Biosynthesis of β-(1→5)-Galactofuranosyl Chains of Fungal-Type and O-Mannose-Type Galactomannans within the Invasive Pathogen Aspergillus fumigatus

Discovery of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in Aspergillus fumigatus mycelium

The cellular surface of the pathogenic filamentous fungus Aspergillus fumigatus is enveloped in a mannose layer, featuring well-established fungal-type galactomannan and O-mannose-type galactomannan. This study reports the discovery of cell wall component in A. fumigatus mycelium, which resembles N-glycan outer chains found in yeast. The glycosyltransferases involved in its biosynthesis in A. fumigatus were identified, with a focus on two key α-(1→2)-mannosyltransferases, Mnn2 and Mnn5, and two α-(1→6)-mannosyltransferases, Mnn9 and Van1. In vitro examination revealed the roles of recombinant Mnn2 and Mnn5 in transferring α-(1→2)-mannosyl residues. Proton nuclear magnetic resonance (1H-NMR) analysis of cell wall extracts from the ∆mnn2∆mnn5 strain indicated the existence of an α-(1→6)-linked mannan backbone in the A. fumigatus mycelium, with Mnn2 and Mnn5 adding α-(1→2)-mannosyl residues to this backbone. The α-(1→6)-linked mannan backbone was absent in strains where mnn9 or van1 was disrupted in the parental ∆mnn2∆mnn5 strain in A. fumigatus. Mnn9 and Van1 functioned as α-(1→6)-linked mannan polymerases in heterodimers when co-expressed in Escherichia coli, indicating their crucial role in biosynthesizing the α-(1→6)-linked mannan backbone. Disruptions of these mannosyltransferases did not affect fungal-type galactomannan biosynthesis. This study provides insights into the complexity of fungal cell wall architecture and a better understanding of mannan biosynthesis in A. fumigatus.

IMPORTANCE

This study unravels the complexities of mannan biosynthesis in A. fumigatus, a key area for antifungal drug discovery. It reveals the presence of α-(1→6)-linked mannan structures resembling yeast N-glycan outer chains in A. fumigatus mycelium, offering fresh insights into the fungal cell wall's design. Key enzymes, Mnn2, Mnn5, Mnn9, and Van1, are instrumental in this process, with Mnn2 and Mnn5 adding specific mannose residues and Mnn9 and Van1 assembling the α-(1→6)-linked mannan structures. Although fungal-type galactomannan's presence in the cell wall is known, the existence of an α-(1→6)-linked mannan adds a new dimension to our understanding. This intricate web of mannan biosynthesis opens avenues for further exploration and enhances our understanding of fungal cell wall dynamics, paving the way for targeted drug development.

Characterization of the need for galactofuranose during the Neurospora crassa life cycle

Galactofuranose is a constituent of the cell walls of filamentous fungi. The galactofuranose can be found as a component of N-linked oligosaccharides, in O-linked oligosaccharides, in GPI-anchored galactomannan, and in free galactomannan. The Neurospora genome contains a single UDP-galactose mutase gene (ugm-1/NCU01824) and two UDP-galactofuranose translocases used to import UDP-galactofuranose into the lumen of the Golgi apparatus (ugt-1/NCU01826 and ugt-2/NCU01456). Our results demonstrate that loss of galactofuranose synthesis or its translocation into the lumen of the secretory pathway affects the morphology and growth rate of the vegetative hyphae, the production of conidia (asexual spores), and dramatically affects the sexual stages of the life cycle. In mutants that are unable to make galactofuranose or transport it into the lumen of the Golgi apparatus, ascospore development is aborted soon after fertilization and perithecium maturation is aborted prior to the formation of the neck and ostiole. The Neurospora genome contains three genes encoding possible galactofuranosyltransferases from the GT31 family of glycosyltransferases (gfs-1/NCU05878, gfs-2/NCU07762, and gfs-3/NCU02213) which might be involved in generating galactofuranose-containing oligosaccharide structures. Analysis of triple KO mutants in GT31 glycosyltransferases shows that these mutants have normal morphology, suggesting that these genes do not encode vital galactofuranosyltransferases.

