Plasma membrane architecture protects Candida albicans from killing by copper

Biocompatible N-carbazoleacetic acid decorated CuxO nanoparticles as self-cascading platforms for synergistic single near-infrared triggered phototherapy treating microbial infections

In this work, positively charged N-carbazoleacetic acid decorated CuxO nanoparticles (CuxO-CAA NPs) as novel biocompatible nanozymes have been successfully prepared through a one-step hydrothermal method. CuxO-CAA can serve as a self-cascading platform through effective GSH-OXD-like and POD-like activities, and the former can induce continuous generation of H2O2 through the catalytic oxidation of overexpressed GSH in the bacterial infection microenvironment, which in turn acts as a substrate for the latter to yield ˙OH via Fenton-like reaction, without introducing exogenous H2O2. Upon NIR irradiation, CuxO-CAA NPs possess a high photothermal conversion effect, which can further improve the enzymatic activity for increasing the production rate of H2O2 and ˙OH. Besides, the photodynamic performance of CuxO-CAA NPs can produce 1O2. The generated ROS and hyperthermia have synergetic effects on bacterial mortality. More importantly, CuxO-CAA NPs are more stable and biosafe than Cu2O, and can generate electrostatic adsorption with negatively charged bacterial cell membranes and accelerate bacterial death. Antibacterial results demonstrate that CuxO-CAA NPs are lethal against methicillin-resistant Staphylococcus aureus (MRSA) and ampicillin-resistant Escherichia coli (AREC) through destroying the bacterial membrane and disrupting the bacterial biofilm formation. MRSA-infected animal wound models show that CuxO-CAA NPs can efficiently promote wound healing without causing toxicity to the organism.

The Candida albicans ζ-crystallin homolog Zta1 promotes resistance to oxidative stress

IMPORTANCE

Candida albicans is an important human pathogen that can cause lethal systemic infections. The ability of C. albicans to colonize and establish infections is closely tied to its highly adaptable nature and capacity to resist various types of stress, including oxidative stress. Previous studies showed that four C. albicans proteins belonging to the flavodoxin-like protein family of quinone reductases are needed for resistance to quinones and virulence. Therefore, in this study, we examined the role of a distinct type of quinone reductase, Zta1, and found that it acts in conjunction with the flavodoxin-like proteins to protect against oxidative stress.

Candida albicans resistance to hypochlorous acid

IMPORTANCE

Hypochlorous acid (HOCl), commonly known as bleach, is generated during the respiratory burst by phagocytes and is a key weapon used to attack Candida albicans and other microbial pathogens. However, the effects of hypochlorous acid on C. albicans have been less well studied than H2O2, a different type of oxidant produced by phagocytes. HOCl kills C. albicans more effectively than H2O2 and results in disruption of the plasma membrane. HOCl induced a very different transcriptional response than H2O2, and there were significant differences in the susceptibility of mutant strains of C. albicans to these oxidants. Altogether, these results indicate that HOCl has distinct effects on cells that could be targeted in novel therapeutic strategies to enhance the killing of C. albicans and other pathogens.

The Candida albicans ζ-crystallin homolog Zta1 promotes resistance to oxidative stress

The fungal pathogen Candida albicans is capable of causing lethal infections in humans. Its pathogenic potential is due in part to the ability to resist various stress conditions in the host, including oxidative stress. Recent studies showed that a family of four flavodoxin-like proteins (Pst1, Pst2, Pst3, Ycp4) that function as quinone reductases promotes resistance to oxidation and is needed for virulence. Therefore, in this study Zta1 was examined because it belongs to a structurally distinct family of quinone reductases that are highly conserved in eukaryotes and have been called the ζ-crystallins. The levels of Zta1 in C. albicans rapidly increased after exposure to oxidants, consistent with a role in resisting oxidative stress. Accumulation of reactive oxygen species was significantly higher in cells lacking ZTA1 upon exposure to quinones and other oxidants. Furthermore, deletion of ZTA1 in a mutant lacking the four flavodoxin-like proteins, resulted in further increased susceptibility to quinones, indicating that these distinct quinone reductases work in combination. These results demonstrate that Zta1 contributes to C. albicans survival after exposure to oxidative conditions, which increases the understanding of how C. albicans resists stressful conditions in the host.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Two distinct lipid transporters together regulate invasive filamentous growth in the human fungal pathogen Candida albicans

