Radiation-induced apoptosis of oligodendrocytes in the adult rat brain

The life, death, and replacement of oligodendrocytes in the adult CNS

Oligodendrocytes (OLs) are mature glial cells that myelinate axons in the brain and spinal cord. As such, they are integral to functional and efficient neuronal signaling. The embryonic lineage and postnatal development of OLs have been well-studied and many features of the process have been described, including the origin, migration, proliferation, and differentiation of precursor cells. Less clear is the extent to which OLs and damaged/dysfunctional myelin are replaced following injury to the adult CNS. OLs and their precursors are very vulnerable to conditions common to CNS injury and disease sites, such as inflammation, oxidative stress, and elevated glutamate levels leading to excitotoxicity. Thus, these cells become dysfunctional or die in multiple pathologies, including Alzheimer's disease, spinal cord injury, Parkinson's disease, ischemia, and hypoxia. However, studies of certain conditions to date have detected spontaneous OL replacement. This review will summarize current information on adult OL progenitors, mechanisms that contribute to OL death, the consequences of their loss and the pathological conditions in which spontaneous oligodendrogenesis from endogenous precursors has been observed in the adult CNS.

Long-term mutagenic effects of ionising radiation on mice which vary in their p53 status

The tumor suppressor gene p53 plays a major role in the maintenance of genomic integrity. The impact that variations in cellular turnover rates and sensitivity to DNA damage will have on the effectiveness of p53 in this role was examined by following the induction and persistence of mutations in the brain and small intestine of mice after exposure to ionising radiation (IR). The examination of mutagenesis was carried out using the pUR288 LacZ plasmid-based mouse model-consisting of mice containing a target gene for mutation analysis integrated into every cell. In addition the mice varied in their p53 status. The tissues were compared at post-irradiation time-points from 24h to 3 months. The mutation frequencies (MFs) in the p53 wildtype and heterozygous brains peaked at 24h post-irradiation, and then returned to background or close to background levels, respectively. The p53 nullizygous brain showed a more fluctuating MF pattern, but returned to background levels by 3 months, indicating that the effect of the loss of p53 did not result in lasting differences in the response to mutation induction in the brain. In the intestine, there was a different pattern; in the wildtype and heterozygous animals, the MFs increased from 24h to a peak at 4 weeks post-irradiation, before decreasing towards background levels at 3 months. The MFs in the intestine from the nullizygous animals did not decrease significantly between 4 weeks and 3 months, illustrating that the loss of p53 had a greater impact in this tissue than the brain. The variation in mutation frequencies and the type of mutations generated after DNA damage suggests that while p53 plays a significant role in the maintenance of genomic integrity, other mechanisms, such as the drive to replicate in progenitor cells, can reduce its effectiveness as the "guardian of the genome".

Quantitative morphologic evaluation of white matter in survivors of childhood medulloblastoma

In survivors of pediatric brain tumors, cranial radiation therapy can cause a debilitating cognitive decline associated with decreased volume in normal-appearing white matter (NAWM). We applied fractal geometry to quantify white matter (WM) integrity in the brain of medulloblastoma survivors. Fractal features of WM were evaluated by indices of fractal dimensions (FDs) of WM intensity and boundary on T1-weighted magnetic resonance images. The FD index of WM intensity was calculated by using a fractional Brownian motion model, and the FD index of WM boundary was calculated by using a box-counting method. Fractal features of WM on 116 magnetic resonance images of 58 patients with medulloblastoma were investigated at the start of therapy (Start TX) and approximately 2 years later (After TX). Patients were assigned to one of two groups based on change in NAWM volumes. Fractal features in patients with decreased NAWM volume were significantly greater After TX, whereas those in patients with increased NAWM volumes were not. Multiple linear regression analysis showed that fractal features were strongly correlated with NAWM volumes After TX in patients with decreased NAWM volume. These results demonstrated significant deficit in NAWM integrity and WM density changes in children treated for medulloblastoma. Multiple regression analysis illustrated that deficits in NAWM integrity in these children may partly explain the decrease in NAWM volume. We conclude that fractal geometry can be used to monitor the morphologic effects of neurotoxicity in brain tumor survivors.

