β-1,6-linked Galactofuranose- rich peptidogalactomannan of Fusarium oxysporum is important in the activation of macrophage mechanisms and as a potential diagnostic antigen

Multifaceted roles of plant glycosyl hydrolases during pathogen infections: more to discover

MAIN CONCLUSION

Carbohydrates are hydrolyzed by a family of carbohydrate-active enzymes (CAZymes) called glycosidases or glycosyl hydrolases. Here, we have summarized the roles of various plant defense glycosidases that possess different substrate specificities. We have also highlighted the open questions in this research field. Glycosidases or glycosyl hydrolases (GHs) are a family of carbohydrate-active enzymes (CAZymes) that hydrolyze glycosidic bonds in carbohydrates and glycoconjugates. Compared to those of all other sequenced organisms, plant genomes contain a remarkable diversity of glycosidases. Plant glycosidases exhibit activities on various substrates and have been shown to play important roles during pathogen infections. Plant glycosidases from different GH families have been shown to act upon pathogen components, host cell walls, host apoplastic sugars, host secondary metabolites, and host N-glycans to mediate immunity against invading pathogens. We could classify the activities of these plant defense GHs under eleven different mechanisms through which they operate during pathogen infections. Here, we have provided comprehensive information on the catalytic activities, GH family classification, subcellular localization, domain structure, functional roles, and microbial strategies to regulate the activities of defense-related plant GHs. We have also emphasized the research gaps and potential investigations needed to advance this topic of research.

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.

Defence response in plants and animals against a common fungal pathogen, Fusarium oxysporum

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Fusarium oxysporum species complex (FOSC) is considered one of the most devastating plant pathogen.

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FOSC is an emerging pathogen of immunocompromised individuals.

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Mycotoxins produced by FOSC predisposes the host to other pathogens.

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Comparative immune reactions in plant and invertebrate show that several antimicrobial peptides (AMPs) and secondary metabolites maybe used as control against Fusarium infection.

Plant pathogens emerging as threat to human and animal health has been a matter of concern within the scientific community. Fusarium oxysporum, predominantly a phytopathogen, can infect both plants and animals. As a plant pathogen, F. oxysporum is one of the most economically damaging pathogen. In humans, F. oxysporum can infect immunocompromised individuals and is increasingly being considered as a problematic pathogen. Mycotoxins produced by F. oxysporum supress the innate immune pathways in both plants and animals. Hence, F. oxysporum is the perfect example for studying similarities and differences between defence strategies adopted by plants and animals. In this review we will discuss the innate immune response of plant and animal hosts for protecting against F. oxysporum infection. Such studies will be helpful for identifying genes, protein and metabolites with antifungal properties suitable for protecting humans.

Galactofuranose (Galf)-containing sugar chain contributes to the hyphal growth, conidiation and virulence of F. oxysporum f.sp. cucumerinum

The ascomycete fungus Fusarium oxysporum f.sp. cucumerinum causes vascular wilt diseases in cucumber. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. BLASTp searches of the Aspergillus fumigatus UgmA and galatofuranosyltransferases (Galf-transferases) sequences in the F. oxysporum genome identified two genes encoding putative UDP-galactopyranose mutase (UGM), ugmA and ugmB, and six genes encoding putative Galf-transferase homologs. In this study, the single and double mutants of the ugmA, ugmB and gfsB were obtained. The roles of UGMs and GfsB were investigated by analyzing the phenotypes of the mutants. Our results showed that deletion of the ugmA gene led to a reduced production of galactofuranose-containing sugar chains, reduced growth and impaired conidiation of F. oxysporum f.sp. cucumerinum. Most importantly, the ugmA deletion mutant lost the pathogenicity in cucumber plantlets. Although deletion of the ugmB gene did not cause any visible phenotype, deletion of both ugmA and ugmB genes caused more severe phenotypes as compared with the ΔugmA, suggesting that UgmA and UgmB are redundant and they can both contribute to synthesis of UDP-Galf. Furthermore, the ΔgfsB exhibited an attenuated virulence although no other phenotype was observed. Our results demonstrate that the galactofuranose (Galf) synthesis contributes to the cell wall integrity, germination, hyphal growth, conidiation and virulence in Fusarium oxysporum f.sp. cucumerinum and an ideal target for the development of new anti-Fusarium agents.