Aspergillus fumigatus transcription factor ZfpA regulates hyphal development and alters susceptibility to antifungals and neutrophil killing during infection

Hyphal growth is essential for host colonization during Aspergillus infection. The transcription factor ZfpA regulates A. fumigatus hyphal development including branching, septation, and cell wall composition. However, how ZfpA affects fungal growth and susceptibility to host immunity during infection has not been investigated. Here, we use the larval zebrafish-Aspergillus infection model and primary human neutrophils to probe how ZfpA affects A. fumigatus pathogenesis and response to antifungal drugs in vivo. ZfpA deletion promotes fungal clearance and attenuates virulence in wild-type hosts and this virulence defect is abrogated in neutrophil-deficient zebrafish. ZfpA deletion also increases susceptibility to human neutrophils ex vivo while overexpression impairs fungal killing. Overexpression of ZfpA confers protection against the antifungal caspofungin by increasing chitin synthesis during hyphal development, while ZfpA deletion reduces cell wall chitin and increases caspofungin susceptibility in neutrophil-deficient zebrafish. These findings suggest a protective role for ZfpA activity in resistance to the innate immune response and antifungal treatment during A. fumigatus infection.

Biochemical characterization of a glycoside hydrolase family 43 β-D-galactofuranosidase from the fungus Aspergillus niger

β-D-Galactofuranose (Galf) and its polysaccharides are found in bacteria, fungi and protozoa but do not occur in mammalian tissues, and thus represent a specific target for anti-pathogenic drugs. Understanding the enzymatic degradation of these polysaccharides is therefore of great interest, but the identity of fungal enzymes with exclusively galactofuranosidase activity has so far remained elusive. Here we describe the identification and characterization of a galactofuranosidase from the industrially important fungus Aspergillus niger. Analysis of glycoside hydrolase family 43 subfamily 34 (GH43_34) members via conserved unique peptide patterns and phylogeny, revealed the occurrence of distinct clusters and, by comparison with specificities of characterized bacterial members, suggested a basis for prediction of enzyme specificity. Using this rationale, in tandem with molecular docking, we identified a putative β-D-galactofuranosidase from A. niger which was recombinantly produced in Escherichia coli. The Galf-specific hydrolase, encoded by xynD demonstrates maximum activity at pH 5, 25 °C towards 4-nitrophenyl-β-galactofuranoside (pNP-β-Galf), with a K_m of 17.9 ± 1.9 mM and V_max of 70.6 ± 5.3 µM min^-1. The characterization of this first fungal GH43 galactofuranosidase offers further molecular insight into the degradation of Galf-containing structures.

Identification of galactofuranose antigens such as galactomannoproteins and fungal-type galactomannan from the yellow koji fungus (Aspergillus oryzae)

Filamentous fungi belonging to the genus Aspergillus are known to possess galactomannan in their cell walls. Galactomannan is highly antigenic to humans and has been reported to be involved in the pathogenicity of pathogenic filamentous fungi, such as A. fumigatus, and in immune responses. In this study, we aimed to confirm the presence of D-galactofuranose-containing glycans and to clarify the biosynthesis of D-galactofuranose-containing glycans in Aspergillus oryzae, a yellow koji fungus. We found that the galactofuranose antigen is also present in A. oryzae. Deletion of ugmA, which encodes UDP-galactopyranose mutase in A. oryzae, suppressed mycelial elongation, suggesting that D-galactofuranose-containing glycans play an important role in cell wall integrity in A. oryzae. Proton nuclear magnetic resonance spectrometry revealed that the galactofuranose-containing sugar chain was deficient and that core mannan backbone structures were present in ΔugmA A. oryzae, indicating the presence of fungal-type galactomannan in the cell wall fraction of A. oryzae. The findings of this study provide new insights into the cell wall structure of A. oryzae, which is essential for the production of fermented foods in Japan.

