Retrograde signaling disruption influences ABC superfamily transporter, ergosterol and chitin levels along with biofilm formation 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.

Role of FoERG3 in Ergosterol Biosynthesis by Fusarium oxysporum and the Associated Regulation by Bacillus subtilis HSY21

Ergosterol is an important component of the fungal cell membrane and represents an effective target of chemical pesticides. However, the current understanding of ergosterol biosynthesis in the soybean root rot pathogen Fusarium oxysporum remains limited. In addition, the regular use of fungicides that inhibit ergosterol synthesis will seriously harm the ecological environment and human health. Bacillus subtilis is gradually replacing chemical control as a safe and effective biological agent; to investigate its effect on ergosterol synthesis of F. oxysporum, we verified the biological function of the FoERG3 gene of F. oxysporum by constructing knockout mutants. The results showed that knocking out FoERG3 blocked ergosterol biosynthesis, restricted mycelial growth, and increased the sensitivity to external stressors (NaCl, D-sorbitol, Congo Red, and H₂O₂). The increased permeability of the cell membrane promoted increased extracellular K⁺ levels and decreased mitochondrial cytochrome C contents. Treatment with suspension of B. subtilis HSY21 cells resulted in similar damage as observed when treating FoERG3-knockout F. oxysporum cells with ergosterol, which was characterised by deformity and swelling of the mycelium surface; increased membrane permeability; decreased pathogenicity to soybeans; and significantly decreased activities of cellulase, β-glucosidase, amylase, and pectin-methyl galactosylase. Notably, deleting FoERG3 resulted in a significant lag in the defense-response time of soybeans. Our results suggest that FoERG3 strongly influences the virulence of F. oxysporum and may be used as a potential antimicrobial target by B. subtilis HSY21 to inhibit ergosterol synthesis, which supports the use of B. subtilis as a biological control agent for protecting against F. oxysporum infection.

Antifungal Annona muricata L. (soursop) extract targets the cell envelope of multi-drug resistant Candida albicans

ETNOPHARMACOLOGICAL RELEVANCE

Annona muricata L. (soursop) is traditionally used in the treatment of inflammatory diseases, cancer, and infections caused by fungi. The therapeutic activity explored by its medicinal use is generally associated with its phytoconstituents, such as acetogenins and alkaloids. However, its potential antifungal bioactivity as well as its mechanism of action remains to be established.

AIM OF THE STUDY

To evaluate the antifungal activity of the ethanolic extract of A. muricata leaves against multidrug-resistant Candida albicans (ATCC® 10231).

MATERIAL AND METHODS

Phytoconstituents were detected by UFLC-QTOF-MS. The minimum inhibitory concentration was determined, followed by the determination of the minimum fungicidal concentration. For planktonic cells, the growth curve and cell density were evaluated. Studies to understand the mechanism of action on the cell envelope involved crystal violet permeability, membrane extravasation, sorbitol protection, exogenous ergosterol binding assay, metabolic activity, and cell viability. Furthermore, mitochondrial membrane potential was assessed.

RESULTS

Our analyses demonstrated a significant inhibitory effect of A. muricata, with the ability to reduce fungal growth by 58% and cell density by 65%. The extract affected both the fungal plasma membrane and cell wall integrity, with significant reduction of the cell viability. Depolarization of the fungal mitochondrial membrane was observed after treatment with A. muricata. Rutin, xi-anomuricine, kaempferol-3O-rutinoside, nornuciferine, xylopine, atherosperminine, caffeic acid, asimilobine, s-norcorydine, loliolide, annohexocin, annomuricin, annopentocin, and sucrose were identified as extract bioactive components.

CONCLUSIONS

Our findings show that the A. muricata extract is a source of chemical diversity, which acts as a potential antifungal agent with promising application to the therapy of infections caused by C. albicans.

