Cell wall maintenance in fungi

Cymbopogom Citratus Essential Oils: A Promising Source of Antifungals Against Panax Notoginseng-Associated Pathogenic Fungi

Due to the great threat of chemical pesticides to the ecosystem environment, it is a long-term goal to find environmentally friendly green pesticides. Essential oils (EOs) are considered weapons in plant chemical defense and are important sources of green pesticides. Therefore, the antifungal effects and action mechanisms of Cymbopogom citratus (C. citratus) EOs against seven kinds of Panax notoginseng (P. notoginseng) pathogenic fungi were investigated. Oxford Cup results showed that C. citratus EOs had an excellent detraction effects against seven fungi of P. notoginseng. Gas chromatography-mass spectrometry (GC-MS) was used to construct the chemical profiles of C. citratus EOs, disclosed that the main categories are terpenes and oxygenated terpenes. In addition, compared with the hymexazol, the minimum inhibitory concentration (MIC) showed that EOs and their main components had strong antifungal activities. Besides, EOs had a synergistic effect with hymexazol (a chemical pesticide). The antifungal mechanism of C. citratus EOs was studied by using Fusarium oxysporum (F. oxysporum) as the dominant pathogen. C. citratus EOs may affect the metabolism of fungi and induce mycotoxins to destroy the cell wall to achieve antifungal effects. Finally, EOs were found to significantly retard P. notoginseng infection by F. oxysporum. According to our research, C. citratus EOs are potential green antifungal agent that can be used in the cultivation of P. notoginseng.

Frass derived from black soldier fly larvae treatment of biodegradable wastes. A critical review and future perspectives

Inadequately treated biodegradable waste is considered an environmental, social and economic threat worldwide, which call for great attention. Waste treatment with larvae of the black soldier fly (BSF, Hermetia illucens) complies with the concepts of circular economy, as it enables the transformation of these wastes into marketable products, closing loops and promoting circularity. The processing residues of the treatment (frass) is constantly generated in waste management facilities in large volumes, and this product can be used as an organic fertilizer in agriculture, stimulating a transition to a circular economy. However, many aspects related to frass are still unknown, such as its varying composition of nutrients, microorganisms and bioactive compounds, its post-processing requirements for improved biological stabilization, its behavior in the soil and action in the plants' metabolism, among other aspects. In this review article, we highlight the potential of frass from BSF larvae treatment of biodegradable waste in the world market regarding its possible use as a fertilizer, summarize recent results with this novel product and point towards future research perspectives.

Proteomics of Two Thermotolerant Isolates of Trichoderma under High-Temperature Stress

Several species of the soil borne fungus of the genus Trichoderma are known to be versatile, opportunistic plant symbionts and are the most successful biocontrol agents used in today's agriculture. To be successful in field conditions, the fungus must endure varying climatic conditions. Studies have indicated that a high atmospheric temperature coupled with low humidity is a major factor in the inconsistent performance of Trichoderma under field conditions. Understanding the molecular modulations associated with Trichoderma that persist and deliver under abiotic stress conditions will aid in exploiting the value of these organisms for such uses. In this study, a comparative proteomic analysis, using two-dimensional gel electrophoresis (2DE) and matrix-assisted laser desorption/time-of-flight (MALDI-TOF-TOF) mass spectrometry, was used to identify proteins associated with thermotolerance in two thermotolerant isolates of Trichoderma: T. longibrachiatum 673, TaDOR673 and T. asperellum 7316, TaDOR7316; with 32 differentially expressed proteins being identified. Sequence homology and conserved domains were used to identify these proteins and to assign a probable function to them. The thermotolerant isolate, TaDOR673, seemed to employ the stress signaling MAPK pathways and heat shock response pathways to combat the stress condition, whereas the moderately tolerant isolate, TaDOR7316, seemed to adapt to high-temperature conditions by reducing the accumulation of misfolded proteins through an unfolded protein response pathway and autophagy. In addition, there were unique, as well as common, proteins that were differentially expressed in the two isolates studied.

A unique cell wall synthetic response evoked by glucosamine determines pathogenicity-associated fungal cellular differentiation

The yeast-to-hypha transition is tightly associated with pathogenicity in many human pathogenic fungi, such as the model fungal pathogen Cryptococcus neoformans, which is responsible for approximately 180,000 deaths annually. In this pathogen, the yeast-to-hypha transition can be initiated by distinct stimuli: mating stimulation or glucosamine (GlcN), the monomer of cell wall chitosan. However, it remains poorly understood how the signal specificity for Cryptococcus morphological transition by disparate stimuli is ensured. Here, by integrating temporal expression signature analysis and phenome-based clustering evaluation, we demonstrate that GlcN specifically triggers a unique cellular response, which acts as a critical determinant underlying the activation of GlcN-induced filamentation (GIF). This cellular response is defined by an unusually hyperactive cell wall synthesis that is highly ATP-consuming. A novel cell surface protein Gis1 was identified as the indicator molecule for the GlcN-induced cell wall response. The Mpk1-directed cell wall pathway critically bridges global cell wall gene induction and intracellular ATP supply, ensuring the Gis1-dependent cell wall response and the stimulus specificity of GIF. We further reveal that the ability of Mpk1 to coordinate the cell wall response and GIF activation is conserved in different Cryptococcus pathogens. Phosphoproteomics-based profiling together with genetic and phenotypic analysis revealed that the Mpk1 kinase mediates the regulatory specificity of GIF through a coordinated downstream regulatory network centered on Skn7 and Crz1. Overall, our findings discover an unprecedented and conserved cell wall biosynthesis-dependent fungal differentiation commitment mechanism, which enables the signal specificity of pathogenicity-related dimorphism induced by GlcN in Cryptococcus pathogens.

