Mathematical Modeling Predicts Enhanced Growth of X-Ray Irradiated Pigmented Fungi

Abstract

Ionizing radiation is known for its cytotoxic and mutagenic properties. However, recent evidence suggests that chronic sub-lethal irradiation stimulates the growth of melanin-pigmented (melanized) fungi, supporting the hypothesis that interactions between melanin and ionizing photons generate energy useful for fungal growth, and/or regulate growth-promoting genes. There are no quantitative models of how fungal proliferation is affected by ionizing photon energy, dose rate, and presence versus absence of melanin on the same genetic background. Here we present such a model, which we test using experimental data on melanin-modulated radiation-induced proliferation enhancement in the fungus Cryptococcus neoformans, exposed to two different peak energies (150 and 320 kVp) over a wide range of X-ray dose rates. Our analysis demonstrates that radiation-induced proliferation enhancement in C. neoformans behaves as a binary “on/off” phenomenon, which is triggered by dose rates <0.002 mGy/h, and stays in the “on” position. A competing dose rate-dependent growth inhibition becomes apparent at dose rates >5000 mGy/h. Proliferation enhancement of irradiated cells compared with unirradiated controls occurs at both X-ray peak energies, but its magnitude is modulated by X-ray peak energy and cell melanization. At dose rates <5000 mGy/h, both melanized and non-melanized cells exposed to 150 kVp X-rays, and non-melanized cells exposed to 320 kVp X-rays, all exhibit the same proliferation enhancement: on average, chronic irradiation stimulates each founder cell to produce 100 (95% CI: 83, 116) extra descendants over 48 hours. Interactions between melanin and 320 kVp X-rays result in a significant (2-tailed p-value = 4.8×10−5) additional increase in the number of radiation-induced descendants per founder cell: by 55 (95% CI: 29, 81). These results show that both melanin-dependent and melanin-independent mechanisms are involved in radiation-induced fungal growth enhancement, and implicate direct and/or indirect interactions of melanin with high energy ionizing photons as an important pro-proliferative factor.

Transcriptomic analysis reveals the relationship of melanization to growth and resistance to gamma radiation in Cryptococcus neoformans

“…In the experiments conducted here, melanin only provided moderate, and variable, protection from γ‐radiation. This observation was somewhat surprising, as other studies have indicated a significant shielding effect of fungal melanin against various IR sources (Dadachova et al ., ; Dadachova and Casadevall, ; Schweitzer et al ., ; Shuryak et al ., ; Pacelli et al ., ). However, inconsistency in the effects of melanin on radiation protection is not unprecedented.…”

“…This dose was far lower what is needed to cause substantial damage in these cells, and in fact cell growth was enhanced, as shown through increases in cell number and size. Significantly, the enhanced growth was seen in both wild type cells and the non‐melanized wdpks1 mutant, contrary to previous studies which only observed enhanced growth in melanized cells (Dadachova et al ., ; Shuryak et al ., ). Still, we found that more than 1500 genes were differentially regulated at least twofold between the irradiated wild type and wdpks1 , suggesting that melanin has an effect on the cellular response to γ‐radiation.…”

Melanin is effective in protecting fast and slow growing fungi from various types of ionizing radiation

“…Radiotropism, the propensity of fungi to grow toward a radiation source, was described in the late 1990s (Vember et al 1999). Since then, increased growth of melanized fungi was demonstrated after irradiation with gamma-rays and X-rays (Dadachova et al, 2007;Robertson et al, 2012;Shuryak et al, 2014) and gammarays have been shown to alter the electronic properties of melanins (Dadachova et al, 2007;Khajo et al 2011;Robertson et al, 2012;Shuryak et al, 2014). In addition, recent transcriptomic investigation revealed that ionizing radiation stimulates protein biosynthesis with input from melanin (Robertson et al, 2012).…”

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

“…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]).…”