In vitro neurotoxic effects of 1 GeV/n iron particles assessed in retinal explants

Effects of low dose ionizing radiation on the brain- a functional, cellular, and molecular perspective

Over the years, the advancement of radio diagnostic imaging tools and techniques has radically improved the diagnosis of different pathophysiological conditions, accompanied by increased exposure to low-dose ionizing radiation. Though the consequences of high dose radiation exposure on humans are very well comprehended, the more publicly relevant effects of low dose radiation (LDR) (≤100 mGy) exposure on the biological system remain ambiguous. The central nervous system, predominantly the developing brain with more neuronal precursor cells, is exceptionally radiosensitive and thus more liable to neurological insult even at low doses, as shown through several rodent studies. Further molecular studies have unraveled the various inflammatory and signaling mechanisms involved in cellular damage and repair that drive these physiological alterations that lead to functional alterations. Interestingly, few studies also claim that LDR exerts therapeutic effects on the brain by initiating an adaptive response. The present review summarizes the current understanding of the effects of low dose radiation at functional, cellular, and molecular levels and the various risks and benefits associated with it based on the evidence available from in vitro, in vivo, and clinical studies. Although the consensus indicates minimum consequences, the overall evidence suggests that LDR can bring about considerable neurological effects in the exposed individual, and hence a re-evaluation of the LDR usage levels and frequency of exposure is required.

Ionizing radiation-induced risks to the central nervous system and countermeasures in cellular and rodent models

PURPOSE

Harmful effects of ionizing radiation on the Central Nervous System (CNS) are a concerning outcome in the field of cancer radiotherapy and form a major risk for deep space exploration. Both acute and chronic CNS irradiation induce a complex network of molecular and cellular alterations including DNA damage, oxidative stress, cell death and systemic inflammation, leading to changes in neuronal structure and synaptic plasticity with behavioral and cognitive consequences in animal models. Due to this complexity, countermeasure or therapeutic approaches to reduce the harmful effects of ionizing radiation include a wide range of protective and mitigative strategies, which merit a thorough comparative analysis.

MATERIALS AND METHODS

We reviewed current approaches for developing countermeasures to both targeted and non-targeted effects of ionizing radiation on the CNS from the molecular and cellular to the behavioral level.

RESULTS

We focus on countermeasures that aim to mitigate the four main detrimental actions of radiation on CNS: DNA damage, free radical formation and oxidative stress, cell death, and harmful systemic responses including tissue death and neuroinflammation. We propose a comprehensive review of CNS radiation countermeasures reported for the full range of irradiation types (photons and particles, low and high linear energy transfer) and doses (from a fraction of gray to several tens of gray, fractionated and unfractionated), with a particular interest for exposure conditions relevant to deep-space environment and radiotherapy. Our review reveals the importance of combined strategies that increase DNA protection and repair, reduce free radical formation and increase their elimination, limit inflammation and improve cell viability, limit tissue damage and increase repair and plasticity.

CONCLUSIONS

The majority of therapeutic approaches to protect the CNS from ionizing radiation have been limited to acute high dose and high dose rate gamma irradiation, and few are translatable from animal models to potential human application due to harmful side effects and lack of blood-brain barrier permeability that precludes peripheral administration. Therefore, a promising research direction would be to focus on practical applicability and effectiveness in a wider range of irradiation paradigms, from fractionated therapeutic to deep space radiation. In addition to discovering novel therapeutics, it would be worth maximizing the benefits and reducing side effects of those that already exist. Finally, we suggest that novel cellular and tissue models for developing and testing countermeasures in the context of other impairments might also be applied to the field of CNS responses to ionizing radiation.

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

Analysis of cell cycle in mouse bone marrow cells following acute in vivo exposure to 56Fe ions

A pilot study was conducted to examine the magnitude of cell-cycle delay and apoptosis in bone marrow (BM) cells collected at 18, 42 and 66 hr from radiosensitive CBA/CaJ mice and radioresistant C57BL/6J mice following a whole-body in vivo exposure to 1 GeV/amu (56)Fe ions or (137)Cs gamma rays. At each sacrifice, BM cells were collected from three mice of each strain per dose of (56)Fe ions (0, 10 and 100 cGy) and two mice of each strain per dose of (137)Cs gamma rays (0, 100 and 300 cGy). A significant G1-arrest (ANOVA, p < 0.05) was observed at 18 hr after exposure of mice to 100 cGy of (56)Fe ions or 300 cGy of (137)Cs gamma rays, relative to their corresponding sham-controls, resulting in a significant decrease in the percentage of cells cycling into S-phase in both strains. The percentage of S-phase cells subsequently increased and persisted up to 66 hr post-irradiation. Significant numbers of G2/M cells were found at 18 and 66 (but not at 42) hr post-irradiation, regardless of radiation-type or mouse-strain. It is likely that BM cells have undergone at least one cell cycle at 66 hr after exposure of mice to either 100 cGy (56)Fe ions or 300 cGy (137)Cs gamma rays. Our study is the first to investigate the in vivo effects of (56)Fe ions (1 GeV/amu) on the cell cycle of mouse BM cells using flow cytometry. The cell-cycle distribution (but not the number of apoptotic cells) was dependent on radiation-dose and harvest-time.