Flippases transport lipids across the membrane bilayer to generate and maintain asymmetry. The human fungal pathogen Candida albicans has 5 flippases, including Drs2, which is critical for filamentous growth and phosphatidylserine (PS) distribution. Furthermore, a drs2 deletion mutant is hypersensitive to the antifungal drug fluconazole and copper ions. We show here that such a flippase mutant also has an altered distribution of phosphatidylinositol 4-phosphate [PI(4)P] and ergosterol. Analyses of additional lipid transporters, i.e. the flippases Dnf1-3, and all the oxysterol binding protein (Osh) family lipid transfer proteins, i.e. Osh2-4 and Osh7, indicate that they are not critical for filamentous growth. However, deletion of Osh4 alone, which exchanges PI(4)P for sterol, in a drs2 mutant can bypass the requirement for this flippase in invasive filamentous growth. In addition, deletion of the lipid phosphatase Sac1, which dephosphorylates PI(4)P, in a drs2 mutant results in a synthetic growth defect, suggesting that Drs2 and Sac1 function in parallel pathways. Together, our results indicate that a balance between the activities of two putative lipid transporters regulates invasive filamentous growth, via PI(4)P. In contrast, deletion of OSH4 in drs2 does not restore growth on fluconazole, nor on papuamide A, a toxin that binds PS in the outer leaflet of the plasma membrane, suggesting that Drs2 has additional role(s) in plasma membrane organization, independent of Osh4. As we show that C. albicans Drs2 localizes to different structures, including the Spitzenkörper, we investigated if a specific localization of Drs2 is critical for different functions, using a synthetic physical interaction approach to restrict/stabilize Drs2 at the Spitzenkörper. Our results suggest that the localization of Drs2 at the plasma membrane is critical for C. albicans growth on fluconazole and papuamide A, but not for invasive filamentous growth.

Lipid Transport by Candida albicans Dnf2 Is Required for Hyphal Growth and Virulence

Candida albicans is a common cause of human mucosal yeast infections, and invasive candidiasis can be fatal. Antifungal medications are limited, but those targeting the pathogen cell wall or plasma membrane have been effective. Therefore, virulence factors controlling membrane biogenesis are potential targets for drug development. P4-ATPases contribute to membrane biogenesis by selecting and transporting specific lipids from the extracellular leaflet to the cytoplasmic leaflet of the bilayer to generate lipid asymmetry. A subset of heterodimeric P4-ATPases, including Dnf1-Lem3 and Dnf2-Lem3 from Saccharomyces cerevisiae, transport phosphatidylcholine (PC), phosphatidylethanolamine (PE), and the sphingolipid glucosylceramide (GlcCer). GlcCer is a critical lipid for Candida albicans polarized growth and virulence, but the role of GlcCer transporters in virulence has not been explored. Here, we show that the Candida albicans Dnf2 (CaDnf2) requires association with CaLem3 to form a functional transporter and flip fluorescent derivatives of GlcCer, PC, and PE across the plasma membrane. Mutation of conserved substrate-selective residues in the membrane domain strongly abrogates GlcCer transport and partially disrupts PC transport by CaDnf2. Candida strains harboring dnf2-null alleles (dnf2ΔΔ) or point mutations that disrupt substrate recognition exhibit defects in yeast-to-hypha growth transition, filamentous growth, and virulence in systemically infected mice. The influence of CaDNF1 deletion on the morphological phenotypes is negligible, although the dnf1ΔΔ dnf2ΔΔ strain was less virulent than the dnf2ΔΔ strain. These results indicate that the transport of GlcCer and/or PC by plasma membrane P4-ATPases is important for the pathogenicity of Candida albicans.

Microdomain Protein Nce102 Is a Local Sensor of Plasma Membrane Sphingolipid Balance

Sphingolipids are essential building blocks of eukaryotic membranes and important signaling molecules that are regulated tightly in response to environmental and physiological inputs. While their biosynthetic pathway has been well-described, the mechanisms that facilitate the perception of sphingolipid levels at the plasma membrane remain to be uncovered. In Saccharomyces cerevisiae, the Nce102 protein has been proposed to function as a sphingolipid sensor as it changes its plasma membrane distribution in response to sphingolipid biosynthesis inhibition. We show that Nce102 redistributes specifically in regions of increased sphingolipid demand, e.g., membranes of nascent buds. Furthermore, we report that the production of Nce102 increases following sphingolipid biosynthesis inhibition and that Nce102 is internalized when excess sphingolipid precursors are supplied. This finding suggests that the total amount of Nce102 in the plasma membrane is a measure of the current need for sphingolipids, whereas its local distribution marks sites of high sphingolipid demand. The physiological role of Nce102 in the regulation of sphingolipid synthesis is demonstrated by mass spectrometry analysis showing reduced levels of hydroxylated complex sphingolipids in response to heat stress in the nce102Δ deletion mutant. We also demonstrate that Nce102 behaves analogously in the widespread human fungal pathogen Candida albicans, suggesting a conserved principle of local sphingolipid control across species. IMPORTANCE Microorganisms are challenged constantly by their rapidly changing environment. To survive, they have developed diverse mechanisms to quickly perceive stressful situations and adapt to them appropriately. The primary site of both stress sensing and adaptation is the plasma membrane. We identified the yeast protein Nce102 as a marker of local sphingolipid levels and fluidity in the plasma membrane. Nce102 is an important structural and functional component of the membrane compartment Can1 (MCC), a plasma membrane microdomain stabilized by a large cytosolic hemitubular protein scaffold, the eisosome. The MCC/eisosomes are widely conserved among fungi and unicellular algae. To determine if Nce102 carries out similar functions in other organisms, we analyzed the human fungal pathogen Candida albicans and found that Nce102 responds to sphingolipid levels also in this organism, which has potential applications for the development of novel therapeutic approaches. The presented study represents a valuable model for how organisms regulate plasma membrane sphingolipids.