Many human fungal pathogens can undergo dimorphic transition between yeast and hyphal forms in response to different external stimuli, and this morphological transition is generally and critically linked with their infections. In Cryptococcus neoformans, a model pathogenic fungus, the yeast-to-hypha transition can be elicited by mating stimulation or glucosamine (GlcN), the monomer of cell wall chitosan. Here, we show that GlcN specifically evokes a unique hyperactive cell wall synthetic response, which determines GlcN-induced filamentation (GIF) as a key commitment event. The Mpk1-directed cell wall signaling pathway as a core and conserved cascade connects the cell wall synthetic response and GIF activation in different Cryptococcus pathogens. Overall, the findings reveal a previously unrecognized function of GlcN in stimulating cell wall signaling and biosynthetic machinery, which enables a unique dimorphism commitment mechanism underlying the signal specificity of the mating-independent yeast-to-hypha transition in Cryptococcus pathogens.

Whole Transcriptome Analysis Provides Insights Into the Molecular Mechanisms of Chlamydospore-Like Cell Formation in Phanerochaete chrysosporium

Phanerochaete chrysosporium is a white rot fungus naturally isolated from hardwoods and widely used in environmental pollution control because it produces extracellular peroxidases. It forms chlamydospores during nitrogen starvation, which naturally occurs in the habitat of P. chrysosporium. Chlamydospores protect fungi against many stresses; the molecular basis underlying chlamydospore formation in basidiomycetes is poorly explored. Chlamydospores in P. chrysosporium have a different cell wall compared with hyphae, as confirmed by cell wall digestion and microscopy. Furthermore, this study investigated the transcriptome of P. chrysosporium in different life stages, including conidium, hypha, and chlamydospore formation, through RNA sequencing. A total of 2215 differentially expressed genes were identified during these processes. The expression patterns of genes involved in several molecular events critical for chlamydospore formation, including starch and sucrose metabolism, phosphatase and kinase, and transcription factors, were determined. This study serves as a basis for further investigating the function of chlamydospore formation in the biotechnologically relevant fungus P. chrysosporium.

Chitin and chitosan remodeling defines vegetative development and Trichoderma biocontrol

Fungal parasitism depends on the ability to invade host organisms and mandates adaptive cell wall remodeling to avoid detection and defense reactions by the host. All plant and human pathogens share invasive strategies, which aid to escape the chitin-triggered and chitin-targeted host immune system. Here we describe the full spectrum of the chitin/chitosan-modifying enzymes in the mycoparasite Trichoderma atroviride with a central role in cell wall remodeling. Rapid adaption to a variety of growth conditions, environmental stresses and host defense mechanisms such as oxidative stress depend on the concerted interplay of these enzymes and, ultimately, are necessary for the success of the mycoparasitic attack. To our knowledge, we provide the first in class description of chitin and associated glycopolymer synthesis in a mycoparasite and demonstrate that they are essential for biocontrol. Eight chitin synthases, six chitin deacetylases, additional chitinolytic enzymes, including six chitosanases, transglycosylases as well as accessory proteins are involved in this intricately regulated process. Systematic and biochemical classification, phenotypic characterization and mycoparasitic confrontation assays emphasize the importance of chitin and chitosan assembly in vegetative development and biocontrol in T. atroviride. Our findings critically contribute to understanding the molecular mechanism of chitin synthesis in filamentous fungi and mycoparasites with the overarching goal to selectively exploit the discovered biocontrol strategies.

Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents

Fungal infections are a worldwide problem associated with high morbidity and mortality. There are relatively few antifungal agents, and resistance has emerged within these pathogens for the newest antifungal drugs. As the fungal cell wall is critical for growth and development, it is one of the most important targets for drug development. In this review, the currently available cell wall inhibitors and suitable drug candidates for the treatment of fungal infections are explored. Future studies of the fungal cell wall and compounds that have detrimental effects on this important outer structural layer could aid in antifungal drug discovery and lead to the development of alternative cell wall inhibitors to fill gaps in clinical therapies for difficult-to-treat fungal infections.

Exophiala dermatitidis: Key issues of an opportunistic fungal pathogen

The black yeast Exophiala dermatitidis is an opportunistic pathogen, causing phaeohyphomycosis in immunosuppressed patients, chromoblastomycosis and fatal infections of the central nervous system in otherwise healthy Asian patients. In addition, it is also regularly isolated from respiratory samples from cystic fibrosis patients, with rates varying between 1% and 19%.Melanin, as part of the cell wall of black yeasts, is one major factor known contributing to the pathogenicity of E. dermatitidis and increased resistance against host defense and anti-infective therapeutics. Further virulence factors, e.g. the capability to adhere to surfaces and to form biofilm were reported. A better understanding of the pathogenicity of E. dermatitidis is essential for the development of novel preventive and therapeutic strategies. In this review, the current knowledge of E. dermatitidis prevalence, clinical importance, diagnosis, microbiological characteristics, virulence attributes, susceptibility, and resistances as well as therapeutically strategies are discussed.

Transformation of Fonsecaea pedrosoi into sclerotic cells links to the refractoriness of experimental chromoblastomycosis in BALB/c mice via a mechanism involving a chitin-induced impairment of IFN-γ production

Fonsecaea pedrosoi (F. pedrosoi) is the most common agent of chromoblastomycosis. Transformation of this fungus from its saprophytic phase into pathogenic sclerotic cells in tissue is an essential link to the refractoriness of this infection. Experimental studies in murine models have shown that the absence of CD4+ T cells impairs host defense against F. pedrosoi infection. Clinical research has also suggested that a relatively low level of the Th1 cytokine INF-γ and inefficient T cell proliferation are simultaneously present in patients with severe chromoblastomycosis upon in vitro stimulation with ChromoAg, an antigen prepared from F. pedrosoi. In the present study, we show that in mice intraperitoneally infected with F. pedrosoi-spores, -hyphae or in vitro-induced sclerotic cells respectively, the transformation of this causative agent into sclerotic cells contributes to a compromised Th1 cytokine production in the earlier stage of infection with impaired generation of neutrophil reactive oxygen species (ROS) and pan-inhibition of Th1/Th2/Th17 cytokine production with disseminated infection in the later stage by using a CBA murine Th1/Th2/Th17 cytokine kit. In addition, we have further demonstrated that intraperitoneal administration of recombinant mouse IFN-γ (rmIFN-γ) effectively reduces the fungal load in the infected mouse spleen, and dampens the peritoneal dissemination of F. pedrosoi-sclerotic cells. Meanwhile, exogeneous rmIFN-γ contributes to the formation and maintenance of micro-abscess and restores the decrease in neutrophil ROS generation in the mouse spleen infected with F. pedrosoi-sclerotic cells. Of note, we have once again demonstrated that it is a chitin-like component, but not β-glucans or mannose moiety, that exclusively accumulates on the outer cell wall of F. pedrosoi-sclerotic cells which were induced in vitro or isolated from the spleens of intraperitoneally infected BALB/c mice. In addition, our results indicate that decreased accumulation of chitin on the surface of live F. pedrosoi-sclerotic cells after chitinase treatment can be self-compensated in a time-dependent manner. Importantly, we have for the first time demonstrated that exclusive accumulation of chitin on the transformed sclerotic cells of F. pedrosoi is involved in an impaired murine Th1 cytokine profile, therefore promoting the refractoriness of experimental murine chromoblastomycosis.

