Differential expression of thechsEgene encoding a chitin synthase ofAspergillus nidulansin response to developmental status and growth conditions

Functional Characterization of Aldehyde Dehydrogenase in Fusarium graminearum

Aldehyde dehydrogenase (ALDH), a common oxidoreductase in organisms, is an aldehyde scavenger involved in various metabolic processes. However, its function in different pathogenic fungi remains unknown. Fusarium graminearum causes Fusarium head blight in cereals, which reduces grain yield and quality and is an important global food security problem. To elucidate the pathogenic mechanism of F. graminearum, seven genes encoding ALDH were knocked out and then studied for their function. Single deletions of seven ALDH genes caused a decrease in spore production and weakened the pathogenicity. Furthermore, these deletions altered susceptibility to various abiotic stresses. FGSG_04194 is associated with a number of functions, including mycelial growth and development, stress sensitivity, pathogenicity, toxin production, and energy metabolism. FGSG_00139 and FGSG_11482 are involved in sporulation, pathogenicity, and SDH activity, while the other five genes are multifunctional. Notably, we found that FGSG_04194 has an inhibitory impact on ALDH activity, whereas FGSG_00979 has a positive impact. RNA sequencing and subcellular location analysis revealed that FGSG_04194 is responsible for biological process regulation, including glucose and lipid metabolism. Our results suggest that ALDH contributes to growth, stress responses, pathogenicity, deoxynivalenol synthesis, and mitochondrial energy metabolism in F. graminearum. Finally, ALDH presents a potential target and theoretical basis for fungicide development.

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

Phosphoglucose Isomerase Is Important for Aspergillus fumigatus Cell Wall Biogenesis

Aspergillus fumigatus is a devastating opportunistic fungal pathogen causing hundreds of thousands of deaths every year. Phosphoglucose isomerase (PGI) is a glycolytic enzyme that converts glucose-6-phosphate to fructose-6-phosphate, a key precursor of fungal cell wall biosynthesis. Here, we demonstrate that the growth of A. fumigatus is repressed by the deletion of pgi, which can be rescued by glucose and fructose supplementation in a 1:10 ratio. Even under these optimized growth conditions, the Δpgi mutant exhibits severe cell wall defects, retarded development, and attenuated virulence in Caenorhabditis elegans and Galleria mellonella infection models. To facilitate exploitation of A. fumigatus PGI as an antifungal target, we determined its crystal structure, revealing potential avenues for developing inhibitors, which could potentially be used as adjunctive therapy in combination with other systemic antifungals. IMPORTANCE Aspergillus fumigatus is an opportunistic fungal pathogen causing deadly infections in immunocompromised patients. Enzymes essential for fungal survival and cell wall biosynthesis are considered potential drug targets against A. fumigatus. PGI catalyzes the second step of the glycolysis pathway, linking glycolysis and the pentose phosphate pathway. As such, PGI has been widely considered as a target for metabolic regulation and therefore a therapeutic target against hypoxia-related diseases. Our study here reveals that PGI is important for A. fumigatus survival and exhibit pleiotropic functions, including development, cell wall glucan biosynthesis, and virulence. We also solved the crystal structure of PGI, thus providing the genetic and structural groundwork for the exploitation of PGI as a potential antifungal target.

Chitin Synthase Genes Are Differentially Required for Growth, Stress Response, and Virulence in Verticillium dahliae

Crop wilt disease caused by Verticillium dahliae usually leads to serious yield loss. Chitin, an important component of most fungal cell walls, functions to maintain the rigidity of cell walls and septa. Chitin synthesis mainly relies on the activity of chitin synthase (CHS). Eight CHS genes have been predicted in V. dahliae. In this study, we characterized the functions of these genes in terms of growth, stress responses, penetration, and virulence. Results showed that VdCHS5 is important for conidia germination and resistance to hyperosmotic stress. Conidial production is significantly decreased in Vdchs1, Vdchs4, and Vdchs8 mutants. VdCHS1, VdCHS2, VdCHS4, VdCHS6, VdCHS7, and VdCHS8 genes are important for cell wall integrity, while all mutants are important for cell membrane integrity. All of the VdCHS genes, except for VdCHS3, are required for the full pathogenicity of V. dahliae to Arabidopsis thaliana and cotton plants. The in vitro and in vivo penetration of Vdchs1, Vdchs4, Vdchs6, and Vdchs7 mutants was impaired, while that of the other mutants was normal. Overall, our results indicate that the VdCHS genes exert diverse functions to regulate the growth and development, conidial germination, conidial production, stress response, penetration, and virulence in V. dahliae.

