The C2H2 transcription factor SltA is required for germination and hyphal development in Aspergillus fumigatus

Recent developments in Aspergillus fumigatus research: diversity, drugs, and disease

SUMMARYAdvances in modern medical therapies for many previously intractable human diseases have improved patient outcomes. However, successful disease treatment outcomes are often prevented due to invasive fungal infections caused by the environmental mold Aspergillus fumigatus. As contemporary antifungal therapies have not experienced the same robust advances as other medical therapies, defining mechanisms of A. fumigatus disease initiation and progression remains a critical research priority. To this end, the World Health Organization recently identified A. fumigatus as a research priority human fungal pathogen and the Centers for Disease Control has highlighted the emergence of triazole-resistant A. fumigatus isolates. The expansion in the diversity of host populations susceptible to aspergillosis and the complex and dynamic A. fumigatus genotypic and phenotypic diversity call for a reinvigorated assessment of aspergillosis pathobiological and drug-susceptibility mechanisms. Here, we summarize recent advancements in the field and discuss challenges in our understanding of A. fumigatus heterogeneity and its pathogenesis in diverse host populations.

A microscopy-based image analysis pipeline for the quantification of germination of filamentous fungi

Germination is the fundamental process whereby fungi transition from the dormant and stress resistant spores into actively replicating cells such as hyphae. Germination is essential for fungal colonization of new environments and pathogenesis, yet this differentiation process remains relatively poorly understood. For filamentous fungi, the study of germination has been limited by the lack of high-throughput, temporal, low cost, and easy-to-use methods of quantifying germination. To this end we have developed an image analysis pipeline to automate the quantification of germination from microscopy images. We have optimized this tool for the fungal pathogen Aspergillus fumigatus and demonstrated its potential applications by evaluating different strains, germination inhibitors, and auxotrophic and antifungal resistant mutants. Finally, we have expanded this tool to a variety of filamentous fungi and developed an easy-to-use web app for the fungal research community.

F-box protein Fbx23 acts as a transcriptional coactivator to recognize and activate transcription factor Ace1

Protein ubiquitination is usually coupled with proteasomal degradation and is crucial in regulating protein quality. The E3 ubiquitin-protein ligase SCF (Skp1-Cullin-F-box) complex directly recognizes the target substrate via interaction between the F-box protein and the substrate. F-box protein is the determinant of substrate specificity. The limited number of identified ubiquitin ligase-substrate pairs is a major bottleneck in the ubiquitination field. Penicillium oxalicum contains many transcription factors, such as BrlA, CreA, XlnR, and Ace1, conserved in filamentous fungi that regulate the fungal development and transcription of (hemi)cellulase genes. Transcription factor Ace1 (also known as SltA) positively correlated with fungal growth and conidiation and negatively correlated with the expression of (hemi)cellulase genes. A ubiquitin ligase-substrate pair, SCFFbx23-Ace1, is identified in P. oxalicum. Most of PoFbx23 is present in free form within the nucleus. A small portion of PoFbx23 associates with Skp1 to form PoFbx23-Skp1 heterodimer or assembles with the three invariable core components (Skp1, Cul1, and Rbx1) of SCF to form the SCFFbx23 complex. Under glucose signal, PoFbx23 absence (Δfbx23) results in decreased transcription levels of the brlA gene which encodes the master regulator for asexual development and six spore pigmentation genes (abrB→abrA→aygB→arpA→arpB→albA) which encode the proteins in the dihydroxynaphthalene-melanin pathway, along with impaired conidiation. Under cellulose signal, transcription levels of (hemi)cellulase genes in the Δfbx23 mutant are significantly upregulated. When PoFbx23 is present, PoAce1 exists as a full-length version and several low-molecular-weight degraded versions. PoAce1 has polyubiquitin modification. Deleting the Pofbx23 gene does not affect Poace1 gene transcription but results in the remarkable accumulation of all versions of the PoAce1 protein. Accumulated PoAce1 protein is a dysfunctional form that no longer binds promoters of the target gene, including the cellulase genes cbh1 and eg1, the hemicellulase gene xyn11A, and the pigmentation-related gene abrB. PoFbx23 acts as a transcriptional coactivator, recognizing and activating PoAce1, allowing the latter to regulate the transcription of target genes with different effects (activating or repressing) under different signals.

Aspergillus fumigatus conidial surface-associated proteome reveals factors for fungal evasion and host immunity modulation

Aspergillus fumigatus causes aspergillosis and relies on asexual spores (conidia) for initiating host infection. There is scarce information about A. fumigatus proteins involved in fungal evasion and host immunity modulation. Here we analysed the conidial surface proteome of A. fumigatus, two closely related non-pathogenic species, Aspergillus fischeri and Aspergillus oerlinghausenensis, as well as pathogenic Aspergillus lentulus, to identify such proteins. After identifying 62 proteins exclusively detected on the A. fumigatus conidial surface, we assessed null mutants for 42 genes encoding these proteins. Deletion of 33 of these genes altered susceptibility to macrophage, epithelial cells and cytokine production. Notably, a gene that encodes a putative glycosylasparaginase, modulating levels of the host proinflammatory cytokine IL-1β, is important for infection in an immunocompetent murine model of fungal disease. These results suggest that A. fumigatus conidial surface proteins are important for evasion and modulation of the immune response at the onset of fungal infection.

