Insights into the pathogenicity of Penicillium marneffei.

Penicillium marneffei is a significant pathogen of AIDS patients in Southeast Asia. This fungus is unique in that it is the only dimorphic member of the genus. Pathogenesis of P. marneffei requires the saprobic mold form to undergo a morphological change upon tissue invasion. The in vivo form of this fungus reproduces as a fission yeast that capably evades the host immune system. The processes that control these morphological changes, better termed as phase transition, can be replicated in vitro by incubation of the mold form at 37 degrees C. The unidentified molecular mechanisms regulating phase transition in this fungus are now being uncovered using modern methodologies and novel strategies. A better comprehension of these underlying regulatory pathways will provide insight into eukaryotic cellular development as well as the potential factors responsible for infections caused by P. marneffei and other fungi. Such knowledge may lead to better chemotherapeutic interventions of fungal diseases.     

The role of the cell wall in fungal pathogenesis

Fungal infections are a serious health problem. In recent years, basic research is focusing on the identification of fungal virulence factors as promising targets for the development of novel antifungals. The wall, as the most external cellular component, plays a crucial role in the interaction with host cells mediating processes such as adhesion or phagocytosis that are essential during infection. Specific components of the cell wall (called PAMPs) interact with specific receptors in the immune cell (called PRRs), triggering responses whose molecular mechanisms are being elucidated. We review here the main structural carbohydrate components of the fungal wall (glucan, mannan and chitin), how their biogenesis takes place in fungi and the specific receptors that they interact with. Different model fungal pathogens are chosen to illustrate the functional consequences of this interaction. Finally, the identification of the key components will have important consequences in the future and will allow better approaches to treat fungal infections.     

The different morphologies of yeast and filamentous fungi trigger distinct killing and feeding mechanisms in a fungivorous amoeba

Size and diverse morphologies pose a primary challenge for phagocytes such as innate immune cells and predatory amoebae when encountering fungal prey. Although filamentous fungi can escape phagocytic killing by pure physical constraints, unicellular spores and yeasts can mask molecular surface patterns or arrest phagocytic processing. Here, we show that the fungivorous amoeba Protostelium aurantium was able to adjust its killing and feeding mechanisms to these different cell shapes. Yeast-like fungi from the major fungal groups of basidiomycetes and ascomycetes were readily internalized by phagocytosis, except for the human pathogen Candida albicans whose mannoprotein coat was essential to escape recognition by the amoeba. Dormant spores of the filamentous fungus Aspergillus fumigatus also remained unrecognized, but swelling and the onset of germination induced internalization and intracellular killing by the amoeba. Mature hyphae of A. fumigatus were mostly attacked from the hyphal tip and killed by an actin-mediated invasion of fungal filaments. Our results demonstrate that predatory pressure imposed by amoebae in natural environments selects for distinct survival strategies in yeast and filamentous fungi but commonly targets the fungal cell wall as a crucial molecular pattern associated to prey and pathogens.     

Systems biology of host-fungus interactions: turning complexity into simplicity

Modeling interactions between fungi and their hosts at the systems level requires a molecular understanding both of how the host orchestrates immune surveillance and tolerance, and how this activation, in turn, affects fungal adaptation and survival. The transition from the commensal to pathogenic state, and the co-evolution of fungal strains within their hosts, necessitates the molecular dissection of fungal traits responsible for these interactions. There has been a dramatic increase in publically available genome-wide resources addressing fungal pathophysiology and host-fungal immunology. The integration of these existing data and emerging large-scale technologies addressing host-pathogen interactions requires novel tools to connect genome-wide data sets and theoretical approaches with experimental validation so as to identify inherent and emerging properties of host-pathogen relationships and to obtain a holistic view of infectious processes. If successful, a better understanding of the immune response in health and microbial diseases will eventually emerge and pave the way for improved therapies.     

