Glucan synthase complex of Aspergillus fumigatus

GH16 and GH81 family β-(1,3)-glucanases in Aspergillus fumigatus are essential for conidial cell wall morphogenesis

The fungal cell wall is a rigid structure because of fibrillar and branched β-(1,3)-glucan linked to chitin. Softening of the cell wall is an essential phenomenon during fungal morphogenesis, wherein rigid cell wall structures are cleaved by glycosylhydrolases. During the search for glycosylhydrolases acting on β-(1,3)-glucan, we identified seven genes in the Aspergillus fumigatus genome coding for potential endo-β-(1,3)-glucanase. ENG1 (previously characterized and named ENGL1, Mouyna et al., ), belongs to the Glycoside-Hydrolase 81 (GH81) family, while ENG2 to ENG7, to GH16 family. ENG1 and four GH16 genes (ENG2-5) were expressed in the resting conidia as well as during germination, suggesting an essential role during A. fumigatus morphogenesis. Here, we report the effect of sequential deletion of AfENG2-5 (GH16) followed by AfENG1 (GH81) deletion in the Δeng2,3,4,5 mutant. The Δeng1,2,3,4,5 mutant showed conidial defects, with linear chains of conidia unable to separate while the germination rate was not affected. These results show, for the first time in a filamentous fungus, that endo β-(1,3)-glucanases are essential for proper conidial cell wall assembly and thus segregation of conidia during conidiation.

Lipid Flippase Subunit Cdc50 Mediates Drug Resistance and Virulence in Cryptococcus neoformans

Cryptococcus neoformans is a human fungal pathogen and a major cause of fungal meningitis in immunocompromised individuals. Treatment options for cryptococcosis are limited. Of the two major antifungal drug classes, azoles are active against C. neoformans but exert a fungistatic effect, necessitating long treatment regimens and leaving open an avenue for emergence of azole resistance. Drugs of the echinocandin class, which target the glucan synthase and are fungicidal against a number of other fungal pathogens, such as Candida species, are ineffective against C. neoformans. Despite the sensitivity of the target enzyme to the drug, the reasons for the innate resistance of C. neoformans to echinocandins remain unknown. To understand the mechanism of echinocandin resistance in C. neoformans, we screened gene disruption and gene deletion libraries for mutants sensitive to the echinocandin-class drug caspofungin and identified a mutation of CDC50, which encodes the β-subunit of membrane lipid flippase. We found that the Cdc50 protein localized to membranes and that its absence led to plasma membrane defects and enhanced caspofungin penetration into the cell, potentially explaining the increased caspofungin sensitivity. Loss of CDC50 also led to hypersensitivity to the azole-class drug fluconazole. Interestingly, in addition to functioning in drug resistance, CDC50 was also essential for fungal resistance to macrophage killing and for virulence in a murine model of cryptococcosis. Furthermore, the surface of cdc50Δ cells contained increased levels of phosphatidylserine, which has been proposed to act as a macrophage recognition signal. Together, these results reveal a previously unappreciated role of membrane lipid flippase in C. neoformans drug resistance and virulence.

Cryptococcus neoformans is a fungal pathogen that is the most common cause of fungal meningitis, causing over 620,000 deaths annually. The treatment options for cryptococcosis are very limited. The most commonly used drugs are either fungistatic (azoles) or highly toxic (amphotericin B). Echinocandins are the newest fungicidal drug class that works well in treating candidiasis and aspergillosis, yet they are ineffective in treating cryptococcosis. In this study, we showed that the regulatory subunit of the lipid translocase (flippase), a protein that regulates the asymmetrical orientation of membrane lipids, is required for C. neoformans resistance to caspofungin, as well as for virulence during infection. This discovery identifies lipid flippase as a potential C. neoformans drug target, which plays an important role in the innate resistance of C. neoformans to echinocandins and in fungal virulence.

Recent advances in the understanding of the Aspergillus fumigatus cell wall

Over the past several decades, research on the synthesis and organization of the cell wall polysaccharides of Aspergillus fumigatus has expanded our knowledge of this important fungal structure. Besides protecting the fungus from environmental stresses and maintaining structural integrity of the organism, the cell wall is also the primary site for interaction with host tissues during infection. Cell wall polysaccharides are important ligands for the recognition of fungi by the innate immune system and they can mediate potent immunomodulatory effects. The synthesis of cell wall polysaccharides is a complicated process that requires coordinated regulation of many biosynthetic and metabolic pathways. Continuous synthesis and remodeling of the polysaccharides of the cell wall is essential for the survival of the fungus during development, reproduction, colonization and invasion. As these polysaccharides are absent from the human host, these biosynthetic pathways are attractive targets for antifungal development. In this review, we present recent advances in our understanding of Aspergillus fumigatus cell wall polysaccharides, including the emerging role of cell wall polysaccharides in the host-pathogen interaction.

Network Modeling Reveals Cross Talk of MAP Kinases during Adaptation to Caspofungin Stress in Aspergillus fumigatus

Mitogen activated protein kinases (MAPKs) are highly conserved in eukaryotic organisms. In pathogenic fungi, their activities were assigned to different physiological functions including drug adaptation and resistance. Aspergillus fumigatus is a human pathogenic fungus, which causes life-threatening invasive infections. Therapeutic options against invasive mycoses are still limited. One of the clinically used drugs is caspofungin, which specifically targets the fungal cell wall biosynthesis. A systems biology approach, based on comprehensive transcriptome data sets and mathematical modeling, was employed to infer a regulatory network and identify key interactions during adaptation to caspofungin stress in A. fumigatus. Mathematical modeling and experimental validations confirmed an intimate cross talk occurring between the cell wall-integrity and the high osmolarity-glycerol signaling pathways. Specifically, increased concentrations of caspofungin promoted activation of these signalings. Moreover, caspofungin affected the intracellular transport, which caused an additional osmotic stress that is independent of glucan inhibition. High concentrations of caspofungin reduced this osmotic stress, and thus decreased its toxic activity. Our results demonstrated that MAPK signaling pathways play a key role during caspofungin adaptation and are contributing to the paradoxical effect exerted by this drug.