Intra- and inter-specific variations in chitin in lichens along a N-deposition gradient

The mechanisms of nitrogen (N) tolerance in lichens are not yet fully understood. Here, we investigated how the increase of chitin content is related with N excess at inter- and intra-specific levels, by using species with differing ecological N tolerances (the tolerant Xanthoria parietina and Parmotrema hypoleucinum and the sensitive Evernia prunastri and Usnea sp.) and thalli of X. parietina and P. hypoleucinum from sites with different availabilities of N of agricultural origin (livestock), as confirmed by lichen N content and δ¹⁵N. Nitrogen, chitin (N-containing compound), and ergosterol contents were measured in lichen thalli. Nitrogen and chitin contents were higher in tolerant species than those in sensitive ones (inter-specific level) and in thalli collected from the N-polluted site than in thalli from the clean site (intra-specific level). We suggest that chitin contributes to N stress tolerance in lichens, and that excess N can be partially stored as chitin (non-toxic form) in the cell walls of tolerant species.

Hypoxia-elicited impairment of cell wall integrity, glycosylation precursor synthesis, and growth in scaled-up high-cell density fed-batch cultures of Saccharomyces cerevisiae

Background

In this study we examine the integrity of the cell wall during scale up of a yeast fermentation process from laboratory scale (10 L) to industrial scale (10,000 L). In a previous study we observed a clear difference in the volume fraction occupied by yeast cells as revealed by wet cell weight (WCW) measurements between these scales. That study also included metabolite analysis which suggested hypoxia during scale up. Here we hypothesize that hypoxia weakens the yeast cell wall during the scale up, leading to changes in cell permeability, and/or cell mechanical resistance, which in turn may lead to the observed difference in WCW. We tested the cell wall integrity by probing the cell wall sensitivity to Zymolyase. Also exometabolomics data showed changes in supply of precursors for the glycosylation pathway.

Results

The results show a more sensitive cell wall later in the production process at industrial scale, while the sensitivity at early time points was similar at both scales. We also report exometabolomics data, in particular a link with the protein glycosylation pathway. Significantly lower levels of Man6P and progressively higher GDP-mannose indicated partially impaired incorporation of this sugar nucleotide during co- or post-translational protein glycosylation pathways at the 10,000 L compared to the 10 L scale. This impairment in glycosylation would be expected to affect cell wall integrity. Although cell viability from samples obtained at both scales were similar, cells harvested from 10 L bioreactors were able to re-initiate growth faster in fresh shake flask media than those harvested from the industrial scale.

Conclusions

The results obtained help explain the WCW differences observed at both scales by hypoxia-triggered weakening of the yeast cell wall during the scale up.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0542-3) contains supplementary material, which is available to authorized users.

The Sclerotinia sclerotiorum Slt2 mitogen-activated protein kinase ortholog, SMK3, is required for infection initiation but not lesion expansion

Mitogen-activated protein kinases (MAPKs) play a central role in transferring signals and regulating gene expression in response to extracellular stimuli. An ortholog of the Saccharomyces cerevisiae cell wall integrity MAPK was identified in the phytopathogenic fungus Sclerotinia sclerotiorum. Disruption of the S. sclerotiorum Smk3 gene severely reduced virulence on intact host plant leaves but not on leaves stripped of cuticle wax. This was attributed to alterations in hyphal apical dominance leading to the inability to aggregate and form infection cushions. The mutation also caused loss of the ability to produce sclerotia, increased aerial hyphae formation, and altered hyphal hydrophobicity and cell wall integrity. Mutants had slower radial expansion rates on solid media but more tolerance to elevated temperatures. Loss of the SMK3 cell wall integrity MAPK appears to have impaired the ability of S. sclerotiorum to sense its surrounding environment, leading to misregulation of a variety of functions. Many of the phenotypes were similar to those observed in S. sclerotiorum adenylate cyclase and SMK1 MAPK mutants, suggesting that these signaling pathways co-regulate aspects of fungal growth, physiology, and pathogenicity.

Preparation of chitooligosaccharides from fungal waste mycelium by recombinant chitinase

This study aimed to develop an enzymatic method for conversion of chitin from fungal waste mycelia to chitooligosaccharides. The recombinant chitinase LlChi18A from Lactococcus lactis was over-expressed by Escherichia coli BL21 (DE3) and purified by affinity chromatography. The enzymatic properties of the purified enzyme were studied by chitin oligosaccharides. Waste mycelium was pre-treated by alkaline. The optimal conditions for hydrolysis of fungal chitin by recombinant chitinase were determined by Schales method. HPLC/ESI-MS was used to determine the content of N-acetylglucosamine and chitooligosaccharides after hydrolysis. The level of reducing sugar released from pretreated mycelium by chitinase increased with the reaction time during 6 days. The main product in the hydrolysates was N,N'-diacetylchitobiose. After hydrolysis by chitinase for 5 d, the yield of N,N'-diacetylchitobiose from waste mycelium was around 10% with estimated purity of around 70%. Combination of chitinase and snailase remarkably increased the yield to 24% with purity of 78%. Fungal mycelium which contains chitin is a new potential source for obtaining food grade chitooligosaccharides.

