Inhibition of autophagy prevents irradiation-induced neural stem and progenitor cell death in the juvenile mouse brain

Microglia in radiation-induced brain injury: Cellular and molecular mechanisms and therapeutic potential

BACKGROUND

Radiation-induced brain injury is a neurological condition resulting from radiotherapy for malignant tumors, with its underlying pathogenesis still not fully understood. Current hypotheses suggest that immune cells, particularly the excessive activation of microglia in the central nervous system and the migration of peripheral immune cells into the brain, play a critical role in initiating and progressing the injury. This review aimed to summarize the latest advances in the cellular and molecular mechanisms and the therapeutic potential of microglia in radiation-induced brain injury.

METHODS

This article critically examines recent developments in understanding the role of microglia activation in radiation-induced brain injury. It elucidates associated mechanisms and explores novel research pathways and therapeutic options for managing this condition.

RESULTS

Post-irradiation, activated microglia release numerous inflammatory factors, exacerbating neuroinflammation and facilitating the onset and progression of radiation-induced damage. Therefore, controlling microglial activation and suppressing the secretion of related inflammatory factors is crucial for preventing radiation-induced brain injury. While microglial activation is a primary factor in neuroinflammation, the precise mechanisms by which radiation prompts this activation remain elusive. Multiple signaling pathways likely contribute to microglial activation and the progression of radiation-induced brain injury.

CONCLUSIONS

The intricate microenvironment and molecular mechanisms associated with radiation-induced brain injury underscore the crucial roles of immune cells in its onset and progression. By investigating the interplay among microglia, neurons, astrocytes, and peripheral immune cells, potential strategies emerge to mitigate microglial activation, reduce the release of inflammatory agents, and impede the entry of peripheral immune cells into the brain.

Evolution of radiation-induced temporal lobe injury after intensity-modulated radiation therapy in nasopharyngeal carcinoma: a large cohort retrospective study

BACKGROUND

Previous studies have demonstrated conflicting findings regarding the initial MRI patterns of radiotherapy-induced temporal lobe injury (RTLI) and the evolution of different RTLI patterns. The aim of this study was to evaluate the initial MRI pattern and evolution of RTLI in patients with nasopharyngeal carcinoma (NPC) by means of a large cohort study.

METHODS

Data of patients with RTLI were retrospectively collected from two hospitals between January 2011 and December 2021. The injured lobes were categorized into three patterns based on initial MRI patterns: isolated white matter lesions (WMLs), isolated contrast-enhanced lesions (CELs), and combined WMLs and CELs. The latency period, MRI appearances, and temporal changes in WMLs and CELs were evaluated.

RESULTS

A total of 913 RTLI patients with 1092 injured lobes were included in this study. The numbers of isolated WMLs, isolated CELs, and combined WMLs and CELs identified at the first MRI detection were 7 (0.6%), 172 (15.8%), and 913 (83.6%), respectively. The evolution of bilateral RTLI was different in the same patient, and that of unilateral RTLI combined with WMLs and CELs also may occur asynchronously. The time intervals from the initial MRI detection of isolated WMLs, isolated CELs, combined WMLs and CELs to the last negative MRI scan were 8.6, 8.9 and 11.0 months, respectively. A significant difference was observed in the time intervals between the three patterns (H = 14.287, P = 0.001). And the time interval was identified as an independent factor influencing the initial MRI pattern of RTLI after Poisson regression (P = 0.002).

CONCLUSION

Both WMLs and CELs could be the initial and only MRI abnormalities in patients with RTLI. This study is of great significance in accurately diagnosing RTLI early and providing timely treatment options. Additionally, it provides clinical evidence for guidelines on NPC, emphasizing the importance of regular follow-up of NPC patients.

