Link between heat shock protein 90 (Hsp90) and the mitochondrial respiratory chain in the caspofungin stress response of Aspergillus fumigatus .

Aspergillus fumigatus is an opportunistic mold responsible of invasive aspergillosis. Triazoles (e.g. voriconazole) represent the first-line treatment, but emerging resistance is of concern. The echinocandin drug caspofungin is used as second line treatment but has limited efficacy. The heat shock protein 90 (Hsp90) orchestrates caspofungin stress response and is the trigger of an adaptive phenomenon called the paradoxical effect (growth recovery at increasing caspofungin concentrations). The aim of this study was to elucidate Hsp90-dependent mechanisms of caspofungin stress response.Transcriptomic profiles of the wild-type A. fumigatus strain (Ku80) were compared to that of a mutant strain with substitution of the native hsp90 promoter by the thiA promoter ( pthiA -hsp90), which lacks the caspofungin paradoxical effect. Caspofungin induced expression of the genes of the mitochondrial respiratory chain (MRC), in particular NADH-ubiquinone oxidoreductases (complex I), in Ku80 but not in pthiA -hsp90. The caspofungin paradoxical effect could be abolished by rotenone (MRC complex I inhibitor) in Ku80, supporting the role of MRC in caspofungin stress response. Fluorescent staining of active mitochondria, measurement of oxygen consumption and of ATP production confirmed the activation of the MRC in Ku80 in response to caspofungin, but this activity was impaired in pthiA -hsp90. Using a bioluminescent reporter for measurement of intracellular calcium, we demonstrated that inhibition of Hsp90 by geldanamycin or MRC complex I by rotenone prevented the increase of intracellular calcium, shown to be essential for caspofungin paradoxical effect. In conclusion, our data support a role of the MRC in caspofungin stress response, which is dependent on Hsp90.