Identifying glycoside hydrolase family 18 genes in the mycoparasitic fungal species Clonostachys rosea

Clonostachysrosea is a mycoparasitic fungal species that is an efficient biocontrol agent against many plant diseases. During mycoparasitic interactions, one of the most crucial steps is the hydrolysis of the prey's fungal cell wall, which mainly consists of glucans, glycoproteins and chitin. Chitinases are hydrolytic enzymes responsible for chitin degradation and it is suggested that they play an important role in fungal-fungal interactions. Fungal chitinases belong exclusively to the glycoside hydrolase (GH) family 18.These GH18 proteins are categorized into three distinct phylogenetic groups (A, B and C), subdivided into several subgroups. In this study, we identified 14 GH18 genes in the C. rosea genome, which is remarkably low compared with the high numbers found in mycoparasitic Trichoderma species. Phylogenetic analysis revealed that C. rosea contains eight genes in group A, two genes in group B, two genes in group C, one gene encoding a putative ENGase (endo-β-N-acetylglucosaminidase) and the ech37 gene, which is of bacterial origin. Gene expression analysis showed that only two genes had higher transcription levels during fungal-fungal interactions, while eight out of 14 GH18 genes were triggered by chitin. Furthermore, deletion of the C group chiC2 gene decreased the growth inhibitory activity of C. rosea culture filtrates against Botrytis cinerea and Rhizoctonia solani, although the biocontrol ability of C. rosea against B. cinerea was not affected. In addition, a potential role of the CHIC2 chitinase in the sporulation process was revealed. These results provide new information about the role of GH18 proteins in mycoparasitic interactions.

Black yeasts and their filamentous relatives: principles of pathogenesis and host defense

Among the melanized fungi, the so-called "black yeasts" and their filamentous relatives are particularly significant as agents of severe phaeohyphomycosis, chromoblastomycosis, and mycetoma in humans and animals. The pathogenicity and virulence of these fungi may differ significantly between closely related species. The factors which probably are of significance for pathogenicity include the presence of melanin and carotene, formation of thick cell walls and meristematic growth, presence of yeast-like phases, thermo- and perhaps also osmotolerance, adhesion, hydrophobicity, assimilation of aromatic hydrocarbons, and production of siderophores. Host defense has been shown to rely mainly on the ingestion and elimination of fungal cells by cells of the innate immune system, especially neutrophils and macrophages. However, there is increasing evidence supporting a role of T-cell-mediated immune responses, with increased interleukin-10 (IL-10) and low levels of gamma interferon (IFN-γ) being deleterious during the infection. There are no standardized therapies for treatment. It is therefore important to obtain in vitro susceptibilities of individual patients' fungal isolates in order to provide useful information for selection of appropriate treatment protocols. This article discusses the pathogenesis and host defense factors for these fungi and their severity, chronicity, and subsequent impact on treatment and prevention of diseases in human or animal hosts.

Isothermal microcalorimetry provides new insight into terrestrial carbon cycling

Energy is continuously transformed in environmental systems through the metabolic activities of living organisms, but little is known about the relationship between the two. In this study, we tested the hypothesis that microbial energetics are controlled by microbial community composition in terrestrial ecosystems. We determined the functional diversity profiles of the soil biota (i.e., multiple substrate-induced respiration and microbial energetics) in soils from an arable ecosystem with contrasting long-term management regimes (54 years). These two functional profiling methods were then related to the soils' microbial community composition. Using isothermal microcalorimetry, we show that direct measures of energetics provide a functional link between energy flows and the composition of below-ground microbial communities at a high taxonomic level (Mantel R = 0.4602, P = 0.006). In contrast, this link was not apparent when carbon dioxide (CO2) was used as an aggregate measure of microbial metabolism (Mantel R = 0.2291, P = 0.11). Our work advocates that the microbial energetics approach provides complementary information to soil respiration for investigating the involvement of microbial communities in below-ground carbon dynamics. Empirical data of our proposed microbial energetics approach can feed into carbon-climate based ecosystem feedback modeling with the suggested conceptual ecological model as a base.

Importance of MAP kinases during protoperithecial morphogenesis in Neurospora crassa

In order to produce multicellular structures filamentous fungi combine various morphogenetic programs that are fundamentally different from those used by plants and animals. The perithecium, the female sexual fruitbody of Neurospora crassa, differentiates from the vegetative mycelium in distinct morphological stages, and represents one of the more complex multicellular structures produced by fungi. In this study we defined the stages of protoperithecial morphogenesis in the N. crassa wild type in greater detail than has previously been described; compared protoperithecial morphogenesis in gene-deletion mutants of all nine mitogen-activated protein (MAP) kinases conserved in N. crassa; confirmed that all three MAP kinase cascades are required for sexual development; and showed that the three different cascades each have distinctly different functions during this process. However, only MAP kinases equivalent to the budding yeast pheromone response and cell wall integrity pathways, but not the osmoregulatory pathway, were essential for vegetative cell fusion. Evidence was obtained for MAP kinase signaling cascades performing roles in extracellular matrix deposition, hyphal adhesion, and envelopment during the construction of fertilizable protoperithecia.

Induction of cell wall thickening by the antifungal compound dihydromaltophilin disrupts fungal growth and is mediated by sphingolipid biosynthesis

Dihydromaltophilin (heat-stable antifungal factor [HSAF]) is an antifungal metabolite produced in Lysobacter enzymogenes biocontrol strain C3. This compound induces cell wall thickening in Aspergillus nidulans. Here we show that the cell wall thickening is a general response to HSAF in diverse fungal species. In the A. nidulans model, the thickened cell wall negatively affects hyphal growth. Growth of HSAF-pre-treated hyphae failed to resume at hyphal tips with thick cell wall and the actin cable could not re-polarize at the thickened region of the cell wall, even after the treated hyphae were transferred to drug-free medium. Moreover, HSAF-induced cell wall thickening is mediated by sphingolipid synthesis: HSAF failed to induce cell wall thickening in the absence of ceramide synthase BarA and the sphingolipid synthesis inhibitor myriocin was able to suppress HSAF-induced cell wall thickening. The thickened cell wall could be digested by chitinase suggesting that chitin contributes to the HSAF-induced thickening. Furthermore, HSAF treatment activated the transcription of two chitin synthase encoding genes chsB and chsC.

