Entry into the stationary phase is associated with a rapid loss of viability and an apoptotic-like phenotype in the opportunistic pathogen Aspergillus fumigatus

Antifungal Activity of Cedrol from Cunninghamia lanceolate var. konishii against Phellinus noxius and Its Mechanism

Phellinus noxius is a highly destructive fungus that causes brown root disease in trees, leading to decay and death. In Taiwan, five prized woods-Taiwania cryptomerioides, Calocedrus macrolepis var. formosana, Cunninghamia lanceolata var. konishii, Chamaecyparis formosensis, and Chamaecyparis obtusa var. formosana-are known for their fragrance and durability. This study aims to explore the anti-brown-root-rot-fungus activity of Cunninghamia lanceolata var. konishii (CL) essential oil (CLOL) and its primary components, while also delving into their mechanisms of action and inhibition pathways. The essential oil (CLOL) from CL wood demonstrated significant efficacy against P. noxius, with an inhibitory concentration (IC50) of 37.5 µg/mL. Cedrol, the major component (78.48%) in CLOL, emerged as a potent antifungal agent, surpassing the reference drug triflumizole. Further assays with cedrol revealed a stronger anti-brown-root-disease activity (IC50 = 15.7 µg/mL) than triflumizole (IC50 = 32.1 µg/mL). Scanning electron microscopy showed deformation and rupture of fungal hyphae treated with CLOL and cedrol, indicating damage to the fungal cell membrane. Cedrol-induced oxidative stress in P. noxius was evidenced by increased reactive oxygen species (ROS) levels, leading to DNA fragmentation, mitochondrial membrane potential reduction, and fungal apoptosis through the mitochondrial pathway. Gel electrophoresis confirmed cedrol-induced DNA fragmentation, whereas TUNEL staining demonstrated increased apoptosis with rising cedrol concentrations. Moreover, protein expression analysis revealed cedrol-triggered release of cytochrome c, activation of caspase-9, and subsequent caspase-3 activation, initiating a caspase cascade reaction. This groundbreaking study establishes cedrol as the first compound to induce apoptosis in P. noxius while inhibiting its growth through oxidative stress, an increase in mitochondrial membrane permeability, and activation of the mitochondrial pathway. The findings offer compelling evidence for cedrol's potential as an effective antifungal agent against the destructive brown root disease caused by P. noxius.

The conservation of IAP-like proteins in fungi, and their potential role in fungal programmed cell death

Programmed cell death (PCD) is a tightly regulated process which is required for survival and proper development of all cellular life. Despite this ubiquity, the precise molecular underpinnings of PCD have been primarily characterized in animals. Attempts to expand our understanding of this process in fungi have proven difficult as core regulators of animal PCD are apparently absent in fungal genomes, with the notable exception of a class of proteins referred to as inhibitors of apoptosis proteins (IAPs). These proteins are characterized by the conservation of a distinct Baculovirus IAP Repeat (BIR) domain and animal IAPs are known to regulate a number of processes, including cellular death, development, organogenesis, immune system maturation, host-pathogen interactions and more. IAP homologs are broadly conserved throughout the fungal kingdom, but our understanding of both their mechanism and role in fungal development/virulence is still unclear. In this review, we provide a broad and comparative overview of IAP function across taxa, with a particular focus on fungal processes regulated by IAPs. Furthermore, their putative modes of action in the absence of canonical interactors will be discussed.

S-ethyl ethanethiosulfinate, a derivative of allicin, induces metacaspase-dependent apoptosis through ROS generation in Penicillium chrysogenum

Allicin can be used as fumigant to protect food and cultural relics from fungal contamination because of its strong antifungal activity and the characteristics of high volatility and no residues. However, the obvious disadvantages such as high minimal inhibitory concentration and instability prevent it from wide application. In this study, a stable derivative of allicin, S-ethyl ethanethiosulfinate (ALE), was synthesized. We further explored its antifungal activity and apoptosis-inducing effect, as well as the underlying mechanism. ALE had an excellent capability of inhibiting spore germination and mycelial growth of Penicillium chrysogenum observed by inverted microscope and scanning electron microscopy. XTT colorimetric assay indicated ALE could reduce the cell viability obviously and IC₅₀ was 0.92 μg/ml, only 1/42 of allicin (38.68 μg/ml). DHR 123 ROS Assay Kit, flow cytometry assay and confocal immunofluorescence revealed intercellular ROS generation and metacaspase-dependent apoptosis triggered by ALE, while antioxidant tocopherol could reverse ALE-induced cytotoxicity effect and metacaspase activation. These results indicate that ALE induces metacaspase-dependent apoptosis through ROS generation, thus possesses an effective antifungal activity. This new derivative of allicin might be developed as a high efficient alternative to the conventional fungicides for food storage and cultural relic protection.

N,N'-diaryl-bishydrazones in a biphenyl platform: Broad spectrum antifungal agents

N,N'-Diaryl-bishydrazones of [1,1'-biphenyl]-3,4'-dicarboxaldehyde, [1,1'-biphenyl]-4,4'-dicarboxaldehyde, and 4,4'-bisacetyl-1,1-biphenyl exhibited excellent antifungal activity against a broad spectrum of filamentous and non-filamentous fungi. These N,N'-diaryl-bishydrazones displayed no antibacterial activity in contrast to previously reported N,N'-diamidino-bishydrazones and N-amidino-N'-aryl-bishydrazones. The leading candidate, 4,4'-bis((E)-1-(2-(4-fluorophenyl)hydrazono)ethyl)-1,1'-biphenyl, displayed less hemolysis of murine red blood cells at concentrations at or below that of a control antifungal agent (voriconazole), was fungistatic in a time-kill study, and possessed no mammalian cytotoxicity and no toxicity with respect to hERG inhibition.

