Nanoscale biophysical properties of the cell surface galactosaminogalactan from the fungal pathogen Aspergillus fumigatus

Cell wall of Aspergillus fumigatus: Variability and response to stress

The fungal cell is surrounded by a thick cell wall which obviously play an essential role in the protection of the fungus against external aggressive environments. In spite of 50 years of studies, the cell wall remains poorly known and especially its constant modifications during growth as well as environmental changes is not well appreciated. This review focus on the cell wall changes seen between different fungal stages and cell populations with a specific view to explain the resistance to stresses.

Could the Lung Be a Gateway for Amphotericin B to Attack the Army of Fungi?

Fungal diseases are a significant cause of morbidity and mortality worldwide, primarily affecting immunocompromised patients. Aspergillus, Pneumocystis, and Cryptococcus are opportunistic fungi and may cause severe lung disease. They can develop mechanisms to evade the host immune system and colonize or cause lung disease. Current fungal infection treatments constitute a few classes of antifungal drugs with significant fungi resistance development. Amphotericin B (AmB) has a broad-spectrum antifungal effect with a low incidence of resistance. However, AmB is a highly lipophilic antifungal with low solubility and permeability and is unstable in light, heat, and oxygen. Due to the difficulty of achieving adequate concentrations of AmB in the lung by intravenous administration and seeking to minimize adverse effects, nebulized AmB has been used. The pulmonary pathway has advantages such as its rapid onset of action, low metabolic activity at the site of action, ability to avoid first-pass hepatic metabolism, lower risk of adverse effects, and thin thickness of the alveolar epithelium. This paper presented different strategies for pulmonary AmB delivery, detailing the potential of nanoformulation and hoping to foster research in the field. Our finds indicate that despite an optimistic scenario for the pulmonary formulation of AmB based on the encouraging results discussed here, there is still no product registration on the FDA nor any clinical trial undergoing ClinicalTrial.gov.

Filamentous fungal biofilms: Conserved and unique aspects of extracellular matrix composition, mechanisms of drug resistance and regulatory networks in Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is an ubiquitous mold that can cause invasive pulmonary infections in immunocompromised patients. Within the lung, A. fumigatus forms biofilms that can enhance resistance to antifungals and immune defenses, highlighting the importance of defining the mechanisms underlying biofilm development and associated emergent properties. A. fumigatus biofilms display a morphology and architecture that is distinct from bacterial and yeast biofilms. Moreover, A. fumigatus biofilms display unique characteristics in the composition of their extracellular matrix (ECM) and the regulatory networks governing biofilm formation. This review will discuss our current understanding of the form and function of A. fumigatus biofilms, including the unique components of ECM matrix, potential drug resistance mechanisms, the regulatory networks governing A. fumigatus biofilm formation, and potential therapeutics targeting these structures.

Conservative production of galactosaminogalactan in Metarhizium is responsible for appressorium mucilage production and topical infection of insect hosts

The exopolysaccharide galactosaminogalactan (GAG) has been well characterized in Aspergilli, especially the human pathogen Aspergillus fumigatus. It has been found that a five-gene cluster is responsible for GAG biosynthesis in Aspergilli to mediate fungal adherence, biofilm formation, immunosuppression or induction of host immune defences. Herein, we report the presence of the conserved GAG biosynthetic gene cluster in the insect pathogenic fungus Metarhizium robertsii to mediate either similar or unique biological functions. Deletion of the gene cluster disabled fungal ability to produce GAG on germ tubes, mycelia and appressoria. Relative to the wild type strain, null mutant was impaired in topical infection but not injection of insect hosts. We found that GAG production by Metarhizium is partially acetylated and could mediate fungal adherence to hydrophobic insect cuticles, biofilm formation, and penetration of insect cuticles. In particular, it was first confirmed that this exopolymer is responsible for the formation of appressorium mucilage, the essential extracellular matrix formed along with the infection structure differentiation to mediate cell attachment and expression of cuticle degrading enzymes. In contrast to its production during A. fumigatus invasive growth, GAG is not produced on the Metarhizium cells harvested from insect hemocoels; however, the polymer can glue germ tubes into aggregates to form mycelium pellets in liquid culture. The results of this study unravel the biosynthesis and unique function of GAG in a fungal system apart from the aspergilli species.

