The Fungal Pathogen Candida glabrata Does Not Depend on Surface Ferric Reductases for Iron Acquisition

Abstract

Iron acquisition is a crucial virulence determinant for many bacteria and fungi, including the opportunistic fungal pathogens Candida albicans and C. glabrata. While the diverse strategies used by C. albicans for obtaining iron from the host are well-described, much less is known about the acquisition of this micronutrient from host sources by C. glabrata – a distant relative of C. albicans with closer evolutionary ties to Saccharomyces cerevisiae, which nonetheless causes severe clinical symptoms in humans. Here we show that C. glabrata is much more restricted than C. albicans in using host iron sources, lacking, for example, the ability to grow on transferrin and hemin/hemoglobin. Instead, C. glabrata is able to use ferritin and non-protein-bound iron (FeCl3) as iron sources in a pH-dependent manner. As in other fungal pathogens, iron-dependent growth requires the reductive high affinity (HA) iron uptake system. Typically highly conserved, this uptake mechanism normally relies on initial ferric reduction by cell-surface ferric reductases. The C. glabrata genome contains only three such putative ferric reductases, which were found to be dispensable for iron-dependent growth. In addition and in contrast to C. albicans and S. cerevisiae, we also detected no surface ferric reductase activity in C. glabrata. Instead, extracellular ferric reduction was found in this and the two other fungal species, which was largely dependent on an excreted low-molecular weight, non-protein ferric reductant. We therefore propose an iron acquisition strategy of C. glabrata which differs from other pathogenic fungi, such as C. albicans, in that it depends on a limited set of host iron sources and that it lacks the need for surface ferric reductases. Extracellular ferric reduction by a secreted molecule possibly compensates for the loss of surface ferric reductase activity in the HA iron uptake system.

Genomic Tandem Quadruplication is Associated with Ketoconazole Resistance in Malassezia pachydermatis

“…Considering that increased expression of ERG11 and ERG4 is one of the common mechanisms of azole resistance in other pathogenic fungi, we concluded that the overexpression of the ERG4 and ERG11 homologs, which may be caused by quadruplication of the genomic region, might be the main cause of ketoconazole resistance in M. pachydermatis KCTC 27587. We note that the genes encoding homologs of ferric reductases, MP87_02943 and MP87_02944, in the quadruplicated region were also more strongly expressed in M. pachydermatis KCTC 27587, which may also influence susceptibility of the yeast to ketoconazole as the enzyme has been shown to contribute to azole resistance [31,32]. Transcript levels of two other genes in the quadruplicated region, encoding a hypothetical protein (MP87_02949) and the homolog of S. cerevisiae Sip1 domain containing protein (MP87_02972), respectively, were also significantly increased by >10-fold.…”

Metals in fungal virulence

“… 2013 ) and C. glabrata (Srivastava, Suneetha and Kaur 2014 ), which heavily rely on the reductive pathway for iron uptake to facilitate growth and virulence (Srivastava, Suneetha and Kaur 2014 ; Gerwien et al . 2016 , 2017 ). In contrast, while A. fumigatus siderophore synthesis mutants were dramatically attenuated in virulence (Hissen et al .…”

“…However, our own work has shown that both Fre6 and Fre8 might have roles other than ferric or cupric reduction in C. glabrata , since this fungus does not exhibit evident surface ferric reductase activity (Gerwien et al . 2017 ). Finally, low-affinity broad-spectrum metal transporters for iron, copper and zinc have been identified in S. cerevisiae (Fet4) (Dix et al .…”

“…Likewise, culture supernatants of C. albicans , C. glabrata and S. cerevisiae show ferric reduction activity, which depends on a so far unknown low-molecular-weight compound (Gerwien et al . 2017 ), and A. terreus has recently been shown to secrete terrein under iron starvation, which acts as a ferric reductant and can partially rescue strains defective in siderophore biosynthesis (Gressler et al . 2015 ).…”

Comparative Transcriptomics Highlights New Features of the Iron Starvation Response in the Human Pathogen Candida glabrata

“…However, several important differences in iron homeostasis control have also been observed between C. glabrata and other model yeast species. For instance, the deletion of the C. glabrata ferric reductase encoding genes FRE8 and FRE6 does not seem to impact the C. glabrata extracellular iron uptake or extracellular ferric reduction activities, in contrast to what has been shown in S. cerevisiae and C. albicans ( Gerwien et al, 2017 ). Still, these two genes are induced by iron starvation ( Nagi et al, 2016 ; Gerwien et al, 2017 ) and regulated by Aft2 (this work) and Aft1 ( Gerwien et al, 2017 ), respectively.…”