Response to Comment on "Sterilizing immunity in the lung relies on targeting fungal apoptosis-like programmed cell death"

Aouacheria et al question the interpretation of contemporary assays to monitor programmed cell death with apoptosis-like features (A-PCD) in Aspergillus fumigatus Although our study focuses on fungal A-PCD for host immune surveillance and infectious outcomes, the experimental approach incorporates multiple independent A-PCD markers and genetic manipulations based on fungal rather than mammalian orthologs to circumvent the limitations associated with any single approach.

Guidelines and recommendations on yeast cell death nomenclature

Elucidating the biology of yeast in its full complexity has major implications for science, medicine and industry. One of the most critical processes determining yeast life and physiology is cel-lular demise. However, the investigation of yeast cell death is a relatively young field, and a widely accepted set of concepts and terms is still missing. Here, we propose unified criteria for the defi-nition of accidental, regulated, and programmed forms of cell death in yeast based on a series of morphological and biochemical criteria. Specifically, we provide consensus guidelines on the differ-ential definition of terms including apoptosis, regulated necrosis, and autophagic cell death, as we refer to additional cell death rou-tines that are relevant for the biology of (at least some species of) yeast. As this area of investigation advances rapidly, changes and extensions to this set of recommendations will be implemented in the years to come. Nonetheless, we strongly encourage the au-thors, reviewers and editors of scientific articles to adopt these collective standards in order to establish an accurate framework for yeast cell death research and, ultimately, to accelerate the pro-gress of this vibrant field of research.

Regulated Forms of Cell Death in Fungi

Cell death occurs in all domains of life. While some cells die in an uncontrolled way due to exposure to external cues, other cells die in a regulated manner as part of a genetically encoded developmental program. Like other eukaryotic species, fungi undergo programmed cell death (PCD) in response to various triggers. For example, exposure to external stress conditions can activate PCD pathways in fungi. Calcium redistribution between the extracellular space, the cytoplasm and intracellular storage organelles appears to be pivotal for this kind of cell death. PCD is also part of the fungal life cycle, in which it occurs during sexual and asexual reproduction, aging, and as part of development associated with infection in phytopathogenic fungi. Additionally, a fungal non-self-recognition mechanism termed heterokaryon incompatibility (HI) also involves PCD. Some of the molecular players mediating PCD during HI show remarkable similarities to major constituents involved in innate immunity in metazoans and plants. In this review we discuss recent research on fungal PCD mechanisms in comparison to more characterized mechanisms in metazoans. We highlight the role of PCD in fungi in response to exogenic compounds, fungal development and non-self-recognition processes and discuss identified intracellular signaling pathways and molecules that regulate fungal PCD.

Hydrogen peroxide-induced secondary necrosis in conidia of Aspergillus fumigatus

Aspergillus fumigatus conidia have been linked to severe aspergillosis in immunocompromised patients. Recently, the cytotoxic effect of secondary metabolites from A. fumigatus conidia was reported. In the present work, a methodology used to detect cell death markers in fungal hyphae was adapted to study conidia cell death. Additionally, the mechanism of H2O2-induced cell death was studied in A. fumigatus conidia for the first time. Data presented in this work reveal that the H2O2-induced conidial cell death was associated with a marked increase of TUNEL- and PI-positive cells. It is therefore suggested that conidia cell death occurs in a dose-dependent manner through a secondary necrosis mechanism. The knowledge of conidia cell death machinery may provide insights into the molecular mechanism of conidia-mediated toxicity to the respiratory tract and may pave the way for improved therapeutic approaches against A. fumigatus conidia-mediated diseases.

Chronological aging in conidia of pathogenic Aspergillus: Comparison between species

Aspergillus fumigatus, Aspergillus flavus, Aspergillus terreus and Aspergillus niger are common airborne fungi, and the most frequent causative agents of human fungal infections. However, the resistance and lifetime persistence of these fungi in the atmosphere, and the mechanism of aging of Aspergillus conidia are unknown.With this work, we intended to study the processes underlying conidial aging of these four relevant and pathogenic Aspergillus species. Chronological aging was therefore evaluated in A. fumigatus, A. flavus, A. terreus and A. niger conidia exposed to environmental and human body temperatures. The results showed that the aging process in Aspergillus conidia involves apoptosis,with metacaspase activation, DNA fragmentation, and reactive oxygen species production, associated with secondary necrosis. Distinct results were observed for the selected pathogenic species. At environmental conditions, A. niger was the species with the highest resistance to aging, indicating a higher adaption to environmental conditions, whereas A. flavus followed by A. terreus were the most sensitive species. At higher temperatures (37 °C), A. fumigatus presented the longest lifespan, in accordance with its good adaptation to the human body temperature. Altogether,with this work new insights regarding conidia aging are provided, which may be useful when designing treatments for aspergillosis.

