Mitogen-activated protein kinases MpkA and MpkB independently affect micafungin sensitivity in Aspergillus nidulans

The mechanism of short hypha formation and high protein production system mediated by cell wall integrity signaling pathway in Aspergillus niger

Aspergillus niger is a cell factory widely used in industries to produce proteases, organic acids, drugs, and other substances. The hyphal morphology of A. niger is a complex differentiated elongated tubular structure, which limits its basic research and application. In this study, the mpkA, bck1, steC, and Tpk2 genes were successfully deleted using a quick way to knock out genes based on the RNP (Ribonucleoprotein) complex. The study showed that the knockout of mpkA and bck1 kinase gene strains resulted in smaller, denser colonies, short rod-shaped hypha, and a significant increase in glucoamylase secretion. The mechanism of short hypha formation and high protein production for A. niger is the cell wall integrity signaling (CWIS) pathway. The CWIS pathway passed through the bck1-mkkA-mpkA tertiary kinase to deliver phosphorylation signals to the rlmA transcription factor, which regulated the expression of the cell wall synthesis gene agsA, thus regulating hyphal morphology. The mpkA kinase regulated the expression of the transcription factor amyR, which affected the expression of the genes glaA and amyA, thus enhancing the expression of proteins in A. niger. This study provides a strategy for the regulation of hyphal morphology and promotes the application of A. niger in industrial production.

Identification, cloning, and characterization of a novel chitinase from leaf-cutting ant Atta sexdens: An enzyme with antifungal and insecticidal activity

Chitinases are enzymes that degrade chitin, a polysaccharide found in the exoskeleton of insects, fungi, yeast, and internal structures of other vertebrates. Although chitinases isolated from bacteria, fungi and plants have been reported to have antifungal or insecticide activities, chitinases from insects with these activities have been seldomly reported. In this study, a leaf-cutting ant Atta sexdens DNA fragment containing 1623 base pairs was amplified and cloned into a vector to express the protein (AsChtII-C4B1) in Pichia pastoris. AsChtII-C4B1, which contains one catalytic domain and one carbohydrate-binding module (CBM), was secreted to the extracellular medium and purified by ammonium sulfate precipitation followed by nickel column chromatography. AsChtII-C4B1 showed maximum activity at pH 5.0 and 55 °C when tested against colloidal chitin substrate and maintained >60% of its maximal activity in different temperatures during 48 h. AsChtII-C4B1 decreased the survival of Spodoptera frugiperda larvae fed with an artificial diet that contained AsChtII-C4B1. Our results have indicated that AsChtII-C4B1 has a higher effect on larva-pupa than larva-larva molts. AsChtII-C4B1 activity targets more specifically the growth of filamentous fungus than yeast. This work describes, for the first time, the obtaining a recombinant chitinase from ants and the characterization of its insecticidal and antifungal activities.

Cell Wall Integrity and Its Industrial Applications in Filamentous Fungi

Signal transduction pathways regulating cell wall integrity (CWI) in filamentous fungi have been studied taking into account findings in budding yeast, and much knowledge has been accumulated in recent years. Given that the cell wall is essential for viability in fungi, its architecture has been analyzed in relation to virulence, especially in filamentous fungal pathogens of plants and humans. Although research on CWI signaling in individual fungal species has progressed, an integrated understanding of CWI signaling in diverse fungi has not yet been achieved. For example, the variety of sensor proteins and their functional differences among different fungal species have been described, but the understanding of their general and species-specific biological functions is limited. Our long-term research interest is CWI signaling in filamentous fungi. Here, we outline CWI signaling in these fungi, from sensor proteins required for the recognition of environmental changes to the regulation of cell wall polysaccharide synthesis genes. We discuss the similarities and differences between the functions of CWI signaling factors in filamentous fungi and in budding yeast. We also describe the latest findings on industrial applications, including those derived from studies on CWI signaling: the development of antifungal agents and the development of highly productive strains of filamentous fungi with modified cell surface characteristics by controlling cell wall biogenesis.

H2O2 Induces Major Phosphorylation Changes in Critical Regulators of Signal Transduction, Gene Expression, Metabolism and Developmental Networks in Aspergillus nidulans

Reactive oxygen species (ROS) regulate several aspects of cell physiology in filamentous fungi including the antioxidant response and development. However, little is known about the signaling pathways involved in these processes. Here, we report Aspergillus nidulans global phosphoproteome during mycelial growth and show that under these conditions, H2O2 induces major changes in protein phosphorylation. Among the 1964 phosphoproteins we identified, H2O2 induced the phosphorylation of 131 proteins at one or more sites as well as the dephosphorylation of a larger set of proteins. A detailed analysis of these phosphoproteins shows that H2O2 affected the phosphorylation of critical regulatory nodes of phosphoinositide, MAPK, and TOR signaling as well as the phosphorylation of multiple proteins involved in the regulation of gene expression, primary and secondary metabolism, and development. Our results provide a novel and extensive protein phosphorylation landscape in A. nidulans, indicating that H2O2 induces a shift in general metabolism from anabolic to catabolic, and the activation of multiple stress survival pathways. Our results expand the significance of H2O2 in eukaryotic cell signaling.