Apoptosis and proliferation of oligodendrocyte progenitor cells in the irradiated rodent spinal cord

PURPOSE

Oligodendrocytes undergo early apoptosis after irradiation. The aim of this study was to determine the relationship between oligodendroglial apoptosis and proliferation of oligodendrocyte progenitor cells (OPC) in the irradiated central nervous system.

METHODS AND MATERIALS

Adult rats and p53 transgenic mice were given single doses of 2 Gy, 8 Gy, or 22 Gy to the cervical spinal cord. Apoptosis was assessed using TUNEL (Tdt-mediated dUTP terminal nick-end labeling) staining or by examining nuclear morphology. Oligodendrocyte progenitor cells were identified with an NG2 antibody or by in situ hybridization for platelet-derived growth factor receptor alpha. Proliferation of OPC was assessed by in vivo bromodeoxyuridine (BrdU) labeling and subsequent immunohistochemistry. Because radiation-induced apoptosis of oligodendroglial cells is p53 dependent, p53 transgenic mice were used to study the relationship between apoptosis and cell proliferation.

RESULTS

Oligodendrocyte progenitor cells underwent apoptosis within 24 h of irradiation in the rat. That did not result in a change in OPC density at 24 h. Oligodendrocyte progenitor cell density was significantly reduced by 2-4 weeks, but showed recovery by 6 weeks after irradiation. An increase in BrdU-labeled cells was observed at 2 weeks after 8 Gy or 22 Gy, and proliferating cells in the rat spinal cord were immunoreactive for NG2. The mouse spinal cord showed a similar early cell proliferation after irradiation. No difference was observed in the proliferation response in the spinal cord of p53 -/- mice compared with wild type animals.

CONCLUSIONS

Oligodendroglial cells undergo early apoptosis and OPC undergo early proliferation after ionizing radiation. However, apoptosis is not likely to be the trigger for early proliferation of OPC in the irradiated central nervous system.

White-matter diffusion anisotropy after chemo-irradiation: a statistical parametric mapping study and histogram analysis

The aim of the study was to evaluate white-matter (WM) diffusion anisotropy in medulloblastoma survivors after cranial irradiation and chemotherapy using voxel-based analysis with SPM99 and fractional anisotropy (FA) histogram-derived indices, and to identify quantitative indices for detecting and monitoring children with treatment-induced white-matter injury. Familywise error rate (FWE) that corrects for multiple comparisons was used to locate statistically significant regions of P < 0.05 in voxel-based analysis. Subsequently, the false discovery rate (FDR) controlling procedure (corrected P < 0.05) was used. FA map histogram analysis of histogram-derived indices, mean FA, mean FA peak height, and peak location was performed. Two-sample t test was used in all analyses. Using FWE-corrected P < 0.05, there was a cluster of reduced anisotropy in the periventricular white matter lateral to the left ventricular atrium. When FDR-corrected P < 0.05 was used, there were multiple clusters of reduced anisotropy in the periventricular white matter, the corpus callosum, and corona radiata. Simplified voxel-based morphometry (VBM)-like analysis of cerebrospinal fluid (CSF) did not show significant differences between patient and control subjects. 'White-matter FA map' histogram showed significant reduction in mean FA and mean FA peak location and significant increase in mean FA peak height in the patient group compared to control subjects (P = 0.003, P = 0.003, and P = 0.014, respectively). This approach of quantifying FA can be applied to characterize anisotropy in the white matter after cranial irradiation and chemotherapy and can potentially be used to detect and monitor treatment-induced neurotoxicity.

Changes in oligodendrocytes and myelin gene expression after radiation in the rodent spinal cord

PURPOSE

The aim of this study was to assess changes in oligodendrocytes (OL), myelin gene expression, and their relationships with late demyelination after irradiation.

