Elsevier

Fungal Biology

Volume 124, Issue 5, May 2020, Pages 368-375
Fungal Biology

Radioadapted Wangiella dermatitidis senses radiation in its environment in a melanin-dependent fashion

https://doi.org/10.1016/j.funbio.2019.10.011Get rights and content

Highlights

  • Radioadapted fungi can be developed by protracted exposure to radiation.

  • Radioadapted fungi respond to radiation in the environment without direct exposure.

  • Radioadapted melanized fungus have increased resistance to H2O2.

Abstract

Black fungi withstand extreme stresses partly due to the presence of melanin. Melanin is associated with structural integrity and resistance to chemical and radiation stress. This results in improved health and fitness, specifically in extreme conditions. Our goal was to exploit the radiation sensing nature of melanized fungus in order to develop a radioadapted strain capable of responding to radiation in the environment. The protracted exposure of a melanized fungus, Wangiella dermatitidis, to a mixed source of radiation altered the electron transport properties. There was no effect in an albino mutant wdpsk1. We then tested the growth response to radiation in the environment, with shielding from direct exposure to the radiation. Gamma radiation caused increased colony growth irrespective of exposure history in melanized fungus. Beta particles produced growth inhibition. The previously exposed melanized strain demonstrated colony growth in response to alpha particles in the environment. Alpha particles have a higher linear energy transfer, which produces more reactive oxygen species. Our previously exposed melanized strain was resistant to the toxic effects of H2O2, while the naïve and non-melanized strains were sensitive. We propose that previous radiation exposure introduces adaptations that equip melanized fungi to tolerate, sense, and respond to radiation byproducts.

Introduction

Fungi grow and thrive in some of the most extreme environments, both on earth and in space, and experience exposure to varied stresses. The presence of the pigment melanin in black fungi imparts an apparent selective advantage in these extreme locations, as a higher incidence of melanized fungi are observed in the Chernobyl atomic energy station (ChAES), can survive the Antarctic desserts, and are resistant to simulated space conditions (Onofri et al., 2008, Selbmann et al., 2015, Dighton et al., 2008). The advantage melanin imparts is through 1) its ability to provide physical and chemical protection from ionizing radiation, through Compton scattering and free-radical scavenging (Revskaya et al., 2012, Malo and Dadachova, 2019, Schweitzer et al., 2009), 2) the structural integrity it provides improving the ability to withstand sparsely and densely ionizing radiation (Malo et al., 2017), and 3) its radiation sensitive electronic properties positioning it for a role in a yet undefined signaling mechanism (Dadachova et al., 2007, Turick et al., 2011, Kim et al., 2017). These advantages manifest in a biological response in the form of increased growth, metabolism, and tropism in response to radiation (Robertson et al., 2012, Dadachova et al., 2007, Zhdanova et al., 2003, Zhdanova et al., 2004, Tugay et al., 2006).

The goal of this study was to develop radioadapted fungal strains in the laboratory through a protracted exposure to Actinium-225 (225Ac) which is a mixed alpha (α)-, beta (β)-, and gamma (γ)-emitter. Such strains would be more sensitive to low levels of radiation, and would possibly develop the ability to discern between qualitatively different forms of radiation. An additional goal was to separate the ability to sense radiation from the response to direct irradiation which would provide a mechanistic explanation of the tropic nature observed in isolates from the ChAES (Zhdanova et al., 2004).

We completed our study with the constitutively melanized pathogenic fungus Wangiella dermatitidis as a model (Szaniszlo, 2002). Previous studies have established that this strain shows stimulation of growth in response to radiation (Dadachova et al., 2007), both by increasing cell number and size, but also by altering DNA synthesis and response to reactive oxygen species (Robertson et al., 2012).

Section snippets

Growth conditions

The wild type (WT) strain of W. dermatitidis 8656 (ATCC34100, Exophiala dermatitidis CBS 525.76) and mutant strain, wdpks1 (Feng et al., 2001) used in this study were kind gifts from Dr. P. Szaniszlo (The University of Texas, Austin TX). Both strains were cultured in a modified Sabouraud Emmons Broth (SAB; 2 % dextrose, 1 % peptone) at 34 °C with shaking at 200 rpm until they reached an approximate concentration of 106 cells/ml. Cells were then transferred into minimal media (MM; 2 g/L KH2PO4,

Results and discussion

In this study we used a naturally pigmented wild type (WT) strain of W. dermatitidis that produces 1,8-dihydroxynaphthalene (1,8-DHN) melanin under normal growth conditions and deposits it in the cell wall (Geis et al., 1984). We used wdpks1as a non-melanized control, a strain with a disruption of the polyketide synthase 1 gene (Feng et al., 2001). While this mutant is still capable of producing melanin, the amounts of melanin produced are severely reduced and the dark pigmentation observed in

Conclusion

The results generated in this study have helped to elucidate the radio-stimulatory response in previously exposed fungus to indirect interaction with ionizing radiation. It has been demonstrated that fungal cultures isolated from sites contaminated with radioisotopes present radiotropic properties (Tugay et al., 2006) (Zhdanova et al., 2003, Zhdanova et al., 2004). These strains grow towards a radiation source while not being directly exposed to the radiation. In these studies the ionizing

Acknowledgements

This research was funded by the Defense Threat Reduction Agency grant HDTRA1-17-1-0020. This article is part of the Fungal Adaptation to Hostile Challenges special issue for the third International Symposium on Fungal Stress (ISFUS), which is supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo grant 2018/20571-6 and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior grant 88881.289327/2018-01.

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