Using non-mammalian hosts to study fungal virulence and host defense

Non-mammalian hosts have been used to study host-fungal interactions. Hosts such as Drosophila melanogaster, Caenorhabditis elegans, Acathamoeba castellanii, Dictyostelium discoideum, and Galleria mellonella have provided means to examine the physical barriers, cellular mechanisms and molecular elements of the host response. The Drosophila host-response to fungi is mediated through the Toll pathway, whereas in C. elegans the host-response is TIR-1-dependent. Virulence traits that are involved in mammalian infection are important for the interaction of fungi with these hosts. Screening of fungal virulence traits using mutagenized fungi to determine changes in fungal infectivity of non-mammalian hosts has been used to identify novel virulence proteins used to infect C. elegans such as Kin1 (a serine/threonine protein kinase) and Rom2 (a Rho1 guanyl-nucleotide exchange factor) from Cryptococcus neoformans. These heterologous non-mammalian hosts highlight the similarities and differences between different hosts in fungal pathogenesis and they complement studies in mammalian systems and those using other genetic approaches.     

Nonpathogenic, environmental fungi induce activation and degranulation of human eosinophils

Eosinophils and their products are probably important in the pathophysiology of allergic diseases, such as bronchial asthma, and in host immunity to certain organisms. An association between environmental fungal exposure and asthma has been long recognized clinically. Although products of microorganisms (e.g., lipopolysaccharides) directly activate certain inflammatory cells (e.g., macrophages), the mechanism(s) that triggers eosinophil degranulation is unknown. In this study we investigated whether human eosinophils have an innate immune response to certain fungal organisms. We incubated human eosinophils with extracts from seven environmental airborne fungi (Alternaria alternata, Aspergillus versicolor, Bipolaris sorokiniana, Candida albicans, Cladosporium herbarum, Curvularia spicifera, and Penicillium notatum). Alternaria and Penicillium induced calcium-dependent exocytosis (e.g., eosinophil-derived neurotoxin release) in eosinophils from normal individuals. Alternaria also strongly induced other activation events in eosinophils, including increases in intracellular calcium concentration, cell surface expression of CD63 and CD11b, and production of IL-8. Other fungi did not induce eosinophil degranulation, and Alternaria did not induce neutrophil activation, suggesting specificity for fungal species and cell type. The Alternaria-induced eosinophil degranulation was pertussis toxin sensitive and desensitized by preincubating cells with G protein-coupled receptor agonists, platelet-activating factor, or FMLP. The eosinophil-stimulating activity in Alternaria extract was highly heat labile and had an M(r) of approximately 60 kDa. Thus, eosinophils, but not neutrophils, possess G protein-dependent cellular activation machinery that directly responds to an Alternaria protein product(s). This innate response by eosinophils to certain environmental fungi may be important in host defense and in the exacerbation of inflammation in asthma and allergic diseases.     

Identification and functional analysis of antifungal immune response genes in Drosophila

Essential aspects of the innate immune response to microbial infection appear to be conserved between insects and mammals. Although signaling pathways that activate NF-kappaB during innate immune responses to various microorganisms have been studied in detail, regulatory mechanisms that control other immune responses to fungal infection require further investigation. To identify new Drosophila genes involved in antifungal immune responses, we selected genes known to be differentially regulated in SL2 cells by microbial cell wall components and tested their roles in antifungal defense using mutant flies. From 130 mutant lines, sixteen mutants exhibited increased sensitivity to fungal infection. Examination of their effects on defense against various types of bacteria and fungi revealed nine genes that are involved specifically in defense against fungal infection. All of these mutants displayed defects in phagocytosis or activation of antimicrobial peptide genes following infection. In some mutants, these immune deficiencies were attributed to defects in hemocyte development and differentiation, while other mutants showed specific defects in immune signaling required for humoral or cellular immune responses. Our results identify a new class of genes involved in antifungal immune responses in Drosophila.     

Ergosterol,an orphan fungal microbe‐associated molecular pattern (MAMP)

Fungal pathogens continue to pose a significant threat to crop production and food supply. The early stages of plant–fungus interactions are mostly mediated by microbe‐associated molecular pattern (MAMP) molecules, perceived by plant pattern recognition receptors (PRRs). Currently, the identified fungal MAMP molecules include chitin, chitosan, β‐glucans, elicitins and ergosterol. Although the molecular battles between host plants and infecting fungal phytopathogens have been studied extensively, many aspects still need to be investigated to obtain a holistic understanding of the intrinsic mechanisms, which is paramount in combating fungal plant diseases. Here, an overview is given of the most recent findings concerning an ‘orphan’ fungal MAMP molecule, ergosterol, and we present what is currently known from a synopsis of different genes, proteins and metabolites found to play key roles in induced immune responses in plant–fungus interactions. Clearly, integrative investigations are still needed to provide a comprehensive systems‐based understanding of the dynamics associated with molecular mechanisms in plant–ergosterol interactions and associated host responses.     

