Radiation-induced diffuse brain injury in the neonatal rat model--radiation-induced apoptosis of oligodendrocytes

Radiation-induced neuropathological changes in the oligodendrocyte lineage with relevant clinical manifestations and therapeutic strategies

PURPOSE

With technological advancements in radiation therapy for tumors of the central nervous system (CNS), high doses of ionizing radiation can be delivered to the tumors with improved accuracy. Despite the reduction of ionizing radiation-induced toxicity to surrounding tissues of the CNS, a wide array of side effects still occurs, particularly late-delayed changes. These alterations, such as white matter damages and neurocognitive impairments, are often debilitative and untreatable, significantly affecting the quality of life of these patients, especially children. Oligodendrocytes, a major class of glial cells, have been identified to be one of the targets of radiation toxicity and are recognized be involved in late-delayed radiation-induced neuropathological changes. These cells are responsible for forming the myelin sheaths that surround and insulate axons within the CNS. Here, the effects of ionizing radiation on the oligodendrocyte lineage as well as the common clinical manifestations resulting from radiation-induced damage to oligodendrocytes will be discussed. Potential prophylactic and therapeutic strategies against radiation-induced oligodendrocyte damage will also be considered.

CONCLUSION

Oligodendrocytes and oligodendrocyte progenitor cells (OPCs) are radiosensitive cells of the CNS. Here, general responses of these cells to radiation exposure have been outlined. However, several findings have not been consistent across various studies. For instance, cognitive decline in irradiated animals was observed to be accompanied by obvious demyelination or white matter changes in several studies but not in others. Hence, further studies have to be conducted to elucidate the level of contribution of the oligodendrocyte lineage to the development of late-delayed effects of radiation exposure, as well as to classify the dose and brain region-specific responses of the oligodendrocyte lineage to radiation. Several potential therapeutic approaches against late-delayed changes have been discussed, such as the transplantation of OPCs into irradiated regions and implementation of exercise. Many of these approaches show promising results. Further elucidation of the mechanisms involved in radiation-induced death of oligodendrocytes and OPCs would certainly aid in the development of novel protective and therapeutic strategies against the late-delayed effects of radiation.

Stem-Cell Therapy as a Potential Strategy for Radiation-Induced Brain Injury

Radiation therapy is a standard and effective non-surgical treatment for primary brain tumors and metastases. However, this strategy inevitably results in damage of normal brain tissue, causing severe complications, especially the late-delayed cognitive impairment. Due to the multifactorial and complex pathological effects of radiation, there is a lack of effective preventative and restorative treatments for the irradiated brain. Stem-cell therapy has held considerable promise for decades in the treatment of central nervous system (CNS) disorders because of its unique capacity for tissue repair and functional integrity. Currently, there is growing interest in using stem cells as a novel option to attenuate the adverse effects of irradiation. In the present review, we discuss recent studies evaluating stem-cell therapies for the irradiated brain and their therapeutic effects on ameliorating radiation-related brain injury as well as their potential challenges in clinical applications. We discuss these works in context of the pathogenesis of radiation-induced injury to CNS tissue in an attempt to elucidate the potential mechanisms of engrafted stem cells to reverse radiation-induced degenerative processes.

Effects of Radiation Therapy on Neural Stem Cells

Brain and nervous system cancers in children represent the second most common neoplasia after leukemia. Radiotherapy plays a significant role in cancer treatment; however, the use of such therapy is not without devastating side effects. The impact of radiation-induced damage to the brain is multifactorial, but the damage to neural stem cell populations seems to play a key role. The brain contains pools of regenerative neural stem cells that reside in specialized neurogenic niches and can generate new neurons. In this review, we describe the advances in radiotherapy techniques that protect neural stem cell compartments, and subsequently limit and prevent the occurrence and development of side effects. We also summarize the current knowledge about neural stem cells and the molecular mechanisms underlying changes in neural stem cell niches after brain radiotherapy. Strategies used to minimize radiation-related damages, as well as new challenges in the treatment of brain tumors are also discussed.

