Radiosensitivity of the granule cell line and other cell types of the immature rat cerebellar cortex

IFN-γ promotes radioresistant Nestin-expressing progenitor regeneration in the developing cerebellum by augmenting Shh ligand production

BACKGROUND

Patients with brain tumors, especially pediatric brain tumors such as cerebellar medulloblastoma, always suffer from the severe side effects of radiotherapy. Regeneration of neural cells in irradiation-induced cerebellar injury has been reported, but the underlying mechanism remains elusive.

METHODS

We established an irradiation-induced developing cerebellum injury model in neonatal mice. Microarray, KEGG analysis and semi in vivo slice culture were performed for mechanistic study.

RESULTS

Nestin-expressing progenitors (NEPs) but not granule neuron precursors (GNPs) were resistant to irradiation and able to regenerate after irradiation. NEPs underwent less apoptosis but similar DNA damage following irradiation compared with GNPs. Subsequently, they started to proliferate and contributed to granule neurons regeneration dependent on the sonic hedgehog (Shh) pathway. In addition, irradiation increased Shh ligand provided by Purkinje cells. And microglia accumulated in the irradiated cerebellum producing more IFN-γ, which augmented Shh ligand production to promote NEP proliferation.

CONCLUSIONS

NEP was radioresistant and regenerative. IFN-γ was increased post irradiation to upregulate Shh ligand, contributing to NEP regeneration. Our study provides insight into the mechanisms of neural cell regeneration in irradiation injury of the developing cerebellum and will help to develop new therapeutic targets for minimizing the side effects of radiotherapy for brain tumors.

Multiple roles of chemokine CXCL12 in the central nervous system: a migration from immunology to neurobiology

Chemotactic cytokines (chemokines) have been traditionally defined as small (10-14kDa) secreted leukocyte chemoattractants. However, chemokines and their cognate receptors are constitutively expressed in the central nervous system (CNS) where immune activities are under stringent control. Why and how the CNS uses the chemokine system to carry out its complex physiological functions has intrigued neurobiologists. Here, we focus on chemokine CXCL12 and its receptor CXCR4 that have been widely characterized in peripheral tissues and delineate their main functions in the CNS. Extensive evidence supports CXCL12 as a key regulator for early development of the CNS. CXCR4 signaling is required for the migration of neuronal precursors, axon guidance/pathfinding and maintenance of neural progenitor cells (NPCs). In the mature CNS, CXCL12 modulates neurotransmission, neurotoxicity and neuroglial interactions. Thus, chemokines represent an inherent system that helps establish and maintain CNS homeostasis. In addition, growing evidence implicates altered expression of CXCL12 and CXCR4 in the pathogenesis of CNS disorders such as HIV-associated encephalopathy, brain tumor, stroke and multiple sclerosis (MS), making them the plausible targets for future pharmacological intervention.

DNA damage and apoptosis in the immature mouse cerebellum after acute exposure to a 1 mT, 60 Hz magnetic field

Several recent studies have reported that whole-body exposure of rodents to power frequency magnetic fields (MFs) can result in DNA single- and double-strand breaks in the brains of these animals. The current study was undertaken to investigate whether an acute 2h exposure of a 1 mT, 60 Hz MF could elicit DNA damage, and subsequently apoptosis, in the brains of immature (10-day-old) mice. DNA damage was quantitated at 0, 2, 4, and 24h after exposure using the alkaline comet assay. Apoptosis was quantitated in the external granule cell layer (EGCL) of the immature mouse cerebellum at 0 and 24h after exposure to MF by the TdT-mediated dUTP nick-end labeling (TUNEL) assay. Four parameters (tail ratio, tail moment, comet length and tail length) were used to assess DNA damage for each comet. While increased DNA damage was detected by tail ratio at 2h after MF exposure, no supporting evidence of increased DNA damage was detected by the other parameters. In addition, no similar differences were observed using these parameters at any of the other post-exposure times. No increase in apoptosis was observed in the EGCL of MF-exposed mice, when compared to sham mice. Taken together, these results do not support the hypothesis that acute MF exposure causes DNA damage in the cerebellums of immature mice.

Neuroanatomical and functional alterations resulting from early postnatal cerebellar insults in rodents

This review examines neuroanatomical and functional alterations in rodents resulting from postnatal insults during cerebellar development. Treatments such as irradiation and methylazoxymethanol (MAM) administration produced near birth (< postnatal day 8 for irradiation treatment and < postnatal day 4 for MAM administration) result in more severe cerebellar damage than do similar treatments administered several days after birth. Prominent among the more severe alterations are foliation abnormalities, misalignment of Purkinje cells and continued multiple innervation of climbing fibers; few or none of these occur as a result of later treatments (> postnatal day 8 for irradiation treatment and > postnatal day 4 for MAM treatment). The functional alterations also differ: insults produced near birth result in hypoactivity, ataxia, tremor and accompanying learning deficits, whereas those produced later result in hyperactivity and few learning deficits. This hyperactivity may have relevance to human disorders. Brief discussions of cerebellar and functional alterations (e.g., hyperactivity) resulting from neonatal infection with the Borna disease virus and induction of hypo- and hyperthyroidism during the preweaning period are also presented.