PsANT, the adenine nucleotide translocase of Puccinia striiformis, promotes cell death and fungal growth

Adenine nucleotide translocase (ANT) is a constitutive mitochondrial component that is involved in ADP/ATP exchange and mitochondrion-mediated apoptosis in yeast and mammals. However, little is known about the function of ANT in pathogenic fungi. In this study, we identified an ANT gene of Puccinia striiformis f. sp. tritici (Pst), designated PsANT. The PsANT protein contains three typical conserved mitochondrion-carrier-protein (mito-carr) domains and shares more than 70% identity with its orthologs from other fungi, suggesting that ANT is conserved in fungi. Immuno-cytochemical localization confirmed the mitochondrial localization of PsANT in normal Pst hyphal cells or collapsed cells. Over-expression of PsANT indicated that PsANT promotes cell death in tobacco, wheat and fission yeast cells. Further study showed that the three mito-carr domains are all needed to induce cell death. qRT-PCR analyses revealed an in-planta induced expression of PsANT during infection. Knockdown of PsANT using a host-induced gene silencing system (HIGS) attenuated the growth and development of virulent Pst at the early infection stage but not enough to alter its pathogenicity. These results provide new insight into the function of PsANT in fungal cell death and growth and might be useful in the search for and design of novel disease control strategies.

Measurement of apoptosis by SCAN©, a system for counting and analysis of fluorescently labelled nuclei

Apoptosis-like programmed cell death (A-PCD) is a universal process common to all types of eukaryotic organisms. Because A-PCD-associated processes are conserved, it is possible to define A-PCD by a standard set of markers. Many of the popular methods to measure A-PCD make use of fluorescent ligands that change in intensity or cellular localization during A-PCD. In single cell organisms, it is possible to quantify levels of A-PCD by scoring the number of apoptotic cells using flow cytometry instruments. In a multicellular organism, quantification of A-PCD is more problematic due to the complex nature of the tissue. The situation is further complicated in filamentous fungi, in which nuclei are divided between compartments, each containing a number of nuclei, which can also migrate between the compartments. We developed SCAN©, a System for Counting and Analysis of Nuclei, and used it to measure A-PCD according to two markers - chromatin condensation and DNA strand breaks. The package includes three modules designed for counting the number of nuclei in multi-nucleated domains, scoring the relative number of nuclei with condensed chromatin, and calculating the relative number of nuclei with DNA strand breaks. The method provides equal or better results compared with manual counting, the analysis is fast and can be applied on large data sets. While we demonstrated the utility of the software for measurement of A-PCD in fungi, the method is readily adopted for measurement of A-PCD in other types of multicellular specimens.

Submicronic fungal bioaerosols: high-resolution microscopic characterization and quantification

Submicronic particles released from fungal cultures have been suggested to be additional sources of personal exposure in mold-contaminated buildings. In vitro generation of these particles has been studied with particle counters, eventually supplemented by autofluorescence, that recognize fragments by size and discriminate biotic from abiotic particles. However, the fungal origin of submicronic particles remains unclear. In this study, submicronic fungal particles derived from Aspergillus fumigatus, A. versicolor, and Penicillium chrysogenum cultures grown on agar and gypsum board were aerosolized and enumerated using field emission scanning electron microscopy (FESEM). A novel bioaerosol generator and a fungal spores source strength tester were compared at 12 and 20 liters min(-1) airflow. The overall median numbers of aerosolized submicronic particles were 2 × 10(5) cm(-2), 2.6 × 10(3) cm(-2), and 0.9 × 10(3) cm(-2) for A. fumigatus, A. versicolor, and P. chrysogenum, respectively. A. fumigatus released significantly (P < 0.001) more particles than A. versicolor and P. chrysogenum. The ratios of submicronic fragments to larger particles, regardless of media type, were 1:3, 5:1, and 1:2 for A. fumigatus, A. versicolor, and P. chrysogenum, respectively. Spore fragments identified by the presence of rodlets amounted to 13%, 2%, and 0% of the submicronic particles released from A. fumigatus, A. versicolor, and P. chrysogenum, respectively. Submicronic particles with and without rodlets were also aerosolized from cultures grown on cellophane-covered media, indirectly confirming their fungal origin. Both hyphae and conidia could fragment into submicronic particles and aerosolize in vitro. These findings further highlight the potential contribution of fungal fragments to personal fungal exposure.

Induction of Rhizopus oryzae germination under starvation using host metabolites increases spore susceptibility to heat stress

Sweetpotato is a nutritional source worldwide. Soft rot caused by Rhizopus spp. is a major limiting factor in the storage of produce, rendering it potentially unsafe for human consumption. In this study, Rhizopus oryzae was used to develop a concept of postharvest disease control by weakening the pathogen through induction of spore germination under starvation conditions. We isolated the sweetpotato active fractions (SPAFs) that induce spore germination and used them at a low dose to enhance spore weakening caused by starvation. Germination in SPAF at 1 mg/ml weakened the pathogen spores by delaying their ability to form colonies on rich media and by increasing their sensitivity to heat stress. The weakening effect was also supported by reduced metabolic activity, as detected by Alarmar Blue fluorescent dye assays. Spores incubated with SPAF at 1 mg/ml showed DNA fragmentation in some of their nuclei, as observed by TUNEL assay. In addition, these spores exhibited changes in ultrastructural morphology (i.e., shrinkage of germ tubes, nucleus deformation, and vacuole formation) which are hallmarks of programmed cell death. We suggest that induction of spore germination under starvation conditions increases their susceptibility to stress and, therefore, might be considered a new strategy for pathogen control.