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

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

The N-mannosyltransferase gene BbAlg9 contributes to cell wall integrity, fungal development and the pathogenicity of Beauveria bassiana

The mannosyltransferase Alg9 plays a vital role in N-linked protein glycosylation in Saccharomyces cerevisiae, but its function in most filamentous fungi is not clear. The present study characterized BbAlg9 (an ortholog of S. cerevisiae Alg9) in Beauveria bassiana to determine the roles of N-mannosyltransferase in biological control potential of the filamentous entomopathogenic fungus. The disruption of BbAlg9 led to slower fungal growth in media with various nutrition compositions. The conidiation of ΔBbAlg9 was less than that of the wild type from the third to the fifth day but showed no significant difference on the sixth day, suggesting that BbAlg9 affects the development of conidia rather than conidial yield of late stage. ΔBbAlg9 showed defects in conidial germination, multiple stress tolerances and the yield of blastospores, with altered size and density, and virulence in hosts infected via the immersion and injection methods. The deletion of BbAlg9 resulted in defects in cell wall integrity, including increased mannoprotein and glucan content and decreased chitin content, which were accompanied by transcriptional activation or suppression of genes related to cell wall component biosynthesis. Notably, deletion of the N-mannosyltransferase BbAlg9 altered the transcription levels of O-mannosyltransferase genes (Pmt and Ktr family). These data show that BbAlg9 is involved in the fungal development, conidial stress tolerance, cell wall integrity and virulence of B. bassiana.

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

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

Hülle Cells of Aspergillus nidulans with Nuclear Storage and Developmental Backup Functions Are Reminiscent of Multipotent Stem Cells

Some aspergilli are among the most cosmopolitan and ecologically dominant fungal species. One pillar of their success is their complex life cycle, which creates specialized cell types for versatile dispersal and regenesis. One of these cell types is unique to aspergilli-the Hülle cells. Despite being known for over a century, the biological and ecological roles of Hülle cells remain largely speculative. Previously reported data on in vivo Hülle cell formation and localization have been conflicting. Our quantification reveals that Hülle cells can occur at all locations on hyphae and that they show cellular activity similar to that seen with adjacent hyphae, indicating that they develop as intricate parts of hyphal tissue. In addition, we show that during sexual development associated with two parental strains, the typically multinucleate Hülle cells can inherit nuclei from both parents, indicating that they may serve as genetic backups. We provide an easy, reproducible method to study Hülle cell biology and germination with which we investigate the 90-year-old puzzle of whether and how Hülle cells germinate. We present clear evidence for the germination of Hülle cells, and we show that Hülle cells grow hyphae that develop into a spore-producing colony. Finally, we show that Hülle cell-derived colonies produce conidiospores faster than spore-derived colonies, providing evidence for an as-yet-undescribed developmental shortcut program in Aspergillus nidulans We propose that Hülle cells represent a unique cell type as specialized hypha-derived sexual tissue with a nucleus storage function and may act as fungal backup stem cells under highly destructive conditions.IMPORTANCE The in vivo identification of Hülle cells in cases of aspergillosis infections in animals and humans illustrates their biological relevance and suggests that they might be involved in pathogenicity. It is striking that aspergilli have developed and maintained a multinucleate nurse cell that is presumably energy-intensive to produce and is usually found only in higher eukaryotes. Our findings shed light on how the understudied Hülle cells might contribute to the success of aspergilli by acting not only as nurse cells under detrimental conditions (sexual development) but also as fungal backup stem cells with the capacity to produce genetically diverse spores in an accelerated manner, thereby substantially contributing to survival in response to predator attack or under otherwise severely destructive conditions. Our study solved the 90-year-old puzzle of Hülle cell germination and provides easy, reproducible methods that will facilitate future studies on biological and ecological roles of Hülle cells in aspergilli.

Protein O-mannosylation affects protein secretion, cell wall integrity and morphogenesis in Trichoderma reesei