Functional Genomic and Biochemical Analysis Reveals Pleiotropic Effect of Congo Red on Aspergillus fumigatus

Inhibition of fungal growth by Congo red (CR) has been putatively associated with specific binding to β-1,3-glucans, which blocks cell wall polysaccharide synthesis. In this study, we searched for transcription factors (TFs) that regulate the response to CR and interrogated their regulon. During the investigation of the susceptibility to CR of the TF mutant library, several CR-resistant and -hypersensitive mutants were discovered and further studied. Abnormal distorted swollen conidia called Quasimodo cells were seen in the presence of CR. Quasimodo cells in the resistant mutants were larger than the ones in the sensitive and parental strains; consequently, the conidia of the resistant mutants absorbed more CR than the germinating conidia of the sensitive or parental strains. Accordingly, this higher absorption rate by Quasimodo cells resulted in the removal of CR from the culture medium, allowing a subset of conidia to germinate and grow. In contrast, all resting conidia of the sensitive mutants and the parental strain were killed. This result indicated that the heterogeneity of the conidial population is essential to promote the survival of Aspergillus fumigatus in the presence of CR. Moreover, amorphous surface cell wall polysaccharides such as galactosaminogalactan control the influx of CR inside the cells and, accordingly, resistance to the drug. Finally, long-term incubation with CR led to the discovery of a new CR-induced growth effect, called drug-induced growth stimulation (DIGS), since the growth of one of them could be stimulated after recovery from CR stress.

Fungal spore adhesion on glycidoxypropyltrimethoxy silane modified silica nanoparticle surfaces as revealed by single cell force spectroscopy

Thin film coatings prepared from commercially available glycidoxypropyltrimethoxysilane (GPS) modified silica nanoparticles (SiNPs) (Bindzil® CC301 and Bindzil® CC302) have previously shown excellent antifouling performance against a broad range of microbes [Molino et al., "Hydration layer structure of biofouling-resistant nanoparticles," ACS Nano 12, 11610 (2018)]. In this work, single cell force spectroscopy (SCFS) was used to measure the biological interactions between Epicoccum nigrum fungal spores and the same silica nanoparticle-based surfaces used in the aforementioned study, including a: glass coverslip, unmodified SiNP coatings, and both low (Bindzil® CC301) and high density (CC302) GPS functionalized SiNP coatings as a function of NaCl concentration. From the SCFS curves, the spore adhesion to the surface was greatest on the glass coverslip (20-80 nN) followed by the unmodified SiNP (3-5 nN) across all salt concentrations. Upon approach to both surfaces, the spores showed a long-range attraction generally with a profile characteristic of biointeractions and likely those of the outer cell wall structures or biological constituents. The attractive force allowed the spores to initially adhere to the surface and was found to be linearly proportional to the spore adhesion. In comparison, both high and low density GPS-SINP significantly reduced the spore adhesion (0.5-0.9 nN). In addition, the spore adhesion on high density GPS-SiNP occurred in only 14%-27% of SCFS curves (40%-48% for low density GPS-SiNP) compared to 83%-97% for the unmodified SiNP, indicating that in most cases the GPS functionalization completely prevented spore adhesion. The GPS-SiNP surfaces conversely showed a long-range electrostatic repulsion at low 1mM NaCl that was replaced by short-range repulsion at the higher salt concentrations. From the findings, it is proposed that the attractive force is a critical step in initial adhesion processes of the spore. The effective antifouling properties of the GPS are attributed to the ability to negate the attractive forces, either through electrostatic repulsion in low salt conditions and primarily from short-range repulsion correlating to the previously reported combined steric-hydration effect of the GPS functionalization on SiNP coatings.

