Peroxiredoxin Asp f3 Is Essential for Aspergillus fumigatus To Overcome Iron Limitation during Infection

Insights and Perspectives on the Role of Proteostasis and Heat Shock Proteins in Fungal Infections

Fungi are a diverse group of eukaryotic organisms that infect humans, animals, and plants. To successfully colonize their hosts, pathogenic fungi must continuously adapt to the host's unique environment, e.g., changes in temperature, pH, and nutrient availability. Appropriate protein folding, assembly, and degradation are essential for maintaining cellular homeostasis and survival under stressful conditions. Therefore, the regulation of proteostasis is crucial for fungal pathogenesis. The heat shock response (HSR) is one of the most important cellular mechanisms for maintaining proteostasis. It is activated by various stresses and regulates the activity of heat shock proteins (HSPs). As molecular chaperones, HSPs participate in the proteostatic network to control cellular protein levels by affecting their conformation, location, and degradation. In recent years, a growing body of evidence has highlighted the crucial yet understudied role of stress response circuits in fungal infections. This review explores the role of protein homeostasis and HSPs in fungal pathogenicity, including their contributions to virulence and host-pathogen interactions, as well as the concerted effects between HSPs and the main proteostasis circuits in the cell. Furthermore, we discuss perspectives in the field and the potential for targeting the components of these circuits to develop novel antifungal therapies.

The role of peroxiredoxins PrxA and PrxB in the antioxidant response, carbon utilization and development in Aspergillus nidulans

In addition to their role in the breakdown of H2O2, some peroxiredoxins (Prxs) have chaperone and H2O2 sensing functions. Acting as an H2O2 sensor, Prx Gpx3 transfers the oxidant signal to the transcription factor Yap1, involved in the antioxidant response in Saccharomyces cerevisiae. We have shown that Aspergillus nidulans Yap1 ortholog NapA is necessary for the antioxidant response, the utilization of arabinose, fructose and ethanol, and for proper development. To address the Prx roles in these processes, we generated and characterized mutants lacking peroxiredoxins PrxA, PrxB, PrxC, or TpxC. Our results show that the elimination of peroxiredoxins PrxC or TpxC does not produce any distinguishable phenotype. In contrast, the elimination of atypical 2-cysteine peroxiredoxins PrxA and PrxB produce different mutant phenotypes. ΔprxA, ΔnapA and ΔprxA ΔnapA mutants are equally sensitive to H2O2 and menadione, while PrxB is dispensable for this. However, the sensitivity of ΔprxA and ΔprxA ΔnapA mutants is increased by the lack of PrxB. Moreover, PrxB is required for arabinose and ethanol utilization and fruiting body cell wall pigmentation. PrxA expression is partially independent of NapA, and the replacement of peroxidatic cysteine 61 by serine (C61S) is enough to cause oxidative stress sensitivity and prevent NapA nuclear accumulation in response to H2O2, indicating its critical role in H2O2 sensing. Our results show that despite their high similarity, PrxA and PrxB play differential roles in Aspergillus nidulans antioxidant response, carbon utilization and development.

Proteomic Perspective of Azole Resistance in Aspergillus fumigatus Biofilm Extracellular Matrix in Response to Itraconazole

Azole-resistant Aspergillus fumigatus makes a major challenge to the chemotherapy for invasive aspergillosis, whereas cyp51A gene mutation is the most dominant mechanism for azole resistance. Moreover, biofilm contributes to drug resistance for A. fumigatus, and extracellular matrix (ECM) is essential to protect live cells from antifungal drugs. Therefore, we performed a comparative proteomic study on the biofilm ECM of both the wild-type and azole-resistant strains of A. fumigatus under azole pressure. In total, 2377 proteins were identified, of which 480 and 604 proteins with differential expression were obtained from the wild-type and azole-resistant A. fumigatus in exposure to itraconazole respectively (fold change > 2 or < 0.5, P-value < 0.05). We found that a high proportion of regulated proteins were located in cytoplasm, nucleus, and mitochondria. Meanwhile, GO and KEGG analyses revealed that metabolic process and ribosome pathway were significantly enriched. Particularly, differentially expressed proteins in response to azole pressure of both the wild-type and resistant strains were further analyzed. Our results indicated that these changes in biofilm ECM proteins were related to ergosterol synthesis, oxidative stress, efflux pumps, DNA repair, DNA replication, and transcription.

Ambient Availability of Amino Acids, Proteins, and Iron Impacts Copper Resistance of Aspergillus fumigatus

The transition metals iron and copper are required by virtually all organisms but are toxic in excess. Acquisition of both metals and resistance to copper excess have previously been shown to be important for virulence of the most common airborne human mold pathogen, Aspergillus fumigatus. Here we demonstrate that the ambient availability of amino acids and proteins increases the copper resistance of A. fumigatus wild type and particularly of the ΔcrpA mutant that lacks export-mediated copper detoxification. The highest-protecting activity was found for L-histidine followed by L-asparagine, L-aspartate, L-serine, L-threonine, and L-tyrosine. Other amino acids and proteins also displayed significant but lower protection. The protecting activity of non-proteinogenic D-histidine, L-histidine-mediated growth inhibition in the absence of high-affinity copper uptake, determination of cellular metal contents, and expression analysis of copper-regulated genes suggested that histidine inhibits low-affinity but not high-affinity copper acquisition by extracellular copper complexation. An increase in the cellular copper content was found to be accompanied by an increase in the iron content, and, in agreement, iron starvation increased copper susceptibility, which underlines the importance of cellular metal balancing. Due to the role of iron and copper in nutritional immunity, these findings are likely to play an important role in the host niche.