Metabolomics combined with physiology and transcriptomics reveals how Citrus grandis leaves cope with copper-toxicity

Limited data are available on metabolic responses of plants to copper (Cu)-toxicity. Firstly, we investigated Cu-toxic effects on metabolomics, the levels of free amino acids, NH₄⁺-N, NO₃^--N, total nitrogen, total soluble proteins, total phenolics, lignin, reduced glutathione (GSH) and malondialdehyde, and the activities of nitrogen-assimilatory enzymes in 'Shatian' pummelo (Citrus grandis) leaves. Then, a conjoint analysis of metabolomics, physiology and transcriptomics was performed. Herein, 59 upregulated [30 primary metabolites (PMs) and 29 secondary metabolites (SMs)] and 52 downregulated (31 PMs and 21 SMs) metabolites were identified in Cu-toxic leaves. The toxicity of Cu to leaves was related to the Cu-induced accumulation of NH₄⁺ and decrease of nitrogen assimilation. Metabolomics combined with physiology and transcriptomics revealed some adaptive responses of C. grandis leaves to Cu-toxicity, including (a) enhancing tryptophan metabolism and the levels of some amino acids and derivatives (tryptophan, phenylalanine, 5-hydroxy-l-tryptophan, 5-oxoproline and GSH); (b) increasing the accumulation of carbohydrates and alcohols and upregulating tricarboxylic acid cycle and the levels of some organic acids and derivatives (chlorogenic acid, quinic acid, d-tartaric acid and gallic acid o-hexoside); (c) reducing phospholipid (lysophosphatidylcholine and lysophosphatidylethanolamine) levels, increasing non-phosphate containing lipid [monoacylglycerol ester (acyl 18:2) isomer 1] levels, and inducing low-phosphate-responsive gene expression; and (d) triggering the biosynthesis of some chelators (total phenolics, lignin, l-trytamine, indole, eriodictyol C-hexoside, quercetin 5-O-malonylhexosyl-hexoside, N-caffeoyl agmatine, N'-p-coumaroyl agmatine, hydroxy-methoxycinnamate and protocatechuic acid o-glucoside) and vitamins and derivatives (nicotinic acid-hexoside, B₁ and methyl nicotinate). Cu-induced upregulation of many antioxidants could not protect Cu-toxic leaves from oxidative damage. To conclude, our findings corroborated the hypothesis that extensive reprogramming of metabolites was carried out in Cu-toxic C. grandis leaves in order to cope with Cu-toxicity.

The Sur7 cytoplasmic C terminus regulates morphogenesis and stress responses in Candida albicans

MCC/eisosome subdomains of the plasma membrane promote proper cell wall morphogenesis that is critical for the fungal pathogen Candida albicans to grow invasively and resist stressful environments in the host. Sur7 localizes to MCC/eisosomes and is needed for their function, so in this work, the role of this tetraspan membrane protein was studied by mutagenesis. Deletion mutant analysis showed that the N-terminal region containing the four transmembrane domains mediates Sur7 localization to MCC/eisosomes. Mutation of 32 conserved residues in the N-terminal region indicated that extracellular loop 1 is important, although these mutants generally displayed weak phenotypes. Surprisingly, two Cys residues in a conserved motif in extracellular loop 1 were not important. However, deletion of the entire 15 amino acid motif revealed that it was needed for proper membrane trafficking of Sur7. Deletion and substitution mutagenesis showed that the C terminus is important for resisting cell wall stress. This is significant as it indicates Sur7 carries out an important role in the cytoplasm. Altogether, these results indicate that the N-terminal region localizes Sur7 to MCC/eisosomes and that the C-terminal domain promotes responses in the cytoplasm needed for cell wall morphogenesis and stress resistance.

