Substantial decrease in cell wall α-1,3-glucan caused by disruption of the kexB gene encoding a subtilisin-like processing protease in Aspergillus oryzae

First Insight into the Degradome of Aspergillus ochraceus: Novel Secreted Peptidases and Their Inhibitors

Aspergillus fungi constitute a pivotal element within ecosystems, serving as both contributors of biologically active compounds and harboring the potential to cause various diseases across living organisms. The organism's proteolytic enzyme complex, termed the degradome, acts as an intermediary in its dynamic interaction with the surrounding environment. Using techniques such as genome and transcriptome sequencing, alongside protein prediction methodologies, we identified putative extracellular peptidases within Aspergillus ochraceus VKM-F4104D. Following manual annotation procedures, a total of 11 aspartic, 2 cysteine, 2 glutamic, 21 serine, 1 threonine, and 21 metallopeptidases were attributed to the extracellular degradome of A. ochraceus VKM-F4104D. Among them are enzymes with promising applications in biotechnology, potential targets and agents for antifungal therapy, and microbial antagonism factors. Thus, additional functionalities of the extracellular degradome, extending beyond mere protein substrate digestion for nutritional purposes, were demonstrated.

Deletion of the Aspergillus niger Pro-Protein Processing Protease Gene kexB Results in a pH-Dependent Morphological Transition during Submerged Cultivations and Increases Cell Wall Chitin Content

There is a growing interest in the use of post-fermentation mycelial waste to obtain cell wall chitin as an added-value product. In the pursuit to identify suitable production strains that can be used for post-fermentation cell wall harvesting, we turned to an Aspergillus niger strain in which the kexB gene was deleted. Previous work has shown that the deletion of kexB causes hyper-branching and thicker cell walls, traits that may be beneficial for the reduction in fermentation viscosity and lysis. Hyper-branching of ∆kexB was previously found to be pH-dependent on solid medium at pH 6.0, but was absent at pH 5.0. This phenotype was reported to be less pronounced during submerged growth. Here, we show a series of controlled batch cultivations at a pH range of 5, 5.5, and 6 to examine the pellet phenotype of ΔkexB in liquid medium. Morphological analysis showed that ΔkexB formed wild type-like pellets at pH 5.0, whereas the hyper-branching ΔkexB phenotype was found at pH 6.0. The transition of phenotypic plasticity was found in cultivations at pH 5.5, seen as an intermediate phenotype. Analyzing the cell walls of ΔkexB from these controlled pH-conditions showed an increase in chitin content compared to the wild type across all three pH values. Surprisingly, the increase in chitin content was found to be irrespective of the hyper-branching morphology. Evidence for alterations in cell wall make-up are corroborated by transcriptional analysis that showed a significant cell wall stress response in addition to the upregulation of genes encoding other unrelated cell wall biosynthetic genes.

Efficient acylation of gastrodin by Aspergillus oryzae whole-cells in non-aqueous media

Gastrodin, a bioactive compound extracted from the plant source of Gastrodia elata Blume, has a wide range of therapeutic effects on central nervous system (CNS) diseases, but suffers from poor brain permeability and short half-life in plasma. In this study, fatty acid esters of gastrodin were successfully synthesized by a whole cell-based biocatalytic method. Aspergillus oryzae cells showed different catalytic activities in the organic solvent systems tested. Tetrahydrofuran was confirmed as the most suitable pure organic solvent, with the highest substrate conversion of 98.0%. Addition of ionic liquids (ILs) into pyridine dramatically accelerated the reaction with conversion increased from 5.9% to 84.2%, and also changed the selectivity of the cells, mainly due to the use of IL-containing systems altering cell permeability and contact of the enzymes with solvent molecules possessing different polarities. The ester products were characterized by 13C-NMR and ESI-MS as gastrodin monoester and diester.

Function and Biosynthesis of Cell Wall α-1,3-Glucan in Fungi

Although α-1,3-glucan is a major cell wall polysaccharide in filamentous fungi, its biological functions remain unclear, except that it acts as a virulence factor in animal and plant pathogenic fungi: it conceals cell wall β-glucan on the fungal cell surface to circumvent recognition by hosts. However, cell wall α-1,3-glucan is also present in many of non-pathogenic fungi. Recently, the universal function of α-1,3-glucan as an aggregation factor has been demonstrated. Applications of fungi with modified cell wall α-1,3-glucan in the fermentation industry and of in vitro enzymatically-synthesized α-1,3-glucan in bio-plastics have been developed. This review focuses on the recent progress in our understanding of the biological functions and biosynthetic mechanism of cell wall α-1,3-glucan in fungi. We briefly consider the history of studies on α-1,3-glucan, overview its biological functions and biosynthesis, and finally consider the industrial applications of fungi deficient in α-1,3-glucan.