The transcription factor Ron1 is required for chitin metabolism, asexual development and pathogenicity in Beauveria bassiana, an entomopathogenic fungus

Ndt80-like transcription factor Ron1 is best known for its essential role in the regulation of N-acetylglucosamine (GlcNAc) catabolism. Ron1 was again found to be essential for sensing GlcNAc in Beauveria bassiana. Importantly, our study revealed that Ron1 is involved in the metabolic processes of chitin and asexual development. To further investigate the novel functions of Ron1 in B. bassiana, extracellular chitinase activity in the ΔRon1 mutant was found to decrease by 84.73% compared with wild type. The deletion of Ron1 made it difficult for the fungus to accumulate intracellular GlcNAc. Furthermore, transcriptomic analysis revealed that Ron1 exerted a significant effect on global transcription and positively regulated genes encoding chitin metabolism in respond to chitin nutrition. Yeast one-hybrid assay confirmed that Ron1 could bind to specific cis-acting elements in the promoters of chitinase and hexokinase. In addition, ΔRon1 displayed an impaired chitin component of the cell wall, with a chitin synthetase (ChsVII) predicted to function downstream of Ron1. Finally, the virulence of ΔRon1 mutant was significantly reduced in the Galleria mellonella insect model through cuticle infection or cuticle bypassing infection. These data functionally characterize Ron1 in B. bassiana and expand our understanding of how the transcription factor Ron1 works in pathogens.

Caenorhabditis elegans as an Infection Model for Pathogenic Mold and Dimorphic Fungi: Applications and Challenges

The threat burden from pathogenic fungi is universal and increasing with alarming high mortality and morbidity rates from invasive fungal infections. Understanding the virulence factors of these fungi, screening effective antifungal agents and exploring appropriate treatment approaches in in vivo modeling organisms are vital research projects for controlling mycoses. Caenorhabditis elegans has been proven to be a valuable tool in studies of most clinically relevant dimorphic fungi, helping to identify a number of virulence factors and immune-regulators and screen effective antifungal agents without cytotoxic effects. However, little has been achieved and reported with regard to pathogenic filamentous fungi (molds) in the nematode model. In this review, we have summarized the enormous breakthrough of applying a C. elegans infection model for dimorphic fungi studies and the very few reports for filamentous fungi. We have also identified and discussed the challenges in C. elegans-mold modeling applications as well as the possible approaches to conquer these challenges from our practical knowledge in C. elegans-Aspergillus fumigatus model.

Bacillus subtilis HSY21 can reduce soybean root rot and inhibit the expression of genes related to the pathogenicity of Fusarium oxysporum

Soybean root rot occurs globally and seriously affects soybean production. To avoid the many disadvantages of chemical fungicides, the addition of Bacillus is gradually becoming an alternative strategy to tackle soybean root rot. However, the molecular mechanism of phytopathogenic fungi in this process by Bacillus inhibition is rarely reported. In this study, we isolated a strain of B. subtilis HSY21 from soybean rhizosphere soil, which had an inhibition rate of 81.30 ± 0.15% (P < 0.05) against Fusarium oxysporum. The control effects of this strain against soybean root rot under greenhouse and field conditions were 63.83% and 57.07% (P < 0.05), respectively. RNA-seq analysis of F. oxysporum after treatment with strain HSY21 revealed 1445 downregulated genes and 1561 upregulated genes. Among them, genes involved in mycelial growth, metabolism regulation, and disease-related enzymes were mostly downregulated. The activities of cellulase, β-glucosidase, α-amylase, and pectin-methyl- galacturonase as well as levels of oxalic acid and ergosterol in F. oxysporum were significantly decreased after HSY21 treatment. These results demonstrated that B. subtilis HSY21 could effectively control F. oxysporum by inhibiting its growth and the expression of pathogenic genes, thus indicating that this strain may be an ideal candidate for the prevention and control of soybean root rot.

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.

Insights into the modulatory effect of magnesium on efflux mechanisms of Candida albicans reveal inhibition of ATP binding cassette multidrug transporters and dysfunctional mitochondria

Candida infections pose a serious hazard to public health followed by widespread and prolonged deployment of antifungal drugs has which has led multidrug resistance (MDR) progress in prevalent human fungal pathogen, Candida albicans. Despite the fact that MDR is multifactorial phenomenon govern by several mechanisms in C. albicans, overexpression of drug efflux transporters by far remains the leading cause of MDR govern by ATP Binding Cassette (ABC) or major facilitator superfamily (MFS) transporters. Hence searching for strategies to target efflux pumps transporter still signifies a promising approach. In this study we analyzed the effect of magnesium (Mg) deprivation, on efflux pump action of C. albicans. We explored that Mg deprivation specially inhibits efflux of transporters (CaCdr1p and CaCdr2p) belonging to ABC superfamily as revealed by rhodamine 6G and Nile red accumulation. Furthermore, Mg deprivation causes mislocalization of CaCdr1p and CaCdr2p and reduced transcripts of CDR1 and CDR2 with no effect on CaMdr1p. Additionally, Mg deprivation causes depletion of ergosterol content in azole sensitive and resistant clinical matched pair of isolates Gu4/Gu5 and F2/F5 of C. albicans. Lastly, we observed that Mg deprivation impairs mitochondrial potential which could be the causal reason for abrogated efflux activity. With growing appreciation of manipulating metal homeostasis to combat MDR, inhibition of efflux activity under Mg deprivation warrants further studies to be utilized as an effective antifungal strategy.