Influence of sorbitol on protein production and glycosylation and cell wall formation in Trichoderma reesei

Sorbitol is often used at 1 mol/liter as an osmotic stabilizer for cultivation of fungi with a fragile cell wall phenotype. On the other hand, at this concentration sorbitol causes an osmotic stress in fungal cells resulting in intensive production of intracellular glycerol. The highly increased consumption of glucose for glycerol synthesis may lead to changes in processes requiring carbohydrate residues. This study provides new information on the consequences of osmotic stress to the cell wall composition, protein production and glycosylation, and cell morphology of Trichoderma reesei. We observed that high osmolarity conditions enhanced biomass production and strongly limited synthesis of cell wall glucans and chitin. Moreover, in these conditions the amount of secreted protein decreased nearly ten-fold and expression of cbh1 and cbh2 genes coding for cellobiohydrolase I and cellobiohydrolase II, the main secretory proteins in T. reesei, was inhibited resulting in a lack of the proteins in the cell and cultivation medium. The activity of DPM synthase, enzyme engaged in both N- and O-glycosylation pathways, was reduced two-fold, suggesting an overall inhibition of protein glycosylation. However, the two modes of glycosylation were affected divergently: O-glycosylation of secreted proteins decreased in the early stages of growth while N-glycosylation significantly increased in the stationary phase.

MAPK cell-cycle regulation in Saccharomyces cerevisiae and Candida albicans

The cell cycle is the sequential set of events that living cells undergo in order to duplicate. This process must be tightly regulated as alterations may lead to diseases such as cancer. The molecular events that control the cell cycle are directional and involve regulatory molecules such as cyclins and cyclin-dependent kinases (CDKs). The budding yeast Saccharomyces cerevisiae has become a model to study this complex system since it shares several mechanisms with higher eukaryotes. Signal transduction pathways are biochemical mechanisms that sense environmental changes and there is recent evidence that they control the progression through the cell cycle in response to several stimuli. In response to pheromone, the budding yeast arrests the cell cycle in the G1 phase at the START stage. Activation of the pheromone response pathway leads to the phosphorylation of Far1, which inhibits the function of complexes formed by G1 cyclins (Cln1 and Cln2) and the CDK (Cdc28), blocking the transition to the S phase. This response prepares the cells to fuse cytoplasms and nuclei to generate a diploid cell. Activation of the Hog1 MAP kinase in response to osmotic stress or arsenite leads to the transient arrest of the cell cycle in G1 phase, which is mediated by direct phosphorylation of the CDK inhibitor, Sic1, and by downregulation of cyclin expression. Osmotic stress also induces a delay in G2 phase by direct phosphorylation of Hsl7 via Hog1, which results in the accumulation of Swe1. As a consequence, cell cycle arrest allows cells to survive upon stress. Finally, cell wall damage can induce cell cycle arrest at G2 via the cell integrity MAPK Slt2. By linking MAPK signal transduction pathways to the cell cycle machinery, a tight and precise control of the cell division takes place in response to environmental changes. Research into similar MAPK-mediated cell cycle regulation in the opportunistic pathogen Candida albicans may result in the development of new antifungal therapies.

A multifunctional mannosyltransferase family in Candida albicans determines cell wall mannan structure and host-fungus interactions

The cell wall proteins of fungi are modified by N- and O-linked mannosylation and phosphomannosylation, resulting in changes to the physical and immunological properties of the cell. Glycosylation of cell wall proteins involves the activities of families of endoplasmic reticulum and Golgi-located glycosyl transferases whose activities are difficult to infer through bioinformatics. The Candida albicans MNT1/KRE2 mannosyl transferase family is represented by five members. We showed previously that Mnt1 and Mnt2 are involved in O-linked mannosylation and are required for virulence. Here, the role of C. albicans MNT3, MNT4, and MNT5 was determined by generating single and multiple MnTDelta null mutants and by functional complementation experiments in Saccharomyces cerevisiae. CaMnt3, CaMnt4, and CaMnt5 did not participate in O-linked mannosylation, but CaMnt3 and CaMnt5 had redundant activities in phosphomannosylation and were responsible for attachment of approximately half of the phosphomannan attached to N-linked mannans. CaMnt4 and CaMnt5 participated in N-mannan branching. Deletion of CaMNT3, CaMNT4, and CaMNT5 affected the growth rate and virulence of C. albicans, affected the recognition of the yeast by human monocytes and cytokine stimulation, and led to increased cell wall chitin content and exposure of beta-glucan at the cell wall surface. Therefore, the MNT1/KRE2 gene family participates in three types of protein mannosylation in C. albicans, and these modifications play vital roles in fungal cell wall structure and cell surface recognition by the innate immune system.

Immunoaffinity purification of the class V chitin synthase of Wangiella (Exophiala) dermatitidis

The class V chitin synthase is unique because it has a myosin motor-like domain fused to its catalytic domain. The biochemical properties of this enzyme and its function remain undefined beyond the knowledge that it is the only single chitin synthase required for sustained cell growth at elevated temperatures and, consequently, virulence. This report describes our successful efforts to isolate and purify an active and soluble form of the enzyme from the cell membranes of Wangiella by using a specific polyclonal antibody. To our knowledge, this is the first purification of a single chitin synthase of a filamentous fungus.