Novel in vivo observations on double acting points of luliconazole on Trichophyton rubrum: an ultrastructural study

Scales from lesional skin of 12 patients with tinea pedis were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to gain an insight into the spatial and morphological changes of dermatophytes after application of a clinical dosage of topical luliconazole 1% cream (Lulicon® cream 1%). In all cases, Trichophyton rubrum was identified. The scales from the lesions collected before and after topical luliconazole application were fixed with glutaraldehyde and subjected to SEM and TEM. For SEM, fixed specimens were first placed in 1N-KOH and then post-fixed and observed. SEM showed a swollen appearance of fungal hyphae as an early change, and then shrinkage of them showing a flattened and twisted appearance as a later change. TEM showed cell wall alterations with initial development of and accumulation of a granular structure in the outermost layer and subsequent amorphous and electron-lucent change of the thickened inner part of the cell wall. This is the first report of dramatic morphological changes of T. rubrum before and after topical luliconazole application in vivo demonstrated by SEM and TEM. We hypothesize that luliconazole has double acting points, on the plasma membrane and cell wall, of dermatophyte hyphae.

Caspofungin Treatment of Aspergillus fumigatus Results in ChsG-Dependent Upregulation of Chitin Synthesis and the Formation of Chitin-Rich Microcolonies

Treatment of Aspergillus fumigatus with echinocandins such as caspofungin inhibits the synthesis of cell wall β-1,3-glucan, which triggers a compensatory stimulation of chitin synthesis. Activation of chitin synthesis can occur in response to sub-MICs of caspofungin and to CaCl₂ and calcofluor white (CFW), agonists of the protein kinase C (PKC), and Ca²⁺-calcineurin signaling pathways. A. fumigatus mutants with the chs gene (encoding chitin synthase) deleted (ΔAfchs) were tested for their response to these agonists to determine the chitin synthase enzymes that were required for the compensatory upregulation of chitin synthesis. Only the ΔAfchsG mutant was hypersensitive to caspofungin, and all other ΔAfchs mutants tested remained capable of increasing their chitin content in response to treatment with CaCl₂ and CFW and caspofungin. The resulting increase in cell wall chitin content correlated with reduced susceptibility to caspofungin in the wild type and all ΔAfchs mutants tested, with the exception of the ΔAfchsG mutant, which remained sensitive to caspofungin. In vitro exposure to the chitin synthase inhibitor, nikkomycin Z, along with caspofungin demonstrated synergistic efficacy that was again AfChsG dependent. Dynamic imaging using microfluidic perfusion chambers demonstrated that treatment with sub-MIC caspofungin resulted initially in hyphal tip lysis. However, thickened hyphae emerged that formed aberrant microcolonies in the continued presence of caspofungin. In addition, intrahyphal hyphae were formed in response to echinocandin treatment. These in vitro data demonstrate that A. fumigatus has the potential to survive echinocandin treatment in vivo by AfChsG-dependent upregulation of chitin synthesis. Chitin-rich cells may, therefore, persist in human tissues and act as the focus for breakthrough infections.

Velvet-mediated repression of β-glucan synthesis in Aspergillus nidulans spores

Beta-glucans are a heterologous group of fibrous glucose polymers that are a major constituent of cell walls in Ascomycetes and Basidiomycetes fungi. Synthesis of β (1,3)- and (1,6)-glucans is coordinated with fungal cell growth and development, thus, is under tight genetic regulation. Here, we report that β-glucan synthesis in both asexual and sexual spores is turned off by the NF-kB like fungal regulators VosA and VelB in Aspergillus nidulans. Our genetic and genomic analyses have revealed that both VosA and VelB are necessary for proper down-regulation of cell wall biosynthetic genes including those associated with β-glucan synthesis in both types of spores. The deletion of vosA or velB results in elevated accumulation of β-glucan in asexual spores. Double mutant analyses indicate that VosA and VelB play an inter-dependent role in repressing β-glucan synthesis in asexual spores. In vivo chromatin immuno-precipitation analysis shows that both VelB and VosA bind to the promoter region of the β-glucan synthase gene fksA in asexual spores. Similarly, VosA is required for proper repression of β-glucan synthesis in sexual spores. In summary, the VosA-VelB hetero-complex is a key regulatory unit tightly controlling proper levels of β-glucan synthesis in asexual and sexual spores.

The Aspergillus fumigatus cell wall integrity signaling pathway: drug target, compensatory pathways, and virulence

Aspergillus fumigatus is the most important airborne fungal pathogen, causing severe infections with invasive growth in immunocompromised patients. The fungal cell wall (CW) prevents the cell from lysing and protects the fungus against environmental stress conditions. Because it is absent in humans and because of its essentiality, the fungal CW is a promising target for antifungal drugs. Nowadays, compounds acting on the CW, i.e., echinocandin derivatives, are used to treat A. fumigatus infections. However, studies demonstrating the clinical effectiveness of echinocandins in comparison with antifungals currently recommended for first-line treatment of invasive aspergillosis are still lacking. Therefore, it is important to elucidate CW biosynthesis pathways and their signal transduction cascades, which potentially compensate the inhibition caused by CW- perturbing compounds. Like in other fungi, the central core of the cell wall integrity (CWI) signaling pathway in A. fumigatus is composed of three mitogen activated protein kinases. Deletion of these genes resulted in severely enhanced sensitivity of the mutants against CW-disturbing compounds and in drastic alterations of the fungal morphology. Additionally, several cross-talk interactions between the CWI pathways and other signaling pathways are emerging, raising the question about their role in the CW compensatory mechanisms. In this review we focused on recent advances in understanding the CWI signaling pathway in A. fumigatus and its role during drug stress response and virulence.