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.

Multiplexed activity-based protein profiling of the human pathogen Aspergillus fumigatus reveals large functional changes upon exposure to human serum

Environmental adaptability is critical for survival of the fungal human pathogen Aspergillus fumigatus in the immunocompromised host lung. We hypothesized that exposure of the fungal pathogen to human serum would lead to significant alterations to the organism's physiology, including metabolic activity and stress response. Shifts in functional pathway and corresponding enzyme reactivity of A. fumigatus upon exposure to the human host may represent much needed prognostic indicators of fungal infection. To address this, we employed a multiplexed activity-based protein profiling (ABPP) approach coupled to quantitative mass spectrometry-based proteomics to measure broad enzyme reactivity of the fungus cultured with and without human serum. ABPP showed a shift from aerobic respiration to ethanol fermentation and utilization over time in the presence of human serum, which was not observed in serum-free culture. Our approach provides direct insight into this pathogen's ability to survive, adapt, and proliferate. Additionally, our multiplexed ABPP approach captured a broad swath of enzyme reactivity and functional pathways and provides a method for rapid assessment of the A. fumigatus response to external stimuli.

Monomeric L-amino acid oxidase-induced mitochondrial dysfunction in Rhizoctonia solani Reveals a novel antagonistic mechanism of Trichoderma harzianum ETS 323

The monomeric L-amino acid oxidase (mTh-LAAO) of Trichoderma harzianum ETS 323 has been suggested to antagonize Rhizoctonia solani by an unknown mechanism. Here, the mTh-LAAO-treated R. solani exhibited hyphal lysis and apoptotic characteristics such as DNA fragmentation, reactive oxygen species (ROS) accumulation, lipid peroxidation, and mitochondrial membrane potential depolarization. This hyphal lysis was suppressed by the mitochondria-dependent apoptosis inhibitor oligomycin while accompanied by reduction of ROS accumulation. This result suggested that mitochondria-mediated apoptosis in R. solani was involved in mTh-LAAO-induced growth inhibition, which was supported by the evidence of cytocheome c release and activation of caspases 9 and 3. Furthermore, the data indicated that the mTh-LAAO-induced fungal cell death was also closely interrelated with the interaction of mTh-LAAO with R. solani hyphal cell wall proteins. These results illuminate the biological function and mechanism underlying the antagonistic action of T. harzianum mTh-LAAO against fungal pathogens.

Lipopeptide induces apoptosis in fungal cells by a mitochondria-dependent pathway

Bacillus amyloliquefaciens WH1 inhibit the growth of fungi by producing a new surfactin called as WH1fungin. WH1fungin plays an anti-fungal role by two models: high concentration to elicit pores on cell membrane and low concentration to induce apoptosis. WH1fungin can also inhibits the glucan synthase resulting in a decreased synthesis of callose on fungal cell wall. Further detection revealed that classical apoptotic markers such as reactive oxygen species (ROS) accumulation, phosphatidylserine (PS) externalization, DNA strand breaks and caspase-like activities could be found in fungal cells after treated by WH1fungin. Oligomycin was used as an inhibitor to block the mitochondria-dependent apoptosis in fungal cells, and results showed it could not inhibit but enhance the apoptosis induced by WH1fungin. After isolation by affinity chromatography, WH1fungin was found to bind with ATPase on the mitochondrial membrane and result in a decreased ATPase activity in fungal cells. This was further verified by treating fungal cells with FITC-labeled WH1fungin, which could bind to the mitochondrial membrane showing green fluorescence in fungal cells. After that, cytochrome C was released from the mitochondria, which then acted with caspase 9 to induce apoptosis by an intracellular pathway. High caspase 8 activity was also detectable in apoptotic fungal cells, indicating that an extracellular pathway might also be responsible for apoptosis induced by WH1fungin. Taken together, we report that lipopeptide can induce apoptosis in fungal cells, and induction of apoptosis by lipopeptide might be a common anti-fungal mechanism of Bacillus in the natural habitat.

Dynamics of extracellular release of Aspergillus fumigatus DNAand galactomannan during growth in blood and serum

Aspergillus fumigatus is the major cause of invasive aspergillosis (IA),a disease associated with high rates of morbidity and mortality in patientsundergoing treatment for haematological malignancies. This study investigated A. fumigatus growth in vitro and in a murine model of IA inorder to provide insights into the dynamics of extracellular DNA and galactomannan (GM)release and their relevance to early diagnosis of IA. Following inoculationof whole blood with 20 A. fumigatus conidia ml−1,DNA that corresponded to the inoculum could be detected by PCR but GM wasnot detected in plasma separated from the blood sample, indicating that thefungus did not grow in whole blood. The quantities of DNA detected by PCR,and GM, were proportional to the amount of fungal biomass present in vitro. Fungal DNA could be detected in the sera of mice experimentally infectedwith A. fumigatus with maximum detection in cyclophosphamide-treatedmice.

