Class V chitin synthase and β(1,3)-glucan synthase co-travel in the same vesicle in Zymoseptoria tritici

Role of chitin synthases CHS1 and CHS2 in biosynthesis of the cyst wall of Cryptocaryon irritans

Cryptocaryon irritans, a protozoan parasite that infects marine fish, is characterized by a complex life cycle that includes a cyst-forming reproductive phase. However, the composition of the cyst wall and mechanism of its formation remain unclear. In this study, we identified chitin as a key component of the cyst wall using calcofluor white and wheat germ agglutinin, with Fourier-transform infrared spectroscopy confirming its β-form structure. Two chitin synthase genes, CHS1 and CHS2, were identified as being expressed throughout the life cycle and show close phylogenetic relationships with chitin synthase from ciliates. Incubation with specific anti-CHS1 and -CHS2 antibodies significantly reduced both the thickness and chitin content of the cyst wall, highlighting the critical role of these enzymes in chitin biosynthesis. Treatment with benzoylureas, which inhibit chitin synthesis, caused thinning of the cyst wall and downregulation of CHS gene expression, resulting in an 84 % reduction in the hatching rate after treatment with 0.01 mM CuSO4 compared with control tomonts. Western blot analysis demonstrated that recombinant CHS proteins are immunogenic, and tomonts from CHS-immunized grouper exhibited reduced size. These findings bridge a crucial knowledge gap in understanding of the C. irritans cyst wall and highlight promising targets for infection prevention and control strategies.

Breaking down barriers: comprehensive functional analysis of the Aspergillus niger chitin synthase repertoire

BACKGROUND

Members of the fungal kingdom are heterotrophic eukaryotes encased in a chitin containing cell wall. This polymer is vital for cell wall stiffness and, ultimately, cell shape. Most fungal genomes contain numerous putative chitin synthase encoding genes. However, systematic functional analysis of the full chitin synthase catalogue in a given species is rare. This greatly limits fundamental understanding and potential applications of manipulating chitin synthesis across the fungal kingdom.

RESULTS

In this study, we conducted in silico profiling and subsequently deleted all predicted chitin synthase encoding genes in the multipurpose cell factory Aspergillus niger. Phylogenetic analysis suggested nine chitin synthases evolved as three distinct groups. Transcript profiling and co-expression network construction revealed remarkably independent expression, strongly supporting specific role(s) for the respective chitin synthases. Deletion mutants confirmed all genes were dispensable for germination, yet impacted colony spore titres, chitin content at hyphal septa, and internal architecture of submerged fungal pellets. We were also able to assign specific roles to individual chitin synthases, including those impacting colony radial growth rates (ChsE, ChsF), lateral cell wall chitin content (CsmA), chemical genetic interactions with a secreted antifungal protein (CsmA, CsmB, ChsE, ChsF), resistance to therapeutics (ChsE), and those that modulated pellet diameter in liquid culture (ChsA, ChsB). From an applied perspective, we show chsF deletion increases total protein in culture supernatant over threefold compared to the control strain, indicating engineering filamentous fungal chitin content is a high priority yet underexplored strategy for strain optimization.

CONCLUSION

This study has conducted extensive analysis for the full chitin synthase encoding gene repertoire of A. niger. For the first time we reveal both redundant and non-redundant functional roles of chitin synthases in this fungus. Our data shed light on the complex, multifaceted, and dynamic role of chitin in fungal growth, morphology, survival, and secretion, thus improving fundamental understanding and opening new avenues for biotechnological applications in fungi.

Turning Inside Out: Filamentous Fungal Secretion and Its Applications in Biotechnology, Agriculture, and the Clinic

Filamentous fungi are found in virtually every marine and terrestrial habitat. Vital to this success is their ability to secrete a diverse range of molecules, including hydrolytic enzymes, organic acids, and small molecular weight natural products. Industrial biotechnologists have successfully harnessed and re-engineered the secretory capacity of dozens of filamentous fungal species to make a diverse portfolio of useful molecules. The study of fungal secretion outside fermenters, e.g., during host infection or in mixed microbial communities, has also led to the development of novel and emerging technological breakthroughs, ranging from ultra-sensitive biosensors of fungal disease to the efficient bioremediation of polluted environments. In this review, we consider filamentous fungal secretion across multiple disciplinary boundaries (e.g., white, green, and red biotechnology) and product classes (protein, organic acid, and secondary metabolite). We summarize the mechanistic understanding for how various molecules are secreted and present numerous applications for extracellular products. Additionally, we discuss how the control of secretory pathways and the polar growth of filamentous hyphae can be utilized in diverse settings, including industrial biotechnology, agriculture, and the clinic.

