Sperm abnormalities in high- and low-dose-rate Gamma-irradiated Korean dark-striped field mice, Apodemus agrarius coreae

Inverse dose protraction effects of low-LET radiation: Evidence and significance

Biological effects of ionizing radiation vary not merely with total dose but also with temporal dose distribution. Sparing dose protraction effects, in which dose protraction reduces effects of radiation have widely been accepted and generally assumed in radiation protection, particularly for stochastic effects (e.g., solid cancer). In contrast, inverse dose protraction effects (IDPEs) in which dose protraction enhances radiation effects have not been well recognized, nor comprehensively reviewed. Here, we review the current knowledge on IDPEs of low linear energy transfer (LET) radiation. To the best of our knowledge, since 1952, 157 biology, epidemiology or clinical papers have reported IDPEs following external or internal low-LET irradiation with photons (X-rays, γ-rays), β-rays, electrons, protons or helium ions. IDPEs of low-LET radiation have been described for biochemical changes in cell-free macromolecules (DNA, proteins or lipids), DNA damage responses in bacteria and yeasts, DNA damage, cytogenetic changes, neoplastic transformation and cell death in mammalian cell cultures of human, rodent or bovine origin, mutagenesis in silkworms, cytogenetic changes, induction of cancer (solid tumors and leukemia) and non-cancer effects (male sterility, cataracts and diseases of the circulatory system), tumor inactivation and survival in non-human mammals (rodents, rabbits, dogs and pigs), and induction of cancer and non-cancer effects (skin changes and diseases of the circulatory system) in humans. In contrast to a growing body of phenomenological evidence for manifestations of IDPEs, there is limited knowledge on mechanistic underpinnings, but proposed mechanisms involve cell cycle-dependent resensitization and low dose hyper-radiosensitivity. These necessitate continued studies for further mechanistic developments and assessment of implications of scientific evidence for radiation protection (e.g., in terms of a dose rate effectiveness factor).

Effects of environmental radiation on testes and spermatogenesis in wild large Japanese field mice (Apodemus speciosus) from Fukushima

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident that occurred after the Great East Japan Earthquake in March 2011 released large quantities of radionuclides to the environment. The long-term effects of radioactive cesium (Cs) on biota are of particular concern. We investigated the accumulation of radioactive Cs derived from the FDNPP accident, and chronic effects of environmental radionuclides on male reproduction, in the large Japanese field mouse (Apodemus speciosus). In 2013 and 2014, wild mice were captured at 2 sites in Fukushima Prefecture and at 2 control sites that were distant from Fukushima. Although the median concentrations of (134)Cs and (137)Cs in the mice from Fukushima exceeded 4,000 Bq/kg, there were no significant differences in the apoptotic cell frequencies or the frequencies of morphologically abnormal sperm among the capture sites. Thus, we conclude that radiation did not cause substantial male subfertility in Fukushima during 2013 and 2014, and radionuclide pollution levels in the study sites would not be detrimental to spermatogenesis of the wild mice in Fukushima.

Low-dose-rate radiation exposure leads to testicular damage with decreases in DNMT1 and HDAC1 in the murine testis

This study examined the effects of continuous low-dose-rate radiation exposure (3.49 mGy/h) of gamma rays on mice testicles. C57BL/6 mice were divided into sham and radiation groups (n = 8 each), and were exposed to either sham irradiation or 2 Gy for 21 days, 0.2 Gy for 2 days, or 0.02 Gy for 6 h of low-dose-rate irradiation. Testicular weight, seminiferous tubular diameter, and seminiferous epithelial depth were significantly decreased in the mice irradiated with 2 Gy at 1 and 9 days after exposure. Moreover, the low-dose-rate radiation exposure induced an increase in malondialdehyde levels, and a decrease in superoxide dismutase activity in the testis of mice irradiated with 2 Gy at 1 and 9 days after exposure. The sperm count and motility in the epididymis also decreased in mice irradiated with 2 Gy at 1 and 9 days after exposure, whereas there was no significant effect on the proportion of abnormal sperm. The expressions of DNA methlytransferases-1 and histone deacetylases 1 in testes irradiated with 2 Gy were significantly decreased compared with the sham group. In conclusion, the damage exerted on the testes and epididymis largely depended on the total dose of low-dose-rate radiation.