The fungivorous amoeba Protostelium aurantium targets redox homeostasis and cell wall integrity during intracellular killing of Candida parapsilosis

Predatory interactions among microbes are major evolutionary driving forces for biodiversity. The fungivorous amoeba Protostelium aurantium has a wide fungal food spectrum including foremost pathogenic members of the genus Candida. Here we show that upon phagocytic ingestion by the amoeba, Candida parapsilosis is confronted with an oxidative burst and undergoes lysis within minutes of processing in acidified phagolysosomes. On the fungal side, a functional genomic approach identified copper and redox homeostasis as primary targets of amoeba predation, with the highly expressed copper exporter gene CRP1 and the peroxiredoxin gene PRX1 contributing to survival when encountered with P. aurantium. The fungicidal activity was largely retained in intracellular vesicles of the amoebae. Following their isolation, the content of these vesicles induced immediate killing and lysis of C. parapsilosis in vitro. Proteomic analysis identified 56 vesicular proteins from P. aurantium. Although completely unknown proteins were dominant, many of them could be categorised as hydrolytic enzymes targeting the fungal cell wall, indicating that fungal cell wall structures are under selection pressure by predatory phagocytes in natural environments. TAKE AWAY: The amoeba Protostelium aurantium feeds on fungi, such as Candida parapsilosis. Ingested yeast cells are exposed to reactive oxygen species. A copper exporter and a peroxiredoxin contribute to fungal defence. Yeast cells undergo intracellular lysis. Lysis occurs via a cocktail of hydrolytic enzymes from intracellular vesicles.

Graphene- and Nanoparticle-Embedded Antimicrobial and Biocompatible Cotton/Silk Fabrics for Protective Clothing

Protection against pathogens using personal protective equipment is essential yet challenging in healthcare settings. Concerns over emerging biothreats and outbreaks of infectious diseases underscore the need for antimicrobial and biocompatible protective clothing to protect patients and staff. Herein, we report the antimicrobial efficacy and cytotoxicity of cotton/silk fabrics containing embedded reduced graphene oxide (RGO) and Ag/Cu nanoparticles (NPs), prepared using a 3-glycidyloxypropyl trimethoxy silane coupling agent followed by chemical reduction and vacuum heat treatment. Embedding NPs on top of the RGO layer substantially increased the antimicrobial activity. All RGO-Ag NPs or RGO-Cu NPs embedded in cotton or silk fabrics reduced the viability of approximately 99% of the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa. RGO-Ag NPs embedded into cotton or silk fabrics reduced the viability of the Gram-positive bacterium Staphylococcus aureus by 78-99%, which was higher than the growth inhibition by RGO-Cu NPs samples against S. aureus. Both silk and cotton containing RGO-Cu NPs produced a greater reduction in the viability of the yeast Candida albicans compared to RGO-Ag NPs fabrics. All RGO-Ag NPs or RGO-Cu NPs embedded in cotton or silk fabrics showed good washing durability by sustaining good bactericidal activity, even on washing up to 10 times. Moreover, none of the RGO-Ag or RGO-Cu fabrics reduced mammalian cells' (HEK293) viability by >30%, suggesting low cytotoxicity and good biocompatibility. These findings show that RGO-NPs embedded in cotton or silk fabrics have great potential for use in protective clothing and medical textiles.

The role of non-standard translation in Candida albicans pathogenesis

Candida albicans typically resides in the human gastrointestinal tract and mucosal membranes as a commensal organism. To adapt and cope with the host immune system, it has evolved a variety of mechanisms of adaptation such as stress-induced mutagenesis and epigenetic regulation. Niche-specific patterns of gene expression also allow the fungus to fine-tune its response to specific microenvironments in the host and switch from harmless commensal to invasive pathogen. Proteome plasticity produced by CUG ambiguity, on the other hand is emerging as a new layer of complexity in C. albicans adaptation, pathogenesis, and drug resistance. Such proteome plasticity is the result of a genetic code alteration where the leucine CUG codon is translated mainly as serine (97%), but maintains some level of leucine (3%) assignment. In this review, we dissect the link between C. albicans non-standard CUG translation, proteome plasticity, host adaptation and pathogenesis. We discuss published work showing how this pathogen uses the fidelity of protein synthesis to spawn novel virulence traits.

