Types and three-dimensional distribution of neuronal ectopias in the brain of mice prenatally subjected to X-irradiation

Prenatal irradiation-induced brain neuropathology and cognitive impairment

Embryo/fetus is much more radiosensitive than neonatal and adult human being. The main potential effects of pre-natal radiation exposure on the human brain include growth retardation, small head/brain size, mental retardation, neocortical ectopias, callosal agenesis and brain tumor which may result in a lifetime poor quality of life. The patterns of prenatal radiation-induced effects are dependent not only on the stages of fetal development, the sensitivity of tissues and organs, but also on radiation sources, doses, dose rates. With the increased use of low dose radiation for diagnostic or radiotherapeutic purposes in recent years, combined with postnatal negative health effect after prenatal radiation exposure to fallout of Chernobyl nuclear power plant accident, the great anxiety and unnecessary termination of pregnancies after the nuclear disaster, there is a growing concern about the health effect of radiological examinations or therapies in pregnant women. In this paper, we reviewed current research progresses on pre-natal ionizing irradiation-induced abnormal brain structure changes. Subsequent postnatal neuropsychological and neurological diseases were provided. Relationship between irradiation and brain aging was briefly mentioned. The relevant molecular mechanisms were also discussed. Future research directions were proposed at the end of this paper. With limited human data available, we hoped that systematical review of animal data could relight research interests on prenatal low dose/dose rate irradiation-induced brain microanatomical changes and subsequent neurological and neuropsychological disorders.

Current Evidence for Developmental, Structural, and Functional Brain Defects following Prenatal Radiation Exposure

Ionizing radiation is omnipresent. We are continuously exposed to natural (e.g., radon and cosmic) and man-made radiation sources, including those from industry but especially from the medical sector. The increasing use of medical radiation modalities, in particular those employing low-dose radiation such as CT scans, raises concerns regarding the effects of cumulative exposure doses and the inappropriate utilization of these imaging techniques. One of the major goals in the radioprotection field is to better understand the potential health risk posed to the unborn child after radiation exposure to the pregnant mother, of which the first convincing evidence came from epidemiological studies on in utero exposed atomic bomb survivors. In the following years, animal models have proven to be an essential tool to further characterize brain developmental defects and consequent functional deficits. However, the identification of a possible dose threshold is far from complete and a sound link between early defects and persistent anomalies has not yet been established. This review provides an overview of the current knowledge on brain developmental and persistent defects resulting from in utero radiation exposure and addresses the many questions that still remain to be answered.

Brain and arterial abnormalities following prenatal X-ray irradiation in mice assessed by magnetic resonance imaging and angiography

The present study aimed to quantitatively characterize changes in the whole brain and arterial morphology in response to prenatal ionizing irradiation. Magnetic resonance imaging (MRI) and angiography (MRA) were used to evaluate brain and arterial abnormalities in 8-week-old male mice prenatally exposed to X-ray radiation at a dose of 0.5 or 1.0 Gy on embryonic day (E) 13. Irradiated mice demonstrated decreased brain volume, increased ventricular volume, and arterial malformation. Additionally, MRA signal intensity and arterial thickness in the anterior cerebral artery, middle cerebral artery, and basilar artery were lower in radiation-exposed mice than in control mice. MRI and MRA are useful tools for assessing brain and arterial abnormalities after prenatal exposure to radiation.

Diffusion tensor imaging of brain abnormalities induced by prenatal exposure to radiation in rodents

We assessed brain abnormalities in rats exposed prenatally to radiation (X-rays) using magnetic resonance imaging (MRI) and histological experiments. Pregnant rats were divided into 4 groups: the control group (n = 3) and 3 groups that were exposed to different radiation doses (0.5, 1.0, or 1.5 Gy; n = 3 each). Brain abnormalities were assessed in 32 neonatal male rats (8 per group). Ex vivo T2-weighted imaging and diffusion tensor imaging (DTI) were performed using 11.7-T MRI. The expression of markers of myelin production (Kluver-Barrera staining, KB), nonpyramidal cells (calbindin-D28k staining, CaBP), and pyramidal cells (staining of the nonphosphorylated heavy-chain neurofilament SMI-32) were histologically evaluated. Decreased brain volume, increased ventricle volume, and thinner cortices were observed by MRI in irradiated rats. However, no abnormalities in the cortical 6-layered structure were observed via KB staining in radiation-exposed rats. The DTI color-coded map revealed a dose-dependent reduction in the anisotropic signal (vertical direction), which did not represent reduced numbers of pyramidal cells; rather, it indicated a signal reduction relative to the vertical direction because of low nerve cell density in the entire cortex. We conclude that DTI and histological experiments are useful tools for assessing cortical and hippocampal abnormalities after prenatal exposure to radiation in rats.