Abnormal sterol-induced cell wall glucan deficiency in yeast is due to impaired glucan synthase transport to the plasma membrane

Abnormal sterols disrupt cellular functions through yet unclear mechanisms. In Saccharomyces cerevisiae, accumulation of Δ8-sterols, the same type of sterols observed in patients of Conradi-Hünermann-Happle syndrome or in fungi after amine fungicide treatment, leads to cell wall weakness. We have studied the influence of Δ8-sterols on the activity of glucan synthase I, the protein synthetizing the main polymer in fungal cell walls, its regulation by the Cell Wall Integrity (CWI) pathway, and its transport from the endoplasmic reticulum to the plasma membrane. We ascertained that the catalytic characteristics were mostly unaffected by the presence of abnormal sterols but the enzyme was partially retained in the endoplasmic reticulum, leading to glucan deficit at the cell wall. Furthermore, we observed that glucan synthase I traveled through an unconventional exocytic route to the plasma membrane that is associated with low density intracellular membranes. Also, we found out that the CWI pathway remained inactive despite low glucan levels at the cell wall. Taken together, these data suggest that Δ8-sterols affect cell walls by inhibiting unconventional secretion of proteins leading to retention and degradation of glucan synthase I, while the compensatory CWI pathway is unable to activate. These results could be instrumental to understand defects of bone development in cholesterol biosynthesis disorders and fungicide mechanisms of action.

Characterization of Extracellular Vesicles Produced by Aspergillus fumigatus Protoplasts

Fungal cells use extracellular vesicles (EVs) to export biologically active molecules to the extracellular space. In this study, we used protoplasts of Aspergillus fumigatus, a major fungal pathogen, as a model to evaluate the role of EV production in cell wall biogenesis. Our results demonstrated that wall-less A. fumigatus exports plasma membrane-derived EVs containing a complex combination of proteins and glycans. Our report is the first to characterize fungal EVs in the absence of a cell wall. Our results suggest that protoplasts represent a promising model for functional studies of fungal vesicles.

Extracellular vesicles (EVs) are membranous compartments produced by yeast and mycelial forms of several fungal species. One of the difficulties in perceiving the role of EVs during the fungal life, and particularly in cell wall biogenesis, is caused by the presence of a thick cell wall. One alternative to have better access to these vesicles is to use protoplasts. This approach has been investigated here with Aspergillus fumigatus, one of the most common opportunistic fungal pathogens worldwide. Analysis of regenerating protoplasts by scanning electron microscopy and fluorescence microscopy indicated the occurrence of outer membrane projections in association with surface components and the release of particles with properties resembling those of fungal EVs. EVs in culture supernatants were characterized by transmission electron microscopy and nanoparticle tracking analysis. Proteomic and glycome analysis of EVs revealed the presence of a complex array of enzymes related to lipid/sugar metabolism, pathogenic processes, and cell wall biosynthesis. Our data indicate that (i) EV production is a common feature of different morphological stages of this major fungal pathogen and (ii) protoplastic EVs are promising tools for undertaking studies of vesicle functions in fungal cells.

IMPORTANCE Fungal cells use extracellular vesicles (EVs) to export biologically active molecules to the extracellular space. In this study, we used protoplasts of Aspergillus fumigatus, a major fungal pathogen, as a model to evaluate the role of EV production in cell wall biogenesis. Our results demonstrated that wall-less A. fumigatus exports plasma membrane-derived EVs containing a complex combination of proteins and glycans. Our report is the first to characterize fungal EVs in the absence of a cell wall. Our results suggest that protoplasts represent a promising model for functional studies of fungal vesicles.