Trifluoromethanesulfonic acid-based proteomic analysis of cell wall and secreted proteins of the ascomycetous fungi Neurospora crassa and Candida albicans

Cell wall proteins from purified Candida albicans and Neurospora crassa cell walls were released using trifluoromethanesulfonic acid (TFMS) which cleaves the cell wall glucan/chitin matrix and deglycosylates the proteins. The cell wall proteins were then characterized by SDS-PAGE and identified by proteomic analysis. The analyses for C. albicans identified 15 cell wall proteins and six secreted proteins. For N. crassa, the analyses identified 26 cell wall proteins and nine secreted proteins. Most of the C. albicans cell wall proteins are found in the cell walls of both yeast and hyphae cells, but some cell type-specific cell wall proteins were observed. The analyses showed that the pattern of cell wall proteins present in N. crassa vegetative hyphae and conidia (asexual spores) are quite different. Almost all of the cell wall proteins identified in N. crassa have close homologs in the sequenced fungal genomes, suggesting that these proteins have important conserved functions within the cell wall.

Aspects of the progesterone response in Hortaea werneckii: Steroid detoxification, protein induction and remodelling of the cell wall

Progesterone in sublethal concentrations temporarily inhibits growth of Hortaea werneckii. This study investigates some of the compensatory mechanisms which are activated in the presence of progesterone and are most probably contributing to escape from growth inhibition. These mechanisms lead on the one hand to progesterone biotransformation/detoxification but, on the other, are suggested to increase the resistance of H. werneckii to the steroid. Biotransformation can detoxify progesterone efficiently in the early logarithmic phase, with mostly inducible steroid transforming enzymes, while progesterone biotransformation/detoxification in the late logarithmic and stationary phases of growth is not very efficient. The relative contribution of constitutive steroid transforming enzymes to progesterone biotransformation is increased in these latter phases of growth. In the presence of progesterone, activation of the cell wall integrity pathway is suggested by the overexpression of Pck2 which was detected in the stationary as well as the logarithmic phase of growth of the yeast. Progesterone treated H. werneckii cells were found to be more resistant to cell lysis than mock treated cells, indicating for the first time changes in the yeast cell wall as a result of treatment with progesterone.

Cytolocalization of the class V chitin synthase in the yeast, hyphal and sclerotic morphotypes of Wangiella (Exophiala) dermatitidis

Wangiella (Exophiala) dermatitidis is a polymorphic fungus that produces polarized yeast and hyphae, as well as a number of non-polarized sclerotic morphotypes. The phenotypic malleability of this agent of human phaeohyphomycosis allows detailed study of its biology, virulence and the regulatory mechanisms responsible for the transitions among the morphotypes. Our prior studies have demonstrated the existence of seven chitin synthase structural genes in W. dermatitidis, each of which encodes an isoenzyme of a different class. Among them, the class V chitin synthase (WdChs5p) is most unique in terms of protein structure, because it has an N-terminal myosin motor-like domain with a P-loop (MMD) fused to its C-terminal chitin synthase catalytic domain (CSCD). However, the exact role played by WdChs5p in the different morphotypes remains undefined beyond the knowledge that it is the only single chitin synthase required for sustained cell growth at 37 degrees C and consequently virulence. This report describes the expression in Escherichia coli of a 12kDa polypeptide (WdMyo12p) of WdChs5p, which was used to raise in rabbits a polyclonal antibody that recognized exclusively its MMD region. Results from the use of the antibody in immunocytolocalization studies supported our previous findings that WdChs5p is critically important at infection temperatures for maintaining the cell wall integrity of developing yeast buds, elongating tips of hyphae, and random sites of expansion in sclerotic forms. The results also suggested that WdChs5p localizes to the regions of cell wall growth in an actin-dependent fashion.

Inhibition of species of the Aspergillus section Nigri and ochratoxin a production in grapes by fusapyrone

Fusapyrone (FP), an antifungal natural compound, was tested against the three main ochratoxigenic species of the Aspergillus section Nigri. The MICs at 24 h were 6.0, 11.6, and 9.9 mug/ml for Aspergillus carbonarius, Aspergillus tubingensis, and Aspergillus niger, respectively. Strong inhibition of growth and morphological changes were still observed at half the MIC after 7 days. The application of a 100 mug/ml FP solution in a laboratory assay on artificially inoculated grapes resulted in a significant reduction (up to 6 orders of magnitude) of A. carbonarius CFU counts. Dramatic reductions of the ochratoxin A (OTA) content, compared to the content of the positive control (average amount of OTA, 112.5 ng/g of grape; three experiments), were obtained with the application of either 100 or 50 mug/ml of FP (0.6 or 5.1 ng/g of grape, respectively).

Chitosan-mediated changes in cell wall composition, morphology and ultrastructure in two wood-inhabiting fungi

The effect of chitosan on cell wall deposition was investigated in the two wood-inhabiting fungal species Trichoderma harzianum (CBS 597.91) and Sphaeropsis sapinea (NZFS 2725). The study used three independent analytical techniques to quantify chitin in the fungal mycelium. A colorimetric method for the detection of D-glucosamine was compared with two gas chromatography-mass spectroscopy (GC-MS) methods employing alditol acetates analysis and pyrolysis. The latter used a stable-isotope-labelled internal standard, d(3)-N-acetyl glucosamine. At least in the case of S. sapinea, the study provided evidence of an increase in the chitin content in the mycelium due to chitosan treatment, indicating that chitosan treatment affected cell wall deposition. Electron microscopy techniques showed alteration in surface morphology and cell wall texture due to chitosan treatment. The implications of these results are discussed with a view to analysing possible mechanisms for growth inhibitory effects of chitosan on fungal hyphae.

basA regulates cell wall organization and asexual/sexual sporulation ratio in Aspergillus nidulans

Sphingolipid C4 hydroxylase catalyzes the conversion of dihydrosphingosine to phytosphingosine. In Saccharomyces cerevisiae, Sur2 is essential for sphingolipid C4 hydroxylation activity but not essential for normal growth. Here we demonstrate that the Aspergillus nidulans Sur2 homolog BasA is also required for phytosphingosine biosynthesis but is also essential for viability. We previously reported that a point missense mutation in basA resulted in aberrant cell wall thickening. Here our data suggest that accumulation of dihydrosphingosine is responsible for this phenotype. In addition, two different mutations in basA consistently accelerated the transition from asexual development to sexual development compared to the wild-type strain. The phenotype could be suppressed by exogenous addition of phytosphingosine. Northern analysis suggests that faster sexual development in the basA mutant might be due to a higher transcription level of ppoA and steA, genes demonstrated to coordinate a balance between asexual and sexual development in A. nidulans. Consistent with these findings, mutations in the ceramide-synthase-encoding genes barA and lagA also caused faster transition from asexual to sexual development, supporting the involvement of sphingolipid metabolism in fungal morphogenesis.

