Role of Ferric Reductases in Iron Acquisition and Virulence in the Fungal Pathogen Cryptococcus neoformans

Abstract

Iron acquisition is critical for the ability of the pathogenic yeast Cryptococcus neoformans to cause disease in vertebrate hosts. In particular, iron overload exacerbates cryptococcal disease in an animal model, defects in iron acquisition attenuate virulence, and iron availability influences the expression of major virulence factors. C. neoformans acquires iron by multiple mechanisms, including a ferroxidase-permease high-affinity system, siderophore uptake, and utilization of both heme and transferrin. In this study, we examined the expression of eight candidate ferric reductase genes and their contributions to iron acquisition as well as to ferric and cupric reductase activities. We found that loss of the FRE4 gene resulted in a defect in production of the virulence factor melanin and increased susceptibility to azole antifungal drugs. In addition, the FRE2 gene was important for growth on the iron sources heme and transferrin, which are relevant for proliferation in the host. Fre2 may participate with the ferroxidase Cfo1 of the high-affinity uptake system for growth on heme, because a mutant lacking both genes showed a more pronounced growth defect than the fre2 single mutant. A role for Fre2 in iron acquisition is consistent with the attenuation of virulence observed for the fre2 mutant. This mutant also was defective in accumulation in the brains of infected mice, a phenotype previously observed for mutants with defects in high-affinity iron uptake (e.g., the cfo1 mutant). Overall, this study provides a more detailed view of the iron acquisition components required for C. neoformans to cause cryptococcosis.

The endosomal sorting complex required for transport machinery influences haem uptake and capsule elaboration in Cryptococcus neoformans

“…We previously demonstrated that defects in the components of the haem uptake system (i.e. Cig1, Rim101 and Vps23) cause reduced susceptibility to MPs (Cadieux et al ., ; Hu et al ., ; Saikia et al ., ). We therefore predicted that the ESCRT‐II and ‐III complexes would also influence susceptibility to MPs if they participated in haem uptake.…”

A P4-ATPase subunit of the Cdc50 family plays a role in iron acquisition and virulence inCryptococcus neoformans

“…Previously, we showed that heme or a siderophore can restore growth on fluconazole for the C . neoformans cfo1 and cft1 mutants defective in high‐affinity iron uptake (Hu et al, ; Jung et al, ; Kim et al, ; Saikia, Oliveira, Hu, & Kronstad, ). This may be due to upregulation of uptake functions in the mutants due to iron starvation or to direct rescue of heme‐containing enzymes for ergosterol biosynthesis.…”

Iron acquisition in fungal pathogens of humans

“…Furthermore, FRE2 is required for robust growth in the presence of heme and transferrin, and for full virulence in a mouse inhalation model of cryptococcosis. 29 Similar to the S. cerevisiae FRE genes, the FRE genes in both C. albicans and C. neoformans also show differential transcriptional regulation in the presence of copper. 29,30 …”

“…In C. neoformans , eight genes encoding putative ferric reductases have been identified ( FRE1–7 , FRE201 ). 29 Notably, the transcription of FRE2 and FRE4 is regulated by hemin or FeCl 3 suggesting a crucial role of these genes in maintaining iron homeostasis during iron-limited conditions. Furthermore, FRE2 is required for robust growth in the presence of heme and transferrin, and for full virulence in a mouse inhalation model of cryptococcosis.…”