Involvement of BbTpc1, an important Zn(II)2Cys6 transcriptional regulator, in chitin biosynthesis, fungal development and virulence of an insect mycopathogen

Chitin is one of the major components of the fungal cell wall and contributes to the mechanical strength and shape of the fungal cell. Zn(II)₂Cys₆ transcription factors are unique to the fungal kingdom and have a variety of functions in some fungi. However, the mechanisms by which Zn(II)₂Cys₆ proteins affect entomopathogenic fungi are largely unknown. Here, we characterized the Zn(II)₂Cys₆ transcription factor BbTpc1 in the insect pathogenic fungus Beauveria bassiana. Disruption of BbTpc1 resulted in a distinct changes in vegetative growth and septation patterns, and a significant decrease in conidia and blastospore yield. The ΔBbTpc1 mutant displayed impaired resistance to chemical stresses and heat shock and attenuated virulence in topical and intrahemocoel injection assays. Importantly, the ΔBbTpc1 mutant had an abnormal cell wall with altered wall thickness and chitin synthesis, which were accompanied by transcriptional repression of the chitin synthetase family genes. In addition, comparative transcriptomics revealed that deletion of BbTpc1 altered fungal asexual reproduction via different genetic pathways. These data revealed that BbTpc1 regulates fungal development, chitin synthesis and biological control potential in B. bassiana.

Antifungal Activity of the Natural Coumarin Scopoletin Against Planktonic Cells and Biofilms From a Multidrug-Resistant Candida tropicalis Strain

Candida tropicalis is one the most relevant biofilm-forming fungal species increasingly associated with invasive mucosal candidiasis worldwide. The amplified antifungal resistance supports the necessity for more effective and less toxic treatment, including the use of plant-derived natural products. Scopoletin, a natural coumarin, has shown antifungal properties against plant yeast pathogens. However, the antifungal activity of this coumarin against clinically relevant fungal species such as C. tropicalis remains to be established. Here, we investigated the potential antifungal properties and mechanisms of action of scopoletin against a multidrug-resistant C. tropicalis strain (ATCC 28707). First, scopoletin was isolated by high-performance liquid chromatography from Mitracarpus frigidus, a plant species (family Rubiaceae) distributed throughout South America. Next, scopoletin was tested on C. tropicalis cultivated for 48h in both planktonic and biofilm forms. Fungal planktonic growth inhibition was analyzed by evaluating minimal inhibitory concentration (MIC), time-kill kinetics and cell density whereas the mechanisms of action were investigated with nucleotide leakage, efflux pumps and sorbitol and ergosterol bioassays. Finally, the scopoletin ability to affect C. tropicalis biofilms was evaluated through spectrophotometric and whole slide imaging approaches. In all procedures, fluconazole was used as a positive control. MIC values for scopoletin and fluconazole were 50 and 250 μg/L respectively, thus demonstrating a fungistatic activity for scopoletin. Scopoletin induced a significant decrease of C. tropicalis growth curves and cell density (91.7% reduction) compared to the growth control. Its action was related to the fungal cell wall, affecting plasma membrane sterols. When associated with fluconazole, scopoletin led to inhibition of efflux pumps at the plasma membrane. Moreover, scopoletin not only inhibited the growth rate of preformed biofilms (68.2% inhibition at MIC value) but also significantly decreased the extent of biofilms growing on the surface of coverslips, preventing the formation of elongated fungal forms. Our data demonstrate, for the first time, that scopoletin act as an effective antifungal phytocompound against a multidrug-resistant strain of C. tropicalis with properties that affect both planktonic and biofilm forms of this pathogen. Thus, the present findings support additional studies for antifungal drug development based on plant isolated-scopoletin to treat candidiasis caused by C. tropicalis.