Aspergillus fumigatus devoid of cell wall β-1,3-glucan is viable, massively sheds galactomannan and is killed by septum formation inhibitors

Echinocandins inhibit β-1,3-glucan synthesis and are one of the few antimycotic drug classes effective against Aspergillus spp. In this study, we characterized the β-1,3-glucan synthase Fks1 of Aspergillus fumigatus, the putative target of echinocandins. Data obtained with a conditional mutant suggest that fks1 is not essential. In agreement, we successfully constructed a viable Δfks1 deletion mutant. Lack of Fks1 results in characteristic growth phenotypes similar to wild type treated with echinocandins and an increased susceptibility to calcofluor white and sodium dodecyl sulfate. In agreement with Fks1 being the only β-1,3-glucan synthase in A. fumigatus, the cell wall is devoid of β-1,3-glucan. This is accompanied by a compensatory increase of chitin and galactosaminogalactan and a significant decrease in cell wall galactomannan due to a massively enhanced galactomannan shedding. Our data furthermore suggest that inhibition of hyphal septation can overcome the limitations of echinocandin therapy. Compounds inhibiting septum formation boosted the antifungal activity of caspofungin. Thus, development of clinically applicable inhibitors of septum formation is a promising strategy to improve existing antifungal therapy.

Sensitivity of Neurospora crassa to a marine-derived Aspergillus tubingensis anhydride exhibiting antifungal activity that is mediated by the MAS1 protein

The fungus Aspergillustubingensis (strain OY907) was isolated from the Mediterranean marine sponge Ircinia variabilis. Extracellular extracts produced by this strain were found to inhibit the growth of several fungi. Among the secreted extract components, a novel anhydride metabolite, tubingenoic anhydride A (1) as well as the known 2-carboxymethyl-3-hexylmaleic acid anhydride, asperic acid, and campyrone A and C were purified and their structure elucidated. Compound 1 and 2-carboxymethyl-3-hexylmaleic acid anhydride inhibited Neurospora crassa growth (MIC = 330 and 207 μM, respectively) and affected hyphal morphology. We produced a N. crassa mutant exhibiting tolerance to 1 and found that a yet-uncharacterized gene, designated mas-1, whose product is a cytosolic protein, confers sensitivity to this compound. The ∆mas-1 strain showed increased tolerance to sublethal concentrations of the chitin synthase inhibitor polyoxin D, when compared to the wild type. In addition, the expression of chitin synthase genes was highly elevated in the ∆mas-1 strain, suggesting the gene product is involved in cell wall biosynthesis and the novel anhydride interferes with its function.

The importance of connections between the cell wall integrity pathway and the unfolded protein response in filamentous fungi

In the external environment, or within a host organism, filamentous fungi experience sudden changes in nutrient availability, osmolality, pH, temperature and the exposure to toxic compounds. The fungal cell wall represents the first line of defense, while also performing essential roles in morphology, development and virulence. A polarized secretion system is paramount for cell wall biosynthesis, filamentous growth, nutrient acquisition and interactions with the environment. The unique ability of filamentous fungi to secrete has resulted in their industrial adoption as fungal cell factories. Protein maturation and secretion commences in the endoplasmic reticulum (ER). The unfolded protein response (UPR) maintains ER functionality during exposure to secretion and cell wall stress. UPR, therefore, influences secretion and cell wall homeostasis, which in turn impacts upon numerous fungal traits important to pathogenesis and biotechnology. Subsequently, this review describes the relevance of the cell wall and UPR systems to filamentous fungal pathogens or industrial microbes and then highlights interconnections between the two systems. Ultimately, the possible biotechnological applications of an enhanced understanding of such regulatory systems in combating fungal disease, or the removal of natural bottlenecks in protein secretion in an industrial setting, are discussed.

The Aspergillus fumigatus β-1,3-glucanosyltransferase Gel7 plays a compensatory role in maintaining cell wall integrity under stress conditions

Aspergillus fumigatus is an opportunistic fungal pathogen that causes fatal invasive aspergillosis among immunocompromised patients. The cell wall β-1,3-glucan is mainly elongated by β-1,3-glucanosyltransferase Gel family, which is vital for growth and virulence of A. fumigatus. Although seven members of Gels have been annotated, only Gel1, Gel2 and Gel4 were characterized. In this study, the function of Gel7 was analyzed for the first time, by constructing Δgel7, Δgel7Δcwh41 and Δgel1Δgel7Δcwh41 separately. Disruption of gel7 alone did not result in any obvious phenotype except an abnormality in conidia formation, whereas Δgel7Δcwh41 and Δgel1Δgel7Δcwh41 exhibited abnormal conidiogenesis, a heat-induced delay of germination and a severe decrease in β-1,3-glucan content. Our results suggested that the A. fumigatus β-1,3-glucanosyltransferase Gel7 was involved in conidiation and was compensated for the cell wall β-1,3-glucan defects when Gel1 and Gel2 lost their functions, especially at an elevated temperature.

in-silico characterization of β-(1, 3)-endoglucanase (ENGL1) from Aspergillus fumigatus by homology modeling and docking studies

During the past few years a significant rise in aspergillosis caused by filamentous fungus Aspergillus fumigatus has been recorded particularly in immunocompromised patients. At present, there are limited numbers of antifungal agents to combat these infections and the situation has become more complex due to emergence of antifungal resistance and side-effects of antifungal drugs. These situations have increased the demand for novel drug targets. Recent studies have revealed that the β-1,3-endoglucanase (ENGL1) plays an essential role in cell wall remodeling that is absolutely required during growth and morphogenesis of filamentous fungi and thus is a promising target for the development of antifungal agents. Unfortunately no structural information of fungal β- glucanases has yet been available in the Protein Databank (PDB). Therefore in the present study, 3D structure of β-(1,3)- endoglucanase (ENGL1) was modeled by using I-TASSER server and validated with PROCHECK and VERIFY 3D. The best model was selected, energy minimized and used to analyze structure function relationship with substrate β-(1,3)-glucan by C-DOCKER (Accelrys DS 2.0). The results indicated that amino acids (GLU 380, GLN 383, ASP 384, TYR 395, SER 712, and ARG 713) present in β-1,3-endoglucanase receptor are of core importance for binding activities and these residues are having strong hydrogen bond interactions with β-(1,3)-glucan. The predicted model and docking studies permits initial inferences about the unexplored 3D structure of the β-(1,3)-endoglucanase and may be promote in relational designing of molecules for structure-function studies.