Dynamic Transcriptomic and Phosphoproteomic Analysis During Cell Wall Stress in Aspergillus nidulans

The fungal cell-wall integrity signaling (CWIS) pathway regulates cellular response to environmental stress to enable wall repair and resumption of normal growth. This complex, interconnected, pathway has been only partially characterized in filamentous fungi. To better understand the dynamic cellular response to wall perturbation, a β-glucan synthase inhibitor (micafungin) was added to a growing A. nidulans shake-flask culture. From this flask, transcriptomic and phosphoproteomic data were acquired over 10 and 120 min, respectively. To differentiate statistically-significant dynamic behavior from noise, a multivariate adaptive regression splines (MARS) model was applied to both data sets. Over 1800 genes were dynamically expressed and over 700 phosphorylation sites had changing phosphorylation levels upon micafungin exposure. Twelve kinases had altered phosphorylation and phenotypic profiling of all non-essential kinase deletion mutants revealed putative connections between PrkA, Hk-8-4, and Stk19 and the CWIS pathway. Our collective data implicate actin regulation, endocytosis, and septum formation as critical cellular processes responding to activation of the CWIS pathway, and connections between CWIS and calcium, HOG, and SIN signaling pathways.

Novel Antifungal Compound Z-705 Specifically Inhibits Protein Kinase C of Filamentous Fungi

The cell wall integrity signaling (CWIS) pathway is involved in fungal cell wall biogenesis. This pathway is composed of sensor proteins, protein kinase C (PKC), and the mitogen-activated protein kinase (MAPK) pathway, and it controls the transcription of many cell wall-related genes. PKC plays a pivotal role in this pathway; deficiencies in PkcA in the model filamentous fungus Aspergillus nidulans and in MgPkc1p in the rice blast fungus Magnaporthe grisea are lethal. This suggests that PKC in filamentous fungi is a potential target for antifungal agents. In the present study, to search for MgPkc1p inhibitors, we carried out in silico screening by three-dimensional (3D) structural modeling and performed growth inhibition tests for M. grisea on agar plates. From approximately 800,000 candidate compounds, we selected Z-705 and evaluated its inhibitory activity against chimeric PKC expressed in Saccharomyces cerevisiae cells in which the kinase domain of native S. cerevisiae PKC was replaced with those of PKCs of filamentous fungi. Transcriptional analysis of MLP1, which encodes a downstream factor of PKC in S. cerevisiae, and phosphorylation analysis of the mitogen-activated protein kinase (MAPK) Mpk1p, which is activated downstream of PKC, revealed that Z-705 specifically inhibited PKCs of filamentous fungi. Moreover, the inhibitory activity of Z-705 was similar to that of a well-known PKC inhibitor, staurosporine. Interestingly, Z-705 inhibited melanization induced by cell wall stress in M. grisea We discuss the relationships between PKC and melanin biosynthesis.IMPORTANCE A candidate inhibitor of filamentous fungal protein kinase C (PKC), Z-705, was identified by in silico screening. A screening system to evaluate the effects of fungal PKC inhibitors was constructed in Saccharomyces cerevisiae Using this system, we found that Z-705 is highly selective for filamentous fungal PKC in comparison with S. cerevisiae PKC. Analysis of the AGS1 mRNA level, which is regulated by Mps1p mitogen-activated protein kinase (MAPK) via PKC, in the rice blast fungus Magnaporthe grisea revealed that Z-705 had a PKC inhibitory effect comparable to that of staurosporine. Micafungin induced hyphal melanization in M. grisea, and this melanization, which is required for pathogenicity of M. grisea, was inhibited by PKC inhibition by both Z-705 and staurosporine. The mRNA levels of 4HNR, 3HNR, and SCD1, which are essential for melanization in M. grisea, were suppressed by both PKC inhibitors.