Protein O-mannosyltransferases (PMTs) initiate O-mannosylation of proteins in the ER. Trichoderma reesei strains displayed a single representative of each PMT subfamily, Trpmt1, Trpmt2 and Trpmt4. In this work, two knockout strains ΔTrpmt1and ΔTrpmt4were obtained. Both mutants showed retarded growth, defective cell walls, reduced conidiation and decreased protein secretion. Additionally, the ΔTrpmt1strain displayed a thermosensitive growth phenotype, while the ΔTrpmt4 strain showed abnormal polarity. Meanwhile, OETrpmt2 strain, in which the Trpmt2 was over-expressed, exhibited increased conidiation, enhanced protein secretion and abnormal polarity. Using a lectin enrichment method and MS/MS analysis, 173 O-glycoproteins, 295 O-glycopeptides and 649 O-mannosylation sites were identified as the targets of PMTs in T. reesei. These identified O-mannoproteins are involved in various physiological processes such as protein folding, sorting, transport, quality control and secretion, as well as cell wall integrity and polarity. By comparing proteins identified in the mutants and its parent strain, the potential specific protein substrates of PMTs were identified. Based on our results, TrPMT1 is specifically involved inO-mannosylation of intracellular soluble proteins and secreted proteins, specially glycosidases. TrPMT2 is involved inO-mannosylation of secreted proteins and GPI-anchor proteins, and TrPMT4 mainly modifies multiple transmembrane proteins. The TrPMT1-TrPMT4 complex is responsible for O-mannosylation of proteins involved in cell wall integrity. Overexpression of TrPMT2 enhances protein secretion, which might be a new strategy to improve expression efficiency in T. reesei.

Characterization of the putative polysaccharide synthase CpsA and its effects on the virulence of the human pathogen Aspergillus fumigatus

The fungus Aspergillus fumigatus is a ubiquitous opportunistic human pathogen capable of causing a life-threatening disease called invasive aspergillosis, or IA, with an associated 40-90% mortality rate in immunocompromised patients. Of the approximately 250 species known in the genus Aspergillus, A. fumigatus is responsible for up to 90% of IA infections. This study focuses on examining the role of the putative polysaccharide synthase cpsA gene in A. fumigatus virulence. Additionally, we evaluated its role in cellular processes that influence invasion and colonization of host tissue. Importantly, our results support that cpsA is indispensable for virulence in A. fumigatus infection of non-neutropenic hosts. Our study revealed that cpsA affects growth and sporulation in this fungus. Absence of cpsA resulted in a drastic reduction in conidiation, and forced overexpression of cpsA produced partially fluffy colonies with low sporulation levels, suggesting that wild-type cpsA expression levels are required for proper conidiation in this fungus. This study also showed that cpsA is necessary for normal cell wall integrity and composition. Furthermore, both deletion and overexpression of cpsA resulted in a reduction in the ability of A. fumigatus to adhere to surfaces, and caused increased sensitivity to oxidative stress. Interestingly, metabolomics analysis indicated that cpsA affects A. fumigatus secondary metabolism. Forced overexpression of cpsA resulted in a statistically significant difference in the production of fumigaclavine A, fumigaclavine B, fumigaclavine C, verruculogen TR-2, and tryprostatin A.

The Aspergillus flavus rtfA Gene Regulates Plant and Animal Pathogenesis and Secondary Metabolism

Aspergillus flavus is an opportunistic fungal plant and human pathogen and a producer of mycotoxins, including aflatoxin B₁ (AFB₁). As part of our ongoing studies to elucidate the biological functions of the A. flavus rtfA gene, we examined its role in the pathogenicity of both plant and animal model systems. rtfA encodes a putative RNA polymerase II (Pol II) transcription elongation factor previously characterized in Saccharomyces cerevisiae, Aspergillus nidulans, and Aspergillus fumigatus, where it was shown to regulate several important cellular processes, including morphogenesis and secondary metabolism. In addition, an initial study in A. flavus indicated that rtfA also influences development and production of AFB₁; however, its effect on virulence is unknown. The current study reveals that the rtfA gene is indispensable for normal pathogenicity in plants when using peanut seed as an infection model, as well as in animals, as shown in the Galleria mellonella infection model. Interestingly, rtfA positively regulates several processes known to be necessary for successful fungal invasion and colonization of host tissue, such as adhesion to surfaces, protease and lipase activity, cell wall composition and integrity, and tolerance to oxidative stress. In addition, metabolomic analysis revealed that A. flavus rtfA affects the production of several secondary metabolites, including AFB₁, aflatrem, leporins, aspirochlorine, ditryptophenaline, and aflavinines, supporting a role of rtfA as a global regulator of secondary metabolism. Heterologous complementation of an A. flavus rtfA deletion strain with rtfA homologs from A. nidulans or S. cerevisiae fully rescued the wild-type phenotype, indicating that these rtfA homologs are functionally conserved among these three species.IMPORTANCE In this study, the epigenetic global regulator rtfA, which encodes a putative RNA-Pol II transcription elongation factor-like protein, was characterized in the mycotoxigenic and opportunistic pathogen A. flavus Specifically, its involvement in A. flavus pathogenesis in plant and animal models was studied. Here, we show that rtfA positively regulates A. flavus virulence in both models. Furthermore, rtfA-dependent effects on factors necessary for successful invasion and colonization of host tissue by A. flavus were also assessed. Our study indicates that rtfA plays a role in A. flavus adherence to surfaces, hydrolytic activity, normal cell wall formation, and response to oxidative stress. This study also revealed a profound effect of rtfA on the metabolome of A. flavus, including the production of potent mycotoxins.