Infection structure-specific expression of β-1,3-glucan synthase is essential for pathogenicity of Colletotrichum graminicola and evasion of β-glucan-triggered immunity in maize

β-1,3-Glucan and chitin are the most prominent polysaccharides of the fungal cell wall. Covalently linked, these polymers form a scaffold that determines the form and properties of vegetative and pathogenic hyphae. While the role of chitin in plant infection is well understood, the role of β-1,3-glucan is unknown. We functionally characterized the β-1,3-glucan synthase gene GLS1 of the maize (Zea mays) pathogen Colletotrichum graminicola, employing RNA interference (RNAi), GLS1 overexpression, live-cell imaging, and aniline blue fluorochrome staining. This hemibiotroph sequentially differentiates a melanized appressorium on the cuticle and biotrophic and necrotrophic hyphae in its host. Massive β-1,3-glucan contents were detected in cell walls of appressoria and necrotrophic hyphae. Unexpectedly, GLS1 expression and β-1,3-glucan contents were drastically reduced during biotrophic development. In appressoria of RNAi strains, downregulation of β-1,3-glucan synthesis increased cell wall elasticity, and the appressoria exploded. While the shape of biotrophic hyphae was unaffected in RNAi strains, necrotrophic hyphae showed severe distortions. Constitutive expression of GLS1 led to exposure of β-1,3-glucan on biotrophic hyphae, massive induction of broad-spectrum defense responses, and significantly reduced disease symptom severity. Thus, while β-1,3-glucan synthesis is required for cell wall rigidity in appressoria and fast-growing necrotrophic hyphae, its rigorous downregulation during biotrophic development represents a strategy for evading β-glucan-triggered immunity.

β-1,3-glucan modifying enzymes in Aspergillus fumigatus

In Aspergillus fumigatus like in other filamentous ascomycetes, β-1,3-glucan constitutes a prominent cell wall component being responsible for rigidity of the cell wall structure. In filamentous fungi, softening of the cell wall is absolutely required during conidial germination and hyphal branching. Because of the central structure of β-1,3-glucans, it is expected that β-1,3-glucanases play a major role in cell wall softening. Based on in silico and experimental data, this review gives an overview of β-1,3-glucan modifying enzymes in A. fumigatus genome and their putative role during morphogenesis.

Expression and phylogenetic analyses of the Gel/Gas proteins of Tuber melanosporum provide insights into the function and evolution of glucan remodeling enzymes in fungi

The β(1,3)-glucanosyltransferases of the GH72 family are redundant enzymes that are essential for the formation and dynamic remodeling of the fungal wall during different stages of the life cycle. Four putative genes encoding glycosylphosphatidylinositol (GPI)-anchored β(1,3)-glucanosyltransferases, designated TmelGEL1, TmelGEL2, TmelGEL4 and TmelGAS4, have been annotated in the genome of Tuber melanosporum, an ectomycorrhizal fungus that also produces a hypogeous fruiting body (FB) of great commercial value (black truffle). This work focuses on the characterization and expression of this multigene family by taking advantage of a laser microdissection (LMD) technology that has been used to separate two distinct compartments in the FB, the hyphae and the asci containing the ascospores. Of the four genes, TmelGEL1 was the most up-regulated in the FB compared to the free-living mycelium. Inside the FB, the expression of TmelGEL1 was restricted to the hyphal compartment. A phylogenetic analysis of the Gel/Gas protein family of T. melanosporum was also carried out. A total of 237 GH72 proteins from 51 Ascomycotina and 3 Basidiomycota (outgroup) species were analyzed. The resulting tree provides insight into the evolution of the T. melanosporum proteins and identifies new GH72 paralogs/subfamilies. Moreover, it represents a starting point to formulate new hypotheses on the significance of the striking GH72 gene redundancy in fungal biology.

Fungal cell wall organization and biosynthesis

The composition and organization of the cell walls from Saccharomyces cerevisiae, Candida albicans, Aspergillus fumigatus, Schizosaccharomyces pombe, Neurospora crassa, and Cryptococcus neoformans are compared and contrasted. These cell walls contain chitin, chitosan, β-1,3-glucan, β-1,6-glucan, mixed β-1,3-/β-1,4-glucan, α-1,3-glucan, melanin, and glycoproteins as major constituents. A comparison of these cell walls shows that there is a great deal of variability in fungal cell wall composition and organization. However, in all cases, the cell wall components are cross-linked together to generate a cell wall matrix. The biosynthesis and properties of each of the major cell wall components are discussed. The chitin and glucans are synthesized and extruded into the cell wall space by plasma membrane-associated chitin synthases and glucan synthases. The glycoproteins are synthesized by ER-associated ribosomes and pass through the canonical secretory pathway. Over half of the major cell wall proteins are modified by the addition of a glycosylphosphatidylinositol anchor. The cell wall glycoproteins are also modified by the addition of O-linked oligosaccharides, and their N-linked oligosaccharides are extensively modified during their passage through the secretory pathway. These cell wall glycoprotein posttranslational modifications are essential for cross-linking the proteins into the cell wall matrix. Cross-linking the cell wall components together is essential for cell wall integrity. The activities of four groups of cross-linking enzymes are discussed. Cell wall proteins function as cross-linking enzymes, structural elements, adhesins, and environmental stress sensors and protect the cell from environmental changes.