The Effects of Whole Body Gamma Irradiation on Mice, Age-Related Behavioral, and Pathophysiological Changes

We designed a study with the objective to determine the long-term radiation effects of gamma rays, originating from a single shot of Co60 at a dose of 2 Gy on the 7-month-old male mice of the ICR line in 30 days after the irradiation. The aim of this study was to characterize the behavior of animals using the Open Field test, immuno-hematological status, and morpho-functional changes in the central nervous system of mice. Irradiated animals displayed significantly different behavior in the OF in comparison with the control group. The radiation damage was confirmed by assessing the ratio of leukocytes in the peripheral blood of mice at a later date after exposure to Co60. After irradiation, a decrease in the glioneuronal complex was observed in the irritated group as well as histological changes of brain cells. To sum up, not only was the hematological status of mice altered upon the total gamma irradiation, but also their behavior, which was most probably due to significant alterations in the CNS. Study of influence of ionizing radiation on female mice, comparison between different age groups. Open Field test on the 30 days after 2 Gy of γ-rays and histological analysis indicated changes in behavioral patterns, leucocytes, and brain tissue.

Cerebellar irradiation does not cause hyperactivity, fear, and anxiety-related disorders in the juvenile rat brain

Background

The cerebellum is involved in hyperactivity, fear, and anxiety disorders that could be induced by whole-brain irradiation (WBI). However, whether cerebellar irradiation alone (CIA) could induce these disorders is unknown. We investigated the effect of CIA in an animal model.

Methods

Eleven-day-old rat pups underwent a single 3-Gy dose of either WBI (n = 28) or CIA (n = 20), while 34 rat pups were sham-irradiated (controls). Cell death was evaluated in the subgranular zone of the hippocampus by counting pyknotic cells after haematoxylin/eosin staining at 6 h after irradiation for 10, 8, and 9 pups, respectively. Behavioural changes were evaluated via open-field test at 6 weeks for 18, 12, and 25 pups, respectively. Unpaired two-tailed t-test and one-way and two-way repeated ANOVA were used.

Results

Massive cell death in cerebellar external granular layer was detected at 6 h after CIA (1,419 ± 211 mm, mean ± S.E.M. versus controls (68 ± 12 mm) (p < 0.001)), while no significant difference between CIA (1,419 ± 211 mm) versus WBI (1,433 ± 107 mm) (p = 0.955) was found. At open-field behavioural test, running distance, activity, wall distance, middle zone visit times, and duration were higher for WBI versus controls (p < 0.010), but no difference between CIA and controls was found (p > 0.05).

Conclusions

Although the cerebellum is involved in hyperactivity, fear, and anxiety disorders, CIA did not induce these disorders, indicating that WBI-induced cerebellar injury does not directly cause these behavioural abnormalities after WBI. Thus, targeting the cerebellum alone may not be enough to rescue or reduce these behavioural abnormalities after WBI.

Pathogenic mechanisms and therapeutic promise of phytochemicals and nanocarriers based drug delivery against radiotherapy-induced neurotoxic manifestations

Radiotherapy is one of the extensively used therapeutic modalities in glioblastoma and other types of cancers. Radiotherapy is either used as a first-line approach or combined with pharmacotherapy or surgery to manage and treat cancer. Although the use of radiotherapy significantly increased the survival time of patients, but its use has been reported with marked neuroinflammation and cognitive dysfunction that eventually reduced the quality of life of patients. Based on the preclinical and clinical investigations, the profound role of increased oxidative stress, nuclear translocation of NF-kB, production of proinflammatory cytokines such as TNF-α, IL-6, IL-β, increased level of MMPs, increased apoptosis, reduced angiogenesis, neurogenesis, and histological aberrations in CA1, CA2, CA3 and DG region of the hippocampus have been reported. Various pharmacotherapeutic drugs are being used as an adjuvant to counteract this neurotoxic manifestation. Still, most of these drugs suffer from systemic adverse effect, causes interference to ongoing chemotherapy, and exhibit pharmacokinetic limitations in crossing the blood-brain barrier. Therefore, various phytoconstituents, their nano carrier-based drug delivery systems and miRNAs have been explored to overcome the aforementioned limitations. The present review is focused on the mechanism and evidence of radiotherapy-induced neuroinflammation and cognitive dysfunction, pathological and molecular changes in the brain homeostasis, available adjuvants, their limitations. Additionally, the potential role and mechanism of neuroprotection of various nanocarrier based natural products and miRNAs have been discussed.