Delayed cognitive deficits can be alleviated by calcium antagonist nimodipine by downregulation of apoptosis following whole brain radiotherapy

Radiation therapy is important for the comprehensive treatment of intracranial tumors. However, the molecular mechanisms underlying the pathogenesis of delayed cognitive dysfunction are not well-defined and effective treatments or prevention measures remain insufficient. In the present study, 60 adult male Wistar rats were randomly divided into three groups, which included a control, whole brain radiotherapy (WBRT) (single dose of 30 Gy of WBRT) and nimodipine (single dose of 30 Gy of WBRT followed by nimodipine injection intraperitoneally) groups. The rats were sacrificed 7 days or 3 months following irradiation. At 3 months, the Morris water maze test was used to assess spatial learning and memory function in rats. The results demonstrated that the WBRT group demonstrated a significantly impaired cognitive performance, decreased numbers of hippocampal Cornu Ammonis (CA)1 neurons and upregulated expression of caspase-3 in the dentate gyrus compared with those in the control and nimodipine groups. Reverse transcription-quantitative polymerase chain reaction analysis demonstrated that the WBRT group exhibited increased ratio of B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 compared with that in control and nimodipine groups on day 7 following irradiation. However, the WBRT group exhibited decreased levels of brain-derived neurotrophic factor (BDNF) compared with that in control and nimodipine groups at 3 months following brain irradiation. The levels of growth-associated protein 43 and amyloid precursor protein between the nimodipine group and WBRT group were not statistically significant. The present study demonstrated that neuron apoptosis may lead to delayed cognitive deficits in the hippocampus, in response to radiotherapy. The cognitive impairment may be alleviated in response to a calcium antagonist nimodipine. The molecular mechanisms involved in nimodipine-mediated protection against cognitive decline may involve the regulation of Bax/Bcl-2 and BDNF in the hippocampus.

Age-Dependent Responses of Brain Myelin Integrity and Behavioral Performance to Radiation in Mice

Radiation therapy is widely used to treat primary and metastatic brain tumors, but it may also lead to delayed neurological complications. Oligodendrocytes in the central nervous system produce myelin, and myelin integrity becomes highly vulnerable after brain irradiation. In this study, mice at different developmental stages were used to test the age-dependent sensitivity of myelin formation and maintenance, as well as behavioral performance after whole-brain irradiation (WBI). Mice at postnatal days 21 and 28 and at 2 months received a single dose of 25 Gy WBI. Behavioral tests for general locomotor activity and motor coordination revealed an age-dependent response after WBI. Quantitative observation revealed a sharp decrease in the number of oligodendrocytes beginning at day 1 after WBI, which recovered during different observation intervals in white matter and gray matter in mice of different ages. Myelin basic protein (MBP) staining revealed disparate quantities in an age- and brain-region-dependent pattern between groups after WBI, which was confirmed using Black-Gold staining. In summary, the response to radiation in mice of different ages provided insight into the potential of oligogenesis in microenvironments at respective stages of myelin regeneration, which may reduce central nervous system impairment and optimize the prognosis after radiation treatment.

Postnatal irradiation-induced hippocampal neuropathology, cognitive impairment and aging

Irradiation of the brain in early human life may set abnormal developmental events into motion that last a lifetime, leading to a poor quality of life for affected individuals. While the effect of irradiation at different early developmental stages on the late human life has not been investigated systematically, animal experimental studies suggest that acute postnatal irradiation with ⩾0.1Gy may significantly reduce neurogenesis in the dentate gyrus and endotheliogenesis in cerebral vessels and induce cognitive impairment and aging. Fractionated irradiation also reduces neurogenesis. Furthermore, irradiation induces hippocampal neuronal loss in CA1 and CA3 areas, neuroinflammation and reduces gliogenesis. The hippocampal neurovascular niche and the total number of microvessels are also changed after radiation exposures. Each or combination of these pathological changes may cause cognitive impairment and aging. Interestingly, acute irradiation of aged brain with a certain amount of radiation has also been reported to induce brain hormesis or neurogenesis. At molecular levels, inflammatory cytokines, chemokines, neural growth factors, neurotransmitters, their receptors and signal transduction systems, reactive oxygen species are involved in radiation-induced adverse effect on brain development and functions. Further study at different omics levels after low dose/dose rate irradiation may not only unravel the mechanisms of radiation-induced adverse brain effect or hormesis, but also provide clues for detection or diagnosis of radiation exposure and for therapeutic approaches to effectively prevent radiation-induced cognitive impairment and aging. Investigation focusing on radiation-induced changes of critical brain development events may reveal many previously unknown adverse effects.

