans biofilm and hyphal formation at relatively high cell density. This prompted us to investigate the antihyphal efficacy of certain bacterial QSMs through virtual docking against seminal drug targets, viz., CYCc and RAS1, that have been reported to be the hallmark players in C. albicans dimorphic virulence cascade. Against this backdrop, 64 QSMs belonging to five different bacterial QS signaling systems were subjected to initial virtual screening with farnesol as reference. Data of the virtual screening unveiled QSMs belonging to diketopiperazines (DKPs), i.e., 3-benzyl-6-isobutylidene-2,5-piperazinedione (QSSM 1157) and cyclo(l-Pro-l-Leu) (QSSM 1112), as potential inhibitors of CYCc and RAS1 with binding energies of -8.2 and -7.3 kcal mol-1, respectively. Further, the molecular dynamics simulations (for 50 ns) of CYCc-QSSM 1157 and RAS1-QSSM 1112 complexes revealed the mean ligand root mean square deviation (RMSD) values of 0.35 and 0.27 Å, respectively, which endorsed the rigid nature, less fluctuation in binding stiffness, and conformation of binding complexes. Furthermore, the identified two QSMs were found to be good in solubility, absorption, and permeation and less toxic in nature, as revealed by pharmacokinetics and toxicity analyses. In addition, the in vitro antihyphal assays using liquid and solid media, germ-tube experiment, and microscopic analysis strongly validated DKP-QSSM 1112 as a promising inhibitor of hyphal transition. Taken together, the present study unequivocally proves that DKPs can be used as potent inhibitors of C. albicans virulence dimorphism.

Characterization of a G protein α subunit encoded gene from the dimorphic fungus-Tremella fuciformis

Tremella fuciformis is a dimorphic fungus which can undertake the reversible transition between yeast and pseudohypha forms. G protein α subunit (Gα) carries different signals to regulate a variety of biological processes in eukaryotes, including fungal dimorphism. In this study, a novel Gα subunit encoded gene, TrGpa1, was firstly cloned from T. fuciformis. The TrGpa1 open reading frame has 1059 nucleotides, and encodes a protein which belongs to the group I of Gα_i superfamily. Furthermore, the role of TrGpa1 in the T. fuciformis dimorphism was analysed by gene overexpression and knockdown. Stable integration of the target gene into the genome was confirmed by PCR and Southern blot hybridization. Transformants with the highest and lowest TrGpa1 expression levels were selected via quantitative real-time PCR analysis and Western blot. Each transformant was compared with the wild-type strain about the morphological change under different environmental factors, including pH values, temperature, cultivation time, inoculum size, and quorum-sensing molecules (farnesol and tyrosol). Comparing with the wild-type strain, the overexpression transformant always had higher ratios of pseudohyphae, while the knockdown transformant had less proportions of pseudohyphae. Therefore, the TrGpa1 is involved in the dimorphism of T. fuciformis and plays a positive role in promoting pseudohyphal growth.

Farnesol: An approach on biofilms and nanotechnology

Biofilms are important virulence factor in infections caused by microorganisms because of its complex structure, which provide resistance to conventional antimicrobials. Strategies involving the use of molecules capable of inhibiting their formation and also act synergistically with conventional drugs have been explored. Farnesol is a molecule present in essential oils and produced by Candida albicans as a quorum sensing component. This sesquiterpene presents inhibitory properties in the formation of microbial biofilms and synergism with antimicrobials used in clinical practice, and can be exploited even for eradication of biofilms formed by drug-resistant microorganisms. Despite this, farnesol has physical and chemical characteristics that can limit its use, such as high hydrophobicity and volatility. Therefore, nanotechnology may represent an option to improve the efficiency of this molecule in high complex environments such as biofilms. Nanostructured systems present important results in the improvement of treatment with different commercial drugs and molecules with therapeutic or preventive potential. The formation of nanoparticles offers advantages such as protection of the incorporated drugs against degradation, improved biodistribution and residence time in specific treatment sites. The combination of farnesol with nanotechnology may be promising for the development of more effective antibiofilm therapies, as it can improve its solubility, reduce volatility, and increase bioavailability. This review summarizes existing data about farnesol, its action on biofilms, and discusses its encapsulation in nanostructured systems.

LAY SUMMARY

Farnesol is a natural compound that inhibits the formation of biofilms from different microbial species. The encapsulation of this molecule in nanoparticles is a promising alternative for the development of more effective therapies against biofilms.

OBP-functionalized/hybrid superparamagnetic nanoparticles for Candida albicans treatment

Infections caused by the opportunistic yeast Candida albicans are one of the major life threats for hospitalized and immunocompromised patients, as a result of antibiotic and long-term antifungal treatment abuse. Odorant binding proteins can be considered interesting candidates to develop systems able to reduce the proliferation and virulence of this yeast, because of their intrinsic antimicrobial properties and complexation capabilities toward farnesol, the major quorum sensing molecule of Candida albicans. In the present study, a hybrid system characterized by a superparamagnetic iron oxide core functionalized with bovine odorant binding protein (bOBP) was successfully developed. The nanoparticles were designed to be suitable for magnetic protein delivery to inflamed areas of the body. The inorganic superparamagnetic core was characterized by an average diameter of 6.5 ± 1.1 nm and a spherical shape. Nanoparticles were functionalized by using 11-phosphonoundecanoic acid as spacer and linked to bOBP via amide bonds, resulting in a concentration level of 26.0 ± 1.2 mg bOBP/g SPIONs. Finally, both the biocompatibility of the developed hybrid system and the fungistatic activity against Candida albicans by submicromolar OBP levels were demonstrated by in vitro experiments.