Revisiting Old Questions and New Approaches to Investigate the Fungal Cell Wall Construction

The beginning of our understanding of the cell wall construction came from the work of talented biochemists in the 70-80's. Then came the era of sequencing. Paradoxically, the accumulation of fungal genomes complicated rather than solved the mystery of cell wall construction, by revealing the involvement of a much higher number of proteins than originally thought. The situation has become even more complicated since it is now recognized that the cell wall is an organelle whose composition continuously evolves with the changes in the environment or with the age of the fungal cell. The use of new and sophisticated technologies to observe cell wall construction at an almost atomic scale should improve our knowledge of the cell wall construction. This essay will present some of the major and still unresolved questions to understand the fungal cell wall biosynthesis and some of these exciting futurist approaches.

Aspergillus fumigatus and Aspergillosis in 2019

Aspergillus fumigatus is a saprotrophic fungus; its primary habitat is the soil. In its ecological niche, the fungus has learned how to adapt and proliferate in hostile environments. This capacity has helped the fungus to resist and survive against human host defenses and, further, to be responsible for one of the most devastating lung infections in terms of morbidity and mortality. In this review, we will provide (i) a description of the biological cycle of A. fumigatus; (ii) a historical perspective of the spectrum of aspergillus disease and the current epidemiological status of these infections; (iii) an analysis of the modes of immune response against Aspergillus in immunocompetent and immunocompromised patients; (iv) an understanding of the pathways responsible for fungal virulence and their host molecular targets, with a specific focus on the cell wall; (v) the current status of the diagnosis of different clinical syndromes; and (vi) an overview of the available antifungal armamentarium and the therapeutic strategies in the clinical context. In addition, the emergence of new concepts, such as nutritional immunity and the integration and rewiring of multiple fungal metabolic activities occurring during lung invasion, has helped us to redefine the opportunistic pathogenesis of A. fumigatus.

Microbial adhesion and ultrastructure from the single-molecule to the single-cell levels by Atomic Force Microscopy

In the last decades, atomic force microscopy (AFM) has evolved towards an accurate and lasting tool to study the surface of living cells in physiological conditions. Through imaging, single-molecule force spectroscopy and single-cell force spectroscopy modes, AFM allows to decipher at multiple scales the morphology and the molecular interactions taking place at the cell surface. Applied to microbiology, these approaches have been used to elucidate biophysical properties of biomolecules and to directly link the molecular structures to their function. In this review, we describe the main methods developed for AFM-based microbial surface analysis that we illustrate with examples of molecular mechanisms unravelled with unprecedented resolution.

Definition of the Anti-inflammatory Oligosaccharides Derived From the Galactosaminogalactan (GAG) From Aspergillus fumigatus

Galactosaminogalactan (GAG) is an insoluble aminosugar polymer produced by Aspergillus fumigatus and has anti-inflammatory properties. Here, the minimum glycosidic sequences required for the induction of IL-1Ra by peripheral blood mononuclear cells (PBMCs) was investigated. Using chemical degradation of native GAG to isolate soluble oligomers, we have found that the de-N-acetylation of galactosamine residues and the size of oligomer are critical for the in vitro immune response. A minimal oligomer size of 20 galactosamine residues is required for the anti-inflammatory response but the presence of galactose residues is not necessary. In a Dextran sulfate induced colitis mouse model, a fraction of de-N-acetylated oligomers of 13 < dp < 20 rescue inflammatory damage like the native GAG polymer in an IL-1Ra dependent pathway. Our results demonstrate the therapeutic suitability of water-soluble GAG oligosaccharides in IL-1 mediated hyper-inflammatory diseases and suggest that α-1,4-galactosamine oligomers chemically synthesized could represent new anti-inflammatory glycodrugs.

The role of Aspergillus fumigatus polysaccharides in host-pathogen interactions

Aspergillus fumigatus is a saprophytic mold that can cause infection in patients with impaired immunity or chronic lung diseases. The polysaccharide-rich cell wall of this fungus is a key point of contact with the host immune system. The availability of purified cell wall polysaccharides and mutant strains deficient in the production of these glycans has revealed that these glycans play an important role in the pathogenesis of A. fumigatus infections. Herein, we review our current understanding of the key polysaccharides present within the A. fumigatus cell wall, and their interactions with host cells and secreted factors during infection.