Role of the Fungal Cell Wall in Pathogenesis and Antifungal Resistance

Study of the fungal cell wall is currently an area of very active research. The relevance of the fungal cell wall for cell survival, and pathogenicity has been well established. The view of the cell wall as a tough and impenetrable structure has been left behind, and it is now conceived as a plastic shield that undergoes structural changes depending on the surrounding environmental conditions and morphological states. The fungal cell wall is also the source of most of the pathogen-associated molecular patterns that immune cells recognize, and thus facilitates establishment of a protective antifungal immunity. Paradoxically, fungi, through their cell wall, possess disguising mechanisms to avoid immune recognition. This review gathers the current knowledge about the cell wall of Candida albicans, Aspergillus fumigatus and Paracoccidioides brasiliensis, stressing the importance of the fungal cell wall for pathogenesis, immune recognition, and as a source of targets for antifungal drugs.     

Molecular basis of resistance to azole antifungals

The increased incidence of invasive mycoses and the emerging problem of antifungal drug resistance has prompted investigations of the underlying molecular mechanisms, particularly for the azole compounds central to current therapy. The target site for the azoles is the ERG11 gene product, the cytochrome P450 lanosterol 14alpha-demethylase, which is part of the ergosterol biosynthetic pathway. The resulting ergosterol depletion renders fungal cells vulnerable to further membrane damage. Development of azole resistance in fungi may occur through increased levels of the cellular target, upregulation of genes controlling drug efflux, alterations in sterol synthesis and decreased affinity of azoles for the cellular target. Here, we review the adaptative changes in fungi, in particular Candida albicans, in response to inhibitors of ergosterol biosynthesis. The molecular mechanisms of azole resistance might help in devising more effective antifungal therapies.     

Metabolic remodeling in iron-deficient fungi

Eukaryotic cells contain dozens, perhaps hundreds, of iron-dependent proteins, which perform critical functions in nearly every major cellular process. Nutritional iron is frequently available to cells in only limited amounts; thus, unicellular and higher eukaryotes have evolved mechanisms to cope with iron scarcity. These mechanisms have been studied at the molecular level in the model eukaryotes Saccharomyces cerevisiae and Schizosaccharomyces pombe, as well as in some pathogenic fungi. Each of these fungal species exhibits metabolic adaptations to iron deficiency that serve to reduce the cell's reliance on iron. However, the regulatory mechanisms that accomplish these adaptations differ greatly between fungal species. This article is part of a Special Issue entitled: Cell Biology of Metals.     

Antifungal Resistance Mechanisms in Dermatophytes

Although fungi do not cause outbreaks or pandemics, the incidence of severe systemic fungal infections has increased significantly, mainly because of the explosive growth in the number of patients with compromised immune system. Thus, drug resistance in pathogenic fungi, including dermatophytes, is gaining importance. The molecular aspects involved in the resistance of dermatophytes to marketed antifungals and other cytotoxic drugs, such as modifications of target enzymes, over-expression of genes encoding ATP-binding cassette (ABC) transporters and stress-response-related proteins are reviewed. Emphasis is placed on the mechanisms used by dermatophytes to overcome the inhibitory action of terbinafine and survival in the host environment. The relevance of identifying new molecular targets, of expanding the understanding about the molecular mechanisms of resistance and of using this information to design new drugs or to modify those that have become ineffective is also discussed.     

Antibody-mediated protective immunity in fungal infections

The host response to fungal infection is the result of a complex interaction between the pathogen and the host's innate and adaptive immune system. Cell-mediated immunity is widely considered to be critical for the successful outcome of fungal infections. However, in recent years numerous studies have established that certain antibodies may play an important role in host immunoprotection against pathogenic fungi, through interaction with different cellular targets, such as mannans, heat shock proteins, capsular polysaccharides, surface proteins, and yeast killer toxin receptors, with mechanisms of action sometimes still undefined. This review summarizes the latest findings on the role of different types of antibodies in the antifungal defense against infections caused by epidemiologically important fungi, such as Candida albicans, Cryptococcus neoformans, Histoplasma capsulatum, and others. New perspectives of antibody-mediated therapy, based on the availability of monoclonal and recombinant antibodies as well as genetically engineered antibody fragments of defined specificity, will be also envisaged and discussed.     