cpsA regulates mycotoxin production, morphogenesis and cell wall biosynthesis in the fungus Aspergillus nidulans

The model fungus Aspergillus nidulans synthesizes numerous secondary metabolites, including sterigmatocystin (ST). The production of this toxin is positively controlled by the global regulator veA. In the absence of veA (ΔveA), ST biosynthesis is blocked. Previously, we performed random mutagenesis in a ΔveA strain and identified revertant mutants able to synthesize ST, among them RM1. Complementation of RM1 with a genomic library revealed that the mutation occurred in a gene designated as cpsA. While in the ΔveA genetic background cpsA deletion restores ST production, in a veA wild-type background absence of cpsA reduces and delays ST biosynthesis decreasing the expression of ST genes. Furthermore, cpsA is also necessary for the production of other secondary metabolites, including penicillin, affecting the expression of PN genes. In addition, cpsA is necessary for normal asexual and sexual development. Chemical and microscopy analyses revealed that CpsA is found in cytoplasmic vesicles and it is required for normal cell wall composition and integrity, affecting adhesion capacity and oxidative stress sensitivity. The conservation of cpsA in Ascomycetes suggests that cpsA homologs might have similar roles in other fungal species.

Chitin synthase gene FgCHS8 affects virulence and fungal cell wall sensitivity to environmental stress in Fusarium graminearum

Fusarium graminearum is the major causal agent of Fusarium head blight (FHB) of wheat and barley and is considered to be one of the most devastating plant diseases worldwide. Chitin is a critical component of the fungal cell wall and is polymerized from UDP-N-acetyl-alpha-D-glucosamine by chitin synthase. We characterized FgCHS8, a new class of the chitin synthase gene in F. graminearum. Disruption of FgCHS8 resulted in reduced accumulation of chitin, decreased chitin synthase activity, and had no effect on conidia growth when compared with the wild-type isolate. ΔFgCHS8 had a growth rate comparable to that of the wild-type isolate in vitro. However, ΔFgCHS8 had reduced growth when grown on agar supplemented with either 0.025% SDS or 0.9 mM salicylic acid. ΔFgCHS8 produced significantly less deoxynivalenol and exhibited reduced pathogenicity in wheat spikes. Re-introduction of a functional FgCHS8 gene into the ΔFgCHS8 mutant strain restored the wild-type phenotypes. Fluorescence microscopy revealed that FgCHS8 protein was initially expressed in the septa zone, and then gradually distributed over the entire cellular membrane, indicating that FgCHS8 was required for cell wall development. Our results demonstrated that FgCHS8 is important for cell wall sensitivity to environmental stress factors and deoxynivalenol production in F. graminearum.

Role of LAMMER Kinase in Cell Wall Biogenesis during Vegetative Growth of Aspergillus nidulans

Depending on the acquisition of developmental competence, the expression of genes for β-1,3-glucan synthase and chitin synthase was affected in different ways by Aspergillus nidulans LAMMER kinase. LAMMER kinase deletion, ΔlkhA, led to decrease in β-1,3-glucan, but increase in chitin content. The ΔlkhA strain was also resistant to nikkomycin Z.

Transcriptional regulation of fksA, a β-1,3-glucan synthase gene, by the APSES protein StuA during Aspergillus nidulans development

The temporal and spatial regulation of β-1,3-glucan synthesis plays an important role in morphogenesis during fungal growth and development. Northern blot analysis showed that the transcription of fksA, the gene encoding β-1,3-glucan synthase in Aspergillus nidulans, was cell-cycle-dependent and increased steadily over the duration of the vegetative period, but its overall expression during the asexual and sexual stages was fairly constant up until the time of transcription cessation. In an A. nidulans strain mutated in the eukaryotic bHLH-like APSES transcription factor stuA1, the transcriptional level of fksA, and consequently the content of alkali-insoluble cell wall β-glucan, significantly increased at the conidial chain formation and maturation stage. Electrophoretic mobility shift assays revealed that StuA was bound to StREs (StuA Response Elements) on the fksA promoter region. Promoter analysis with sGFP-fusion constructs also indicated the negative regulation of fksA expression by StuA, especially during asexual development. Taken together, these data suggest that StuA plays an important role in cell wall biogenesis during the development of A. nidulans, by controlling the transcription level of fksA.