Longer Ubiquinone Side Chains Contribute to Enhanced Farnesol Resistance in Yeasts

Ubiquinones (UQ) are intrinsic lipid components of many membranes. Besides their role in electron-transfer reactions there is evidence for them acting as free radical scavengers, yet their other roles in biological systems have received little study. The dimorphic fungal pathogen Candida albicans secretes farnesol as both a virulence factor and a quorum-sensing molecule. Thus, we were intrigued by the presence of UQ9 isoprenologue in farnesol-producing Candida species while other members of this genera harbor UQ7 as their major electron carrier. We examined the effect of UQ side chain length in Saccharomyces cerevisiae and C. albicans with a view towards identifying the mechanisms by which C. albicans protects itself from the high levels of farnesol it secretes, levels that are toxic to many other fungi including S. cerevisiae. In this study, we identify UQ9 as the major UQ isoprenoid in C. albicans, regardless of growth conditions or cell morphology. A S. cerevisiae model yeast engineered to make UQ9 instead of UQ6 was 4-5 times more resistant to exogenous farnesol than the parent yeast and this resistance was accompanied by greatly reduced reactive oxygen species (ROS) production. The resistance provided by UQ9 is specific for farnesol in that it does not increase resistance to high salt (1M NaCl) or other oxidants (5 mM H₂O₂ or 1 mM menadione). Additionally, the protection provided by UQ9 appears to be structural rather than transcriptional; UQ9 does not alter key transcriptional responses to farnesol stress. Here, we propose a model in which the longer UQ side chains are more firmly embedded in the mitochondrial membrane making them harder to pry out, so that in the presence of farnesol they remain functional without producing excess ROS. C. albicans and Candida dubliniensis evolved to use UQ9 rather than UQ7 as in other Candida species or UQ6 as in S. cerevisiae. This adaptive mechanism highlights the significance of UQ side chains in farnesol production and resistance quite apart from being an electron carrier in the respiratory chain.

Intraspecies cell-cell communication in yeast

Although yeasts are unicellular microorganisms that can live independently, they can also communicate with other cells, in order to adapt to the environment. Two yeast species, the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe, engage in various kinds of intraspecies cell-cell communication using peptides and chemical molecules that they produce, constituting a sort of 'language'. Cell-cell communication is a fundamental biological process, and its ultimate purpose is to promote survival by sexual reproduction and acquisition of nutrients from the environment. This review summarizes what is known about intraspecies cell-cell communication mediated by molecules including mating pheromones, volatile gases, aromatic alcohols and oxylipins in laboratory strains of S. cerevisiae and S. pombe.

Farnesol-Containing Macromolecular Systems for Antibiofilm Strategies

Farnesol is a natural sesquiterpenoid and an interesting quorum-sensing molecule. Its insolubility in water is the biggest obstacle to its application for bacterial biofilm treatments since it compromises the bioavailability. Recently, an increasing interest in farnesol encapsulation or loading in polymeric materials may be noted due to the prolonged action of the active macromolecular systems. In this short review, we present an overview of methods leading to improved interactions between farnesol and microbial biofilms.

Farnesol and Tyrosol: Secondary Metabolites with a Crucial quorum-sensing Role in Candida Biofilm Development

When living in biological and interactive communities, microorganisms use quorum-sensing mechanisms for their communication. According to cell density, bacteria and fungi can produce signaling molecules (e.g., secondary metabolites), which participate, for example, in the regulation of gene expression and coordination of collective behavior in their natural niche. The existence of these secondary metabolites plays a main role in competence, colonization of host tissues and surfaces, morphogenesis, and biofilm development. Therefore, for the design of new antibacterials or antifungals and understanding on how these mechanisms occur, to inhibit the secretion of quorum-sensing (e.g., farnesol and tyrosol) molecules leading the progress of microbial infections seems to be an interesting option. In yeasts, farnesol has a main role in the morphological transition, inhibiting hyphae production in a concentration-dependent manner, while tyrosol has a contrary function, stimulating transition from spherical cells to germ tube form. It is beyond doubt that secretion of both molecules by fungi has not been fully described, but specific meaning for their existence has been found. This brief review summarizes the important function of these two compounds as signaling chemicals participating mainly in Candida morphogenesis and regulatory mechanisms.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

Candida albicans Zn Cluster Transcription Factors Tac1 and Znc1 Are Activated by Farnesol To Upregulate a Transcriptional Program Including the Multidrug Efflux Pump CDR1