Quantitative assessment of central nervous system disorder induced by prenatal X-ray exposure using diffusion and manganese-enhanced MRI

Prenatal radiation exposure induces various central nervous system (CNS) disorders depending on the dose, affected region and gestation period. The goal of this study was to assess noninvasively a CNS development disorder induced by prenatal X-ray exposure using quantitative manganese-enhanced MRI (MEMRI) as well as apparent diffusion coefficient (ADC) and transverse relaxation time (T(2)) maps in comparison with immunohistological staining. The changes in ΔR(1) (increase in the longitudinal relaxation rate (R(1)) from before and after MnCl(2) administration.) induced by the Mn(2+) contrast agent were evaluated in the CNS of normal and prenatally irradiated rats. ADC and T(2) were also compared with the histological results obtained using hematoxylin and eosin (to estimate cell density), activated caspase-3 (apoptotic cells) and glial fibrillary acidic protein (proliferation of astrocytes/astroglia). We found the following: (i) the decreased Mn(2+) uptake (indicated by a smaller ΔR(1)) for radiation-exposed rats was predominantly correlated with a decrease in cell viability (apoptotic cytopathogenicity) and CNS cell density after prenatal radiation exposure; (ii) the longer T(2) and ADC were associated with a decrease in CNS cell density and apoptotic alteration after radiation exposure. In addition to the slight proliferation of astroglia (+58%), there was a substantial decrease in cell density (-78%) and an excessive increase in apoptotic cells (+613%) in our prenatal radiation exposure model. The results suggest that MEMRI in the prenatal X-ray exposure model predominantly reflected the decrease in cell density and viability rather than the proliferation of astroglia. In conclusion, quantitative MEMRI with ADC/T(2) mapping provides objective information for the in vivo assessment of cellular level alterations by prenatal radiation exposure, and has the potential to be used as a standard approach for the evaluation of the cellular damage of radiotherapy.

A role for endogenous and radiation-induced DNA double-strand breaks in p53-dependent apoptosis during cortical neurogenesis

Prenatal exposure to low-dose radiation increases the risk of microcephaly and/or mental retardation. Microcephaly is also associated with genetic mutations that affect the non-homologous end-joining pathway of DNA double-strand break repair. To examine the link between these two causal factors, we characterized the neural developmental effects of acute radiation exposure in mouse littermate embryos harboring mutations in the Ku70 and p53 genes. Both low-dose radiation exposure and Ku70 deficiency induced morphologically indistinguishable cortical neuronal apoptosis. Irradiated Ku70-deficient embryos displayed anatomical damage indicative of increased radiosensitivity in the developing cerebral cortex. Deleting the p53 gene not only rescued cortical neuronal apoptosis at all levels but also restored the in vitro growth of Ku70-deficient embryonic fibroblasts despite the presence of unrepaired DNA/chromosomal breaks. The results confirm the role of DNA double-strand breaks as a common causative agent of apoptosis in the developing cerebral cortex. Furthermore, the findings suggest a disease mechanism by which the presence of endogenous DNA double-strand breaks in the newly generated cortical neurons becomes radiomimetic when DNA end joining is defective. This in turn activates p53-dependent neuronal apoptosis and leads to microcephaly and mental retardation.

Spatial learning and expression of neural cell adhesion molecule L1 in rats X-irradiated prenatally

The present study was designed to present evidence to clarify the relationships between learning ability, neuronal cell adhesion molecule L1 expression and hippocampal structural changes in the rat model received X-irradiation at an embryonic stage (E15). Water maze task indicated that all of the irradiated rats failed to learn the task in the whole training procedure. Their latency to the platform and swimming distance were significant differences from those sham-treated controls. Histological studies showed that the hippocampal ectopias induced by X-rays in the CA1 were involved in the spatial learning impairment, in which they hampered normal processes in learning development and transmission of information. Number, size and positions of the ectopias in the dorsal parts of the hippocampus were confirmed to be related to degrees of spatial learning impairment. On the other hand, L1 expression in the hippocampus was examined with Western blot analysis. The results indicated a lower content of L1 in the irradiated rats. A decrease in L1 might be one of reasons to cause disorganization of the septohippocampal pathways. These findings suggest some mechanisms of spatial learning impairment can be attributed to the formation of the hippocampal ectopias and redaction of L1 following prenatal exposure to X-irradiation.

Ectopic neurons in the hippocampus may be a cause of learning disability after prenatal exposure to X-rays in rats

The relationship between an impairment of spatial navigation and an incidence of ectopic neurons in the dorsal hippocampus was investigated in adult rats that were prenatally exposed to X-ray irradiation. Adult rats which had received 1.5 Gy X-rays at embryonic day 15 (E15) showed significant learning disability in the water-maze task. According to the mean value of the swimming time, we categorized the irradiated adult rats into the following three groups: slightly damaged group, mildly damaged group and severely damaged group. No significant difference in the brain weight was found between the three categorized groups. Ectopic neurons appearing at abnormal places were prominently observed in the dorsal hippocampus of the severely damaged group with a remarkable learning disturbance, while no ectopia in the hippocampus was observed in the slightly damaged group. This may suggest that the cognitive dysfunction induced by prenatal exposure to X-ray irradiation may be, at least in part, attributable to ectopic neurons of the hippocampus.