The response of the melanized yeast Exophiala dermatitidis to gamma radiation exposure

Abstract

Summary The melanized yeast Exophiala dermatitidis is resistant to many environmental stresses and is used as a model for understanding the diverse roles of melanin in fungi. Here, we describe the extent of resistance of E. dermatitidis to acute γ‐radiation exposure and the major mechanisms it uses to recover from this stress. We find that melanin does not protect E. dermatitidis from γ‐radiation. Instead, environmental factors such as nutrient availability, culture age and culture density are much greater determinants of cell survival after exposure. We also observe a dramatic transcriptomic response to γ‐radiation that mobilizes pathways involved in morphological development, protein degradation and DNA repair, and is unaffected by the presence of melanin. Together, these results suggest that the ability of E. dermatitidis to survive γ‐radiation exposure is determined by the prior and the current metabolic state of the cells as well as DNA repair mechanisms, and that small changes in these conditions can lead to large effects in radiation resistance, which should be taken into account when understanding how diverse fungi recover from this unique stress.

The transcriptomic and phenotypic response of the melanized yeast Exophiala dermatitidis to ionizing particle exposure

“…To address this, we focused on getting more information about the genes were differentially expressed in each group, particularly those that did not have informative annotations. We therefore compiled lists of genes that were differentially regulated more than 5-fold (FDR < 0.05) in both γ-radiation and particle both -irradiation datasets and compared these lists [31]. Notably, we observed less overlap among these two sets than we expected, with only approximately 12% of the genes shared overall (Fig.…”

“…PE, though, was also smaller for Δpks than WT, which may suggest that only some selected "healthy" sub-population of Δpks cells was able to grow even without radiation, and possibly this selection was correlated with improved radioresistance as well. Indeed, Δpks cells have been shown to clump together, possibly due to a decreased ability to completely cleave apart after cell division, which can affect plating e ciency [31,43]. In all, though, there was no clear protection, or difference at all, in WT cells compared to those without melanin.…”

“…1 and Materials and Methods), including: the lower penetration of these particles into materials; the RBE of each particle type in comparison with γ-IR [41,42]; and the shape of previously constructed survival curves exhibited by E. dermatitidis after irradiation. At doses much greater than 2000 Gray (Gy, J/kg), for example, survival decreases rapidly, and differences between strains become harder to interpret due to the low percentage of recovering colonies produced [31]. Effects of interest, such as would be imparted by melanin protection, are therefore best detected at moderate exposures, so we chose to irradiate each strain with doses equivalent to 500, 1000, and 2000 Gy of γ-IR.…”

“…More recently, the observation that dozens of mold species could be isolated in and around the damaged nuclear reactor at Chernobyl [15] has sparked interest in fungi outside of Saccharomyces cerevisiae for radiobiological studies, a current that has been bolstered by their ease of transformation, their relatively small genome sizes , and the relative simplicity of fungal growth and irradiation methods [16][17][18][19][20]. Signi cant ndings from studies in these organisms include the stimulation of growth of some fungi by low dose IR [20][21][22]; the possibility that melanin in the cell walls of certain fungi can protect against IR through shielding, free-radical scavenging, or a physiological mechanism [21,23]; the high IR resistance of organisms such as Ustilago maydis [24][25][26][27], Cryptococcus neoformans [19,23,28,29], Cryomyces antarcticus [30], and Exophiala dermatitidis [31]; and the discovery of novel IR-resistanceassociated proteins through transcriptomics and targeted mutagenesis [19,28,31]. A portion of the radioresistance of fungi can be explained by their genome size, which is approximately two orders of magnitude smaller than mammals, and therefore sustains less damage per unit dose of IR, but within fungi there is also considerable variation in resistance that is not explained by genome size or composition (for instance, see [12,[31][32][33]).…”

“…We are interested in the factors that mediate the response to IR in fungi, and have previously characterized how melanin affects the response of E. dermatitidis to γ-IR [31,38]. E. dermatitidis is useful for such studies, as it is genetically tractable, and due to its ability to thrive in austere environments, is a candidate for bioremediation and sensing of radionuclide contamination [39].…”