Phosphoproteomic and transcriptomic analyses reveal multiple functions for Aspergillus nidulans MpkA independent of cell wall stress

The protein kinase MpkA plays a prominent role in the cell wall integrity signaling (CWIS) pathway, acting as the terminal MAPK activating expression of genes which encode cell wall biosynthetic enzymes and other repair functions. Numerous studies focus on MpkA function during cell wall perturbation. Here, we focus on the role MpkA plays outside of cell wall stress, during steady state growth. In an effort to seek other, as yet unknown, connections to this pathway, an mpkA deletion mutant (ΔmpkA) was subjected to phosphoproteomic and transcriptomic analysis. When compared to the control (isogenic parent of ΔmpkA), there is strong evidence suggesting MpkA is involved with maintaining cell wall strength, branching regulation, and the iron starvation pathway, among others. Particle-size analysis during shake flask growth revealed ΔmpkA mycelia were about 4 times smaller than the control strain and more than 90 cell wall related genes show significantly altered expression levels. The deletion mutant had a significantly higher branching rate than the control and phosphoproteomic results show putative branching-regulation proteins, such as CotA, LagA, and Cdc24, have a significantly different level of phosphorylation. When grown in iron limited conditions, ΔmpkA had no difference in growth rate or production of siderophores, whereas the control strain showed decreased growth rate and increased siderophore production. Transcriptomic data revealed over 25 iron related genes with altered transcript levels. Results suggest MpkA is involved with regulation of broad cellular functions in the absence of stress.

Molecular Mass and Localization of α-1,3-Glucan in Cell Wall Control the Degree of Hyphal Aggregation in Liquid Culture of Aspergillus nidulans

α-1,3-Glucan is one of the main polysaccharides in the cell wall of filamentous fungi. Aspergillus nidulans has two α-1,3-glucan synthase genes, agsA and agsB. We previously revealed that AgsB is a major α-1,3-glucan synthase in vegetative hyphae, but the function of AgsA remained unknown because of its low expression level and lack of phenotypic alteration upon gene disruption. To clarify the role of α-1,3-glucan in hyphal aggregation, we constructed strains overexpressing agsA (agsA^OE) or agsB (agsB^OE), in which the other α-1,3-glucan synthase gene was disrupted. In liquid culture, the wild-type and agsB^OE strains formed tightly aggregated hyphal pellets, whereas agsA^OE hyphae aggregated weakly. We analyzed the chemical properties of cell wall α-1,3-glucan from the agsA^OE and agsB^OE strains. The peak molecular mass of α-1,3-glucan from the agsA^OE strain (1,480 ± 80 kDa) was much larger than that from the wild type (147 ± 52 kDa) and agsB^OE (372 ± 47 kDa); however, the peak molecular mass of repeating subunits in α-1,3-glucan was almost the same (after Smith degradation: agsA^OE, 41.6 ± 5.8 kDa; agsB^OE, 38.3 ± 3.0 kDa). We also analyzed localization of α-1,3-glucan in the cell wall of the two strains by fluorescent labeling with α-1,3-glucan-binding domain–fused GFP (AGBD-GFP). α-1,3-Glucan of the agsB^OE cells was clearly located in the outermost layer, whereas weak labeling was detected in the agsA^OE cells. However, the agsA^OE cells treated with β-1,3-glucanase were clearly labeled with AGBD-GFP. These observations suggest that β-1,3-glucan covered most of α-1,3-glucan synthesized by AgsA, although a small amount of α-1,3-glucan was still present in the outer layer. We also constructed a strain with disruption of the amyG gene, which encodes an intracellular α-amylase that synthesizes α-1,4-glucooligosaccharide as a primer for α-1,3-glucan biosynthesis. In this strain, the hyphal pellets and peak molecular mass of α-1,3-glucan (94.5 ± 1.4 kDa) were smaller than in the wild-type strain, and α-1,3-glucan was still labeled with AGBD-GFP in the outermost layer. Overall, these results suggest that hyphal pellet formation depends on the molecular mass and spatial localization of α-1,3-glucan as well as the amount of α-1,3-glucan in the cell wall of A. nidulans.

Heterologous Production of a Novel Cyclic Peptide Compound, KK-1, in Aspergillus oryzae

A novel cyclic peptide compound, KK-1, was originally isolated from the plant-pathogenic fungus Curvularia clavata. It consists of 10 amino acid residues, including five N-methylated amino acid residues, and has potent antifungal activity. Recently, the genome-sequencing analysis of C. clavata was completed, and the biosynthetic genes involved in KK-1 production were predicted by using a novel gene cluster mining tool, MIDDAS-M. These genes form an approximately 75-kb cluster, which includes nine open reading frames, containing a non-ribosomal peptide synthetase (NRPS) gene. To determine whether the predicted genes were responsible for the biosynthesis of KK-1, we performed heterologous production of KK-1 in Aspergillus oryzae by introduction of the cluster genes into the genome of A. oryzae. The NRPS gene was split in two fragments and then reconstructed in the A. oryzae genome, because the gene was quite large (approximately 40 kb). The remaining seven genes in the cluster, excluding the regulatory gene kkR, were simultaneously introduced into the strain of A. oryzae in which NRPS had already been incorporated. To evaluate the heterologous production of KK-1 in A. oryzae, gene expression was analyzed by RT-PCR and KK-1 productivity was quantified by HPLC. KK-1 was produced in variable quantities by a number of transformed strains, along with expression of the cluster genes. The amount of KK-1 produced by the strain with the greatest expression of all genes was lower than that produced by the original producer, C. clavata. Therefore, expression of the cluster genes is necessary and sufficient for the heterologous production of KK-1 in A. oryzae, although there may be unknown factors limiting productivity in this species.