Human embryonic stem cell-derived oligodendrocyte progenitors remyelinate the brain and rescue behavioral deficits following radiation

Radiation therapy to the brain is a powerful tool in the management of many cancers, but it is associated with significant and irreversible long-term side effects, including cognitive decline and impairment of motor coordination. Depletion of oligodendrocyte progenitors and demyelination are major pathological features that are particularly pronounced in younger individuals and severely limit therapeutic options. Here we tested whether human ESC-derived oligodendrocytes can functionally remyelinate the irradiated brain using a rat model. We demonstrate the efficient derivation and prospective isolation of human oligodendrocyte progenitors, which, upon transplantation, migrate throughout the major white matter tracts resulting in both structural and functional repair. Behavioral testing showed complete recovery of cognitive function while additional recovery from motor deficits required concomitant transplantation into the cerebellum. The ability to repair radiation-induced damage to the brain could dramatically improve the outlook for cancer survivors and enable more effective use of radiation therapies, especially in children.

Lhermitte's Sign following VMAT-Based Head and Neck Radiation-Insights into Mechanism

Purpose/Objectives

We observed a number of patients who developed Lhermitte’s sign (LS) following radiation to the head and neck (H/N), since instituting volumetric modulated arc therapy (VMAT). We aimed to investigate the incidence of LS following VMAT-based RT without chemotherapy, and determine the dosimetric parameters that predict its development. We explored whether the role of inhomogeneous dose distribution across the spinal cord, causing a “bath-and-shower” effect, explains this finding.

Methods and Materials

From 1/20/2010–12/9/2013, we identified 33 consecutive patients receiving adjuvant RT using VMAT to the H/N without chemotherapy at our institution. Patients’ treatment plans were analyzed for dosimetric parameters, including dose gradients along the anterior, posterior, right, and left quadrants at each cervical spine level. Institutional Review Board approval was obtained.

Results

5 out of 33 (15.2%) patients developed LS in our patient group, all of whom had RT to the ipsilateral neck only. LS patients had a steeper dose gradient between left and right quadrants across all cervical spine levels (repeated-measures ANOVA, p = 0.030). Within the unilateral treatment group, LS patients received a higher mean dose across all seven cervical spinal levels (repeated-measures ANOVA, p = 0.046). Dose gradients in the anterior-posterior direction and mean doses to the cord were not significant between LS and non-LS patients.

Conclusions

Dose gradients along the axial plane of the spinal cord may contribute to LS development; however, a threshold dose within the high dose region of the cord may still be required. This is the first clinical study to suggest that inhomogeneous dose distributions in the cord may be relevant in humans. Further investigation is warranted to determine treatment-planning parameters associated with development of LS.

Radiation-Induced Alteration of the Brain Proteome: Understanding the Role of the Sirtuin 2 Deacetylase in a Murine Model

Whole brain radiotherapy (WBRT) produces unwanted sequelae, albeit via unknown mechanisms. A deacetylase expressed in the central nervous system, Sirtuin 2 (SIRT2), has been linked to neurodegeneration. Therefore, we sought to challenge the notion that a single disease pathway is responsible for radiation-induced brain injury in Sirt2 wild-type (WT) and knockout (KO) mice at the proteomic level. We utilized isobaric tag for relative and absolute quantitation to analyze brain homogenates from Sirt2 WT and KO mice with and without WBRT. Selected proteins were independently verified, followed by ingenuity pathway analysis. Canonical pathways for Huntington's, Parkinson's, and Alzheimer's were acutely affected by radiation within 72 h of treatment. Although loss of Sirt2 preferentially affected both Huntington's and Parkinson's pathways, WBRT most significantly affected Huntington's-related proteins in the absence of Sirt2. Identical protein expression patterns were identified in Mog following WBRT in both Sirt2 WT and KO mice, revealing a proteomic radiation signature; however, long-term radiation effects were found to be associated with altered levels of a small number of key neurodegeneration-related proteins, identified as Mapt, Mog, Snap25, and Dnm1. Together, these data demonstrate the principle that the presence of Sirt2 can have significant effects on the brain proteome and its response to ionizing radiation.