MeaB-dependent nutrition sensing regulates autolysis in carbon starved Aspergillus nidulans cultures

Carbon starvation induced autolysis is an active process of self-digestion and is under complex regulation in Aspergillus nidulans. In this study we investigated how autolysis depends on the composition of the culture medium, especially on the presence of yeast extract. We demonstrated that the rate of autolytic cell wall degradation as well as the extracellular chitinase and proteinase productions significantly decreased in the presence of this nutrient. The effect of yeast extract on carbon starved cultures was independent of loss-of-function mutations in the carbon and nitrogen regulatory genes creA and areA and in the heterotrimeric G protein signalling genes fadA and ganB. In contrast, the nitrogen regulating transcription factor MeaB was involved in the yeast-extract-mediated repression of autolysis. Reverse transcriptase - polymerase chain reaction (RT-PCR) experiments demonstrated that MeaB affects the FluG-BrlA sporulation regulatory pathway by affecting transcription of brlA, a gene also initiating the autolytic cell wall degradation in this fungus.

Fungal apoptosis: function, genes and gene function

Cells of all living organisms are programmed to self-destruct under certain conditions. The most well known form of programmed cell death is apoptosis, which is essential for proper development in higher eukaryotes. In fungi, apoptotic-like cell death occurs naturally during aging and reproduction, and can be induced by environmental stresses and exposure to toxic metabolites. The core apoptotic machinery in fungi is similar to that in mammals, but the apoptotic network is less complex and of more ancient origin. Only some of the mammalian apoptosis-regulating proteins have fungal homologs, and the number of protein families is drastically reduced. Expression in fungi of animal proteins that do not have fungal homologs often affects apoptosis, suggesting functional conservation of these components despite the absence of protein-sequence similarity. Functional analysis of Saccharomyces cerevisiae apoptotic genes, and more recently of those in some filamentous species, has revealed partial conservation, along with substantial differences in function and mode of action between fungal and human proteins. It has been suggested that apoptotic proteins might be suitable targets for novel antifungal treatments. However, implementation of this approach requires a better understanding of fungal apoptotic networks and identification of the key proteins regulating apoptotic-like cell death in fungi.

Structural and nanomechanical properties of Termitomyces clypeatus cell wall and its interaction with chromium(VI)

Alterations of cell surface properties accompanying the complex life cycle of Termitomyces clypeatus have been monitored using atomic force microscopy (AFM). A new hyphae/mycelium is developed on cell division, and the cell wall of the mycelium undergoes a process of internal reorganization (or maturation) followed by morphological and chemical alterations. The changes of the surface ultrastructures during the growth process are correlated to the corresponding changes in relative viscoelasticity and rigidity of the cell wall by employing force spectroscopy. The cell wall rigidity and elasticity are found to be 0.34+/-0.02 N/m and 27.5+/-2.1 MPa, respectively, at the early logarithmic phase, on maturation increase to reach 0.81+/-0.08 N/m and 92.5+/-12 MPa, respectively, at the stationary phase, and thereafter decrease to 0.62+/-0.06 N/m and 61.6+/-6.6 MPa at the death phase. The alterations of the ultrastructural and nanomechanical properties of the cell surface as functions of growth phases affect the interaction involving chromium and T. clypeatus.

Apoptosis pathways in fungal growth, development and ageing

Apoptosis is one type of programmed cell death with great importance for development and homeostasis of multicellular organisms. Unexpectedly, during the past decade, evidence has been obtained for the existence of a basal apoptosis machinery in yeast, as unicellular fungus, and in some filamentous fungi, a group of microorganisms that are neither true unicellular nor true multicellular biological systems but something in between. Here, we review evidence for a role of apoptotic processes in fungal pathogenicity, competitiveness, propagation, ageing and lifespan control.

Regulation of autolysis in Aspergillus nidulans

In terms of cell physiology, autolysis is the centerpiece of carbon-starving fungal cultures. In the filamentous fungus model organism Aspergillus nidulans, the last step of carbon-starvation-triggered autolysis was the degradation of the cell wall of empty hyphae, and this process was independent of concomitantly progressing cell death at the level of regulation. Autolysis-related proteinase and chitinase activities were induced via FluG signaling, which initiates sporulation and inhibits vegetative growth in surface cultures of A. nidulans. Extracellular hydrolase production was also subjected to carbon repression, which was only partly dependent on CreA, the main carbon catabolite repressor in this fungus. These data support the view that one of the main functions of autolysis is supplying nutrients for sporulation, when no other sources of nutrients are available. The divergent regulation of cell death and cell wall degradation provides the fungus with the option to keep dead hyphae intact to help surviving cells to absorb biomaterials from dead neighboring cells before these are released into the extracellular space. The industrial significance of these observations is also discussed in this paper.

Deletion of putative apoptosis factors leads to lifespan extension in the fungal ageing model Podospora anserina

Podospora anserina is a filamentous fungus with a limited lifespan. After a strain-specific period of growth, cultures turn to senescence and ultimately die. Here we provide evidence that the last step in the ageing of P. anserina is not accidental but programmed. In this study, PaAMID1, a homologue of a mammalian 'AIF-homologous mitochondrion-associated inducer of death', was analysed as a putative member of a caspase-independent signalling pathway. In addition, two metacaspases, PaMCA1 and PaMCA2, were investigated. While deletion of PaAmid1 as well as of PaMca2 was found to result in a moderate lifespan extension (59% and 78%, respectively), a 148% increase in lifespan was observed after deletion of PaMca1. Measurement of arginine-specific protease activity demonstrates a metacaspase-dependent activity in senescent but not in juvenile cultures, pointing to an activation of these proteases in the senescent stage of the life cycle. Moreover, treatment of juvenile wild-type cultures with hydrogen peroxide leads to a PaMCA1-dependent activity. The presented data strongly suggest that death of senescent wild-type cultures is triggered by an apoptotic programme induced by an age-dependent increase of reactive oxygen species during ageing of cultures and is executed after metacaspase activation.

