Induction of Aspergillus fumigatus zinc cluster transcription factor OdrA/Mdu2 provides combined cellular responses for oxidative stress protection and multiple antifungal drug resistance

IMPORTANCE

An overexpression screen of 228 zinc cluster transcription factor encoding genes of A. fumigatus revealed 11 genes conferring increased tolerance to antifungal drugs. Out of these, four oxidative stress and drug tolerance transcription factor encoding odr genes increased tolerance to oxidative stress and antifungal drugs when overexpressed. This supports a correlation between oxidative stress response and antifungal drug tolerance in A. fumigatus. OdrA/Mdu2 is required for the cross-tolerance between azoles, polyenes, and oxidative stress and activates genes for detoxification. Under oxidative stress conditions or when overexpressed, OdrA/Mdu2 accumulates in the nucleus and activates detoxifying genes by direct binding at their promoters, as we describe with the mdr1 gene encoding an itraconazole specific efflux pump. Finally, this work gives new insights about drug and stress resistance in the opportunistic pathogenic fungus A. fumigatus.

Fungal COP9 signalosome assembly requires connection of two trimeric intermediates for integration of intrinsic deneddylase

The conserved eight-subunit COP9 signalosome (CSN) is required for multicellular fungal development. The CSN deneddylase cooperates with the Cand1 exchange factor to control replacements of E3 ubiquitin cullin RING ligase receptors, providing specificity to eukaryotic protein degradation. Aspergillus nidulans CSN assembles through a heptameric pre-CSN, which is activated by integration of the catalytic CsnE deneddylase. Combined genetic and biochemical approaches provided the assembly choreography within a eukaryotic cell for native fungal CSN. Interactomes of functional GFP-Csn subunit fusions in pre-CSN deficient fungal strains were compared by affinity purifications and mass spectrometry. Two distinct heterotrimeric CSN subcomplexes were identified as pre-CSN assembly intermediates. CsnA-C-H and CsnD-F-G form independently of CsnB, which connects the heterotrimers to a heptamer and enables subsequent integration of CsnE to form the enzymatically active CSN complex. Surveillance mechanisms control accurate Csn subunit amounts and correct cellular localization for sequential assembly since deprivation of Csn subunits changes the abundance and location of remaining Csn subunits.

Verticillium dahliae transcription factors Som1 and Vta3 control microsclerotia formation and sequential steps of plant root penetration and colonisation to induce disease

Verticillium dahliae nuclear transcription factors Som1 and Vta3 can rescue adhesion in a FLO8-deficient Saccharomyces cerevisiae strain. Som1 and Vta3 induce the expression of the yeast FLO1 and FLO11 genes encoding adhesins. Som1 and Vta3 are sequentially required for root penetration and colonisation of the plant host by V. dahliae. The SOM1 and VTA3 genes were deleted and their functions in fungus-induced plant pathogenesis were studied using genetic, cell biology, proteomic and plant pathogenicity experiments. Som1 supports fungal adhesion and root penetration and is required earlier than Vta3 in the colonisation of plant root surfaces and tomato plant infection. Som1 controls septa positioning and the size of vacuoles, and subsequently hyphal development including aerial hyphae formation and normal hyphal branching. Som1 and Vta3 control conidiation, microsclerotia formation, and antagonise in oxidative stress responses. The molecular function of Som1 is conserved between the plant pathogen V. dahliae and the opportunistic human pathogen Aspergillus fumigatus. Som1 controls genes for initial steps of plant root penetration, adhesion, oxidative stress response and VTA3 expression to allow subsequent root colonisation. Both Som1 and Vta3 regulate developmental genetic networks required for conidiation, microsclerotia formation and pathogenicity of V. dahliae.