Transcriptomic analysis of Sur7-mediated response of Beauveria bassiana to different nutritional conditions

Integrity of the cell wall is requisite for fungal growth and function. Sur7 governs cell wall composition, and affects conidial sporulation and germination in Beauveria bassiana, a filamentous entomopathogenic fungus. The role of Sur7 in fungal growth on various nutrients remains unclear. We have previously reported that Sur7 deletion results in the attenuation of B. bassiana growth on supplemented Sabouraud dextrose agar (SDAY) and minimal Czapek-Dox agar (CDA) compared to wild type (WT). Here, we used transcriptomic analysis to compare WT and Sur7 mutant (ΔSur7) responses to CDA and SDAY. Growth on CDA, compared with that on SDAY, affected the expression of more genes in the WT than in the mutant. Differentially expressed genes were enriched for transportation process terms in the ΔSur7 mutant and metabolic process terms in the WT. Different processes were repressed in the ΔSur7 (metabolic process) and WT (ribosome synthesis) cells. Despite the shared enrichment of nitrogen metabolism genes, differentially expressed genes were enriched in distinct saccharide-energy metabolism terms in each strain. We conclude that Sur7 ensures the growth of B. bassiana in a minimal medium by influencing the expression of genes involved in the consumption of sucrose via specific energy metabolism pathways.

Copper in infectious disease: Using both sides of the penny

The transition metal Cu is an essential micronutrient that serves as a co-factor for numerous enzymes involved in redox and oxygen chemistry. However, Cu is also a potentially toxic metal, especially to unicellular microbes that are in direct contact with their environment. Since 400 BCE, Cu toxicity has been leveraged for its antimicrobial properties and even today, Cu based materials are being explored as effective antimicrobials against human pathogens spanning bacteria, fungi, and viruses, including the SARS-CoV-2 agent of the 2019-2020 pandemic. Given that Cu has the double-edged property of being both highly toxic and an essential micronutrient, it plays an active and complicated role at the host-pathogen interface. Humans have evolved methods of incorporating Cu into innate and adaptive immune processes and both sides of the penny (Cu toxicity and Cu as a nutrient) are employed. Here we review the evolution of Cu in biology and its multi-faceted roles in infectious disease, from the viewpoints of the microbial pathogens as well as the animal hosts they infect.

Bioluminescence as a sensitive electroporation indicator in sub-microsecond and microsecond range of electrical pulses

The cell membrane permeabilization in electroporation studies is usually quantified using fluorescent markers such as propidium iodide (PI) or YO-PRO, while Chinese Hamster Ovary cell line frequently serves as a model. In this work, as an alternative, we propose a sensitive methodology for detection and analysis of electroporation phenomenon based on bioluminescence. Luminescent mice myeloma SP2/0 cells (transfected using Luciferase-pcDNA3 plasmid) were used as a cell model. Electroporation has been studied using the 0.1-5 μs × 250 and 100 μs × 1-8 pulsing protocols in 1-2.5 kV/cm PEF range. It was shown that the bioluminescence response is dependent on the cell permeabilization state and can be effectively used to detect even weak permeabilization. During saturated permeabilization the methodology accurately predicts the losses of cell viability due to irreversible electroporation. The results have been superpositioned with permeabilization and pore resealing (1 h post-treatment) data using PI. Also, the viability of the cells was evaluated. Lastly, the SP2/0 tumors have been developed in BALB/C mice and the methodology has been tested in vivo using electrochemotherapy with bleomycin.

The Composition and the Structure of MCC/Eisosomes in Neurospora crassa

MCC/eisosomes are protein-organized domains in the plasma membrane of fungi and algae. However, the composition and function(s) of MCC/eisosomes in the filamentous fungus Neurospora crassa were previously unknown. To identify proteins that localize to MCC/eisosomes in N. crassa, we isolated proteins that co-purified with the core MCC/eisosome protein LSP-1, which was tagged with GFP. Proteins that co-fractionated with LSP-1:GFP were then identified by mass spectrometry. Eighteen proteins were GFP-tagged and used to identify six proteins that highly colocalized with the MCC/eisosome marker LSP-1:RFP, while five other proteins showed partial overlap with MCC/eisosomes. Seven of these proteins showed amino acid sequence homology with proteins known to localize to MCC/eisosomes in the yeast Saccharomyces cerevisiae. However, homologs of three proteins known to localize to MCC/eisosomes in S. cerevisiae (Can1, Pkh1/2, and Fhn1) were not found to colocalize with MCC/eisosome proteins in N. crassa by fluorescence microscopy. Interestingly, one new eisosome protein (glutamine-fructose-6-phosphate aminotransferase, gene ID: NCU07366) was detected in our studies. These findings demonstrate that there are interspecies differences of the protein composition of MCC/eisosomes. To gain further insight, molecular modeling and bioinformatics analysis of the identified proteins were used to propose the organization of MCC/eisosomes in N. crassa. A model will be discussed for how the broad range of functions predicted for the proteins localized to MCC/eisosomes, including cell wall synthesis, response and signaling, transmembrane transport, and actin organization, suggests that MCC/eisosomes act as organizing centers in the plasma membrane.