Farnesol, a quorum-sensing molecule, inhibits Candida albicans hyphal formation, affects its biofilm formation and dispersal, and impacts its stress response. Several aspects of farnesol's mechanism of action remain incompletely uncharacterized. Among these are a thorough accounting of the cellular receptors and transporters for farnesol. This work suggests these processes are linked through the Zn cluster transcription factors Tac1 and Znc1 and their induction of the multidrug efflux pump Cdr1. Specifically, we have demonstrated that Tac1 and Znc1 are functionally activated by farnesol through a mechanism that mimics other means of hyperactivation of Zn cluster transcription factors. This is consistent with our observation that many genes acutely induced by farnesol are dependent on TAC1, ZNC1, or both. A related molecule, 1-dodecanol, invokes a similar TAC1-ZNC1 response, while several other proposed C. albicans quorum-sensing molecules do not. Tac1 and Znc1 both bind to and upregulate the CDR1 promoter in response to farnesol. Differences in inducer and DNA binding specificity lead to Tac1 and Znc1 having overlapping, but nonidentical, regulons. Induction of genes by farnesol via Tac1 and Znc1 was inversely related to the level of CDR1 present in the cell, suggesting a model in which induction of CDR1 by Tac1 and Znc1 leads to an increase in farnesol efflux. Consistent with this premise, our results show that CDR1 expression, and its regulation by TAC1 and ZNC1, facilitates growth in the presence of high farnesol concentrations in C. albicans and in certain strains of its close relative, C. dubliniensis.

White-to-opaque switching is involved in the phospholipase B production of Candida dubliniensis on Price's egg yolk agar

Measuring the production of Candida dubliniensis (C. dubliniensis) phospholipase B (PLase B) by the Price's method has long been considered to be unattainable because the levels of PLase produced are undetectable. In this study, C. dubliniensis, C. glabrata, C. guilliermondii, C. krusei, C. parapsilosis and C. tropicalis were shown to produce PLase B and form clear white zones around their colonies when peptone, a component of the original Price's egg yolk (OP) agar, is replaced with a yeast nitrogen base (YNB). This new medium is named modified Price's (MP) agar. Based on this finding, we propose a new modified Price's (NMP) agar containing 0.75% peptone and 0.25% YNB, which enabled measurement of PLase B production by C. dubliniensis and C. albicans with results consistent with those obtained for C. albicans grown on OP agar. We strongly believe that the MP and NMP agars are very useful for screening PLase B production by C. dubliniensis and non-albicans Candida spp. Moreover, the addition of several bioactive agents (the proteinase inhibitors pepstatin A and saquinavir, the calcineurin inhibitors cyclosporine A and tacrolimus, the cell-permeable cAMP analog dBcAMP, and the quorum-sensing molecule farnesol) to the OP agar enhanced PLase B production by C. dubliniensis. During the course of our study to clarify the reason why PLase B was not produced, we found that C. dubliniensis cells grown on OP agar undergo a white-to-opaque transition, which may explain why they showed minimal production of PLase B on this medium.

New approaches in bioprocess-control: Consortium guidance by synthetic cell-cell communication based on fungal pheromones

Bioconversions in industrial processes are currently dominated by single-strain approaches. With the growing complexity of tasks to be carried out, microbial consortia become increasingly advantageous and eventually may outperform single-strain fermentations. Consortium approaches benefit from the combined metabolic capabilities of highly specialized strains and species, and the inherent division of labor reduces the metabolic burden for each strain while increasing product yields and reaction specificities. However, consortium-based designs still suffer from a lack of available tools to control the behavior and performance of the individual subpopulations and of the entire consortium. Here, we propose to implement novel control elements for microbial consortia based on artificial cell-cell communication via fungal mating pheromones. Coupling to the desired output is mediated by pheromone-responsive gene expression, thereby creating pheromone-dependent communication channels between different subpopulations of the consortia. We highlight the benefits of artificial communication to specifically target individual subpopulations of microbial consortia and to control e.g. their metabolic profile or proliferation rate in a predefined and customized manner. Due to the steadily increasing knowledge of sexual cycles of industrially relevant fungi, a growing number of strains and species can be integrated into pheromone-controlled sensor-actor systems, exploiting their unique metabolic properties for microbial consortia approaches.

An Improved Total RNA Extraction Method for White Jelly Mushroom Tremella fuciformis Rich in Polysaccharides

An improved method for extracting high quality and quantity RNA from a jelly mushroom and a dimorphic fungus-Tremella fuciformis which is especially rich in polysaccharides, is described. RNA was extracted from T. fuciformis mycelium M1332 and its parental monokaryotic yeast-like cells Y13 and Y32. The A260/280 and A260/230 ratios were both approximately 2, and the RNA integrity number was larger than 8.9. The yields of RNA were between 108 and 213 µg/g fresh wt. Downstream molecular applications including reverse transcriptional PCR and quantitative real-time PCR were also performed. This protocol is reliable and may be widely applicable for total RNA extraction from other jelly mushrooms or filamentous fungi rich in polysaccharides.