Radiation-Induced Juvenile Brain Injury

IntroductionModern therapeutic strategies cure about 80% of all children with tumors,and radiotherapy is one of the most effective tools for treating children with primary and metastatic brain tumors or leukemia/lymphoma that involves the central nervous system.However,radiotherapy has both acute and long-lasting adverse effects resulting in intellectual impairment and both endocrine and metabolic sequelae.These have significant impacts on the patient's quality of life and are important measurements of brain tumor therapy outcomes,second only to survival.Currently,there are no successful treatments or prevention strategies for radiation-induced late effects.Recent studies showed that radiation to specific brain regions,including the hippocampus,subventricular zone,and cerebellum results in differences in neurocognitive performance.However,another clinical study showed an association between radiation to the hippocampus and temporal lobes and neurocognitive deficits,but not radiation to the subventricular zone.The cerebellum has traditionally been linked to motor function,but recent evidence has shown an important link between the cerebellum and cognitive functions.In particular,radiation-induced cerebellar vermis injury is correlated with neurocognitive dysfunction.Radiation-induced hypothalamic–pituitary axis dysfunction and endocrine and metabolic complications occur in up to 80% of childhood cancer survivors.The mechanisms of radiation-induced late effects are not fully understood,although radiation-induced oxidative stress,stem/progenitor cell death,neuroinflammation,and neurovascular damage are related to radiation-induced late effects and are potential therapeutic targets for treating or preventing radiationinduced long-term impairments.Radiation-induced oxidative stress can trigger a variety of cellular responses,including cell death.Stem/progenitor cells are proliferative and,therefore,highly vulnerable to irradiation,and radiation-induced proliferated cell death is to a large extent dependent on apoptosis-related mechanisms.Recent studies from us and others have shown that prevention of radiation-induced newborn cell death can ameliorate cognitive dysfunction.Radiation-induced cerebral inflammation,including both acute and prolonged response,is characterized by the production of cytokines,microglia activation,and T-cell infiltration.Activated microglia upregulate the expression of cytokines and chemokines,which are important mediators of stem cell dysfunction and persistent brain injury.Vascular damage is another important contributor to brain injury and the long-term side effects after irradiation.Radiation significantly reduces endothelial cell density by increasing endothelial cell death and inhibiting proliferation.Lithium,the prototype drug used for the treatment of bipolar disorder,has been shown to possess neuroprotective properties,protecting against apoptosis-induced neuronal death,glutamate-induced excitotoxicity and brain irradiation in mature brain models.Yet,despite being used for decades to treat bipolar disorder,and despite multiple findings of effects at the molecular,cellular and behavioral levels,the molecular mechanism of lithium's beneficial effects is yet unraveled.We will use a rodent model to study the mechanisms of both acute and chronic brain injury after radiation exposure and to develop novel strategies for preventing and treating radiotherapy-induced late neurocognitive impairments and endocrine and metabolic disorders in pediatric brain tumor survivors.Part?: The effects of cranial irradiation on juvenile rat cerebellumBackgroundPosterior fossa tumors are the most common childhood intracranial tumors,and radiotherapy is one of the most effective treatments.However,irradiation induces longterm adverse effects that can have significant negative impacts on the patient's quality of life.The purpose of this study was to characterize irradiation-induced cellular and molecular changes in the cerebellum.MethodsMale Wistar rats were subjected to a single dose of 6 Gy whole brain irradiation on postnatal day 11,and were assigned randomly into irradiation and non-irradiation groups.Brd U were injected before irradiation.Newborn cell death and stem and progenitor cell proliferation and differentiation were assessed in the cerebellum.ResultsWe found that irradiation-induced cell death occurred mainly in the external germinal layer(EGL)of the juvenile rat cerebellum.The number of proliferating cells in the EGL decreased,and 82.9% of them died within 24 h after irradiation.Furthermore,irradiation induced oxidative stress,microglia