Plasma Membrane MCC/Eisosome Domains Promote Stress Resistance in Fungi

There is growing appreciation that the plasma membrane orchestrates a diverse array of functions by segregating different activities into specialized domains that vary in size, stability, and composition. Studies with the budding yeast Saccharomyces cerevisiae have identified a novel type of plasma membrane domain known as the MCC (membrane compartment of Can1)/eisosomes that correspond to stable furrows in the plasma membrane. MCC/eisosomes maintain proteins at the cell surface, such as nutrient transporters like the Can1 arginine symporter, by protecting them from endocytosis and degradation. Recent studies from several fungal species are now revealing new functional roles for MCC/eisosomes that enable cells to respond to a wide range of stressors, including changes in membrane tension, nutrition, cell wall integrity, oxidation, and copper toxicity. The different MCC/eisosome functions are often intertwined through the roles of these domains in lipid homeostasis, which is important for proper plasma membrane architecture and cell signaling. Therefore, this review will emphasize the emerging models that explain how MCC/eisosomes act as hubs to coordinate cellular responses to stress. The importance of MCC/eisosomes is underscored by their roles in virulence for fungal pathogens of plants, animals, and humans, which also highlights the potential of these domains to act as novel therapeutic targets.

Candida albicans Agar Invasion Assays

The ability of the human fungal pathogen Candida albicans to disseminate into tissues is promoted by a switch from budding to invasive hyphal growth. This morphological transition is stimulated by multiple environmental factors that can vary at different sites of infection. To identify genes that promote invasive growth, C. albicans mutants can be screened for defects in growing invasively into solid agar medium as a substitute for studying tissue invasion. This in vitro approach has advantages in that it permits the media conditions to be varied to mimic different host environments. In addition, the concentration of agar can be varied to determine the effects of altering the rigidity of the matrix into which the cells invade, as this provides a better indicator of invasive growth than the ability to form hyphae in a liquid culture. Testing under multiple conditions can be used to identify mutant cells with the strongest defects. Therefore, protocols and media for analyzing invasive growth of C. albicans under different conditions will be described that are appropriate for testing a single strain or high-throughput analysis of a collection of mutant C. albicans strains.

Fungal plasma membrane domains

The plasma membrane (PM) performs a plethora of physiological processes, the coordination of which requires spatial and temporal organization into specialized domains of different sizes, stability, protein/lipid composition and overall architecture. Compartmentalization of the PM has been particularly well studied in the yeast Saccharomyces cerevisiae, where five non-overlapping domains have been described: The Membrane Compartments containing the arginine permease Can1 (MCC), the H+-ATPase Pma1 (MCP), the TORC2 kinase (MCT), the sterol transporters Ltc3/4 (MCL), and the cell wall stress mechanosensor Wsc1 (MCW). Additional cortical foci at the fungal PM are the sites where clathrin-dependent endocytosis occurs, the sites where the external pH sensing complex PAL/Rim localizes, and sterol-rich domains found in apically grown regions of fungal membranes. In this review, we summarize knowledge from several fungal species regarding the organization of the lateral PM segregation. We discuss the mechanisms of formation of these domains, and the mechanisms of partitioning of proteins there. Finally, we discuss the physiological roles of the best-known membrane compartments, including the regulation of membrane and cell wall homeostasis, apical growth of fungal cells and the newly emerging role of MCCs as starvation-protective membrane domains.

Yeast Sphingolipid-Enriched Domains and Membrane Compartments in the Absence of Mannosyldiinositolphosphorylceramide

The relevance of mannosyldiinositolphosphorylceramide [M(IP)₂C] synthesis, the terminal complex sphingolipid class in the yeast Saccharomyces cerevisiae, for the lateral organization of the plasma membrane, and in particular for sphingolipid-enriched gel domains, was investigated by fluorescence spectroscopy and microscopy. We also addressed how changing the complex sphingolipid profile in the plasma membrane could influence the membrane compartments (MC) containing either the arginine/ H⁺ symporter Can1p (MCC) or the proton ATPase Pma1p (MCP). To achieve these goals, wild-type (wt) and ipt1Δ cells, which are unable to synthesize M(IP)₂C accumulating mannosylinositolphosphorylceramide (MIPC), were compared. Living cells, isolated plasma membrane and giant unilamellar vesicles reconstituted from plasma membrane lipids were labelled with various fluorescent membrane probes that report the presence and organization of distinct lipid domains, global order, and dielectric properties. Can1p and Pma1p were tagged with GFP and mRFP, respectively, in both yeast strains, to evaluate their lateral organization using confocal fluorescence intensity and fluorescence lifetime imaging. The results show that IPT1 deletion strongly affects the rigidity of gel domains but not their relative abundance, whereas no significant alterations could be perceived in ergosterol-enriched domains. Moreover, in these cells lacking M(IP)₂C, a clear alteration in Pma1p membrane distribution, but no significant changes in Can1p distribution, were observed. Thus, this work reinforces the notion that sphingolipid-enriched domains distinct from ergosterol-enriched regions are present in the S. cerevisiae plasma membrane and suggests that M(IP)₂C is important for a proper hydrophobic chain packing of sphingolipids in the gel domains of wt cells. Furthermore, our results strongly support the involvement of sphingolipid domains in the formation and stability of the MCP, possibly being enriched in this compartment.