METHODS AND MATERIALS

Adult rats were given single doses of 8 or 22 Gy to the cervical spinal cord. Immunohistochemistry for APC or GST-pi was used to identify OL. Changes in myelin gene expression were assessed using RT-PCR for proteolipid protein (PLP). Luxol fast blue staining was used to assess demyelination. CNP-(beta)geo transgenic mice were used to confirm some of the results of the rat model. Cells of the oligodendroglial lineage in these animals express beta-galactosidase (beta-gal).

RESULTS

Early apoptosis of APC, GST-pi, and beta-galactosidase positive cells was observed in the spinal cord of rats and CNP-(beta)geo mice. At 24 h after 22 Gy, there was a significant decrease in OL density. Cell density continued to decline thereafter after both 8 and 22 Gy, and a reduction in PLP expression was observed at 4-5 weeks. A further decrease in PLP expression was seen beginning at 18 weeks after 22 Gy only. Demyelination was observed at 19 weeks after 22 Gy.

CONCLUSIONS

Apoptosis of OL and changes in OL density and PLP gene expression were observed early after both 8 and 22 Gy. This suggests that these early changes are unlikely to be directly related to the late demyelination observed.

X‐irradiation of adult spinal cord increases its capacity to support neurite regeneration in vitro

Previous in vitro studies have shown that X-irradiation during early postnatal life can change the environment of CNS tissue in later adult life such that it becomes more supportive of neurite regeneration from adult dorsal root ganglion (DRG) neurons than non-irradiated tissue. The question arises whether or not x-irradiation during adult life can alter the CNS environment such that it also becomes more supportive of neurite regeneration. This was investigated by exposing portions of the spinal cord of adult rats to 10, 20 or 40 Gray of X-irradiation and later using this tissue to prepare cryosections suitable for use as a substrate in a cryoculture assay. Fixed cryocultures were immunolabelled using anti-glial fibrillary acidic protein (GFAP) to visualise the tissue sections and anti-growth associated protein (GAP-43) to visualise the regenerating neurites. Tissue sections from sham-irradiated animals and from those irradiated with 10 Gray did not support the regeneration of neurites. However, sections of spinal cords from rats treated with either 20 or 40 Gray of X-irradiation 4 or 32 days prior to sampling were found to support a certain degree of neurite regeneration. It is concluded that X-irradiation of adult CNS tissue can alter its environment such that it becomes more supportive of neurite regeneration and it is speculated that this change may be the result of alterations in the glial cell populations in the post-irradiated tissues.

Effects of single low dose irradiation on subventricular zone cells in juvenile rat brain

Although the juvenile human brain is relatively radioresistant, irradiation can result in brain growth retardation, progressive mental disturbance, and neurologic abnormalities. As neural stem cells or progenitor cells may be a target of radiation injury and may play an important role in the brain's functional recovery, we examined the effects of whole brain irradiation on these cells in juvenile rat. Six-week-old Wistar rats, where the brain is still growing, were irradiated with single doses of 1, 2, or 3 Gy X-ray. We measured their body and brain weights at 30 or 60 days after irradiation. The chronological changes of the subventricular zone (SVZ) were examined at 6 h, 2, 7, 14, 30, or 60 days after irradiation by immunohistochemistry, specifically looking at the neural stem cells or progenitor cells using anti-nestin antibodies specific for these cells. The rate of brain weight gain of irradiated rats significantly decreased in comparison to controls, although that of body weight gain was similar among them. Multiple apoptotic cells appeared in the SVZ at 6 h after irradiation with simultaneous reduction in nestin-positive cells (69% of the control). The cell levels recovered within a week, with the nestin-positive cells reaching maximal numbers (182%) on Day 14. Nestin-positive cells returned to baseline levels within 30 days (96%) and remained unchanged for the subsequent 60 days. The X-ray dosage did not affect these findings. Our findings revealed that single low dose X-ray administration reversibly affected the levels of neural stem and progenitor cells in the SVZ region. These results suggest that continuous multiple administrations of X-rays in clinical treatment may affect irreversible changes on neural stem or progenitor cells, causing brain growth retardation, or dysfunction.