Highly conserved,but highly specific: Somatic cell–cell fusion in filamentous fungi

The development of ascomycete fungal colonies involves cell–cell fusion at different growth stages. In the model fungus Neurospora crassa, communication of two fusing cells is mediated by an unusual signaling mechanism, in which the two partners take turns in signal sending and receiving. In recent years, the molecular basis of this unusual cellular behavior has started to unfold, indicating the presence of an excitable signaling network. New evidence suggests that this communication system is highly conserved in ascomycete fungi and, unexpectedly, even mediates interspecies interactions. At the same time, intricate allorecognition mechanisms were identified, which prevent the fusion of genetically unlike individuals. These observations suggest that signal specificity during fungal social behavior has not evolved on the level of signals and receptors, but is achieved at downstream checkpoints. Despite growing insight into the molecular mechanisms controlling self and non-self fungal interactions, their role in natural environments remains largely unknown.     

Dectin-1: a signalling non-TLR pattern-recognition receptor

Dectin-1 is a natural killer (NK)-cell-receptor-like C-type lectin that is thought to be involved in innate immune responses to fungal pathogens. This transmembrane signalling receptor mediates various cellular functions, from fungal binding, uptake and killing, to inducing the production of cytokines and chemokines. These activities could influence the resultant immune response and can, in certain circumstances, lead to autoimmunity and disease. As I discuss here, understanding the molecular mechanisms behind these functions has revealed new concepts, including collaborative signalling with the Toll-like receptors (TLRs) and the use of spleen tyrosine kinase (SYK), that have implications for the role of other non-TLR pattern-recognition receptors in immunity.     

Fungal factors involved in host immune evasion,modulation and exploitation during infection

Human and plant pathogenic fungi have a major impact on public health and agriculture. Although these fungi infect very diverse hosts and are often highly adapted to specific host niches, they share surprisingly similar mechanisms that mediate immune evasion, modulation of distinct host targets and exploitation of host nutrients, highlighting that successful strategies have evolved independently among diverse fungal pathogens. These attributes are facilitated by an arsenal of fungal factors. However, not a single molecule, but rather the combined effects of several factors enable these pathogens to establish infection. In this review, we discuss the principles of human and plant fungal pathogenicity mechanisms and discuss recent discoveries made in this field.     

Mechanisms of immune evasion in fungal pathogens

The incidence of life-threatening fungal infections has continued to increase in recent years, predominantly in patients debilitated by iatrogenic interventions or immunological dysfunctions. While the picture of the immunology of fungal infections grows increasingly complex, it is clear that the phagocyte-pathogen interaction is a critical determinant of establishing an infection. About 10 years ago, genome-scale approaches began to elucidate the intricate and extensive fungal response to phagocytosis and in the last few years it has become clear that some of this response actively modulates immune cell function. Fungal pathogens avoid detection by masking pathogen-associated molecular patterns, such as cell wall carbohydrates, and by downregulating the complement cascade. Once detected, various species interfere with phagocytosis and intracellular trafficking, and can repress production of antimicrobials like nitric oxide (NO). For the most part, the molecular mechanisms behind these behaviors are not yet known. This review discusses recent discoveries and insights into how fungi manipulate the host-pathogen interaction.     

踏破铁鞋无觅处——一类新型抗真菌剂的发现

病原微生物通过其特有的机制破坏植物的防御屏障, 引发病害, 给农业生产造成损失。研究病菌致病机制, 能够启发人们探索病害防控的新思路。四川农业大学陈学伟团队阐明了稻瘟病菌的一种特殊结构——侵染钉的发生机制, 发现超长碳链脂肪酸合成酶在此过程中发挥重要作用。以超长碳链脂肪酸合成酶为靶点, 该团队寻找到了抑制超长碳链脂肪酸生物合成, 进而抑制侵染结构发生的化合物。这些化合物可广谱抑制多种病原真菌在动物和植物宿主上的致病力, 为创制新型农药开拓了新思路。