Intracellular potassium: 40K as a primordial gene irradiator

Reducing the ionizing radiation background does not significantly affect the evolution of Escherichia coli populations over 500 generations

Over millennia, life has been exposed to ionizing radiation from cosmic rays and natural radioisotopes. Biological experiments in underground laboratories have recently demonstrated that the contemporary terrestrial radiation background impacts the physiology of living organisms, yet the evolutionary consequences of this biological stress have not been investigated. Explaining the mechanisms that give rise to the results of underground biological experiments remains difficult, and it has been speculated that hereditary mechanisms may be involved. Here, we have used evolution experiments in standard and very low-radiation backgrounds to demonstrate that environmental ionizing radiation does not significantly impact the evolutionary trajectories of E. coli bacterial populations in a 500 generations evolution experiment.

Platinum nanopetal-based potassium sensors for acute cell death monitoring

A novel potassium-selective electrode based on Pt nanopetals has been used for monitoring potassium efflux from cells as due to two death mechanisms: osmotic shock in DI water and necro-apoptosis by drug overdose.

⁹⁹mTcO₄--, auger-mediated thyroid stunning: dosimetric requirements and associated molecular events

Low-energy Auger and conversion electrons deposit their energy in a very small volume (a few nm³) around the site of emission. From a radiotoxicological point of view the effects of low-energy electrons on normal tissues are largely unknown, understudied, and generally assumed to be negligible. In this context, the discovery that the low-energy electron emitter, ^99mTc, can induce stunning on primary thyrocytes in vitro, at low absorbed doses, is intriguing. Extrapolated in vivo, this observation suggests that a radioisotope as commonly used in nuclear medicine as ^99mTc may significantly influence thyroid physiology. The aims of this study were to determine whether ^99mTc pertechnetate (^99mTcO₄⁻) is capable of inducing thyroid stunning in vivo, to evaluate the absorbed dose of ^99mTcO₄⁻ required to induce this stunning, and to analyze the biological events associated/concomitant with this effect. Our results show that ^99mTcO₄⁻–mediated thyroid stunning can be observed in vivo in mouse thyroid. The threshold of the absorbed dose in the thyroid required to obtain a significant stunning effect is in the range of 20 Gy. This effect is associated with a reduced level of functional Na/I symporter (NIS) protein, with no significant cell death. It is reversible within a few days. At the cellular and molecular levels, a decrease in NIS mRNA, the generation of double-strand DNA breaks, and the activation of the p53 pathway are observed. Low-energy electrons emitted by ^99mTc can, therefore, induce thyroid stunning in vivo in mice, if it is exposed to an absorbed dose of at least 20 Gy, a level unlikely to be encountered in clinical practice. Nevertheless this report presents an unexpected effect of low-energy electrons on a normal tissue in vivo, and provides a unique experimental setup to understand the fine molecular mechanisms involved in their biological effects.

Radioactivity as a significant energy source in prebiotic synthesis

Radioactivity in the continental crust (due mainly to the isotopes 238U, 235U, 232Th and 40K), as a energy source for chemical evolution in the early Archean (between 3.5 and approximately 4 Ga bp), is reviewed. The most important radioactive source in the continental crust is due to the production and accumulation of radioactive gases within the crust voids (porosity). The study of such mechanism has allowed us to reach a deeper understanding about the nature of the radioactive source and to describe its behavior, particularly with regard to prebiotic chemical evolution. An effective total energy of 3 x 10(18) Ja-1 has been obtained for a depth of 1 km, 4 Ga ago. If a depth of 30 km is taken, the obtained value is almost equal to the UV solar energy radiation (lambda < 150 nm). Within the voids the radioactive source of the continental crust played a relevant role in prebiotic synthesis. In uranium deposits of the same age, the role of radioactivity must have been even more relevant in favoring chemical evolution.

Modeling radioprotective mechanisms in the dose effect relation at low doses and low dose rates of ionizing radiation

A new model (Random Coincidence Model--Radiation Adapted (RCM-RA)) is proposed which explains a possible pseudo threshold for stochastic radiation effects. It describes the formation of cancer in the case of multistep fixation of lesions in the critical regions of tumor associated genes such as proto-oncogenes or tumor-suppressor genes. The RCM-RA contains two different possibilities of DNA damage to complementary nucleotides. The damage may be caused either by radiation or by natural processes such as cellular radicals or thermal damage or by chemical cytotoxins. The model is based on the premise that radiation initially is bionegative, damaging organisms at their different levels of organization. The radiation, however, also induces various cellular radioprotective mechanisms which decrease the damage by natural processes. Considering both effects together, the theory explains apparent thresholds in the dose-response relation for radiation carcinogenesis without contradiction to the classical assumption that radiation is predominantly bionegative at doses typically found in occupational exposures.

Radionuclide-induced evolution of DNA and the origin of life

Artesian groundwaters of high radionuclide concentration are ubiquitous and may have provided the large, sustained energy sources that were required to drive the multistage process of DNA and primordial cell evolution. The rapid, early development of the genetic code as well as its degeneracy can be attributed to exceptionally high radiation-induced mutation rates in this unique environment. The ability of double-strand DNA to direct enzymatic repair of radiation damage to single strands contributed importantly to its selective evolution. It is postulated that the polymerization of nucleotides took place at elevated temperatures within alpha-particle tracks of high ion and free-radical density, followed by rapid quenching to ambient conditions. It also is evident that radiation resistance and ploidy were important selection factors in cellular evolution.

Radiation hormesis: an evolutionary expectation and the evidence

Fundamental to evolutionary biology is the tendency for organisms to become progressively adapted to those environments to which they are most frequently exposed. Since background radiation is a universal component of our environment, radiation hormesis is an evolutionary expectation. Whole-of-life experiments and more numerous (and evolutionarily more artificial) shorter duration and higher dose-rate experiments are consistent with this expectation. Additional research is needed especially at the molecular level to search for underlying mechanisms, and implications for radiation protection guidelines need to be analyzed.

Biological effects of background radiation: mutagenicity of 40K

The naturally occurring radioactive isotope 40K is the single largest contributor to the internal background radiation dose in living organisms. We examined cell growth and mutation rate or frequency in several strains of Escherichia coli in (i) media containing the natural content of 40K, (ii) media containing potassium from which essentially all of the 40K had been removed by isotope separation, and (iii) media highly enriched in 40K. Growth rates (doubling times) were identical in the present or absence of 40K. In more than 40 chemostat experiments, we were unable to detect any significant differences in mutation rate to bacteriophage T5 resistance or in mutation frequency to valine resistance or tryptophan prototrophy attributable to 40K. We conclude that, in the bacterial systems we have studied, 40K does not make a significant contribution to spontaneous mutation.