Aspergillus fumigatus transcription factor ZfpA regulates hyphal development and alters susceptibility to antifungals and neutrophil killing during infection

Hyphal growth is essential for host colonization during Aspergillus infection. The transcription factor ZfpA regulates A. fumigatus hyphal development including branching, septation, and cell wall composition. However, how ZfpA affects fungal growth and susceptibility to host immunity during infection has not been investigated. Here, we use the larval zebrafish- Aspergillus infection model and primary human neutrophils to probe how ZfpA affects A. fumigatus pathogenesis and response to antifungal drugs in vivo . ZfpA deletion promotes fungal clearance and attenuates virulence in wild-type hosts and this virulence defect is abrogated in neutrophil-deficient zebrafish. ZfpA deletion also increases susceptibility to human neutrophils ex vivo while overexpression impairs fungal killing. Overexpression of ZfpA confers protection against the antifungal caspofungin by increasing chitin synthesis during hyphal development, while ZfpA deletion reduces cell wall chitin and increases caspofungin susceptibility in neutrophil-deficient zebrafish. These findings suggest a protective role for ZfpA activity in resistance to the innate immune response and antifungal treatment during A. fumigatus infection.

Author Summary

Aspergillus fumigatus is a common environmental fungus that can infect immunocompromised people and cause a life-threatening disease called invasive aspergillosis. An important step during infection is the development of A. fumigatus filaments known as hyphae. A. fumigatus uses hyphae to acquire nutrients and invade host tissues, leading to tissue damage and disseminated infection. In this study we report that a regulator of gene transcription in A. fumigatus called ZfpA is important for hyphal growth during infection. We find that ZfpA activity protects the fungus from being killed by innate immune cells and decreases the efficacy of antifungal drugs during infection by regulating construction of the cell wall, an important protective layer for fungal pathogens. Our study introduces ZfpA as an important genetic regulator of stress tolerance during infection that protects A. fumigatus from the host immune response and antifungal drugs.

Identification of an α-(16)-Mannosyltransferase Contributing To Biosynthesis of the Fungal-Type Galactomannan α-Core-Mannan Structure in Aspergillus fumigatus

Fungal-type galactomannan, a cell wall component of Aspergillus fumigatus, is composed of α-(1→2)-/α-(1→6)-linked mannan and β-(1→5)-/β-(1→6)-linked galactofuran side chains. Recently, CmsA and CmsB were identified as the α-(1→2)-mannosyltransferases involved in the biosynthesis of the α-core-mannan. However, the α-(1→6)-mannosyltransferase involved in the biosynthesis of the α-core-mannan has not been identified yet. In this study, we analyzed 9 putative α-(1→6)-mannosyltransferase gene disruption strains of A. fumigatus. The ΔanpA strain resulted in decreased mycelial elongation and reduced conidia formation. Proton nuclear magnetic resonance analysis revealed that the ΔanpA strain failed to produce the α-core-mannan of fungal-type galactomannan. We also found that recombinant AnpA exhibited much stronger α-(1→6)-mannosyltransferase activity toward α-(1→2)-mannobiose than α-(1→6)-mannobiose in vitro. Molecular simulations corroborated the fact that AnpA has a structure that can recognize the donor and acceptor substrates suitable for α-(1→6)-mannoside bond formation and that its catalytic activity would be specific for the elongation of the α-core-mannan structure in vivo. The identified AnpA is similar to Anp1p, which is involved in the elongation of the N-glycan outer chain in budding yeast, but the building sugar chain structure is different. The difference was attributed to the difference in substrate recognition of AnpA, which was clarified by simulations based on protein conformation. Thus, even proteins that seem to be functionally identical due to amino acid sequence similarity may be glycosyltransferase enzymes that make different glycans upon detailed analysis. This study describes an example of such a case. IMPORTANCE Fungal-type galactomannan is a polysaccharide incorporated into the cell wall of filamentous fungi belonging to the subphylum Pezizomycotina. Biosynthetic enzymes of fungal-type galactomannan are potential targets for antifungal drugs and agrochemicals. In this study, we identified an α-(1→6)-mannosyltransferase responsible for the biosynthesis of the α-core-mannan of fungal-type galactomannan, which has not been known for a long time. The findings of this study shed light on processes that shape this cellular structure while identifying a key enzyme essential for the biosynthesis of fungal-type galactomannan.