The EH domain-containing protein, EdeA, is involved in endocytosis, cell wall integrity, and pathogenicity in Aspergillus fumigatus

Endocytosis has been extensively studied in yeasts, where it plays crucial roles in growth, signaling regulation, and cell-surface receptor internalization. However, the biological functions of endocytosis in pathogenic filamentous fungi remain largely unexplored. In this study, we aimed to functionally characterize the roles of EdeA, an ortholog of the Saccharomyces cerevisiae endocytic protein Ede1, in Aspergillus fumigatus. EdeA was observed to be distributed as patches on the plasma membrane and concentrated in the subapical collar of hyphae, a localization characteristic of endocytic proteins. Loss of edeA caused defective hyphal polarity, reduced conidial production, and fewer sites of endocytosis initiations than that of the parental wild type. Notably, the edeA null mutant exhibited increased sensitivity to cell wall-disrupting agents, indicating a role for EdeA in maintaining cell wall integrity in A. fumigatus. This observation was further supported by the evidence showing that the thickness of the cell wall in the ΔedeA mutant increased, accompanied by abnormal activation of MpkA, a key component in the cell wall integrity pathway. Additionally, the ΔedeA mutant displayed increased pathogenicity in the Galleria mellonella wax moth infection model, possibly due to alterations in cell wall morphology. Site-directed mutagenesis identified the conserved residue E348 within the third EH (Eps15 homology) domain of EdeA as crucial for its subcellular localization and functions. In conclusion, our results highlight the involvement of EdeA in endocytosis, hyphal polarity, cell wall integrity, and pathogenicity in A. fumigatus.

IMPORTANCE

Aspergillus fumigatus is a significant human pathogenic fungus known to cause invasive aspergillosis, a disease with a high mortality rate. Understanding the basic principles of A. fumigatus pathogenicity is crucial for developing effective strategies against this pathogen. Previous research has underscored the importance of endocytosis in the infection capacity of pathogenic yeasts; however, its biological function in pathogenic mold remains largely unexplored. Our characterization of EdeA in A. fumigatus sheds light on the role of endocytosis in the development, stress response, and pathogenicity of pathogenic molds. These findings suggest that the components of the endocytosis process may serve as potential targets for antifungal therapy.

Interconnections between the Cation/Alkaline pH-Responsive Slt and the Ambient pH Response of PacC/Pal Pathways in Aspergillus nidulans

In the filamentous ascomycete Aspergillus nidulans, at least three high hierarchy transcription factors are required for growth at extracellular alkaline pH: SltA, PacC and CrzA. Transcriptomic profiles depending on alkaline pH and SltA function showed that pacC expression might be under SltA regulation. Additional transcriptional studies of PacC and the only pH-regulated pal gene, palF, confirmed both the strong dependence on ambient pH and the function of SltA. The regulation of pacC expression is dependent on the activity of the zinc binuclear (C6) cluster transcription factor PacX. However, we found that the ablation of sltA in the pacX- mutant background specifically prevents the increase in pacC expression levels without affecting PacC protein levels, showing a novel specific function of the PacX factor. The loss of sltA function causes the anomalous proteolytic processing of PacC and a reduction in the post-translational modifications of PalF. At alkaline pH, in a null sltA background, PacC72kDa accumulates, detection of the intermediate PacC53kDa form is extremely low and the final processed form of 27 kDa shows altered electrophoretic mobility. Constitutive ubiquitination of PalF or the presence of alkalinity-mimicking mutations in pacC, such as pacCc14 and pacCc700, resembling PacC53kDa and PacC27kDa, respectively, allowed the normal processing of PacC but did not rescue the alkaline pH-sensitive phenotype caused by the null sltA allele. Overall, data show that Slt and PacC/Pal pathways are interconnected, but the transcription factor SltA is on a higher hierarchical level than PacC on regulating the tolerance to the ambient alkalinity in A. nidulans.

Transcriptomic and biochemical analyses revealed antifungal mechanism of trans-anethole on Aspergillus flavus growth

Plant volatile compounds have great potential for preventing and controlling fungal spoilage in post-harvest grains. Recently, we have reported the antifungal effects of trans-anethole, the main volatile constituent of the Illicium verum fruit, on Aspergillus flavus. In this study, the inhibitory mechanisms of trans-anethole against the growth of A. flavus mycelia were investigated using transcriptomic and biochemical analyses. Biochemical and transcriptomic changes in A. flavus mycelia were evaluated after exposure to 0.2 μL/mL trans-anethole. Scanning electron microscopy showed that trans-anethole treatment resulted in the surface wrinkling of A. flavus mycelia, and calcofluor white staining confirmed that trans-anethole treatment disrupted the mycelial cell wall structure. Annexin V-fluorescein isothiocyanate/propidium iodide double staining suggested that trans-anethole induced apoptosis in A. flavus mycelia. Reduced mitochondrial membrane potential and DNA damage were observed in trans-anethole-treated A. flavus mycelia using 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethyl-imidacarbocyanine and 4',6-diamidino-2-phenylindole staining, respectively. 2',7'- Dichloro-dihydro-fluorescein diacetate staining and biochemical assays demonstrated that trans-anethole treatment cause the accumulation of reactive oxygen species in the A. flavus mycelia. Transcriptome results showed that 1673 genes were differentially expressed in A. flavus mycelia exposed to trans-anethole, which were mainly associated with multidrug transport, oxidative phosphorylation, citric acid cycle, ribosomes, and cyclic adenosine monophosphate signaling. We propose that trans-anethole can inhibit the growth of A. flavus mycelia by disrupting the cell wall structure, blocking the multidrug transport process, disturbing the citric acid cycle, and inducing apoptosis. This study provides new insights into the inhibitory mechanism of trans-anethole on A. flavus mycelia and will be helpful for the development of natural fungicides. KEY POINTS: • Biochemical analyses of A. flavus mycelia exposed to trans-anethole were performed • Transcriptomic changes in trans-anethole-treated A. flavus mycelia were analyzed • An inhibitory mechanism of trans-anethole on the growth of A. flavus mycelia was proposed.