Galactosaminogalactan (GAG) and its multiple roles in Aspergillus pathogenesis

Aspergillus spp and particularly the species Aspergillus fumigatus are the causative agents of invasive aspergillosis, a progressive necrotizing pneumonia that occurs in immunocompromised patients. The limited efficacy of currently available antifungals has led to interest in a better understanding of the molecular mechanisms underlying the pathogenesis of invasive aspergillosis in order to identify new therapeutic targets for this devastating disease. The Aspergillus exopolysaccharide galactosaminogalactan (GAG) plays an important role in the pathogenesis of experimental invasive aspergillosis. The present review article summarizes our current understanding of GAG composition and synthesis and the molecular mechanisms whereby GAG promotes virulence. Promising directions for future research and the prospect of GAG as both a therapy and therapeutic target are reviewed.

The puzzling construction of the conidial outer layer of Aspergillus fumigatus

If the mycelium of Aspergillus fumigatus is very short-lived in the laboratory, conidia can survive for years. This survival capacity and extreme resistance to environmental insults is a major biological characteristic of this fungal species. Moreover, conidia, which easily reach the host alveola, are the infective propagules. Earlier studies have shown the role of some molecules of the outer conidial layer in protecting the fungus against the host defense. The outer layer of the conidial cell wall, directly in contact with the host cells, consists of α-(1,3)-glucan, melanin, and proteinaceous rodlets. This study is focused on the global importance of this outer layer. Single and multiple mutants without one to three major components of the outer layer were constructed and studied. The results showed that the absence of the target molecules resulting from multiple gene deletions led to unexpected phenotypes without any logical additivity. Unexpected compensatory cell wall surface modifications were indeed observed, such as the synthesis of the mycelial virulence factor galactosaminogalactan, the increase in chitin and glycoprotein concentration or particular changes in permeability. However, sensitivity of the multiple mutants to killing by phagocytic host cells confirmed the major importance of melanin in protecting conidia.

Atomic force microscopy for imaging and nanomechanical characterisation of live nematode epicuticle: A comparative Caenorhabditis elegans and Turbatrix aceti study

Atomic force microscopy (AFM), a powerful tool in interdisciplinary biomedical research, has been applied here to investigate the surface of live nematodes epicuticle. We have used AFM in PeakForce Tapping non-resonant imaging and nanomechanical characterisation mode to investigate and compare the surface features of epicuticle of two free-living microscopic nematodes, Caenorhabditis elegans and Turbatrix aceti. We have successfully immobilised live anesthetized adult nematodes on glass supports using either layer-by-layer-deposited polyelectrolyte films or bioadhesive coatings, which allowed for imaging the living nematodes in native environment. We have obtained AFM images and corresponding nanomechanical maps of annular rings and furrows, demonstrating the differences in topography and structure between the species. Our results demonstrate that AFM in PeakForce Tapping mode can be used to image and characterise surfaces of relatively-large live immobilised multicellular organisms, which can be further applied to a number of invertebrates.

Nematode Epicuticle Visualisation by PeakForce Tapping Atomic Force Microscopy

The free-living soil nematode Caenorhabditis elegans has become an iconic experimental model animal in biology. This transparent animal can be easily imaged using optical microscopy to visualise its organs, tissues, single cells and subcellular events. The epicuticle of C. elegans nematodes has been studied at nanoscale using transmission and scanning (SEM) electron microscopies. As a result, imaging artefacts can appear due to embedding the worms into resins or coating the worms with a conductive gold layer. In addition, fixation and contrasting may also damage the cuticle. Conventional tapping mode atomic force microscopy (AFM) can be applied to image the cuticle of the dried nematodes in air, however this approach also suffers from imaging defects. Ideally, the nematodes should be imaged under conditions resembling their natural environment. Recently, we reported the use of PeakForce Tapping AFM mode for the successful visualisation and numerical characterisation of C. elegans nematode cuticle both in air and in liquid ( Fakhrullina et al., 2017 ). We imaged the principal nematode surface structures and characterised mechanical properties of the cuticle. This protocol provides the detailed description of AFM imaging of liquid-immersed C. elegans nematodes using PeakForce Tapping atomic force microscopy.