Mnt1, an α-(1 → 2)-mannosyltransferase responsible for the elongation of N-glycans and O-glycans in Aspergillus fumigatus

The fungal cell wall is necessary for survival as it serves a barrier for physical protection. Therefore, glycosyltransferases responsible for the synthesis of cell wall polysaccharides may be suitable targets for drug development. Mannose is a monosaccharide that is commonly found in sugar chains in the walls of fungi. Mannose residues are present in fungal-type galactomannan, O-glycans, N-glycans, glycosylphosphatidylinositol anchors, and glycosyl inositol phosphorylceramides in Aspergillus fumigatus. Three genes that are homologous to α-(1 → 2)-mannosyltransferase genes and belong to the glycosyltransferase family 15 were found in the A. fumigatus strain, Af293/A1163, genome: cmsA/ktr4, cmsB/ktr7, and mnt1. It is reported that the mutant ∆mnt1 strain exhibited a wide range of properties that included high temperature and drug sensitivity, reduced conidia formation, leakage at the hyphal tips, and attenuation of virulence. However, it is unclear whether Mnt1 is a bona fide α-(1 → 2)-mannosyltransferase and which mannose residues are synthesized by Mnt1 in vivo. In this study, we elucidated the structure of the Mnt1 reaction product, the structure of O-glycan in the Δmnt1 strain. In addition, the length of N-glycans attached to invertase was evaluated in the Δmnt1 strain. The results indicated that Mnt1 functioned as an α-(1 → 2)-mannosyltransferase involved in the elongation of N-glycans and synthesis of the second mannose residue of O-glycans. The widespread abnormal phenotype caused by the disruption of the mnt1 gene is the combined result of the loss of mannose residues from O-glycans and N-glycans. We also clarified the enzymatic properties and substrate specificity of Mnt1 based on its predicted protein structure.

Genetic validation of Aspergillus fumigatus phosphoglucomutase as a viable therapeutic target in invasive aspergillosis

Aspergillus fumigatus is the causative agent of invasive aspergillosis, an infection with mortality rates of up to 50%. The glucan-rich cell wall of A. fumigatus is a protective structure that is absent from human cells and is a potential target for antifungal treatments. Glucan is synthesized from the donor uridine diphosphate glucose, with the conversion of glucose-6-phosphate to glucose-1-phosphate by the enzyme phosphoglucomutase (PGM) representing a key step in its biosynthesis. Here, we explore the possibility of selectively targeting A. fumigatus PGM (AfPGM) as an antifungal treatment strategy. Using a promoter replacement strategy, we constructed a conditional pgm mutant and revealed that pgm is required for A. fumigatus growth and cell wall integrity. In addition, using a fragment screen, we identified the thiol-reactive compound isothiazolone fragment of PGM as targeting a cysteine residue not conserved in the human ortholog. Furthermore, through scaffold exploration, we synthesized a para-aryl derivative (ISFP10) and demonstrated that it inhibits AfPGM with an IC50 of 2 μM and exhibits 50-fold selectivity over the human enzyme. Taken together, our data provide genetic validation of PGM as a therapeutic target and suggest new avenues for inhibiting AfPGM using covalent inhibitors that could serve as tools for chemical validation.

Identification and characterization of β-d-galactofuranosidases from Aspergillus nidulans and Aspergillus fumigatus

Although β-d-galactofuranosidases (Galf-ases) that hydrolyze β-d-galactofuranose (Galf)-containing oligosaccharides have been characterized in various organisms, to date no Galf-specific Galf-ase-encoding genes have been reported in Aspergillus fungi. Based on the amino acid sequences of previously identified bacterial Galf-ases, here we found two candidate Galf-specific Galf-ase genes AN2395 (gfgA) and AN3200 (gfgB) in the genome of Aspergillus nidulans. Indeed, recombinant GfgA and GfgB proteins exhibited Galf-specific Galf-ase activity, but no detectable α-l-arabinofuranosidase (Araf-ase) activity. Phylogenetic analysis of GfgA and GfgB orthologs indicated that there are two types of Aspergillus species: those containing one ortholog each for GfgA and GfgB; and those containing only one ortholog in total, among which Aspergillus fumigatus there is a representative with a single ortholog Galf-ase Afu2g14520. Unlike GfgA and GfgB, the recombinant Afu2g14520 protein showed higher Araf-ase activity than Galf-ase activity. An assay of substrate specificity revealed that although GfgA and GfgB are both exo-type Galf-ases and hydrolyze β-(1,5) and β-(1,6) linkages, GfgA hydrolyzes β-(1,6)-linked Galf-oligosaccharide more effectively as compared with GfgB. Collectively, our findings indicate that Galf-ases in Aspergillus species may have a role in cooperatively degrading Galf-containing oligosaccharides depending on environmental conditions.