accumulation,and inflammation in the cerebellum.Interestingly,blood-brain barrier damage and blood flow reduction was considerably more pronounced in the cerebellum compared to other brain regions.The cerebellar volume decreased by 39% and the migration of proliferating cells to the internal granule layer decreased by 87.5% at 16 weeks after irradiation.ConclusionsIn the light of recent studies demonstrating that the cerebellum is important not only for motor functions,but also for cognition,and since treatment of posterior fossa tumors in children typically results in debilitating cognitive deficits,this differential susceptibility of the cerebellum to irradiation should be taken into consideration for future protective strategies.Part?: The effects of cranial irradiation on juvenile rat cerebellumCranial radiotherapy is one of the most effective tools for treating children with brain tumors.However,radiotherapy-induced late-onset side effects have significant impacts on patients' quality of life.The purpose of this study was to investigate the effects of irradiation on metabolism and the possible mechanism behind such effects.MethodsFemale Wistar rats were subjected to a single dose of 6 Gy whole brain irradiation on postnatal day 11.The animals were sacrificed 6 h or 20 weeks after irradiation.Cell death and proliferation,microglial activation,and inflammation were analyzed and RNA sequencing was performed.Results1.Juvenile irradiation induced higher body weight gain from 15 weeks(P < 0.05),body mass index and abdominal circumference.The fasting blood insulin level and insulin resistance were increased,and the oral glucose tolerance was reduced in the irradiated rats compared with their non-irradiated littermates.2.Compared to controls,irradiation group had increased the weights of subcutaneous white adipose tissue,mesenteric adipose tissue,and gonadal adipose tissue,the adipose cells were significantly larger,adipose tissue derived hormone adiponectin and triglyceride also had increased3.We further checked cellular changes in the hypothalamus and found that irradiation induced cell death and microglia activation as well as inflammation and long-term proliferation of astrocytes,and these changes were likely related to the delayed metabolic disturbances.Hypothalamic transcriptome analysis by RNA sequencing at 20 weeks after irradiation showed that 30 genes were down-regulated in the irradiated group,and most of these genes were related to metabolism.ConclusionsOur findings demonstrate that ionizing radiation to the immature brain induces hypothalamic damage that is likely to be associated with delayed metabolic abnormalities,and this sensitive vulnerability of the hypothalamus to irradiation should be taken into consideration in the development of future protective strategies for use in radiation therapy.Part?: The effects of cranial irradiation on juvenile rat hippocampus and protection of lithiumCranial radiotherapy in children typically causes delayed and progressive cognitive dysfunction and there is no effective preventive strategy for radiation-induced cognitive impairments.MethodsMale Wistar rats were subjected to a single dose of 6 Gy whole brain irradiation on postnatal day 11.Brd U were injected before irradiation.Newborn cell death,stem and progenitor cell proliferation and differentiation in the hippocampus,behavioral functions as well as hypothalamus-pituitary hormones were assessed in lithium and vehicle treated animalsResults1.Irradiation-induced progenitor cell death number was 19.8% lower after lithium treatment in the subgranular zone of the hippocampus,and the total number of surviving Brd U-labelled cells after irradiation was 50.1% higher in the lithium-treated group compared to the vehicle-treated group.2.Lithium treatment normalised the time spent in the open central zone in irradiated rats at P60,and there was a substantial decrease in memory function in irradiated rats at P60,and this deficit was normalized by lithium treatment.3.Growth-related increase in pituitary hormones was reduced or even abolished in irradiated rats,and lithium treatment restored TSH and GH levels,and reduced the injury-induced BDNF increase.4.The survival of tumour cells decreased dramatically when the lithium concentration was more than 12.8 m M,but no significant change for the neural progenitor cells was observed,and in the medulloblastoma cell line which is resistant to irradiation,cell viability decreased significantly in the presence of 10 m M lithium.ConclusionsLithium can be safely administered to prevent both short-and long-term injury to the juvenile brain caused by ionizing radiation.