Bcl-2 proteins link programmed cell death with growth and morphogenetic adaptations in the fungal plant pathogen Colletotrichum gloeosporioides

Proteins belonging to the Bcl-2 family regulate apoptosis in mammals by controlling mitochondria efflux of cytochrome c and other apoptosis-related proteins. Homologues of human Bcl-2 proteins are found in different metazoan organisms where they play a similar role, while they seem to be absent in plants and fungi. Nonetheless, Bcl-2 protein members can induce or prevent yeast cell death, suggesting that enough functional conservation exists between apoptotic machineries of mammals and fungi. Here we show that induction or prevention of apoptosis by Bcl-2 proteins in the fungal plant pathogen Colletotrichum gloeosporioides is tightly linked with growth and morphogenetic adaptation that occur throughout the entire fungal life cycle. Isolates expressing the pro-apoptotic Bax protein underwent cell death with apoptotic characteristics, and showed alterations in conidial germination that are associated with pathogenic and non-pathogenic life styles. Isolates expressing the anti-apoptotic Bcl-2 protein had prolonged longevity, were protected from Bax-induced cell death, and exhibited enhanced stress resistance. These isolates also had enhanced mycelium and conidia production, and were hyper virulent to host plants. Our findings show that apoptotic-associated machinery regulates morphogenetic switches, which are critical for proper responses and adaptation fungi to different environments.

TheAspergillus fumigatusmetacaspases CasA and CasB facilitate growth under conditions of endoplasmic reticulum stress

We have examined the contribution of metacaspases to the growth and stress response of the opportunistic human mould pathogen, Aspergillus fumigatus, based on increasing evidence implicating the yeast metacaspase Yca1p in apoptotic-like programmed cell death. Single metacaspase-deficient mutants were constructed by targeted disruption of each of the two metacaspase genes in A. fumigatus, casA and casB, and a metacaspase-deficient mutant, DeltacasA/DeltacasB, was constructed by disrupting both genes. Stationary phase cultures of wild-type A. fumigatus were associated with the appearance of typical markers of apoptosis, including elevated proteolytic activity against caspase substrates, phosphatidylserine exposure on the outer leaflet of the membrane, and loss of viability. By contrast, phosphatidylserine exposure was not observed in stationary phase cultures of the DeltacasA/DeltacasB mutant, although caspase activity and viability was indistinguishable from wild type. The mutant retained wild-type virulence and showed no difference in sensitivity to a range of pro-apoptotic stimuli that have been reported to initiate yeast apoptosis. However, the DeltacasA/DeltacasB mutant showed a growth detriment in the presence of agents that disrupt endoplasmic reticulum homeostasis. These findings demonstrate that metacaspase activity in A. fumigatus contributes to the apoptotic-like loss of membrane phospholipid asymmetry at stationary phase, and suggest that CasA and CasB have functions that support growth under conditions of endoplasmic reticulum stress.

Non-self recognition and programmed cell death in filamentous fungi

Non-self recognition resulting in programmed cell death is a ubiquitous phenomenon in filamentous ascomycete fungi and is termed heterokaryon incompatibility (HI). Recent analyses show that genes containing predicted HET domains are often involved in HI; however, the function of the HET domain is unknown. Autophagy is induced as a consequence of HI, whereas the presence of a predicted transcription factor, VIB-1, is required for HI. Morphological features associated with apoptosis in filamentous fungi are induced by various stresses and drugs, and also during HI. Future analyses will reveal whether common or different genetic mechanisms trigger death by non-self recognition and death by various environmental onslaughts.

VIB-1 is required for expression of genes necessary for programmed cell death in Neurospora crassa

Nonself recognition during somatic growth is an essential and ubiquitous phenomenon in both prokaryotic and eukaryotic species. In filamentous fungi, nonself recognition is also important during vegetative growth. Hyphal fusion between genetically dissimilar individuals results in rejection of heterokaryon formation and in programmed cell death of the fusion compartment. In filamentous fungi, such as Neurospora crassa, nonself recognition and heterokaryon incompatibility (HI) are regulated by genetic differences at het loci. In N. crassa, mutations at the vib-1 locus suppress nonself recognition and HI mediated by genetic differences at het-c/pin-c, mat, and un-24/het-6. vib-1 is a homolog of Saccharomyces cerevisiae NDT80, which is a transcriptional activator of genes during meiosis. For this study, we determined that vib-1 encodes a nuclear protein and showed that VIB-1 localization varies during asexual reproduction and during HI. vib-1 is required for the expression of genes involved in nonself recognition and HI, including pin-c, tol, and het-6; all of these genes encode proteins containing a HET domain. vib-1 is also required for the production of downstream effectors associated with HI, including the production of extracellular proteases upon carbon and nitrogen starvation. Our data support a model in which mechanisms associated with starvation and nonself recognition/HI are interconnected. VIB-1 is a major regulator of responses to nitrogen and carbon starvation and is essential for the expression of genes involved in nonself recognition and death in N. crassa.

