The Mannoprotein Cig1 Supports Iron Acquisition From Heme and Virulence in the Pathogenic Fungus Cryptococcus neoformans

Abstract

Iron acquisition is critical for virulence of the human pathogenic fungus Cryptococcus neoformans. The cryptococcal transcript for the extracellular mannoprotein Cig1 is highly regulated by iron and abundant in iron-starved cells, suggesting a role in iron acquisition. Indeed, loss of Cig1 resulted in delayed growth on heme at physiological pH. Expression of CIG1 is regulated by the pH-responsive transcription factor Rim101, and loss of Rim101 also impaired growth on heme. A cig1Δ mutant was less susceptible than the wild-type strain to noniron metalloporphyrins, further indicating a role for Cig1 in heme uptake. Recombinant Cig1 exhibited the absorbance spectrum of a heme-binding protein upon heme titration, and Cig1 may therefore function as a hemophore at the cell surface. Cig1 contributed to virulence in a mouse model of cryptococcosis but only in a mutant that also lacked the high-affinity iron uptake system. Overall, Cig1-mediated heme uptake is a potential therapeutic target in C. neoformans.

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

“…This suggests the presence of other systems for acquisition of iron from hemin at higher concentrations. A similar conclusion was recently reported during the analysis of Cig1 (6).…”

“…An analysis of the fungal burden in infected mice also showed that deletion of FRE2 impaired the dissemination of the mutant from lungs to the brain and/or colonization of the brain. This defect is reminiscent of the reduced fungal burden observed for mutants lacking the Cft1 and Cfo1 components of the high-affinity uptake system in C. neoformans (6,7,8). Since the fre2⌬ mutant showed a defect in the use of hemin as an iron source, it is likely that hemin may be important for the growth of C. neoformans during infection.…”

“…Although less is known about the use of heme in parasites and pathogenic fungi, Leishmania amazonensis was recently found to have a heme receptor that was required for heme utilization, and the pathogenic fungi C. albicans and Histoplasma capsulatum utilize heme and/or hemoglobin via cell surface receptors (9,10,56,57). C. neoformans also uses heme and hemoglobin as iron sources, and the mannoprotein Cig1 was recently implicated in heme uptake (6). Cig1 shows heme-binding properties, and its loss delayed growth in hemin and reduced susceptibility to noniron metalloporphyrins.…”

“…In addition, both C. neoformans and C. albicans are able to utilize heme as an iron source. The cell wall mannoproteins Cig1 (in C. neoformans) and Rbt5 and Rbt51 (in C. albicans), as well as the heme oxygenase Hmx1 (in C. albicans), are involved in acquiring iron from heme (6)(7)(8)(9)(10)(11). The reductive high-affinity iron uptake system is an important virulence determinant in C. neoformans and C. albicans but not in A. fumigatus, and this mechanism is employed by these two pathogens to acquire iron from transferrin in mammalian hosts (7,8,12).…”

The Cryptococcus neoformans Rim101 Transcription Factor Directly Regulates Genes Required for Adaptation to the Host

“…In addition to exhibiting altered cell wall composition and defects in the capsule, the rim101⌬ mutant is sensitive to low iron levels, high salt concentrations, and alkaline pHs (8,23,24). Many of these mutant phenotypes are conserved among rim101⌬ mutants in other fungi, including the pathogens Candida albicans, Aspergillus fumigatus, and Ustilago maydis (25)(26)(27)(28)(29)(30)(31).…”