Putative PmrA and PmcA are important for normal growth, morphogenesis and cell wall integrity, but not for viability in Aspergillus nidulans

P-type Ca(2+)-transporting ATPases are Ca(2+) pumps, extruding cytosolic Ca(2+) to the extracellular environment or the intracellular Ca(2+) store lumens. In budding yeast, Pmr1 (plasma membrane ATPase related), and Pmc1 (plasma membrane calcium-ATPase) cannot be deleted simultaneously for it to survive in standard medium. Here, we deleted two putative Ca(2+) pumps, designated AnPmrA and AnPmcA, from Aspergillus nidulans, and obtained the mutants ΔanpmrA and ΔanpmcA, respectively. Then, using ΔanpmrA as the starting strain, the promoter of its anpmcA was replaced with the alcA promoter to secure the mutant ΔanpmrAalcApmcA or its anpmcA was deleted completely to produce the mutant ΔanpmrAΔpmcA. Different from the case in Saccharomyces cerevisiae, double deletion of anpmrA and anpmcA was not lethal in A. nidulans. In addition, deletion of anpmrA and/or anpmcA had produced growth defects, although overexpression of AnPmc1 in ΔanpmrAalcApmcA could not restore the growth defects that resulted from the loss of AnPmrA. Moreover, we found AnPmrA was indispensable for maintenance of normal morphogenesis, especially in low-Ca(2+)/Mn(2+) environments. Thus, our findings suggest AnPmrA and AnPmcA might play important roles in growth, morphogenesis and cell wall integrity in A. nidulans in a different way from that in yeasts.

Three α-1,2-mannosyltransferases contribute differentially to conidiation, cell wall integrity, multistress tolerance and virulence of Beauveria bassiana

Members of α-1,2-mannosyltransferase (Ktr) family are required for protein O-mannosylation for the elongation of Ser/Thr mannose residues in yeasts but functionally unknown in most filamentous fungi. Here we characterized the functions of the Ktr orthologues Ktr1, Ktr4 and Kre2/Mnt1 in Beauveria bassiana, a filamentous enotmopathogen, and found that they were positive, but differential, mediators of many biological traits. Inactivation of Ktr4 and Kre2 resulted in 92% reduction of conidial yield on a standard medium and growth defects on substrates with altered carbon or nitrogen sources and availability, accompanied with reduced conidial size and complexity. This contrasts to the dispensability of Ktr1 for fungal growth and conidiation. More cell wall damage occurred in Δktr4 and Δkre2 than in Δktr1, including altered contents of the cell wall components mannoproteins, α-glucans and chitin, more carbohydrate epitopes changed on conidial surfaces, much lower conidial hydrophobicity, and thinner cell walls. Consequently, Δktr4 and Δkre2 became more sensitive to oxidation and cell wall perturbation than Δktr1 during colony growth or conidial germination despite less difference in their sensitivities to two osmotic agents. Conidial thermotolerance, UV-B resistance and virulence were all lowered greatly in Δktr4 and Δkre2 but only the thermotolerance decreased in Δktr1. All the phenotypical changes were well restored to wild-type levels by the complementation of each target gene. Our results indicate that Ktr4 and Kre2 contribute more to the biocontrol potential of B. bassiana than Ktr1 although all of them are significant contributors.

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.

N-Glycosylation of Gel1 or Gel2 is vital for cell wall β-glucan synthesis in Aspergillus fumigatus

Fungal cell wall is a dynamic structure that communicates with and protects the cell from outside stress. In Aspergillus fumigatus, the cell wall β-glucans are mainly elongated by β-1,3-glucanosyltransferases Gels, which consist of seven family members (Gel1-7) utilizing β-1,3-glucan chains as substrates. Previously, we have shown that the mutant deficient of N-glycan processing displays a reduction in the cell wall β-glucans, suggesting that N-glycosylation is required for the proper function of β-1,3-glucanosyltransferase. To verify this hypothesis, in this study, the gene encoding β-1,3-glucanosyltransferase Gel1 or Gel2 was deleted in the Δcwh41 mutant to construct a double-mutant Δgel1Δcwh41 or Δgel2Δcwh41. The growth phenotypes of both double mutants were similar to the single-mutant Δcwh41, suggesting that Gel1 and Gel2 are proteins that are mainly affected by deficient N-glycan processing in Δcwh41. Furthermore, the mutant Δgel1(Gel1-NM) or Δgel2(Gel2-NM), in which all potential N-glycosylation sites on Gel1 or Gel2 were removed by site-directed mutagenesis, showed phenotypes similar to the single-mutant Δgel1 or Δgel2. Biochemical analysis revealed that N-glycosylation was essential for the function of Gel1 or Gel2 and thus required for β-glucan synthesis in A. fumigatus.