The Role of Autophagy in Childhood Central Nervous System Tumors

OPINION STATEMENT

Autophagy is a physiological process that occurs in normal tissues. Under external environmental pressure or internal environmental changes, cells can digest part of their contents through autophagy in order to reduce metabolic pressure or remove damaged organelles. In cancer, autophagy plays a paradoxical role, acting as a tumor suppressor-by removing damaged organelles and inhibiting inflammation or by promoting genome stability and the tumor-adaptive responses-as a pro-survival mechanism to protect cells from stress. In this article, we review the autophagy-dependent mechanisms driving childhood central nervous system tumor cell death, malignancy invasion, chemosensitivity, and radiosensitivity. Autophagy inhibitors and inducers have been developed, and encouraging results have been achieved in autophagy modulation, suggesting that these might be potential therapeutic agents for the treatment of pediatric central nervous system (CNS) tumors.

Autophagy Inhibition Reduces Irradiation-Induced Subcortical White Matter Injury Not by Reducing Inflammation, but by Increasing Mitochondrial Fusion and Inhibiting Mitochondrial Fission

Radiotherapy is an effective tool in the treatment of malignant brain tumors, but irradiation-induced late-onset toxicity remains a major problem. The purpose of this study was to investigate if genetic inhibition of autophagy has an impact on subcortical white matter development in the juvenile mouse brain after irradiation. Ten-day-old selective neural Atg7 knockout (KO) mice and wild-type (WT) littermates were subjected to a single 6-Gy dose of whole-brain irradiation and evaluated at 5 days after irradiation. Neural Atg7 deficiency partially prevented myelin disruption compared to the WT mice after irradiation, as indicated by myelin basic protein staining. Irradiation induced oligodendrocyte progenitor cell loss in the subcortical white matter, and Atg7 deficiency partly prevented this. There was no significant change between the KO and WT mice in the number of microglia and astrocytes in the subcortical white matter after irradiation. Transcriptome analysis showed that the GO mitochondrial gene expression pathway was significantly enriched in the differentially expressed genes between the KO and WT group after irradiation. Compared with WT mice, expression of the mitochondrial fusion protein OPA1 and phosphorylation of the mitochondrial fission protein DRP1 (P-DRP1) were dramatically decreased in KO mice under physiological conditions. The protein levels of OPA1and P-DRP1 showed no differences in WT mice between the non-irradiated group and the irradiated group but had remarkably increased levels in the KO mice after irradiation. These results indicate that inhibition of autophagy reduces irradiation-induced subcortical white matter injury not by reducing inflammation, but by increasing mitochondrial fusion and inhibiting mitochondrial fission.

Partial-Brain Radiation-Induced Microvascular Cognitive Impairment in Juvenile Murine Unilateral Hippocampal Synaptic Plasticity

PURPOSE

Radiation-induced cognitive deficits have a severe negative impact on pediatric brain tumor patients. The severity of cognitive symptoms is related to the age of the child when radiation was applied, with the most severe effects seen in the youngest. Previous studies using whole-brain irradiation in mice confirmed these findings. To understand ipsilateral and contralateral changes in the hippocampus after partial-brain radiation therapy (PBRT) of the left hemisphere, we assessed the neuroplasticity and changes in the microvasculature of the irradiated and nonirradiated hippocampus in juvenile mice.

METHODS AND MATERIALS

The left hemispheres of 5-week-old mice were irradiated with 2, 8, and 20 Gy and a fractionated dose of 8 Gy in 2 fractions using a computed tomography image guided small animal radiation research platform. Long-term potentiation (LTP) has been monitored ex vivo in the hippocampal cornu ammonis 1 (CA1) region and was assessed 3 days and 5 and 10 weeks after PBRT in both hemispheres and compared to a sham group. Irradiation effects on the hippocampus microvasculature were quantified by efficient tissue clearing and multiorgan volumetric imaging.

RESULTS

LTP in irradiated hippocampal slices of juvenile mice declines 3 days after radiation, lasts up to 10 weeks in the irradiated part of the hippocampus, and correlates with a significantly reduced microvasculature length. Specifically, LTP inhibition is sustained in the irradiated (20 Gy, 8 Gy in 2 fractions, 8 Gy, 2 Gy) hippocampus, whereas the contralateral hippocampus remains unaffected after PBRT. LTP inhibition in the irradiated hemisphere after PBRT might be associated with an impaired microvascular network.