Characterization of Cell Wall α-1,3-Glucan-Deficient Mutants in Aspergillus oryzae Isolated by a Screening Method Based on Their Sensitivities to Congo Red or Lysing Enzymes

We previously reported that sensitivity to Congo Red (CR) or Lysing Enzymes (LE) is affected by the loss of cell-wall α-1,3-glucan (AG) in Aspergillus nidulans. We found that the amount of CR adsorbed to AG was significantly less than the amount adsorbed to β-1,3-glucan (BG) or chitin, suggesting that loss of cell-wall AG would increase exposure of BG on the cell surface, and thereby increase the sensitivity to CR. Generally, fungal BGs are known as biological response modifiers because of their recognition by Dectin-1 receptors in human immune systems. Therefore, isolation of AG-deficient mutants in Aspergillus oryzae has been used in the Japanese fermentation industry to create strains with increased ability to promote immune responses. Here, we aimed to isolate AG-deficient strains by mutagenizing A. oryzae conidia with chemical mutagens. Based on the increased sensitivity to CR in AG-deficient strains of A. nidulans and A. oryzae, we established a screening method for isolation of AG-deficient strains. Several candidate AG-deficient mutants of A. oryzae were isolated using the screening method; these strains showed increased sensitivity to CR and/or LE. Cytokine production was increased in the dendritic cells co-incubated with germinated conidia of the AG-deficient mutants. Furthermore, according to a Dectin-1 NFAT (nuclear factor of activator T cells)-GFP (green fluorescent protein) reporter assay, Dectin-1 response levels in the AG-deficient mutants were higher than those in wild-type A. oryzae. These results suggest that we successfully isolated AG-deficient mutants of A. oryzae with immunostimulatory effects.

Comparative morphology and transcriptome analysis reveals distinct functions of the primary and secondary laticifer cells in the rubber tree

Laticifers are highly specialized cells that synthesize and store natural rubber. Rubber trees (Hevea brasiliensis Muell. Arg.) contain both primary and secondary laticifers. Morphological and functional differences between the two types of laticifers are largely unknown, but such information is important for breeding and cultivation practices. Morphological comparison using paraffin sections revealed only distribution differences: the primary laticifers were distributed randomly, while the secondary laticifers were distributed in concentric rings. Using isolated laticifer networks, the primary laticifers were shown to develop via intrusive "budding" and formed necklace-like morphology, while the secondary laticifers developed straight and smooth cell walls. Comparative transcriptome analysis indicated that genes involved in cell wall modification, such as pectin esterase, lignin metabolic enzymes, and expansins, were highly up-regulated in the primary laticifers and correspond to its necklace-like morphology. Genes involved in defense against biotic stresses and rubber biosynthesis were highly up-regulated in the primary laticifers, whereas genes involved in abiotic stresses and dormancy were up-regulated in the secondary laticifers, suggesting that the primary laticifers are more adequately prepared to defend against biotic stresses, while the secondary laticifers are more adequately prepared to defend against abiotic stresses. Therefore, the two types of laticifers are morphologically and functionally distinct.

Deletion of admB gene encoding a fungal ADAM affects cell wall construction in Aspergillus oryzae

Mammals possess a unique signaling system based on the proteolytic mechanism of a disintegrin and metalloproteinases (ADAMs) on the cell surface. We found two genes encoding ADAMs in Aspergillus oryzae and named them admA and admB. We produced admA and admB deletion strains to elucidate their biological function and clarify whether fungal ADAMs play a similar role as in mammals. The ∆admA∆admB and ∆admB strains were sensitive to cell wall-perturbing agents, congo red, and calcofluor white. Moreover, the two strains showed significantly increased weights of total alkali-soluble fractions from the mycelial cell wall compared to the control strain. Furthermore, ∆admB showed MpkA phosphorylation at lower concentration of congo red stimulation than the control strain. However, the MpkA phosphorylation level was not different between ∆admB and the control strain without the stimulation. The results indicated that A. oryzae AdmB involved in the cell wall integrity without going through the MpkA pathway.

Increased enzyme production under liquid culture conditions in the industrial fungus Aspergillus oryzae by disruption of the genes encoding cell wall α-1,3-glucan synthase

Under liquid culture conditions, the hyphae of filamentous fungi aggregate to form pellets, which reduces cell density and fermentation productivity. Previously, we found that loss of α-1,3-glucan in the cell wall of the fungus Aspergillus nidulans increased hyphal dispersion. Therefore, here we constructed a mutant of the industrial fungus A. oryzae in which the three genes encoding α-1,3-glucan synthase were disrupted (tripleΔ). Although the hyphae of the tripleΔ mutant were not fully dispersed, the mutant strain did form smaller pellets than the wild-type strain. We next examined enzyme productivity under liquid culture conditions by transforming the cutinase-encoding gene cutL1 into A. oryzae wild-type and the tripleΔ mutant (i.e. wild-type-cutL1, tripleΔ-cutL1). A. oryzae tripleΔ-cutL1 formed smaller hyphal pellets and showed both greater biomass and increased CutL1 productivity compared with wild-type-cutL1, which might be attributable to a decrease in the number of tripleΔ-cutL1 cells under anaerobic conditions.