Long-term impact of radiation on the stem cell and oligodendrocyte precursors in the brain

Background

The cellular basis of long term radiation damage in the brain is not fully understood.

Methods and Findings

We administered a dose of 25Gy to adult rat brains while shielding the olfactory bulbs. Quantitative analyses were serially performed on different brain regions over 15 months. Our data reveal an immediate and permanent suppression of SVZ proliferation and neurogenesis. The olfactory bulb demonstrates a transient but remarkable SVZ-independent ability for compensation and maintenance of the calretinin interneuron population. The oligodendrocyte compartment exhibits a complex pattern of limited proliferation of NG2 progenitors but steady loss of the oligodendroglial antigen O4. As of nine months post radiation, diffuse demyelination starts in all irradiated brains. Counts of capillary segments and length demonstrate significant loss one day post radiation but swift and persistent recovery of the vasculature up to 15 months post XRT. MRI imaging confirms loss of volume of the corpus callosum and early signs of demyelination at 12 months. Ultrastructural analysis demonstrates progressive degradation of myelin sheaths with axonal preservation. Areas of focal necrosis appear beyond 15 months and are preceded by widespread demyelination. Human white matter specimens obtained post-radiation confirm early loss of oligodendrocyte progenitors and delayed onset of myelin sheath fragmentation with preserved capillaries.

Conclusions

This study demonstrates that long term radiation injury is associated with irreversible damage to the neural stem cell compartment in the rodent SVZ and loss of oligodendrocyte precursor cells in both rodent and human brain. Delayed onset demyelination precedes focal necrosis and is likely due to the loss of oligodendrocyte precursors and the inability of the stem cell compartment to compensate for this loss.

Age-dependent sensitivity of the developing brain to irradiation is correlated with the number and vulnerability of progenitor cells

In a newly established model of unilateral, irradiation (IR)-induced injury we compared the outcome after IR to the immature and juvenile brain, using rats at postnatal days 9 or 23, respectively. We demonstrate that (i) the immature brains contained more progenitors in the subventricular zone (SVZ) and subgranular zone (SGZ) compared with the juvenile brains; (ii) cellular injury, as judged by activation of caspase 3 and p53, as well as nitrotyrosine formation, was more pronounced in the SVZ and SGZ in the immature brains 6 h after IR; (iii) the number of progenitor and immature cells in the SVZ and SGZ decreased 6 h and 7 days post-IR, corresponding to acute and subacute effects in humans, respectively, these effects were more pronounced in immature brains; (iv) myelination was impaired after IR at both ages, and much more pronounced after IR to immature brains; (v) the IR-induced changes remained significant for at least 10 weeks, corresponding to late effects in humans, and were most pronounced after IR to immature brains. It appears that IR induces both an acute loss of progenitors through apoptosis and a perturbed microenvironment incompatible with normal proliferation and differentiation, and that this is more pronounced in the immature brain.

Immunohistochemical analysis of myelination following hemicranial irradiation in neonatal rats

The mechanism of radiation-induced diffuse brain injury was investigated in a neonatal rat hemicranial irradiation model using immunohistochemistry. Neonatal Fischer 344 rats received hemicranial irradiation with a single dose of 15 Gy, and appropriate combinations of myelin markers were used to assess the myelin damage at various stages of myelin development. Antibodies against myelin basic protein, 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin oligodendrocyte glycoprotein were used, and the density of the antibody-positive fibers was classified into five categories. Statistical analysis showed significant differences between irradiated and unirradiated hemispheres. The differences decreased and myelination approached normality by postnatal day 70. These results show that myelination in the neonatal rat can recover from the developmental delay caused by a single 15 Gy dose of hemicranial irradiation.