MAP kinase pathways as regulators of fungal virulence

MAP kinases are dual phosphorylated protein kinases, present in eukaryotes, which mediate differentiation programs and immune responses in mammalian cells. In pathogenic fungi, MAP kinases are key elements that control adaptation to environmental stress. Recent studies have shown that these pathways have an essential role in the control of essential virulence factors such as capsule biogenesis in Cryptococcus neoformans or morphogenesis, invasion and oxidative stress in Candida albicans. Although MAP kinases sense different activating signals, there is a considerable degree of crosstalk and/or overlap, which enables them to integrate, amplify and modulate the appropriate protective and adaptive response. MAP kinases behave as a 'functional nervous system' that controls virulence and influences the progression of the disease.

Biosynthesis and function of GPI proteins in the yeast Saccharomyces cerevisiae

Like most other eukaryotes, Saccharomyces cerevisiae harbors a GPI anchoring machinery and uses it to attach proteins to membranes. While a few GPI proteins reside permanently at the plasma membrane, a majority of them gets further processed and is integrated into the cell wall by a covalent attachment to cell wall glucans. The GPI biosynthetic pathway is necessary for growth and survival of yeast cells. The GPI lipids are synthesized in the ER and added onto proteins by a pathway comprising 12 steps, carried out by 23 gene products, 19 of which are essential. Some of the estimated 60 GPI proteins predicted from the genome sequence serve enzymatic functions required for the biosynthesis and the continuous shape adaptations of the cell wall, others seem to be structural elements of the cell wall and yet others mediate cell adhesion. Because of its genetic tractability S. cerevisiae is an attractive model organism not only for studying GPI biosynthesis in general, but equally for investigating the intracellular transport of GPI proteins and the peculiar role of GPI anchoring in the elaboration of fungal cell walls.

The antifungal protein AFP from Aspergillus giganteus inhibits chitin synthesis in sensitive fungi

The antifungal protein AFP from Aspergillus giganteus is highly effective in restricting the growth of major human- and plant-pathogenic filamentous fungi. However, a fundamental prerequisite for the use of AFP as an antifungal drug is a complete understanding of its mode of action. In this study, we performed several analyses focusing on the assumption that the chitin biosynthesis of sensitive fungi is targeted by AFP. Here we show that the N-terminal domain of AFP (amino acids 1 to 33) is sufficient for efficient binding of AFP to chitin but is not adequate for inhibition of the growth of sensitive fungi. AFP susceptibility tests and SYTOX Green uptake experiments with class III and class V chitin synthase mutants of Fusarium oxysporum and Aspergillus oryzae showed that deletions made the fungi less sensitive to AFP and its membrane permeabilization effect. In situ chitin synthase activity assays revealed that chitin synthesis is specifically inhibited by AFP in sensitive fungi, indicating that AFP causes cell wall stress and disturbs cell integrity. Further evidence that there was AFP-induced cell wall stress was obtained by using an Aspergillus niger reporter strain in which the cell wall integrity pathway was strongly induced by AFP.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

The Aspergillus niger MADS-box transcription factor RlmA is required for cell wall reinforcement in response to cell wall stress

In Aspergillus niger, the genes coding for glutamine:fructose-6-phosphate amidotransferase (gfaA) and alpha-1,3-glucan synthase (agsA) are induced in response to cell wall stress. In silico analysis of the promoter region of the two genes revealed the presence of putative DNA binding sites for transcription factors involved in stress responses, including sites identical to the Saccharomyces cerevisiae Rlm1p and Msn2p/Msn4p transcription factors. Promoter analysis indicated that the induction of the agsA gene in response to cell wall stress is fully dependent on a putative Rlm1p binding site in its promoter region. Database searches revealed the presence of S. cerevisiae Rlm1p homologues in most filamentous fungi examined, including A. niger. Deletion of the RLM1 homologue, named rlmA in A. niger, completely eliminated the induction of agsA and resulted in a twofold reduced induction of gfaA during Calcofluor White-induced cell wall stress. The rise in cell wall chitin in the presence of Calcofluor White was also affected in the rlmA deletion strain. In addition, the deletion strain was more sensitive towards cell wall stress agents. Our results indicate that A. niger responds to cell wall stress by transcriptional activation of cell wall reinforcing genes including agsA and gfaA through an Rlm1p-like transcription factor. We propose that such a cell wall salvage mechanism is wide spread in filamentous fungi.

Cell wall integrity signaling in Saccharomyces cerevisiae

The yeast cell wall is a highly dynamic structure that is responsible for protecting the cell from rapid changes in external osmotic potential. The wall is also critical for cell expansion during growth and morphogenesis. This review discusses recent advances in understanding the various signal transduction pathways that allow cells to monitor the state of the cell wall and respond to environmental challenges to this structure. The cell wall integrity signaling pathway controlled by the small G-protein Rho1 is principally responsible for orchestrating changes to the cell wall periodically through the cell cycle and in response to various forms of cell wall stress. This signaling pathway acts through direct control of wall biosynthetic enzymes, transcriptional regulation of cell wall-related genes, and polarization of the actin cytoskeleton. However, additional signaling pathways interface both with the cell wall integrity signaling pathway and with the actin cytoskeleton to coordinate polarized secretion with cell wall expansion. These include Ca(2+) signaling, phosphatidylinositide signaling at the plasma membrane, sphingoid base signaling through the Pkh1 and -2 protein kinases, Tor kinase signaling, and pathways controlled by the Rho3, Rho4, and Cdc42 G-proteins.