Biosynthesis of the fungal cell wall polysaccharide galactomannan requires intraluminal GDP-mannose

Fungal cell walls frequently contain a polymer of mannose and galactose called galactomannan. In the pathogenic filamentous fungus Aspergillus fumigatus, this polysaccharide is made of a linear mannan backbone with side chains of galactofuran and is anchored to the plasma membrane via a glycosylphosphatidylinositol or is covalently linked to the cell wall. To date, the biosynthesis and significance of this polysaccharide are unknown. The present data demonstrate that deletion of the Golgi UDP-galactofuranose transporter GlfB or the GDP-mannose transporter GmtA leads to the absence of galactofuran or galactomannan, respectively. This indicates that the biosynthesis of galactomannan probably occurs in the lumen of the Golgi apparatus and thus contrasts with the biosynthesis of other fungal cell wall polysaccharides studied to date that takes place at the plasma membrane. Transglycosylation of galactomannan from the membrane to the cell wall is hypothesized because both the cell wall-bound and membrane-bound polysaccharide forms are affected in the generated mutants. Considering the severe growth defect of the A. fumigatus GmtA-deficient mutant, proving this paradigm might provide new targets for antifungal therapy.

Caspofungin for the treatment of immunocompromised and severely ill children and neonates with invasive fungal infections

Caspofungin is the first member of the echinocandin class of antifungals to receive an indication for the use in infants, children, and adolescents from the United States Food and Drug Administration. Daily doses of 50 mg/m(2) result in pharmacokinetic parameters that are similar to those observed in adults. Although fewer data are available, the response rates in pediatric patients who received caspofungin either as treatment or empiric therapy in clinical trials are similar to those reported in adults. In addition, caspofungin appears to be generally safe and well tolerated in this population. This represents a significant step forward in the treatment of invasive fungal infections within this population, as caspofungin is associated with few clinically significant drug-interactions and toxicities compared to other antifungals, such as the azoles and amphotericin B.

Reconstitution of high-level micafungin resistance detected in a clinical isolate of Candida glabrata identifies functional homozygosity in glucan synthase gene expression

OBJECTIVES

A mechanism for the acquisition of high-level echinocandin resistance in Candida glabrata was investigated. FKS mutants were constructed to: determine whether clinically significant micafungin resistance requires a hot-spot mutation in FKS1 and a premature stop codon in FKS2, as was observed in a clinical isolate; select for variants with reduced susceptibility and locate mutations in FKS genes; and assess the roles of FKS1 and FKS2.

METHODS

A panel of FKS mutants was constructed using micafungin-susceptible parents by site-directed mutagenesis. Drug susceptibility, gene expression and glucan synthase activities were compared between mutants. Mutations acquired by selection were identified by DNA sequence analysis of FKS genes from selected variants. Single FKS deletants were constructed and their phenotypes examined.

RESULTS

Introduction of the hot-spot mutation in FKS1 alone conferred an intermediate reduction in susceptibility, and the premature stop codon in FKS2 alone had no effect on susceptibility, while severely reduced susceptibility equivalent to that of the clinical isolate required both mutations. Exposure of susceptible strains to micafungin yielded variants with an intermediate reduction in susceptibility that possessed a hot-spot mutation in FKS1. Further exposure to micafungin yielded variants with severely reduced susceptibility that acquired various single mutations in FKS2. The phenotypes of Δfks1 and Δfks2 mutants indicate that the two FKS genes are functionally redundant, while deletion of both FKS1 and FKS2 conferred synthetic lethality.

CONCLUSIONS

In the laboratory mutants of C. glabrata, clinically significant reduced susceptibility to micafungin required single nucleotide changes in both FKS1 and FKS2, and both genes encoded β-1,3-glucan synthase catalytic subunits.

Rediscovery by Whole Genome Sequencing: Classical Mutations and Genome Polymorphisms in Neurospora crassa

Classical forward genetics has been foundational to modern biology, and has been the paradigm for characterizing the role of genes in shaping phenotypes for decades. In recent years, reverse genetics has been used to identify the functions of genes, via the intentional introduction of variation and subsequent evaluation in physiological, molecular, and even population contexts. These approaches are complementary and whole genome analysis serves as a bridge between the two. We report in this article the whole genome sequencing of eighteen classical mutant strains of Neurospora crassa and the putative identification of the mutations associated with corresponding mutant phenotypes. Although some strains carry multiple unique nonsynonymous, nonsense, or frameshift mutations, the combined power of limiting the scope of the search based on genetic markers and of using a comparative analysis among the eighteen genomes provides strong support for the association between mutation and phenotype. For ten of the mutants, the mutant phenotype is recapitulated in classical or gene deletion mutants in Neurospora or other filamentous fungi. From thirteen to 137 nonsense mutations are present in each strain and indel sizes are shown to be highly skewed in gene coding sequence. Significant additional genetic variation was found in the eighteen mutant strains, and this variability defines multiple alleles of many genes. These alleles may be useful in further genetic and molecular analysis of known and yet-to-be-discovered functions and they invite new interpretations of molecular and genetic interactions in classical mutant strains.

Repression of N-glycosylation triggers the unfolded protein response (UPR) and overexpression of cell wall protein and chitin in Aspergillus fumigatus

Aspergillus fumigatus is the most common airborne fungal pathogen, causing fatal invasive aspergillosis in immunocompromised patients. The crude mortality is 60-90 % and remains around 29-42 % even with treatment. The main reason for patient death is the low efficiency of the drug therapies. As protein N-glycosylation is involved in cell wall biogenesis in A. fumigatus, a deeper understanding of its role in cell wall biogenesis will help to develop new drug targets. The Afstt3 gene encodes the essential catalytic subunit of oligosaccharyltransferase, an enzyme complex responsible for the transfer of the N-glycan to nascent polypeptides. To evaluate the role of N-glycosylation in cell wall biosynthesis, we constructed the conditional mutant strain CPR-stt3 by replacing the endogenous promoter of Afstt3 with the nitrogen-dependent niiA promoter. Repression of the Afstt3 gene in the CPR-stt3 strain led to a severe retardation of growth and a slight defect in cell wall integrity (CWI). One of the most interesting findings was that upregulation of the cell wall-related genes was not accompanied by an activation of the MpkA kinase, which has been shown to be a central element in the CWI signalling pathway in both Saccharomyces cerevisiae and A. fumigatus. Considering that the unfolded protein response (UPR) was found to be activated, which might upregulate the expression of cell wall protein and chitin, our data suggest that the UPR, instead of the MpkA-dependent CWI signalling pathway, is the major compensatory mechanism induced by repression but not abolition of N-glycosylation in A. fumigatus. Our finding is a key to understanding the complex compensatory mechanisms of cell wall biosynthesis and may provide a new strategy for drug development.