Aspergillus fumigatus MADS-Box Transcription Factor rlmA Is Required for Regulation of the Cell Wall Integrity and Virulence

The Cell Wall Integrity (CWI) pathway is the primary signaling cascade that controls the de novo synthesis of the fungal cell wall, and in Saccharomyces cerevisiae this event is highly dependent on the RLM1 transcription factor. Here, we investigated the function of RlmA in the fungal pathogen Aspergillus fumigatus. We show that the ΔrlmA strain exhibits an altered cell wall organization in addition to defects related to vegetative growth and tolerance to cell wall-perturbing agents. A genetic analysis indicated that rlmA is positioned downstream of the pkcA and mpkA genes in the CWI pathway. As a consequence, rlmA loss-of-function leads to the altered expression of genes encoding cell wall-related proteins. RlmA positively regulates the phosphorylation of MpkA and is induced at both protein and transcriptional levels during cell wall stress. The rlmA was also involved in tolerance to oxidative damage and transcriptional regulation of genes related to oxidative stress adaptation. Moreover, the ΔrlmA strain had attenuated virulence in a neutropenic murine model of invasive pulmonary aspergillosis. Our results suggest that RlmA functions as a transcription factor in the A. fumigatus CWI pathway, acting downstream of PkcA-MpkA signaling and contributing to the virulence of this fungus.

Cell wall structure and biogenesis in Aspergillus species

Aspergillus species are among the most important filamentous fungi from the viewpoints of industry, pathogenesis, and mycotoxin production. Fungal cells are exposed to a variety of environmental stimuli, including changes in osmolality, temperature, and pH, which create stresses that primarily act on fungal cell walls. In addition, fungal cell walls are the first interactions with host cells in either human or plants. Thus, understanding cell wall structure and the mechanism of their biogenesis is important for the industrial, medical, and agricultural fields. Here, we provide a systematic review of fungal cell wall structure and recent findings regarding the cell wall integrity signaling pathways in aspergilli. This accumulated knowledge will be useful for understanding and improving the use of industrial aspergilli fermentation processes as well as treatments for some fungal infections.

Signaling pathways for stress responses and adaptation in Aspergillus species: stress biology in the post-genomic era

Aspergillus species are among the most important filamentous fungi in terms of industrial use and because of their pathogenic or toxin-producing features. The genomes of several Aspergillus species have become publicly available in this decade, and genomic analyses have contributed to an integrated understanding of fungal biology. Stress responses and adaptation mechanisms have been intensively investigated using the accessible genome infrastructure. Mitogen-activated protein kinase (MAPK) cascades have been highlighted as being fundamentally important in fungal adaptation to a wide range of stress conditions. Reverse genetics analyses have uncovered the roles of MAPK pathways in osmotic stress, cell wall stress, development, secondary metabolite production, and conidia stress resistance. This review summarizes the current knowledge on the stress biology of Aspergillus species, illuminating what we have learned from the genomic data in this "post-genomic era."

Expression of ustR and the Golgi protease KexB are required for ustiloxin B biosynthesis in Aspergillus oryzae

Ustiloxin B, originally isolated from the fungus Ustilaginoidea virens, is a known inhibitor of microtubule assembly. Ustiloxin B is also produced by Aspergillus flavus and is synthesized through the ribosomal peptide synthesis pathway. In A. flavus, the gene cluster associated with ustiloxin B production contains 15 genes including those encoding a fungal C6-type transcription factor and ustiloxin B precursor. Although the koji mold Aspergillus oryzae, which is genetically close to A. flavus, has the corresponding gene cluster, it does not produce ustiloxin B, which may be explained by the fact that the gene encoding the transcription factor UstR is not expressed. Here, to investigate whether ustiloxin B can be produced by expressing ustR in A. oryzae, we constructed ustR expression (ustR (EX)) strains and analyzed ustiloxin B production. In the ustR (EX) strains, all genes in the cluster were up-regulated, in line with expression of ustR, and ustiloxin B produced. To elucidate whether the KexB protease is involved in the processing of the ustiloxin B precursor protein UstA, which has repeats of basic amino acid doublets resembling KexB target sites, we also constructed a ustR (EX) strain with the ∆kexB genotype. Although ustR was expressed in this strain, ustiloxin B was barely detectable. This finding strongly suggests that KexB is required for ustiloxin B production.