Enzyme encapsulation in permeabilized Saccharomyces cerevisiae cells

The Saccharomyces cerevisiae cell wall provides a semipermeable barrier that can retain intracellular proteins but still permits small molecules to pass through. When S. cerevisiae cells expressing E. coli lacZ are treated with detergent to extract the cell membrane, beta-galactosidase activity in the permeabilized cells is approximately 40% of the activity of the protein in cell extract. However, the permeabilized cells can easily be collected and reused over 15 times without appreciable loss in activity. Cell wall composition and thickness can be modified using different cell strains for enzyme expression or by mutating genes involved in cell wall biosynthesis or degradation. The Sigma1278b strain cell wall is less permeable than the walls of BY4742 and W303 cells, and deleting EXG1, which encodes a 1,3-beta-glucanase, can further reduce permeability. A short Zymolyase treatment can increase cell wall permeability without rupturing the cells. Encapsulating multiple enzymes in permeabilized cells can offer kinetic advantages over the same enzymes in solution. Regeneration of ATP from AMP by adenylate kinase and pyruvate kinase encapsulated in the same cell proceeded more rapidly than regeneration using a cell extract. Combining permeabilized cells containing adenylate kinase with permeabilized cells containing pyruvate kinase can also regenerate ATP from AMP, but the kinetics of this reaction are slower than regeneration using cell extract or permeabilized cells expressing both enzymes.

Activity of hydrolytic enzymes in fungi isolated from diabetic pregnant women: is there any relationship between fungal alkaline and acid phosphatase activity and glycemic control?

Ability to respond to environmental changes and secretion of hydrolases are considered to be important for Candida virulence. In this study we determined and compared the activities of 19 different hydrolases of the fungal strains isolated from diabetic and non-diabetic pregnant women. We also looked for the presence of a relationship between hydrolase activities and glycemic control, and, furthermore, evaluated the influence of gestational age on the activity of hydrolases. Mycological examinations were performed for 119 diabetic pregnant women: 47 with diabetes mellitus type I (DM), 72 with gestational diabetes (GDM), and for 132 healthy women (CON). Samples were collected from the vagina, rectum and oral cavity and cultured on Sabouraud media. The fungal hydrolase activities were evaluated using the API ZYM test (bioMerieux). For the 19 different fungal hydrolases tested, 13 activities were present in the isolated fungal strains. The activity of alkaline phosphatase (ALP) in vaginal strains (p=0.028) and acid phosphatase (ACP) in strains from the vagina (p=0.006) and rectum (p=0.049) was significantly lower in DM than in GDM and CON women. In conclusion, we describe for the first time that fungi isolated from pregnant diabetic women have lower activity of both phosphatases compared to fungi isolated from healthy women. Furthermore, similar differences of mean ALP and ACP activities were observed in the course of pregnancy in strains from the vagina and rectum of DM and CON women. However, strains from DM had lower activity at each stage of pregnancy. The highest activity of ALP and ACP was detected at the beginning, then declined, and had the lowest values between the 24(th) and 33(rd) week of gestation. After that period the activity of both phosphatases increased.

Fungal cell wall septation and cytokinesis are inhibited by bleomycins

When the essential and distinctive cell walls of either pathogenic or nonpathogenic fungi break, cytoplasmic membranes rupture and fungi die. This fungicidal activity was discovered previously on nonproliferating Saccharomyces cerevisiae cells treated briefly with the oxidative tool and anticancer drug family of bleomycins. The present studies investigated effects of bleomycin on growing fungal organisms. These included the medically important Aspergillus fumigatus and Cryptococcus neoformans, as well as the emerging human pathogen and fungal model, S. cerevisiae. Bleomycin had its highest potency against A. fumigatus. Scanning electron microscopy and thin-section transmission electron microscopy were used to study morphological growth characteristics. Killing and growth inhibition were also measured. Long, thin, and segmented hyphae were observed when A. fumigatus was grown without bleomycin but were never observed when the mold was grown with the drug. Bleomycin arrested conidial germination, hyphal development, and the progression and completion of cell wall septation. Similarly, the drug inhibited the construction of yeast cell wall septa, preventing cytokinesis and progression in the cell division cycle of S. cerevisiae. Even when cytoplasms of mother and daughter cells separated, septation and cell division did not necessarily occur. Bizarre cell configurations, abnormally thickened cell walls at mother-daughter necks, abnormal polarized growth, large undivided cells, fragmented cells, and empty cell ghosts were also produced. This is the first report of a fungicidal agent that arrests fungal growth and development, septum formation, and cytokinesis and that also preferentially localizes to cell walls and alters isolated cell walls as well as intact cell walls on nongrowing cells.

Structure and biosynthesis of fungal cell walls: Methodological approaches

Fungal cell walls possess a characteristic chemical composition differentiating fungal cells from other cell types. For this reason, the mechanisms involved in cell-wall formation represent a potential target for selective antifungal drugs. Understanding the structure and biosynthesis of fungal cell walls opens the ways for design of effective drugs for treating fungal diseases. This article reviews the history methods employed in chemical and structural analysis of fungal cell walls and in studies concerning their formation.

Sir3p phosphorylation by the Slt2p pathway effects redistribution of silencing function and shortened lifespan

An organism's lifespan is modulated by environmental conditions. When nutrients are abundant, the metabolism of many organisms shifts to growth or reproduction at the expense of longer lifespan, whereas a scarcity of nutrients reverses this shift. These correlations suggest that organisms respond to environmental changes by altering their metabolism to promote either reproduction and growth or long life. The only previously reported signaling mechanism involved in this response is the nutrient-responsive insulin/insulin-like growth factor-1 receptor pathway. Here we report another pathway that controls the length of yeast lifespan. Commitment to cell growth activates the Slt2p MAP kinase pathway, which phosphorylates the transcriptional silencing protein Sir3p, resulting in a shorter lifespan. Elimination of the Sir3p phosphorylation site at Ser275 extended lifespan by 38%. Lifespan extension occurs by a mechanism that is independent of suppressing rDNA recombination. Thus, Slt2p is an enzymatic regulator of silencing function that couples commitment to cell growth and shorter lifespan.