Contribution of the gas1 gene of the entomopathogenic fungus Beauveria bassiana, encoding a putative glycosylphosphatidylinositol-anchored beta-1,3-glucanosyltransferase, to conidial thermotolerance and virulence

Beauveria bassiana is a mycoinsecticide alternative to chemicals for use in biological pest control. The fungus-insect interaction is also an emerging model system to examine unique aspects of the development, pathogenesis, and diversity of fungal lifestyles. The glycoside hydrolase 72 (GH72) family includes β-1,3-glucanosyltransferases that are glycosylphosphatidylinositol (GPI)-anchored cell wall-modeling enzymes affecting fungal physiology. A putative B. bassiana GPI-anchored β-1,3-glucanosyltransferase (Bbgas1) was isolated and characterized. B. bassiana targeted gene knockouts lacking Bbgas1 were affected in Congo red and salt sensitivity but displayed minor growth defects in the presence of sorbitol, SDS, or calcofluor white. Lectin and antibody mapping of surface carbohydrates revealed increased exposure of carbohydrate epitopes, including β-1,3-glucans, in the ΔBbgas1 strain. Transmission electron micrographs revealed localized destabilization of the cell wall in ΔBbgas1 conidia, in which fraying of the outer cell wall was apparent. Heat shock temperature sensitivity profiling showed that in contrast to the wild-type parent, ΔBbgas1 conidial spores displayed decreased germination after 1 to 4 h of heat shock at temperatures >40°C, and propidium iodide exclusion assays revealed decreased membrane stability in the knockout strain at temperatures >50°C. The ΔBbgas1 knockout showed reduced virulence in Galleria mellonella insect bioassays in both topical and intrahemocoel-injection assays. B. bassiana ΔBbgas1 strains complemented with the complete Bbgas1 open reading frame were indistinguishable from the wild-type parent in all phenotypes examined. The Bbgas1 gene did not complement the phenotype of a Saccharomyces cerevisiae β-1,3-glucanosyltransferase Δgas1 mutant, indicating that this family of enzymes likely possess discrete cellular functions.

Binding specificity of the recombinant cytoplasmic domain of Cordyceps militaris β-1,3-glucan synthase catalytic subunit

The cytoplasmic domain of the medicinal mushroom Cordyceps militaris β-1,3-glucan synthase catalytic subunit Fks1 was expressed as a fusion protein with an N-terminal hexahistidine tag and glutathione S-transferase in an Escherichia coli cell-free translation system, and was assayed for binding specificity. The recombinant cytoplasmic domain bound specifically to UDP-agarose and lichenan (β-glucan), but not to ADP-agarose, GDP-agarose, or other carbohydrates.

Potential physiological functions of acceptor products of dextransucrase with cellobiose as an inhibitor of mutansucrase and fungal cell synthase

A series of oligosaccharides (cellobio-oligosaccharides) ranging from degrees of polymer 3 to 6 were synthesized by Leuconostoc mesenteroides B-512 FMCM in the presence of cellobiose. The major oligosaccharides were the trisaccharides, α-D-glucopyranosyl-(1 → 2)-β-D-glucopyranosyl-(1 → 4)-D-glucopyranose and α-D-glucopyranosyl-(1 → 6)-β-D-glucopyranosyl-(1 → 4)-D-glucopyranose. These cellobio-oligosaccharides were inhibitory on mutansucrase, an enzyme that causes dental caries. They were also found to be effective antifungal agents against Aspergillus terreus acting by inhibiting β-(1 → 3)-glucan synthase, which is required for fungal cell wall formation.

Cell wall glucan synthases and GTPases in Paracoccidioides brasiliensis

In this report we identified orthologues of fungal AGS1, RHO1, RHO2, RAC1 and CDC42 genes in the dimorphic fungus Paracoccidioides brasiliensis. Based on its homology to known fungal sequences, P. brasiliensis Ags1 was identified as an alpha-1,3-glucan synthase, while Rho1, Rho2, Rac1 and Cdc42 proteins were classified into the Rho1, Rho2, Rac1 and Cdc42 subgroups of fungal Rho GTPases, respectively. Of them, Rho1 is one of two subunits of a putative beta-1,3-glucan synthase complex, the other being the synthase itself (Fks1), while Rho2 has been associated to the alpha-1,3-glucan synthase (Ags1). Expression studies showed that mRNAs levels of RHO2 and AGS1 kept a direct relationship but the levels of RHO1 and FKS1 did not. P. brasiliensis RHO1 successfully restored growth of Saccharomyces cerevisiae rho1 mutant under restrictive temperature conditions. Chemical analyses of P. brasiliensis alpha-1,3-glucan, synthesized by Ags1p, indicated that it is essentially a linear polysaccharide, with <3% of alpha-1,4-linked glucose branches, occasionally attached as single units to the alpha-1,3-backbone.

Secretion stress and antifungal resistance: an Achilles' heel of Aspergillus fumigatus?

