A multidisciplinary approach unravels early and persistent effects of X-ray exposure at the onset of prenatal neurogenesis

Abstract

BackgroundIn humans, in utero exposure to ionising radiation results in an increased prevalence of neurological aberrations, such as small head size, mental retardation and decreased IQ levels. Yet, the association between early damaging events and long-term neuronal anomalies remains largely elusive.MethodsMice were exposed to different X-ray doses, ranging between 0.0 and 1.0 Gy, at embryonic days (E) 10, 11 or 12 and subjected to behavioural tests at 12 weeks of age. Underlying mechanisms of irradiation at E11 were further unravelled using magnetic resonance imaging (MRI) and spectroscopy, diffusion tensor imaging, gene expression profiling, histology and immunohistochemistry.ResultsIrradiation at the onset of neurogenesis elicited behavioural changes in young adult mice, dependent on the timing of exposure. As locomotor behaviour and hippocampal-dependent spatial learning and memory were most particularly affected after irradiation at E11 with 1.0 Gy, this condition was used for further mechanistic analyses, focusing on the cerebral cortex and hippocampus. A classical p53-mediated apoptotic response was found shortly after exposure. Strikingly, in the neocortex, the majority of apoptotic and microglial cells were residing in the outer layer at 24 h after irradiation, suggesting cell death occurrence in differentiating neurons rather than proliferating cells. Furthermore, total brain volume, cortical thickness and ventricle size were decreased in the irradiated embryos. At 40 weeks of age, MRI showed that the ventricles were enlarged whereas N-acetyl aspartate concentrations and functional anisotropy were reduced in the cortex of the irradiated animals, indicating a decrease in neuronal cell number and persistent neuroinflammation. Finally, in the hippocampus, we revealed a reduction in general neurogenic proliferation and in the amount of Sox2-positive precursors after radiation exposure, although only at a juvenile age.ConclusionsOur findings provide evidence for a radiation-induced disruption of mouse brain development, resulting in behavioural differences. We propose that alterations in cortical morphology and juvenile hippocampal neurogenesis might both contribute to the observed aberrant behaviour. Furthermore, our results challenge the generally assumed view of a higher radiosensitivity in dividing cells. Overall, this study offers new insights into irradiation-dependent effects in the embryonic brain, of relevance for the neurodevelopmental and radiobiological field.Electronic supplementary materialThe online version of this article (doi:10.1186/1866-1955-7-3) contains supplementary material, which is available to authorized users.

A detailed characterization of congenital defects and mortality following moderate X‐ray doses during neurulation

“…Previous studies have to a certain extent investigated the radiosensitivity and underlying mechanisms behind both preneurulation and postneurulation development (e.g., neurogenesis) (Quintens et al, ; Verreet et al, ), but knowledge on the mechanisms underlying radiosensitivity during neurulation is limited at best. In addition, differences in irradiation conditions, mouse strain dependency (Streffer & Müller, ) and variations in developmental stages make it difficult to form a unified picture of the phenotypes resulting from in utero irradiation during mid‐gestation.…”

“…To this end we decided that it was necessary to expand on the aforementioned studies and use shorter and better defined time intervals for radiation sensitivity characterization, whilst also using radiation doses at levels more relevant to potential exposures of pregnant women in situations of medicine (e.g., CT) and accidents. Furthermore, due to the focus of most recent studies on radiation‐induced defects at later developmental stages, that is, during neurogenesis (E11–E17) (Quintens et al, ; Verreet et al, ; Verreet, Rangarajan et al, ; Verreet, Verslegers et al, ), there is a general lack of information on the impact of irradiation during neurulation, which starts at E7.5.…”

p53 drives premature neuronal differentiation in response to radiation-induced DNA damage during early neurogenesis

“…At one week after irradiation, the presence of apoptotic cells was no longer evident (data not shown), highlighting the transient nature of the immediate and direct effects of acute DNA damage. However, we previously showed that mice exposed to radiation at E11 have a reduced cortical thickness at this stage 29 . To identify neuronal subtypes that were lost after irradiation at E11, we performed immunostainings for markers of the different neocortical layers in brains of P2 mice.…”

“…The activation of cell cycle arrest and apoptosis following DNA damage pointed to the involvement of p53 in the early response to radiation 29,31 . Consequently, we hypothesized that genetic inactivation of Trp53 would abrogate the negative effects of radiation on brain development.…”

Progress in low dose health risk research

“…More recently, it has been shown that cognitive effects of neonatal irradiated mice are accompanied by changed plasticity, adult neurogenesis and neuroinflammation [231,232] and that low dose IR affects mitochondria and synaptic signalling pathways in the murine hippocampus and cortex [233][234][235]. Also, high doses (1 Gy) of Xirradiation revealed short and long-term effects in terms of gene expression profiling and immune-histochemical alterations after IR exposure during early mouse brain development [236]. When assessing the long term behavioural effects of 1 Gy X-ray exposures in mice, it appeared that the effects observed reflect initially induced DNA damage, apoptosis and inflammation.…”