Comparative analysis of programmed cell death pathways in filamentous fungi

BACKGROUND

Fungi can undergo autophagic- or apoptotic-type programmed cell death (PCD) on exposure to antifungal agents, developmental signals, and stress factors. Filamentous fungi can also exhibit a form of cell death called heterokaryon incompatibility (HI) triggered by fusion between two genetically incompatible individuals. With the availability of recently sequenced genomes of Aspergillus fumigatus and several related species, we were able to define putative components of fungi-specific death pathways and the ancestral core apoptotic machinery shared by all fungi and metazoa.

RESULTS

Phylogenetic profiling of HI-associated proteins from four Aspergilli and seven other fungal species revealed lineage-specific protein families, orphan genes, and core genes conserved across all fungi and metazoa. The Aspergilli-specific domain architectures include NACHT family NTPases, which may function as key integrators of stress and nutrient availability signals. They are often found fused to putative effector domains such as Pfs, SesB/LipA, and a newly identified domain, HET-s/LopB. Many putative HI inducers and mediators are specific to filamentous fungi and not found in unicellular yeasts. In addition to their role in HI, several of them appear to be involved in regulation of cell cycle, development and sexual differentiation. Finally, the Aspergilli possess many putative downstream components of the mammalian apoptotic machinery including several proteins not found in the model yeast, Saccharomyces cerevisiae.

CONCLUSION

Our analysis identified more than 100 putative PCD associated genes in the Aspergilli, which may help expand the range of currently available treatments for aspergillosis and other invasive fungal diseases. The list includes species-specific protein families as well as conserved core components of the ancestral PCD machinery shared by fungi and metazoa.

The appearances of autolytic and apoptotic markers are concomitant but differently regulated in carbon-starvingAspergillus nidulanscultures

In ageing, carbon-depleted cultures of Aspergillus nidulans strain FGSC 26 progressing apoptotic-type cell death was detected, characterised by increasing numbers of Annexin V and TUNEL stained cells after protoplastation. DAPI staining of autolysing mycelia revealed numerous nuclei with elongated, stick-like morphology, which was not observed in surviving hyphal fragments representing a cell population adapted to carbon starvation. Apoptotic cell death was also progressing in aging cultures of the non-autolysing loss-of-function fluG and DeltabrlA mutants, indicating that apoptotic cell death and autolysis were regulated independently. In accordance with this, sphingosine derivatives added to A. nidulans cultures increased cell death rates without influencing autolytic biomass losses and hydrolase production.

Antifungal protein PAF severely affects the integrity of the plasma membrane of Aspergillus nidulans and induces an apoptosis-like phenotype

The small, basic, and cysteine-rich antifungal protein PAF is abundantly secreted into the supernatant by the beta-lactam producer Penicillium chrysogenum. PAF inhibits the growth of various important plant and zoopathogenic filamentous fungi. Previous studies revealed the active internalization of the antifungal protein and the induction of multifactorial detrimental effects, which finally resulted in morphological changes and growth inhibition in target fungi. In the present study, we offer detailed insights into the mechanism of action of PAF and give evidence for the induction of a programmed cell death-like phenotype. We proved the hyperpolarization of the plasma membrane in PAF-treated Aspergillus nidulans hyphae by using the aminonaphtylethenylpyridinium dye di-8-ANEPPS. The exposure of phosphatidylserine on the surface of A. nidulans protoplasts by Annexin V staining and the detection of DNA strand breaks by TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) gave evidence for a PAF-induced apoptotic-like mechanism in A. nidulans. The localization of reactive oxygen species (ROS) by dichlorodihydrofluorescein diacetate and the abnormal cellular ultrastructure analyzed by transmission electron microscopy suggested that ROS-elicited membrane damage and the disintegration of mitochondria played a major role in the cytotoxicity of PAF. Finally, the reduced PAF sensitivity of A. nidulans strain FGSC1053, which carries a dominant-interfering mutation in fadA, supported our assumption that G-protein signaling was involved in PAF-mediated toxicity.

