Cryptococcus neoformans is an environmental fungus and an opportunistic human pathogen. Previous studies have demonstrated major alterations in its transcriptional profile as this microorganism enters the hostile environment of the human host. To assess the role of chromatin remodeling in host-induced transcriptional responses, we identified the C. neoformans Gcn5 histone acetyltransferase and demonstrated its function by complementation studies of Saccharomyces cerevisiae. The C. neoformans gcn5⌬ mutant strain has defects in high-temperature growth and capsule attachment to the cell surface, in addition to increased sensitivity to FK506 and oxidative stress. Treatment of wild-type cells with the histone acetyltransferase inhibitor garcinol mimics cellular effects of the gcn5⌬ mutation. Gcn5 regulates the expression of many genes that are important in responding to the specific environmental conditions encountered by C. neoformans inside the host. Accordingly, the gcn5⌬ mutant is avirulent in animal models of cryptococcosis. Our study demonstrates the importance of chromatin remodeling by the conserved histone acetyltransferase Gcn5 in regulating the expression of specific genes that allow C. neoformans to respond appropriately to the human host.

Aspergillus fumigatus is the causal agent of the life-threatening disease invasive aspergillosis. A. fumigatus laeA deletants, aberrant in toxin biosynthesis and spore development, are decreased in virulence. Among other characteristics, the decreased virulence is associated with increased spore susceptibility to macrophage phagocytosis. Three characteristics, cell wall microbe-associated molecular patterns (MAMPs), secreted metabolites, and rodlet content, thought to be important in macrophage-Aspergillus spore interactions were examined. Flow cytometry analysis of wild-type and ⌬laeA spores did not reveal any differences in surface-accessible MAMPs, including ␤-(1,3)-glucan, ␣-mannose, chitin, and other carbohydrate ligands. Blocking experiments with laminarin and mannan supported the conclusion that differences in cell wall carbohydrates were not responsible for enhanced ⌬laeA spore phagocytosis. Aspergillus spores have been reported to secrete metabolites affecting phagocytosis. Neither spent culture exchange, transwell, nor coincubation internalization experiments supported a role for secreted metabolites in the differential uptake of wild-type and ⌬laeA spores. However, sonication assays implicated a role for surface rodlet protein/hydrophobin (RodAp) in differential spore phagocytosis. A possible role of RodAp in enhanced ⌬laeA spore uptake was further assessed by RodAp extraction and quantification, where wild-type spores were found to contain 60% more RodAp than ⌬laeA spores. After removal of the surface rodlet layer, wild-type spores were phagocytosed at similar rates as ⌬laeA spores. We conclude that increased uptake of ⌬laeA resting spores is not associated with changes in secreted metabolite production of this mutant or surface carbohydrate availability but, rather, due to a decrease in the surface RodAp content of ⌬laeA spores. We theorize that RodAp acts as an antiphagocytic molecule, possibly via physicochemical means and/or by impeding MAMP recognition by macrophage receptors.

Secondary metabolite (SM) production by fungi is hypothesized to provide some fitness attribute for the producing organisms. However, most SM clusters are "silent" when fungi are grown in traditional laboratory settings, and it is difficult to ascertain any function or activity of these SM cluster products. Recently, the creation of a chromatin remodeling mutant in Aspergillus nidulans induced activation of several cryptic SM gene clusters. Systematic testing of nine purified metabolites from this mutant identified an emodin derivate with efficacy against both human fungal pathogens (inhibiting both spore germination and hyphal growth) and several bacteria. The ability of catalase to diminish this antimicrobial activity implicates reactive oxygen species generation, specifically, the generation of hydrogen peroxide, as the mechanism of emodin hydroxyl activity.