Stress granules sense metabolic stress at the plasma membrane and potentiate recovery by storing active Pkc1

As the physical barrier between the cell and the outside environment, the plasma membrane is well-positioned to be the first responder to stress. The membrane is also highly vulnerable to many types of perturbation, including heat, force, osmotic pressure, lipid shortage, and starvation. To determine whether the structural changes in the plasma membrane of Saccharomyces cerevisiae brought about by nutrient stress can be communicated to regulatory networks within the cell, we identified proteins that interact with stress granules (SGs), subcellular structures composed of proteins, and nontranslated RNAs that form when cells are stressed. We found that SG proteins interacted with components of eisosomes, which are subcortical membrane structures with a distinct lipid and protein composition. In response to starvation-triggered phosphorylation of eisosome proteins, eisosomes clustered and recruited SG components, including active Pkc1. The absence of eisosomes impaired SG formation, resulting in delayed recovery from nutrient deprivation. Thus, eisosome clustering is an example of interdomain communication in response to stress and identifies a previously unknown mechanism of SG regulation.

Candida albicans rvs161Δ and rvs167Δ Endocytosis Mutants Are Defective in Invasion into the Oral Cavity

Oropharyngeal candidiasis (OPC) is a common fungal infection that is associated with severe morbidity. Another concern is that patients at risk for developing OPC often take long courses of antifungal drugs, which can lead to the emergence of drug-resistant C. albicans strains. We therefore identified nine mutants with defects in undergoing invasive hyphal growth in the oral cavity, increasing the number of genes known to be involved in OPC by more than 30%. The two strongest mutants, rvs161Δ and rvs167Δ, have defects in endocytosis. The rvsΔ mutants appear to have a specific defect in initiating invasive growth, as preinducing the cells to form hyphae prior to infection restored their ability to cause OPC. These results indicate that blocking endocytosis could have therapeutic value in preventing the initiation of OPC without leading to development of resistance against drugs currently used to treat fungal infections.

Invasive growth in tissues by the human fungal pathogen Candida albicans is promoted by a switch from budding to hyphal morphogenesis that is stimulated by multiple environmental factors that can vary at different sites of infection. To identify genes that promote invasive growth in the oral cavity to cause oropharyngeal candidiasis (OPC), we first identified C. albicans mutants that failed to invade agar medium. Analysis of nine severely defective mutants in a mouse model of OPC revealed that the strongest defects were seen for the rvs161Δ and rvs167Δ mutants, which lack amphiphysin proteins needed for endocytosis. The rvsΔ mutants initially adhered to the tongue but failed to invade efficiently and were lost from the oral cavity. Previous studies indicated that rvsΔ mutants formed filamentous hyphae in the kidney albeit with morphological abnormalities, suggesting that the rvsΔ mutants were influenced by factors that vary at different sites of infection. Consistent with this, increasing concentrations of CO₂, an inducer of hyphal growth that is more abundant in internal organs than air, partially rescued the invasive-growth defects of the rvsΔ mutants in vitro. Interestingly, preinduction of the rvsΔ mutants to form hyphae prior to introduction into the oral cavity restored their ability to cause OPC, identifying a key role for endocytosis in initiating invasive hyphal growth. These results highlight the influence of distinct environmental factors in promoting invasive hyphal growth in the oral cavity and indicate that blocking endocytosis could have therapeutic value in preventing the initiation of OPC.

Role of lipid transporters in fungal physiology and pathogenicity

The fungal cell wall and membrane are the most common targets of antifungal agents, but the potential of membrane lipid organization in regulating drug-target interactions has yet to be investigated. Energy-dependent lipid transporters have been recently associated with virulence and drug resistance in many pathogenic fungi. To illustrate this view, we discuss (i) the structural and biological aspects of ATP-driven lipid transporters, comprising P-type ATPases and ATP-binding cassette transporters, (ii) the role of these transporters in fungal physiology and virulence, and (iii) the potential of lipid transporters as targets for the development of novel antifungals. These recent observations indicate that the lipid-trafficking machinery in fungi is a promising target for studies on physiology, pathogenesis and drug development.