Genes and molecules involved in Aspergillus fumigatus virulence

Aspergillus fumigatus causes a wide range of diseases that include mycotoxicosis, allergic reactions and systemic diseases (invasive aspergillosis) with high mortality rates. Pathogenicity depends on immune status of patients and fungal strain. There is no unique essential virulence factor for development of this fungus in the patient and its virulence appears to be under polygenetic control. The group of molecules and genes associated with the virulence of this fungus includes many cell wall components, such as beta-(1-3)-glucan, galactomannan, galactomannanproteins (Afmp1 and Afmp2), and the chitin synthetases (Chs; chsE and chsG), as well as others. Some genes and molecules have been implicated in evasion from the immune response, such as the rodlets layer (rodA/hyp1 gene) and the conidial melanin-DHN (pksP/alb1 gene). The detoxifying systems for Reactive Oxygen Species (ROS) by catalases (Cat1p and Cat2p) and superoxide dismutases (MnSOD and Cu, ZnSOD), had also been pointed out as essential for virulence. In addition, this fungus produces toxins (14 kDa diffusible substance from conidia, fumigaclavin C, aurasperon C, gliotoxin, helvolic acid, fumagilin, Asp-hemolysin, and ribotoxin Asp fI/mitogilin F/restrictocin), allergens (Asp f1 to Asp f23), and enzymatic proteins as alkaline serin proteases (Alp and Alp2), metalloproteases (Mep), aspartic proteases (Pep and Pep2), dipeptidyl-peptidases (DppIV and DppV), phospholipase C and phospholipase B (Plb1 and Plb2). These toxic substances and enzymes seems to be additive and/or synergistic, decreasing the survival rates of the infected animals due to their direct action on cells or supporting microbial invasion during infection. Adaptation ability to different trophic situations is an essential attribute of most pathogens. To maintain its virulence attributes A. fumigatus requires iron obtaining by hydroxamate type siderophores (ornitin monooxigenase/SidA), phosphorous obtaining (fos1, fos2, and fos3), signal transductional falls that regulate morphogenesis and/or usage of nutrients as nitrogen (rasA, rasB, rhbA), mitogen activated kinases (sakA codified MAP-kinase), AMPc-Pka signal transductional route, as well as others. In addition, they seem to be essential in this field the amino acid biosynthesis (cpcA and homoaconitase/lysF), the activation and expression of some genes at 37 degrees C (Hsp1/Asp f12, cgrA), some molecules and genes that maintain cellular viability (smcA, Prp8, anexins), etc. Conversely, knowledge about relationship between pathogen and immune response of the host has been improved, opening new research possibilities. The involvement of non-professional cells (endothelial, and tracheal and alveolar epithelial cells) and professional cells (natural killer or NK, and dendritic cells) in infection has been also observed. Pathogen Associated Molecular Patterns (PAMP) and Patterns Recognizing Receptors (PRR; as Toll like receptors TLR-2 and TLR-4) could influence inflammatory response and dominant cytokine profile, and consequently Th response to infec tion. Superficial components of fungus and host cell surface receptors driving these phenomena are still unknown, although some molecules already associated with its virulence could also be involved. Sequencing of A. fumigatus genome and study of gene expression during their infective process by using DNA microarray and biochips, promises to improve the knowledge of virulence of this fungus.

Apoptosis in yeast

Apoptosis is a highly regulated cellular suicide program crucial for metazoan development. However, dysfunction of apoptosis also leads to several diseases. Yeast undergoes apoptosis after application of acetic acid, sugar- or salt-stress, plant antifungal peptides, or hydrogen peroxide. Oxygen radicals seem to be key elements of apoptotic execution, conserved during evolution. Furthermore, several yeast orthologues of central metazoan apoptotic regulators have been identified, such as a caspase and a caspase-regulating serine protease. In addition, physiological occurrence of cell death has been detected during aging and mating in yeast. The finding of apoptosis in yeast, other fungi and parasites is not only of great medical relevance but will also help to understand some of the still unknown molecular mechanisms at the core of apoptotic execution.

Oxidative and amphotericin B-mediated cell death in the opportunistic pathogen Aspergillus fumigatus is associated with an apoptotic-like phenotype

When protoplasts of the opportunistic fungal pathogen Aspergillus fumigatus were treated with low but toxic levels of hydrogen peroxide (0.1 mM) or amphotericin B (0.5 microg ml(-1)), loss of cell viability and death were associated with a number of phenotypic changes characteristic of apoptosis. The percentage of protoplasts staining positive with annexin V-FITC, an indicator of the externalization of phosphatidylserine and an early marker of apoptosis, rose to approximately 55 % within 1 h. This was followed by a similar increase in apoptotic DNA fragmentation detected by the TUNEL assay, and led to a loss of cell permeability and death in approximately 90 % of protoplasts, as indicated by the uptake of propidium iodide. The development of an apoptotic phenotype was blocked when protoplasts were pre-treated with the protein synthesis inhibitor cycloheximide, indicating active participation of the cell in the process. However, no significant activity against synthetic caspase substrates was detected, and the inclusion of the cell-permeant broad-spectrum caspase inhibitor Z-VAD-fmk did not block the development of the apoptotic-like phenotype. Higher concentrations of H(2)O(2) (1.8 mM) and amphotericin B (1 microg ml(-1)) caused protoplasts to die without inducing an apoptotic phenotype. As predicted, the fungistatic antifungal agent itraconazole, which inhibits growth without causing immediate cell death, did not induce an apoptotic-like phenotype.

Aspergillus nidulans as a model system to characterize the DNA damage response in eukaryotes

Interest in DNA repair in Aspergillus nidulans had mainly grown out of studies of three different biological processes, namely mitotic recombination, inducible responses to detrimental environmental changes, and genetic control of the cell cycle. Ron Morris started the investigation of the genetic control of the cell cycle by screening hundreds of cell cycle temperature sensitive Aspergillus mutants. The sequencing and innovative analysis of these genes revealed not only several components of the cell cycle machinery that are directly involved in checkpoint response, but also components required for DNA replication and DNA damage response machinery. Here, we will provide an overview about currently known aspects of the DNA damage response in A. nidulans. Emphasis is put on analyzed mutants that are available and review epistatic relationships and other interactions among them. Furthermore, a comprehensive list of A. nidulans genes involved in different processes of the DNA damage response, as identified by homology of genome sequences with well-characterized human and yeast DNA repair genes, is shown.