CONCLUSION

PBRT induces a long-lasting impairment in neuroplasticity and the microvessel network of the irradiated hippocampus, whereas the contralateral hippocampus remains unaffected. These findings provide insight into the design of PBRT strategies to better protect the young developing brain from cognitive decline.

Activation of Autophagy Ameliorates Age-Related Neurogenesis Decline and Neurodysfunction in Adult Mice

Adult neurogenesis is the ongoing generation of functional new neurons from neural progenitor cells (NPCs) in the mammalian brain. However, this process declines with aging, which is implicated in the recession of brain function and neurodegeneration. Understanding the mechanism of adult neurogenesis and stimulating neurogenesis will benefit the mitigation of neurodegenerative diseases. Autophagy, a highly conserved process of cellular degradation, is essential for maintaining cellular homeostasis and normal function. Whether and how autophagy affects adult neurogenesis remains poorly understood. In present study, we revealed a close connection between impaired autophagy and adult neurogenetic decline. Expression of autophagy-related genes and autophagic activity were significantly declined in the middle-adult subventricular/subgranular zone (SVZ/SGZ) homogenates and cultured NPCs, and inhibiting autophagy by siRNA interference resulted in impaired proliferation and differentiation of NPCs. Conversely, stimulating autophagy by rapamycin not only revitalized the viability of middle-adult NPCs, but also facilitated the neurogenesis in middle-adult SVZ/SGZ. More importantly, autophagic activation by rapamycin also ameliorated the olfactory sensitivity and cognitional capacities in middle-adult mice. Taken together, our results reveal that compromised autophagy is involved in the decline of adult neurogenesis, which could be reversed by autophagy activation. It also shed light on the regulation of adult neurogenesis and paves the way for developing a therapeutic strategy for aging and neurodegenerative diseases.

Research progress on mechanism and imaging of temporal lobe injury induced by radiotherapy for head and neck cancer

Radiotherapy (RT) is an effective treatment for head and neck cancer (HNC). Radiation-induced temporal lobe injury (TLI) is a serious complication of RT. Late symptoms of radiation-induced TLI are irreversible and manifest as memory loss, cognitive impairment, and even temporal lobe necrosis (TLN). It is currently believed that the mechanism of radiation-induced TLI involves microvascular injury, neuron and neural stem cell injury, glial cell damage, inflammation, and the production of free radicals. Significant RT-related structural changes and dose-dependent changes in gray matter (GM) and white matter (WM) volume and morphology were observed through computed tomography (CT) and magnetic resonance imaging (MRI) which were common imaging assessment tools. Diffusion tensor imaging (DTI), dispersion kurtosis imaging (DKI), susceptibility-weighted imaging (SWI), resting-state functional magnetic resonance (rs-fMRI), magnetic resonance spectroscopy (MRS), and positron emission tomography (PET) can be used for early diagnosis and prognosis evaluation according to functional, molecular, and cellular processes of TLI. Early diagnosis of TLI is helpful to reduce the incidence of TLN and its related complications. This review summarizes the clinical features, mechanisms, and imaging of radiation-induced TLI in HNC patients. KEY POINTS: • Radiation-induced temporal lobe injury (TLI) is a clinical complication and its symptoms mainly include memory impairment, headache, and cognitive impairment. • The mechanisms of TLI include microvascular injury, cell injury, and inflammatory and free radical injury. Significant RT-related structural changes and dose-dependent changes in TL volume and morphology were observed through CT and MRI. • SWI, MRS, DTI, and DKI and other imaging examinations can detect anatomical and functional, molecular, and cellular changes of TLI.