Transcriptome and biochemical analysis reveals that suppression of GPI-anchor synthesis leads to autophagy and possible necroptosis in Aspergillus fumigatus

Previously, it has been shown that GPI proteins are required for cell wall synthesis and organization in Aspergillus fumigatus, a human opportunistic pathogen causing life-threatening invasive aspergillosis (IA) in immunocompromised patients. Blocking GPI anchor synthesis leads to severe phenotypes such as cell wall defects, increased cell death, and attenuated virulence. However, the mechanism by which these phenotypes are induced is unclear. To gain insight into global effects of GPI anchoring in A. fumigatus, in this study a conditional expression mutant was constructed and a genome wide transcriptome analysis was carried out. Our results suggested that suppression of GPI anchor synthesis mainly led to activation of phosphatidylinositol (PtdIns) signaling and ER stress. Biochemical and morphological evidence showed that autophagy was induced in response to suppression of the GPI anchor synthesis, and also an increased necroptosis was observed. Based on our results, we propose that activation of PtdIns3K and increased cytosolic Ca²⁺, which was induced by both ER stress and PtdIns signaling, acted as the main effectors to induce autophagy and possible necroptosis.

Different chitin synthase genes are required for various developmental and plant infection processes in the rice blast fungus Magnaporthe oryzae

Chitin is a major component of fungal cell wall and is synthesized by chitin synthases (Chs). Plant pathogenic fungi normally have multiple chitin synthase genes. To determine their roles in development and pathogenesis, we functionally characterized all seven CHS genes in Magnaporthe oryzae. Three of them, CHS1, CHS6, and CHS7, were found to be important for plant infection. While the chs6 mutant was non-pathogenic, the chs1 and chs7 mutants were significantly reduced in virulence. CHS1 plays a specific role in conidiogenesis, an essential step for natural infection cycle. Most of chs1 conidia had no septum and spore tip mucilage. The chs6 mutant was reduced in hyphal growth and conidiation. It failed to penetrate and grow invasively in plant cells. The two MMD-containing chitin synthase genes, CHS5 and CHS6, have a similar expression pattern. Although deletion of CHS5 had no detectable phenotype, the chs5 chs6 double mutant had more severe defects than the chs6 mutant, indicating that they may have overlapping functions in maintaining polarized growth in vegetative and invasive hyphae. Unlike the other CHS genes, CHS7 has a unique function in appressorium formation. Although it was blocked in appressorium formation by germ tubes on artificial hydrophobic surfaces, the chs7 mutant still produced melanized appressoria by hyphal tips or on plant surfaces, indicating that chitin synthase genes have distinct impacts on appressorium formation by hyphal tip and germ tube. The chs7 mutant also was defective in appressorium penetration and invasive growth. Overall, our results indicate that individual CHS genes play diverse roles in hyphal growth, conidiogenesis, appressorium development, and pathogenesis in M. oryzae, and provided potential new leads in the control of this devastating pathogen by targeting specific chitin synthases.

Regulation of expression, activity and localization of fungal chitin synthases

The fungal cell wall represents an attractive target for pharmacologic inhibition, as many of the components are fungal-specific. Though targeted inhibition of β-glucan synthesis is effective treatment for certain fungal infections, the ability of the cell wall to dynamically compensate via the cell wall integrity pathway may limit overall efficacy. To date, chitin synthesis inhibitors have not been successfully deployed in the clinical setting. Fungal chitin synthesis is a complex and highly regulated process. Regulation of chitin synthesis occurs on multiple levels, thus targeting of these regulatory pathways may represent an exciting alternative approach. A variety of signaling pathways have been implicated in chitin synthase regulation, at both transcriptional and post-transcriptional levels. Recent research suggests that localization of chitin synthases likely represents a major regulatory mechanism. However, much of the regulatory machinery is not necessarily shared among different chitin synthases. Thus, an in-depth understanding of the precise roles of each protein in cell wall maintenance and repair will be essential to identifying the most likely therapeutic targets.