Peptidogalactomannan from Histoplasma capsulatum yeast cell wall: role of the chemical structure in recognition and activation by peritoneal macrophages

Histoplasma capsulatum is the causative agent of histoplasmosis, a systemic disease responsible for most reported causes of morbidity and mortality among immunosuppressed individuals. Peptidogalactomannan (pGM) was purified from the yeast cell wall of H. capsulatum isolated from bats, and its structure and involvement in modulating the host immune response were evaluated. Gas chromatography, methylation analysis, and two-dimensional nuclear magnetic resonance (2D-NMR) were used for the structural characterization of pGM. Methylation and 2D-NMR data revealed that pGM comprises a main chain containing α-D-Manp (1 → 6) residues substituted at O-2 by α-D-Manp (1 → 2)-linked side chains, non-reducing end units of α-D-Galf, or β-D-Galp linked (1→ 6) to α-D-Manp side chains. The involvement of H. capsulatum pGM in antigenic reactivity and in interactions with macrophages was demonstrated by ELISA and phagocytosis assay, respectively. The importance of the carbohydrate and protein moieties of pGM in sera reactivity was evaluated. Periodate oxidation abolished much pGM antigenic reactivity, suggesting that the sugar moiety is the most immunogenic part of pGM. Reactivity slightly decreased in pGM treated with proteinase K, suggesting that the peptide moiety plays a minor role in pGM antigenicity. In vitro experiments suggested that pGM is involved in the phagocytosis of H. capsulatum yeast and induction of IL-10 and IFN-γ secretion by peritoneal macrophages from C57BL/6 mice. These findings demonstrated the role of pGM in the H. capsulatum-host interaction.

A retrospective cohort of invasive fusariosis in the era of antimould prophylaxis

Mould-active prophylaxis is affecting the epidemiology of invasive mycoses in the form of a shift toward less common entities such as fusariosis. We analyze the characteristics of invasive fusariosis and its association to antifungal prophylaxis in a retrospective cohort (2004-2017) from a tertiary hospital in Madrid, Spain. Epidemiological, clinical, microbiological, and antifungal consumption data were retrieved. Isolates were identified to molecular level, and antifungal susceptibility was tested. Eight cases of invasive fusariosis were diagnosed. Three periods were identified according to incidence: <2008 (three cases), 2008-2013 (zero cases), >2014 (five cases). All except one case involved breakthrough fusariosis. During the earliest period, the episodes occurred while the patient was taking itraconazole (two) or fluconazole (one); more recently, while on micafungin (three) or posaconazole (one). Early cases involved acute leukemia at induction/consolidation, recent cases relapsed/refractory disease (P = .029). Main risk factor for fusariosis (62.5%) was prolonged neutropenia (median 44 days). Galactomannan and beta-D-glucan were positive in 37.5% and 100% of cases, respectively. All isolates except F. proliferatum presented high minimal inhibitory concentrations (MICs) against the azoles and lower MIC to amphotericin B. Most patients received combined therapy. Mortality at 42 days was 62.5%. Resolution of neutropenia was associated with survival (P = .048). Invasive fusariosis occurs as breakthrough infection in patients with hematologic malignancy, prolonged neutropenia, and positive fungal biomarkers. Recent cases were diagnosed in a period of predominant micafungin use in patients who had more advanced disease and protracted neutropenia and for whom mortality was extremely high. Resolution of neutropenia was a favorable prognostic factor.

Peptidorhamanomannan: A surface fungal glycoconjugate from Scedosporium aurantiacum and Scedosporium minutisporum and its recognition by macrophages

The genus Scedosporium is composed of clinically relevant fungal species, such as Scedosporium aurantiacum, Scedosporium apiospermum, and Scedosporium boydii. Surface molecules have been described that play crucial roles in fungi-macrophage interaction, and many of them are pathogen-associated molecular patterns (PAMPs). The present study aims to characterize peptidoglycans obtained from Scedosporium aurantiacum and Scedosporium minutisporum, a clinical and an environmental isolate, respectively, and compare their roles in pathogen-host interaction. Both molecules were characterized as peptidorhamnomannans (PRMs), similar to what has been already described for other Scedosporium species. Rabbit immune sera obtained by injecting whole cells from each species recognized both fungal cells and purified PRMs, suggesting that a cross-reaction occur between both fungi. Immunofluorescent microscopy revealed that PRMs are exposed on fungal surface. Prior incubation of purified molecules with immune sera before adding to cells led to loss of fluorescent, indicating that PRM is a major molecule recognized by immune sera. Fungi-macrophage interaction revealed that S. aurantiacum is able to survive more inside phagocytic cells than S. minutisporum, and PRM from both fungi plays a role in phagocytosis when the purified molecule is pre-incubated with macrophage. In addition, PRM induce nitric oxide release by macrophages. Our data indicate that PRM is an important PAMP exposed on fungal surface with the potential of immune modulation.