Aspergillus fumigatus morphology and dynamic host interactions

Aspergillus fumigatus is an environmental filamentous fungus that can cause life-threatening disease in immunocompromised individuals. The interactions between A. fumigatus and the host environment are dynamic and complex. The host immune system needs to recognize the distinct morphological forms of A. fumigatus to control fungal growth and prevent tissue invasion, whereas the fungus requires nutrients and needs to adapt to the hostile environment by escaping immune recognition and counteracting host responses. Understanding these highly dynamic interactions is necessary to fully understand the pathogenesis of aspergillosis and to facilitate the design of new therapeutics to overcome the morbidity and mortality caused by A. fumigatus. In this Review, we describe how A. fumigatus adapts to environmental change, the mechanisms of host defence, and our current knowledge of the interplay between the host immune response and the fungus.

The Cell Wall of the Human Fungal Pathogen Aspergillus fumigatus: Biosynthesis, Organization, Immune Response, and Virulence

More than 90% of the cell wall of the filamentous fungus Aspergillus fumigatus comprises polysaccharides. Biosynthesis of the cell wall polysaccharides is under the control of three types of enzymes: transmembrane synthases, which are anchored to the plasma membrane and use nucleotide sugars as substrates, and cell wall-associated transglycosidases and glycosyl hydrolases, which are responsible for remodeling the de novo synthesized polysaccharides and establishing the three-dimensional structure of the cell wall. For years, the cell wall was considered an inert exoskeleton of the fungal cell. The cell wall is now recognized as a living organelle, since the composition and cellular localization of the different constitutive cell wall components (especially of the outer layers) vary when the fungus senses changes in the external environment. The cell wall plays a major role during infection. The recognition of the fungal cell wall by the host is essential in the initiation of the immune response. The interactions between the different pattern-recognition receptors (PRRs) and cell wall pathogen-associated molecular patterns (PAMPs) orientate the host response toward either fungal death or growth, which would then lead to disease development. Understanding the molecular determinants of the interplay between the cell wall and host immunity is fundamental to combatting Aspergillus diseases.

Fungal biofilm composition and opportunities in drug discovery

Biofilm infections are exceptionally recalcitrant to antimicrobial treatment or clearance by host immune responses. Within biofilms, microbes form adherent multicellular communities that are embedded in an extracellular matrix. Many prescribed antifungal drugs are not effective against biofilm infections owing to several protective factors including poor diffusion of drugs through biofilms as well as specific drug-matrix interactions. Despite the key roles that biofilms play in infections, there is little quantitative information about their composition and structural complexity because of the analytical challenge of studying these dense networks using traditional techniques. Within this review, recent work to elucidate fungal biofilm composition is discussed, with particular attention given to the challenges of annotation and quantification of matrix composition.

Alarmin(g) the innate immune system to invasive fungal infections

Fungi encounter numerous stresses in a mammalian host, including the immune system, which they must adapt to in order to grow and cause disease. The host immune system tunes its response to the threat level posed by the invading pathogen. We discuss recent findings on how interleukin (IL)-1 signaling is central to tuning the immune response to the virulence potential of invasive fungi, as well as other pathogens. Moreover, we discuss fungal factors that may drive tissue invasion and destruction that regulate IL-1 cytokine release. Moving forward understanding the mechanisms of fungal adaption to the host, together with understanding how the host innate immune system recognizes invading fungal pathogens will increase our therapeutic options for treatment of invasive fungal infections.

Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis

The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae.

Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.

Updates on Therapeutic Strategies Against Candida (and Aspergillus) Biofilm Related Infections

Fungal biofilm related infections are commonly associated with medical devices with biofilms contributing to the virulence of the involved fungal species. If infection does occur, removal of medical device is often warranted. However, this is not always possible. Moreover, biofilm associated infections are often resistant to antifungals and host immunity. Therefore, a need for new agents and strategies to combat these devastating infections is needed. Although no randomized clinical trials have been conducted or are likely to be conducted in the future, the Infectious Disease Society of America (IDSA) and the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) utilized available published data and clinical experience of the infectious disease community to propose strategies to treat biofilm associated devise infections. In this chapter we describe the emerging therapies for biofilm related infections.