Using genetically encoded heme sensors to probe the mechanisms of heme uptake and homeostasis in Candida albicans

Abstract

Candida albicans is a major fungal pathogen that can utilise hemin and haemoglobin as iron sources in the iron‐scarce host environment. While C. albicans is a heme prototroph, we show here that it can also efficiently utilise external heme as a cellular heme source. Using genetically encoded ratiometric fluorescent heme sensors, we show that heme extracted from haemoglobin and free hemin enter the cells with different kinetics. Heme supplied as haemoglobin is taken up via the Common in Fungal Extracellular Membrane (CFEM) hemophore cascade, and reaches the cytoplasm over several hours, whereas entry of free hemin via CFEM‐dependent and independent pathways is much faster, less than an hour. To prevent an influx of extracellular heme from reaching toxic levels in the cytoplasm, the cells deploy Hmx1, a heme oxygenase. Hmx1 was previously suggested to be involved in utilisation of haemoglobin and hemin as iron sources, but we find that it is primarily required to prevent heme toxicity. Taken together, the combination of novel heme sensors with genetic analysis revealed new details of the fungal mechanisms of heme import and homeostasis, necessary to balance the uses of heme as essential cofactor and potential iron source against its toxicity.

Molecular Epidemiology, Antifungal Susceptibility, and Virulence Evaluation of Candida Isolates Causing Invasive Infection in a Tertiary Care Teaching Hospital

“…Hemolytic activity is another virulence factor exhibited by pathogenic microorganisms, which enables fungal pathogens to utilize hemoglobin as an iron source for growth in the iron-scarce host environment ( Weissman et al., 2021 ). Previous studies have shown a close relationship between cellular iron and drug susceptibility in C .…”

“…Hemolytic activity is another virulence factor exhibited by pathogenic microorganisms, which enables fungal pathogens to utilize hemoglobin as an iron source for growth in the ironscarce host environment (Weissman et al, 2021). Previous studies have shown a close relationship between cellular iron and drug susceptibility in C. albicans (Prasad et al, 2006), and iron uptake mechanisms have also been reported to be necessary for the virulence of C. glabrata (Figueiredo-Carvalho et al, 2017).…”

Ferric reductase-related proteins mediate fungal heme acquisition

“…higher cytoplasmic heme concentrations. To expand the range of testable concentrations, a lower-affinity cytochrome domain mutant (M7A) can be used alongside the high-affinity original sensor (HS1) (Hanna et al, 2016; Weissman et al, 2021).…”

Heme Oxygenase-1 (HMX1) Loss of Function Increases the In-Host Fitness of the Saccharomyces ‘boulardii’ Probiotic Yeast in a Mouse Fungemia Model

“…In C. albicans , it has been reported that the activity of HMX1 has a major contribution to its pathogenicity. This may be in relation to the utilization of iron directly, from host heme and heme proteins such as hemoglobin [ 28 ], but recently, CaHmx1’s role was found to be more centered around heme detoxification in C. albicans [ 29 ]. The localization of CaHmx1 in the cell is still unclear, and other enzymes involved in free heme and hemoglobin heme utilization have not yet been determined either ( Figure 1 a) [ 29 , 30 , 31 ].…”

“…This may be in relation to the utilization of iron directly, from host heme and heme proteins such as hemoglobin [ 28 ], but recently, CaHmx1’s role was found to be more centered around heme detoxification in C. albicans [ 29 ]. The localization of CaHmx1 in the cell is still unclear, and other enzymes involved in free heme and hemoglobin heme utilization have not yet been determined either ( Figure 1 a) [ 29 , 30 , 31 ]. The binding and uptake of free heme and hemoglobin-heme is possible through CFEM (common in several fungal extracellular membrane proteins) domain-containing proteins, namely Rbt5, Rbt51/Pga10, Pga7, and Csa2 [ 30 , 31 , 32 ].…”