The ability of Aspergillus fumigatus to establish and maintain an infection requires a continuous supply of nutrients to fuel energy production and growth. Like other filamentous fungi, A. fumigatus acquires nutrients by absorption, a mode of nutrition that depends upon the secretion of extracellular hydrolases to degrade the complex organic polymers in host tissues into reduced forms of carbon and nitrogen. If the folding capacity of the endoplasmic reticulum (ER) is exceeded during periods of high secretory activity, a signaling pathway known as the unfolded protein response (UPR) is activated to relieve the stress on the ER. Current evidence indicates that A. fumigatus relies upon this pathway to sustain the high rate of protease secretion needed to grow optimally in mammalian tissue. In addition, the UPR strengthens the ability of the secretory system to deliver cell wall and membrane components to the hyphal apex, which promotes the invasive growth of the expanding hyphae and protects the fungus from damage caused by antifungal drugs. The important contribution of UPR-dependent functions to the pathogenesis of invasive aspergillosis and antifungal susceptibility suggests that components of this pathway could be promising new targets for antifungal therapy.

beta(1-3)Glucanosyltransferase Gel4p is essential for Aspergillus fumigatus

The beta(1-3)glucanosyltransferase GEL family of Aspergillus fumigatus contains 7 genes, among which only 3 are expressed during mycelial growth. The role of the GEL4 gene was investigated in this study. Like the other Gelps, it encodes a glycosylphosphatidylinositol (GPI)-anchored protein. In contrast to the other beta(1-3)glucanosyltransferases analyzed to date, it is essential for this fungal species.

AfMkk2 is required for cell wall integrity signaling, adhesion, and full virulence of the human pathogen Aspergillus fumigatus

The cell wall integrity (CWI) pathway, best characterized in S. cerevisiae, is strikingly conserved in Aspergillus species. We analyzed the importance of AfMkk2, a CWI signaling kinase, for virulence and antifungal therapy in the human pathogen A. fumigatus. A mutant lacking AfMkk2 is less adherent to glass and plastic surfaces and shows increased sensitivity to alkaline pH stress and antifungals. Rather than AfMpkA, the target kinase of AfMkk2, AfMpkB is activated in the mutant under cell wall stress. Interestingly, the mutant lacking AfMkk2 shows an enhanced sensitivity to posaconazole and voriconazole. And in agreement with its sensitivity to moderate temperatures, it is less virulent in a murine infection model. Our data underline the importance of mkk2 for the fitness, but also for the pathogenicity of A. fumigatus.

Fungal echinocandin resistance

The echinocandins are the newest class of antifungal agents in the clinical armory. These secondary metabolites are non-competitive inhibitors of the synthesis of beta-(1,3)-glucan, a major structural component of the fungal cell wall. Recent work has shown that spontaneous mutations can arise in two hot spot regions of Fks1 the target protein of echinocandins that reduce the enzyme's sensitivity to the drug. However, other strains have been isolated in which the sequence of FKS1 is unaltered yet the fungus has decreased sensitivity to echinocandins. In addition it has been shown that echinocandin-treatment can induce cell wall salvage mechanisms that result in the compensatory upregulation of chitin synthesis in the cell wall. This salvage mechanism strengthens cell walls damaged by exposure to echinocandins. Therefore, fungal resistance to echinocandins can arise due to the selection of either stable mutational or reversible physiological alterations that decrease susceptibility to these antifungal agents.

Our current understanding of fungal biofilms

Fungal biofilms are an escalating clinical problem associated with significant rates of mortality. Candida albicans is the most notorious of all fungal biofilm formers. However, non-Candida species, yeasts such as Cryptococcus neoformans, and filamentous moulds such as Aspergillus fumigatus, have been shown to be implicated in biofilm-associated infections. Fungal biofilms have distinct developmental phases, including adhesion, colonisation, maturation and dispersal, which are governed by complex molecular events. Recalcitrance to antifungal therapy remains the greatest threat to patients with fungal biofilms. This review discusses our current understanding of the basic biology and clinical implications associated with fungal biofilms.

Functional stratification of the Spitzenkörper of Neurospora crassa

GS-1 (ncu04189) is a protein required for the synthesis of beta-1,3-glucan in Neurospora crassa. As chitin, beta-1,3-glucan is a morphogenetically relevant component of the fungal cell wall. Previously, we showed that chitin synthases are delivered to the growing hyphal tip of N. crassa by secretory microvesicles that follow an unconventional route and accumulate in the core of the Spitzenkörper (Spk). Tagged with the green fluorescent protein (GFP), GS-1 accumulated in the hyphal apex forming a dynamic and pleomorphic ring-like structure ('Spitzenring') that corresponded to the Spk outer macrovesicular stratum and surrounded the inner core of chitin synthase-containing microvesicles. TIRF microscopy revealed that GS-1-GFP reached the hyphal apex as a population of heterogeneous-size particles that moved along defined paths. On sucrose density gradients, GS-1-associated particles mainly sedimented in a high density range 1.1272-1.2124 g ml(-1). Clearly, GS-1 and chitin synthases of N. crassa are contained in two different types of secretory vesicles that accumulate in different strata of the Spk, a differentiation presumably related to the spatial control of cell-wall synthesis.

[The mechanisms of resistance to echinocandin class of antifungal drugs]

The echinocandin (candin) class of antifungal drugs inhibit beta-1,3-glucan synthase and block synthesis of beta-1,3-glucan , an important polysaccharide in fungal cell walls. Candins are used widely for treatment of systemic infections caused by Candida and Aspergillus because of their high potency and low toxicity to humans. The incidence of candin resistance has been rare compared to that of azole resistance, although candin-resistant clinical isolates of C. albicans, C. glabrata, C. krusei and C. tropicalis have been reported in the USA and Europe in recent years. These isolates possess hundred-fold higher MIC values for candins than sensitive strains, as well as candin-resistant beta-1,3-glucan synthase activities. Their candin resistance is associated with amino acid substitutions in the echinocandin resistant region (Ech) of the FKS gene that encodes a catalytic subunit of the beta-1,3-glucan synthase. However, the effect of these amino acid substitutions on the drug-protein interaction and the molecular basis for the resistance is unknown. The exposure of fungi to candin drugs induces stress responses that activate networks involving transcriptional regulators and components controlling signal transduction of the pathways responsible for maintenance of fungal cell wall integrity. The fungal cell wall is still an attractive drug target and further investigation into the mechanisms of candin resistance and structural analysis of the beta-1,3-glucan synthase protein complex will facilitate the development of broad spectrum inhibitors of fungal cell wall synthesis.