Copper blocks V-ATPase activity and SNARE complex formation to inhibit yeast vacuole fusion

The accumulation of copper in organisms can lead to altered functions of various pathways and become cytotoxic through the generation of reactive oxygen species. In yeast, cytotoxic metals such as Hg⁺ , Cd²⁺ and Cu²⁺ are transported into the lumen of the vacuole through various pumps. Copper ions are initially transported into the cell by the copper transporter Ctr1 at the plasma membrane and sequestered by chaperones and other factors to prevent cellular damage by free cations. Excess copper ions can subsequently be transported into the vacuole lumen by an unknown mechanism. Transport across membranes requires the reduction of Cu²⁺ to Cu⁺ . Labile copper ions can interact with membranes to alter fluidity, lateral phase separation and fusion. Here we found that CuCl₂ potently inhibited vacuole fusion by blocking SNARE pairing. This was accompanied by the inhibition of V-ATPase H⁺ pumping. Deletion of the vacuolar reductase Fre6 had no effect on the inhibition of fusion by copper. This suggests that Cu²⁺ is responsible for the inhibition of vacuole fusion and V-ATPase function. This notion is supported by the differential effects of chelators. The Cu²⁺ -specific chelator triethylenetetramine rescued fusion, whereas the Cu⁺ -specific chelator bathocuproine disulfonate had no effect on the inhibited fusion.

Epoxy-amine oligomers from terpenes with applications in synergistic antifungal treatments

A bis-epoxide monomer was synthesised in two steps from (R)-carvone, a terpenoid renewable feedstock derived from spearmint oil, and used to prepare β-aminoalcohol oligomers in polyaddition reactions with bis-amines without requiring solvent or catalyst. A sub-set of the resultant materials were readily water soluble and were investigated for antifungal activity in combination with the fungicide iodopropynyl-butylcarbamate (IPBC) or the antifungal drug amphotericin B. The oligo-(β-aminoalcohol)s alone were inactive against Trichoderma virens and Candida albicans but in combination with IPBC and amphotericin B demonstrated synergistic growth-inhibition of both fungi. Quantitative analysis showed that the presence of the terpene-based oligomers decreased the minimum inhibitory concentration (MIC) of IPBC by up to 64-fold and of amphotericin B by 8-fold. The efficacy of the combined formulation was further demonstrated with agar disk diffusion assays, which revealed that IPBC and amphotericin B reduced the growth of the fungi, as shown by zones of inhibition, to a greater extent when in the presence of the oligo-(β-aminoalcohol)s. These data suggest potential future use of these renewable feedstock derived oligomers in antifungal material and related biomedical applications.

Role of MCC/Eisosome in Fungal Lipid Homeostasis

One of the best characterized fungal membrane microdomains is the MCC/eisosome. The MCC (membrane compartment of Can1) is an evolutionarily conserved ergosterol-rich plasma membrane domain. It is stabilized on its cytosolic face by the eisosome, a hemitubular protein complex composed of Bin/Amphiphysin/Rvs (BAR) domain-containing Pil1 and Lsp1. These two proteins bind directly to phosphatidylinositol 4,5-bisphosphate and promote the typical furrow-like shape of the microdomain, with highly curved edges and bottom. While some proteins display stable localization in the MCC/eisosome, others enter or leave it under particular conditions, such as misbalance in membrane lipid composition, changes in membrane tension, or availability of specific nutrients. These findings reveal that the MCC/eisosome, a plasma membrane microdomain with distinct morphology and lipid composition, acts as a multifaceted regulator of various cellular processes including metabolic pathways, cellular morphogenesis, signalling cascades, and mRNA decay. In this minireview, we focus on the MCC/eisosome's proposed role in the regulation of lipid metabolism. While the molecular mechanisms of the MCC/eisosome function are not completely understood, the idea of intracellular processes being regulated at the plasma membrane, the foremost barrier exposed to environmental challenges, is truly exciting.

Candida albicans Interactions with Mucosal Surfaces during Health and Disease

Flexible adaptation to the host environment is a critical trait that underpins the success of numerous microbes. The polymorphic fungus Candida albicans has evolved to persist in the numerous challenging niches of the human body. The interaction of C. albicans with a mucosal surface is an essential prerequisite for fungal colonisation and epitomises the complex interface between microbe and host. C. albicans exhibits numerous adaptations to a healthy host that permit commensal colonisation of mucosal surfaces without provoking an overt immune response that may lead to clearance. Conversely, fungal adaptation to impaired immune fitness at mucosal surfaces enables pathogenic infiltration into underlying tissues, often with devastating consequences. This review will summarise our current understanding of the complex interactions that occur between C. albicans and the mucosal surfaces of the human body.