Fludioxonil is employed as an agricultural fungicide to control plant-pathogenic fungi such as Botrytis cinerea. Cryptococcus neoformans is a basidiomycetous human fungal pathogen that causes fatal disease in immunocompromised hosts. This paper demonstrates that three different signalling cascades regulate sensitivity of C. neoformans to fludioxonil. Fludioxonil inhibited growth of the serotype A sequence reference strain H99 but not that of the sequenced serotype D strain JEC21. In the drug-sensitive wild-type strain, fludioxonil exposure activated the Hog1 osmosensing pathway, and hog1Δ mutations conferred fludioxonil resistance. Fludioxonil treatment caused cell growth inhibition following cell swelling and cytokinesis defects in the sensitive wild-type but not in a hog1Δ mutant strain, suggesting that Hog1 activation results in morphological cellular defects. Fludioxonil exerted a fungistatic effect on the wild-type strain H99, but exhibited fungicidal activity against calcineurin mutant strains, indicating that the calcineurin pathway contributes to drug resistance in this fungus. Combination of fludioxonil and the calcineurin inhibitor FK506 synergistically inhibited C. neoformans growth. mpk1Δ MAPK mutant strains exhibited fludioxonil hypersensitivity, indicating that this pathway also contributes to drug resistance. These studies provide evidence that the broad-spectrum antifungal drug fludioxonil exerts its action via activation of the Hog1 MAPK pathway and provide insight into novel targets for synergistic antifungal drug combinations.
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