Positive visual phenomena in space: A scientific case and a safety issue in space travel

Most astronauts on Apollo, Skylab, and MIR reported 'flashes of light' occurring in different shapes and apparently moving across the visual field, in the absence of auditory, somatosensory, or olfactory abnormal percepts. A temporal correlation with heavy nuclei or protons has been documented in space and comparable phosphenes were observed by volunteers whose eyes were exposed to accelerated heavy ions at intensities below the threshold for Cerenkov visible radiation. An interaction between heavy ions and the retina was suggested. However, the biophysics of heavy ions or protons action remains undefined, the effects on photoreceptors and neuroretina have not been differentiated, and some direct action on the visual cortex never ruled out. Phosphenes are common in migraine and are known to occur also in response to the electrical stimulation of ganglion cells (in retinas without photoreceptors), optic pathways or visual cortex, with mechanisms that bypass the chemically gated channels. Intrinsic photosensitive ganglion cells exist in the retina of teleost fish and mammals. In the hypothesis of a peculiar sensitivity to subatomic particles of the visual system, phosphenes due to the activation of processes by-passing the photoreceptors would raise questions about human safety in space. The issue is particularly relevant with experiments of increasing duration being now operative in the International Space Station (ISS) and with plans of space travel outside the geomagnetic shield. Research is in progress both in the ISS and on animal models, in the framework of the NASA/ESA actions to improve the astronauts' health in space.

Effects of HZE particle on the nigrostriatal dopaminergic system in a future Mars mission

Because of long duration travel outside the Earth's magnetic field, the effect of iron-rich high charge and energy (HZE) particles in Galactic Cosmic Rays on human body is the major concern in radiation protection. Recently attention has been directed to effects on the central nervous system in addition to mutagenic effects. In particular, a reduction in striatal dopamine content on nigrostriatal dopaminergic system has been reported by investigators using accelerated iron ions in ground-based mammalian studies. In addition, studies of the pathophysiology of Parkinson's disease demonstrated that excess iron cause a reduction in the dopamine content in the substantia nigra. This suggests an intriguing possibility to explain the selective detrimental effects of HZE particles on the dopaminergic system. Should these particles have biochemical effects, possible options for countermeasures are: (1) nutritional prevention, (2) medication, and (3) surgical placement of a stimulator electrode at a specific anatomic site in the basal ganglia.

The impact of the new biology on radiation risks in space

Radiation is considered to be one of three or four major hazards for personnel in space and has emerged as the most critical issue to be resolved for long-term missions, both orbital and interplanetary. Space habitats are stressful and dangerous environments. Health and medical consequences arising from microgravity, stress, and trauma include weakened immune systems, increased viral activity, and loss of bone mass. The greatest risks from radiation are generally assumed to be cancers and possibly damage to the central nervous system. Synergistic effects arising from the other environmental hazards along with abscopal and exogenic factors are likely. Space programs represent an exceptional opportunity for examining the biological consequences of low-dose exposures of humans to radiation at every level of progression. Although astronauts are a relatively small population, they are healthy, physically active volunteers who undergo extensive testing and medical examinations before, during, and after protracted exposures with periodic follow-up examinations. The radiation environments along with other hazards are likewise monitored and documented. Extensive international research programs are in progress. Seven years ago the U.S. National Aeronautics and Space Administration established the National Space Biomedical Research Institute through a cooperative agreement with a consortium of research and academic institutions in order to address radiation issues through a concerted, programmatic effort. Advanced technologies are rapidly being incorporated into these programs to determine the significance of new biological data and to evaluate the interplay among the different medical hazards. Programmatic in vivo and in vitro studies of the processes leading to carcinogenesis are in progress. Drugs and dietary supplements are being examined at the cellular and in vivo levels to assess their potential as dose-modifying agents. The infrastructure of this new approach, recent results, and research in progress are reviewed and discussed.

An algorithm for neurite outgrowth reconstruction

We present a numerical method which provides the ability to analyze digitized microscope images of retinal explants and quantify neurite outgrowth. Few parameters are required as input and limited user interaction is necessary to process an entire experiment of images. This eliminates fatigue related errors and user-related bias common to manual analysis. The method does not rely on stained images and handles images of variable quality. The algorithm is used to determine time and dose dependent, in vitro, neurotoxic effects of 1 GeV per nucleon iron particles in retinal explants. No neurotoxic effects are detected until 72 h after exposure; at 72 h, significant reductions of neurite outgrowth occurred at doses higher than 10 cGy.