Heavy Metal-Induced Expression of PcaA Provides Cadmium Tolerance to Aspergillus fumigatus and Supports Its Virulence in the Galleria mellonella Model

Most of the metal transporters in Aspergillus fumigatus are yet uncharacterized. Their role in fungal metabolism and virulence remains unclear. This paper describes the novel PIB-type cation ATPase PcaA, which links metal homeostasis and heavy metal tolerance in the opportunistic human pathogen A. fumigatus. The protein possesses conserved ATPase motif and shares 51% amino acid sequence identity with the Saccharomyces cerevisiae cadmium exporter Pca1p. A pcaA deletion, an overexpression and a gfp-pcaA complementation strain of A. fumigatus were constructed and their heavy metal susceptibilities were studied. The pcaA knock out strain showed drastically decreased cadmium tolerance, however, its growth was not affected by the exposure to high concentrations of copper, iron, zinc, or silver ions. Although the lack of PcaA had no effect on copper adaption, we demonstrated that not only cadmium but also copper ions are able to induce the transcription of pcaA in A. fumigatus wild type Af293. Similarly, cadmium and copper ions could induce the copper exporting ATPase crpA. These data imply a general response on the transcriptomic level to heavy metals in A. fumigatus through the induction of detoxification systems. Confocal microscopy of the gfp-pcaA complementation strain expressing functional GFP-PcaA supports the predicted membrane localization of PcaA. The GFP-PcaA fusion protein is located in the plasma membrane of A. fumigatus in the presence of cadmium ions. Virulence assays support a function of PcaA for virulence of A. fumigatus in the Galleria mellonella wax moth larvae model, which might be linked to the elimination of reactive oxygen species.

Study on the glutathione metabolism of the filamentous fungus Aspergillus nidulans

Yeast protein sequence-based homology search for glutathione (GSH) metabolic enzymes and GSH transporters demonstrated that Aspergillus nidulans has a robust GSH uptake and metabolic system with several paralogous genes. In wet laboratory experiments, two key genes of GSH metabolism, gcsA, and glrA, encoding γ-l-glutamyl-l-cysteine synthetase and glutathione reductase, respectively, were deleted. The gene gcsA was essential, and the ΔgcsA mutant required GSH supplementation at considerably higher concentration than the Saccharomyces cerevisiae gsh1 mutant (8-10 mmol l^-1 vs. 0.5 μmol l^-1). In addition to some functions known previously, both genes were important in the germination of conidiospores, and both gene deletion strains required the addition of extra GSH to reach wild-type germination rates in liquid cultures. Nevertheless, the supplementation of cultures with 10 mmol l^-1 GSH was toxic for the control and ΔglrA strains especially during vegetative growth, which should be considered in future development of high GSH-producer fungal strains. Importantly, the ΔglrA strain was characterized by increased sensitivity toward a wide spectrum of osmotic, cell wall integrity and antimycotic stress conditions in addition to previously reported temperature and oxidative stress sensitivities. These novel phenotypes underline the distinguished functions of GSH and GSH metabolic enzymes in the stress responses of fungi.

Extracellular proteinase formation in carbon starving Aspergillus nidulans cultures--physiological function and regulation

Extracellular proteinase formation in carbon depleted cultures of the model filamentous fungus Aspergillus nidulans was studied to elucidate its regulation and possible physiological function. As demonstrated by gene deletion, culture optimization, microbial physiological and enzymological experiments, the PrtA and PepJ proteinases of A. nidulans did not appear to play a decisive role in the autolytic decomposition of fungal cells under the conditions we tested. However, carbon starvation induced formation of the proteinases observable in autolytic cultures. Similar to other degradative enzymes, production of proteinase was regulated by FluG-BrlA asexual developmental signaling and modulated by PacC-dependent pH-responsive signaling. Under the same carbon starved culture conditions, alterations of CreA, MeaB or heterotrimeric G protein mediated signaling pathways caused less significant changes in the formation of extracellular proteinases. Taken together, these results indicate that while the accumulation of PrtA and PepJ is tightly coupled to the initiation of autolysis, they are not essential for autolytic cell wall degradation in A. nidulans. Thus, as Aspergillus genomes contain a large group of genes encoding proteinases with versatile physiological functions, selective control of proteinase production in fungal cells is needed for the improved industrial use of fungi.