Kinetics of radiation-induced apoptosis in the cerebellum of 14-day-old rats after acute or during continuous exposure

We have studied, by histological methods, cytological progression, frequency and distribution of apoptosis in the external granular layer of the cerebellum after whole-body irradiation of 14-day-old rats by gamma-rays from 60 Co. After acute exposure to 0.25, 0.5, 1.5 and 3 Gy (18 cGy/min), the duration of the apoptotic process gradually increased with dose from 6-9 h after 0.25 Gy, to > 24 h after 3 Gy. Up to 1 Gy, maximal frequency was found 6 h after exposures, and at this postirradiation time a linear increase in apoptosis with dose was observed. No effect of dose-rate on apoptosis induction could be demonstrated 6 h after delivering 1 Gy at dose-rates from 2.2 to 18 cGy/min. Continuous irradiation at 1.8 cGy/h induced a gradual increase of apoptosis that remained at a plateau value of about 3% from 15 to 29 h (controls 0.12%, SD = 0.07) and then gradually decreased to 1% at 53 h. At this time the mitotic index was similar to that measured in controls. Apoptosis occurring 3 h after acute irradiation, confined to proliferative cells, was only observed for doses of 1.5 and 3 Gy.

Morphological changes in cultures of hippocampus following prenatal irradiation in the rat

The effect of prenatal irradiation was studied in organotypic cultures of hippocampus, prepared from newborn rats that had been exposed to whole-body irradiation of 1 Gy from a 60Co-source at day 13 of pregnancy. Light and electron microscopic observations showed remarkable damage to neuronal mitochondria accompanied by extensive swelling, vacuolation of the Golgi complex, and formation of multilamellar bodies and vesicles of the lysosomal type. In contrast to neuronal alterations, no delay in synaptogenesis or onset of myelination was observed based upon the absence of significant morphological changes in synapses and myelin sheaths. Using this tissue culture model it could be confirmed that prenatal exposure to irradiation, even at low doses, induces specific morphological changes in the brain.

Influence of cis-dichlorodiammineplatinum on the structure of the immature rat cerebellum

Groups of 6- to 7-day-old and 10- to 11-day-old rats received cis-dichlorodiammineplatinum (cis-DDP) or inactive trans-DDP subcutaneously and were killed after 1, 6, 10, 15, or 21 days. In both age groups the acute effect (postinjection day 1) was most obvious in the germinal external granular layer (EGL), where many cells underwent shrinkage necrosis (pyknosis, apoptosis); the latter were more frequent within the fissures than on the surface of the cerebellar folia. Cell debris were often seen to be engulfed by macrophages, Bergmann cell fibers, and meningocytes. Bergmann cell bodies were swollen and the nuclei of Purkinje cells as well as of surviving cells of the EGL were enlarged. No comparable effects were observed in animals that had received an injection of trans-DDP in the same dose. Toward postinjection (p.i.) day 6, pyknotic cells disappeared from the EGL or, after the larger doses of cis-DDP, substantially decreased in number. Small nests of pyknotic cells appeared, however, at some places of the internal granular layer (IGL). The EGL was discontinuous or thinner in both age groups. In the IGL and molecular layer (ML) multiple focal fresh hemorrhages appeared together with some macrophages. The packing cell density in the IGL was less than in the controls, especially at the top of the cerebellar folia, i.e., contrary to the distribution of the primary damage in the EGL. Later, at p.i. day 10, ectopic nests of the IGL occurred occasionally in animals injected when 10 to 11 days old and severe atrophy of the ML and Purkinje cell population was observed at some places. At p.i. days 15 to 21, invasion of microglia-like cells appeared in the IGL and in some regions of the ML. Occasionally, subpial hemorrhages occurred at this interval. The acute damage caused by cis-DDP is thus similar to the effect of X rays or some, but not all, drugs with a cytostatic action. In addition, a more profound influence on dividing cells of the EGL within the fissures and distinct capillary lesions, indicated by hemorrhages, were found after cis-DDP. As in other experimental models, the acute cis-DDP damage of the immature cerebellum was partly repaired within a few p.i. days. Spatial difference of the repair process was inferred from the packing cell density in the IGL measured at the top and bottom of the cerebellar folia.

Early effects of gamma radiation on the developing rat cerebellum. Induction of cell pyknosis

Radiation induced cell pyknosis was analysed in the cerebellar cortex of developing rats. Rats of 1, 5 or 10 days of age were irradiated with 60Co and pyknotic cells were evaluated with respect to time after irradiation with 0.1 Gy and with respect to dose in the range 0.05 to 1.0 Gy. In all ages pyknotic cells began to appear about 2 hours after irradiation, a maximum was reached at about 8 hours. With a dose of 0.1 Gy, control values were obtained after 24 hours. The effects varied linearly with dose up to 1 Gy and no threshold seemed to exist at low doses.

Effects of low doses of gamma radiation on DNA synthesis in the developing rat brain

Rats of one or ten days of age were irradiated with low doses of gamma radiation, and synthesis of DNA was examined by the incorporation of 3H-thymidine in the cerebellum and the rest of the brain in vivo. DNA synthesis was depressed in both parts of the brain but the effects were larger in cerebellum. A minimum was found about 10 hours after irradiation in the older rats and later (18 h) in the younger ones. The dose response, in 10-day-old rats, was biphasic and showed that cerebellum was more affected. Autoradiographs showed that fewer cells entered the cycle and those synthesizing showed a depressed rate of synthesis. These findings are discussed in relation to induction of cell death.