Coordination of secondary metabolism and development in fungi: the velvet family of regulatory proteins

Filamentous fungi produce a number of small bioactive molecules as part of their secondary metabolism ranging from benign antibiotics such as penicillin to threatening mycotoxins such as aflatoxin. Secondary metabolism can be linked to fungal developmental programs in response to various abiotic or biotic external triggers. The velvet family of regulatory proteins plays a key role in coordinating secondary metabolism and differentiation processes such as asexual or sexual sporulation and sclerotia or fruiting body formation. The velvet family shares a protein domain that is present in most parts of the fungal kingdom from chytrids to basidiomycetes. Most of the current knowledge derives from the model Aspergillus nidulans where VeA, the founding member of the protein family, was discovered almost half a century ago. Different members of the velvet protein family interact with each other and the nonvelvet protein LaeA, primarily in the nucleus. LaeA is a methyltransferase-domain protein that functions as a regulator of secondary metabolism and development. A comprehensive picture of the molecular interplay between the velvet domain protein family, LaeA and other nuclear regulatory proteins in response to various signal transduction pathway starts to emerge from a jigsaw puzzle of several recent studies.

The cellular basis of chitin synthesis in fungi and insects: common principles and differences

Chitin is a polymer of N-acetylglucosamine, which assembles into microfibrils of about 20 sugar chains. These microfibrils serve as a structural component of natural biocomposites found in cell walls and specialized extracellular matrices such as cuticles and peritrophic membranes. Chitin synthesis is performed by a wide range of organisms including fungi and insects. The underlying biosynthetic machinery is highly conserved and involves several enzymes, of which the chitin synthase is the key enzyme. This membrane integral glycosyltransferase catalyzes the polymerization reaction. Most of what we know about chitin synthesis derives from studies of fungal and insect systems. In this review, common principles and differences will be worked out at the levels of gene organization, enzymatic properties, cellular localization and regulation.

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.

Fusarium verticillioides chitin synthases CHS5 and CHS7 are required for normal growth and pathogenicity

Fusarium verticillioides is both an endophyte and a pathogen of maize and is a health threat in many areas of the world because it can contaminate maize with fumonisins, a toxic secondary metabolite. We identified eight putative chitin synthase (CHS) genes in F. verticillioides genomic sequence, and phylogenetic evidence shows that they group into seven established CHS gene classes. We targeted two CHSs (CHS5 and CHS7) for deletion analysis and found that both are required for normal hyphal growth and maximal disease of maize seedlings and ears. CHS5 and CHS7 encode a putative class V and class VII fungal chitin synthase, respectively; they are located adjacent to each other and are divergently transcribed. Fluorescent microscopy found that both CHS deficient strains produce balloon-shaped hyphae, while growth assays indicated that they were more sensitive to cell wall stressing compounds (e.g., the antifungal compound Nikkomycin Z) than wild type. Pathogenicity assays on maize seedlings and ears indicated that both strains were significantly reduced in their ability to cause disease. Our results demonstrate that both CHS5 and CHS7 are necessary for proper hyphal growth and pathogenicity of F. verticillioides on maize.

Characterization of the Aspergillus fumigatus phosphomannose isomerase Pmi1 and its impact on cell wall synthesis and morphogenesis

Phosphomannose isomerase (PMI) is an enzyme catalysing the interconversion of mannose 6-phosphate (Man-6-P) and fructose 6-phosphate (Fru-6-P). The reaction catalysed by PMI is the first committed step in the synthesis of mannose-containing sugar chains and provides a link between glucose metabolism and mannosylation. In this study, the pmi1 gene was identified to encode PMI in the human fungal pathogen Aspergillus fumigatus. Characterization of A. fumigatus Pmi1 expressed in Escherichia coli revealed that this PMI mainly catalysed the conversion of Fru-6-P to Man-6-P and that its binding affinity for Man-6-P was similar to that of yeast PMIs, but different to those of PMIs from bacteria or animals. Loss of pmi1 was lethal unless Man was provided in the growth medium. However, a Δpmi1 mutant cell showed a significantly reduced growth rate at a high concentration of Man. Biochemical analysis revealed that both inadequate and replete Man led to an accumulation of intracellular Man-6-P and a reduction in the amount of α-glucan in the cell wall. Uncoupling of the link between energy production and glycosylation by deletion of the pmi1 gene led to phenotypes such as defects in cell wall integrity, abnormal morphology and reduced conidiation. Our results reveal that PMI activity is essential for viability and plays a central regulatory role in both cell wall synthesis and energy production in A. fumigatus.

