Siroheme Is Essential for Assimilation of Nitrate and Sulfate as Well as Detoxification of Nitric Oxide but Dispensable for Murine Virulence of Aspergillus fumigatus.

Anna-Maria Dietl,Ulrike Binder,Hubertus Haas,Yana Shadkchan,Nir Osherov

doi:10.3389/fmicb.2018.02615

Abstract

The saprophytic mold Aspergillus fumigatus is the most common airborne fungal pathogen causing severe invasive fungal infections in immunocompromised patients. Siroheme is a heme-like prosthetic group used by plants and microorganisms for sulfate and nitrate assimilation but is absent in higher eukaryotes. Here, we investigated the role of siroheme in A. fumigatus by deletion of the gene encoding the bifunctional dehydrogenase/ferrochelatase enzyme Met8. Met8-deficiency resulted in the inability to utilize sulfate and nitrate as sulfur and nitrogen sources, respectively. These results match previous data demonstrating that siroheme is an essential cofactor for nitrite and sulfite reductases. Moreover, Met8-deficiency caused significantly decreased resistance against nitric oxide (NO) underlining the importance of nitrite reductase in NO detoxification. Met8-deficiency did not affect virulence in murine models for invasive aspergillosis indicating that neither NO-detoxification nor assimilation of sulfate and nitrate play major roles in virulence in this host. Interestingly, Met8-deficiency resulted in mild virulence attenuation in the Galleria mellonella infection model revealing differences in interaction of A. fumigatus with G. mellonella and mouse.

Highlights

Aspergillus fumigatus is a major fungal pathogen causing a wide range of invasive and noninvasive infections
We investigated the role of siroheme in sulfate assimilation, nitrate assimilation, nitric oxide (NO) detoxification and virulence in A. fumigatus
Aspergillus fumigatus strains were generally cultured in Aspergillus minimal medium according to Pontecorvo et al (1953), containing 1% glucose as carbon source and 20 mM L-glutamine or 70 mM NaNO3 as nitrogen source, respectively, or in complex medium containing 2 g/L peptone and 1 g/L yeast extract, trace elements and salts (Pontecorvo et al, 1953) at 37 or 30◦C

Summary

Introduction

Aspergillus fumigatus is a major fungal pathogen causing a wide range of invasive and noninvasive infections. While a healthy human immune system is able to efficiently eliminate daily inhaled A. fumigatus spores, in immunocompromised patients, conidia are able to germinate in the alveoli and subsequently cause the life-threatening disease invasive aspergillosis (Brakhage and Langfelder, 2002; Tekaia and Latge, 2005; Dagenais and Keller, 2009). Treatment possibilities for invasive fungal infections remain limited, due to the fact that fungi are eukaryotes and share the majority of metabolic pathways with mammals. In the search for new antifungal targets, taking into account protein domains and domain architecture, the cofactor siroheme was identified as a potential target for antifungal drugs as it is absent in mammals (Barrera et al, 2014). We characterized the role of the heme-like tetrapyrrole siroheme in A. fumigatus.

Methods

Results

Conclusion