Aspergillus fumigatus: cell wall polysaccharides, their biosynthesis and organization

Aspergillus fumigatus is the most prevalent thermophilic inhabitants of decaying vegetation and one of the most important human opportunistic fungal pathogens. Like other fungi, A. fumigatus cells are covered by a cell wall, which is both a protective, rigid exoskeleton and a dynamic structure, undergoing constant modification depending on its environment. The cell wall, in the majority of fungi, is composed of polysaccharides, and understanding the biochemical organization and biogenesis of an A. fumigatus cell wall is essential as this envelop is continuously in contact with the environment/host cell and acts as a sieve and reservoir for molecules, such as enzymes and toxins that play an active role during infection. This article is intended to give an overview of the biosynthesis of constituent cell wall polysaccharides and their postsynthetic modification in A. fumigatus, it also discusses the antifungal drugs that affect cell wall polysaccharide biosynthesis.

Strain-dependent variation in 18S ribosomal DNA Copy numbers in Aspergillus fumigatus

Enumerating Aspergillus fumigatus CFU can be challenging since CFU determination by plate count can be difficult. CFU determination by quantitative real-time PCR (qPCR), however, is becoming increasingly common and usually relies on detecting one of the subunits of the multicopy rRNA genes. This study was undertaken to determine if ribosomal DNA (rDNA) copy number was constant or variable among different A. fumigatus isolates. FKS1 was used as a single-copy control gene and was validated against single-copy (pyrG and ARG4) and multicopy (arsC) controls. The copy numbers of the 18S rDNA subunit were then determined for a variety of isolates and were found to vary with the strain, from 38 to 91 copies per genome. Investigation of the stability of the 18S rDNA copy number after exposure to a number of different environmental and growth conditions revealed that the copy number was stable, varying less than one copy across all conditions, including in isolates recovered from an animal model. These results suggest that while the ribosomal genes are excellent targets for enumeration by qPCR, the copy number should be determined prior to using them as targets for quantitative analysis.

Micafungin in the treatment of invasive candidiasis and invasive aspergillosis

Micafungin is an echinocandin antifungal agent available for clinical use in Japan, Europe, and the United States. Through inhibition of β-1,3-glucan production, an essential component of the fungal cell wall, micafungin exhibits potent antifungal activity against key pathogenic fungi, including Candida and Aspergillus species, while contributing minimal toxicity to mammalian cells. This activity is maintained against polyene and azole-resistant isolates. Pharmacokinetic and pharmacodynamic studies have demonstrated linear kinetics both in adults and children with concentration-dependent activity observed both in vitro and in vivo. Dosage escalation studies have also demonstrated that doses much higher than those currently recommended may be administered without serious adverse effects. Clinically, micafungin has been shown to be efficacious for the treatment of invasive candidiasis and invasive aspergillosis. Furthermore, the clinical effectiveness of micafungin against these infections occurs without the drug interactions that occur with the azoles and the nephrotoxicity observed with amphotericin B formulations. This review will focus on the pharmacology, clinical microbiology, mechanisms of resistance, safety, and clinical efficacy of micafungin in the treatment of invasive candidiasis and invasive aspergillosis.

A role for the unfolded protein response (UPR) in virulence and antifungal susceptibility in Aspergillus fumigatus

Filamentous fungi rely heavily on the secretory pathway, both for the delivery of cell wall components to the hyphal tip and the production and secretion of extracellular hydrolytic enzymes needed to support growth on polymeric substrates. Increased demand on the secretory system exerts stress on the endoplasmic reticulum (ER), which is countered by the activation of a coordinated stress response pathway termed the unfolded protein response (UPR). To determine the contribution of the UPR to the growth and virulence of the filamentous fungal pathogen Aspergillus fumigatus, we disrupted the hacA gene, encoding the major transcriptional regulator of the UPR. The ΔhacA mutant was unable to activate the UPR in response to ER stress and was hypersensitive to agents that disrupt ER homeostasis or the cell wall. Failure to induce the UPR did not affect radial growth on rich medium at 37°C, but cell wall integrity was disrupted at 45°C, resulting in a dramatic loss in viability. The ΔhacA mutant displayed a reduced capacity for protease secretion and was growth-impaired when challenged to assimilate nutrients from complex substrates. In addition, the ΔhacA mutant exhibited increased susceptibility to current antifungal agents that disrupt the membrane or cell wall and had attenuated virulence in multiple mouse models of invasive aspergillosis. These results demonstrate the importance of ER homeostasis to the growth and virulence of A. fumigatus and suggest that targeting the UPR, either alone or in combination with other antifungal drugs, would be an effective antifungal strategy.

The pathogenic mold Aspergillus fumigatus is the leading cause of airborne fungal infections in immunocompromised patients. The fungus normally resides in compost, an environment that challenges the organism to obtain nutrients by degrading complex organic polymers. This is accomplished by secreted enzymes, some of which may also contribute to nutrient acquisition during infection. Extracellular enzymes are folded in the endoplasmic reticulum (ER) prior to secretion. If the folding capacity of the ER is overwhelmed by increased secretory demand, the resulting ER stress triggers an adaptive response termed the unfolded protein response (UPR). In this study, we uncover a previously unknown function for the master transcriptional regulator of the UPR, HacA, in fungal virulence. In the absence of HacA, A. fumigatus was unable to secrete high levels of proteins and had reduced virulence in mice. In addition, loss of HacA caused a cell wall defect and increased susceptibility to two major classes of antifungal drugs used for the treatment of aspergillosis. These findings demonstrate that A. fumigatus relies on HacA for growth in the host environment and suggest that therapeutic targeting of the UPR could have merit against A. fumigatus, as well as other eukaryotic pathogens with highly developed secretory systems.