Class IIC alpha-mannosidase AfAms1 is required for morphogenesis and cellular function in Aspergillus fumigatus

The mammalian ER/cytosolic alpha-mannosidase (Man2C1p), yeast vacuolar alpha-mannosidase (Ams1p) and the Aspergillus nidulans alpha-mannosidase are members of Class IIC subgroup, which is involved in oligosaccharide catabolism and N-glycan processing. Unlike their mammalian counterparts, the yeast Ams1p and A. nidulans Class IIC alpha-mannosidase are not essential for morphogenesis and cellular function. In this study, the Afams1, a gene encoding a member of Class IIC alpha-mannosidases, was identified in the opportunistic pathogen Aspergillus fumigatus. Deletion of the Afams1 led to a severe defect in conidial formation, especially at a higher temperature. In addition, abnormalities of polarity and septation were associated with the DeltaAfams1 mutant. Our results showed that the Afams1 gene, in contrast to its homolog in yeast or A. nidulans, was required for morphogenesis and cellular function in A. fumigatus.

O-Mannosyltransferase 1 in Aspergillus fumigatus (AfPmt1p) is crucial for cell wall integrity and conidium morphology, especially at an elevated temperature

Protein O-mannosyltransferases initiate O mannosylation of secretory proteins, which are of fundamental importance in eukaryotes. In this study, the PMT gene family of the human fungal pathogen Aspergillus fumigatus was identified and characterized. Unlike the case in Saccharomyces cerevisiae, where the PMT family is highly redundant, only one member of each PMT subfamily, namely, Afpmt1, Afpmt2, and Afpmt4, is present in A. fumigatus. Mutants with a deletion of Afpmt1 are viable. In vitro and in vivo activity assays confirmed that the protein encoded by Afpmt1 acts as an O-mannosyltransferase (AfPmt1p). Characterization of the DeltaAfpmt1 mutant showed that a lack of AfPmt1p results in sensitivity to elevated temperature and defects in growth and cell wall integrity, thereby affecting cell morphology, conidium formation, and germination. In a mouse model, Afpmt1 was not required for the virulence of A. fumigatus under the experimental conditions used.

Glycosylphosphatidylinositol (GPI) anchor is required in Aspergillus fumigatus for morphogenesis and virulence

In yeast, glycosylphosphatidylinositol (GPI) is essential for viability and plays an important role in biosynthesis and organization of cell wall. Initiation of the GPI anchor biosynthesis is catalysed by the GPI-N-acetylglucosaminyltransferase complex (GPI-GnT). The GPI3 (SPT14) gene is thought to encode the catalytic subunit of GPI-GnT complex. In contrast to Saccharomyces cerevisiae, little is known about the GPI biosynthesis in filamentous fungi. In this study, the afpig-a gene was identified as the homologue of the GPI3/pig-A gene in Aspergillus fumigatus, an opportunistic fungal pathogen. By replacement of the afpig-a gene with a pyrG gene, we obtained the null mutants. Although the Deltaafpig-a mutant exhibited a significant increased cell lysis instead of temperature-sensitive or conditional lethal phenotype associated to the GPI3 mutant of yeast, they could survive at temperatures from 30 degrees C to 50 degrees C. The analysis of the mutants showed that a completely blocking of the GPI anchor synthesis in A. fumigatus led to cell wall defect, abnormal hyphal growth, rapid conidial germination and aberrant conidiation. In vivo assays revealed that the mutant exhibited a reduced virulence in immunocompromised mice. The GPI anchor was not essential for viability, but required for the cell wall integrity, morphogenesis and virulence in A. fumigatus.

FvVE1 regulates filamentous growth, the ratio of microconidia to macroconidia and cell wall formation in Fusarium verticillioides

The velvet gene, veA, co-ordinates asexual and sexual development in the homothallic fungal species Aspergillus nidulans. Studies in Aspergillus parasiticus and Aspergillus fumigatus demonstrated that veA also regulates morphological differentiation in these species. Whether veA has the same role in morphogenesis in other fungal genera has not been investigated. In this work, we studied the role of the veA homologue, FvVE1, in the heterothallic fungus Fusarium verticillioides. Deletion of FvVE1 suppressed aerial hyphal growth and reduced colony surface hydrophobicity on solid media. In submerged cultures, FvVE1 deletion caused alterations in hyphal polarity, marked activation of conidiation and yeast-like growth. The latter was promoted by shaking to increase aeration of cultures. In addition, FvVE1 deletion markedly increased the ratio of macroconidia to microconidia. Supplementation of osmotic stabilizers restored the wild-type phenotype to deletion mutants, suggesting phenotypic alterations caused by FvVE1 deletion are related to cell wall defects. This is consistent with the hypersensitivity of FvVE1 deletion mutants to SDS and with the significant reduction in the mannoprotein content of mutants compared with the wild-type strain. However, no dramatic cell wall alterations were observed when mutants were examined by transmission electron microscopy. Our data strongly suggest that FvVE1 is important for cell wall integrity, cell surface hydrophobicity, hyphal polarity and conidiation pattern.