Astroglial Connexins in Neurological and Neuropsychological Disorders and Radiation Exposure

Radiotherapy is a common treatment for brain and spinal cord tumors and also a risk factor for neuropathological changes in the brain leading to different neurological and neuropsychological disorders. Astroglial connexins are involved in brain inflammation, development of Alzheimer's Disease (AD), depressive, epilepsy, and amyotrophic lateral sclerosis, and are affected by radiation exposure. Therefore, it is speculated that radiation-induced changes of astroglial connexins may be related to the brain neuropathology and development of neurological and neuropsychological disorders. In this paper, we review the functional expression and regulation of astroglial connexins expressed between astrocytes and different types of brain cells (including oligodendrocytes, microglia, neurons and endothelial cells). The roles of these connexins in the development of AD, depressive, epilepsy, amyotrophic lateral sclerosis and brain inflammation have also been summarized. The radiation-induced astroglial connexins changes and development of different neurological and neuropsychological disorders are then discussed. Based on currently available data, we propose that radiation-induced astroglial connexins changes may be involved in the genesis of different neurological and neuropsychological disorders which depends on the age, brain regions, and radiation doses/dose rates. The abnormal astroglial connexins may be novel therapeutic targets for the prevention of radiation-induced cognitive impairment, neurological and neuropsychological disorders.

Hypothyroidism Induces Interleukin-1-Dependent Autophagy Mechanism as a Key Mediator of Hippocampal Neuronal Apoptosis and Cognitive Decline in Postnatal Rats

Thyroid hormone (TH) is essential for brain development, and hypothyroidism induces cognitive deficits in children and young adults. However, the participating mechanisms remain less explored. Here, we examined the molecular mechanism, hypothesizing the involvement of a deregulated autophagy and apoptosis pathway in hippocampal neurons that regulate cognitive functions. Therefore, we used a rat model of developmental hypothyroidism, generated through methimazole treatment from gestation until young adulthood. We detected that methimazole stimulated the autophagy mechanism, characterized by increased LC3B-II, Beclin-1, ATG7, and ATG5-12 conjugate and decreased p-mTOR/mTOR and p-ULK1/ULK1 autophagy regulators in the hippocampus of developing and young adult rats. This methimazole-induced hippocampal autophagy could be inhibited by thyroxine treatment. Subsequently, probing the upstream mediators of autophagy revealed an increased hippocampal neuroinflammation, marked by upregulated interleukin (IL)-1alpha and beta and activated microglial marker, Iba1, promoting neuronal IL-1 receptor-1 expression. Hence, IL-1R-antagonist (IL-1Ra), which reduced hippocampal neuronal IL-1R1, also inhibited the enhanced autophagy in hypothyroid rats. We then linked these events with hypothyroidism-induced apoptosis and loss of hippocampal neurons, where we observed that like thyroxine, IL-1Ra and autophagy inhibitor, 3-methyladenine, reduced the cleaved caspase-3 and TUNEL-stained apoptotic neurons and enhanced Nissl-stained neuronal count in methimazole-treated rats. We further related these molecular results with cognition through Y-maze and passive avoidance tests, demonstrating an IL-1Ra and 3-methyladenine-mediated improvement in learning-memory performances of the hypothyroid rats. Taken together, our study enlightens the critical role of neuroinflammation-dependent autophagy mechanism in TH-regulated hippocampal functions, disrupted in developmental hypothyroidism.

Cranial irradiation alters neuroinflammation and neural proliferation in the pituitary gland and induces late-onset hormone deficiency

Cranial radiotherapy induces endocrine disorders and reproductive abnormalities, particularly in long-term female cancer survivors, and this might in part be caused by injury to the pituitary gland, but the underlying mechanisms are unknown. The aim of this study was to investigate the influence of cranial irradiation on the pituitary gland and related endocrine function. Female Wistar rat pups on postnatal day 11 were subjected to a single dose of 6 Gy whole-head irradiation, and hormone levels and organ structure in the reproductive system were examined at 20 weeks after irradiation. We found that brain irradiation reduced cell proliferation and induced persistent inflammation in the pituitary gland. The whole transcriptome analysis of the pituitary gland revealed that apoptosis and inflammation-related pathways were up-regulated after irradiation. In addition, irradiation led to significantly decreased levels of the pituitary hormones, growth hormone, adrenocorticotropic hormone, thyroid-stimulating hormone and the reproductive hormones testosterone and progesterone. To conclude, brain radiation induces reduction of pituitary and reproduction-related hormone secretion, this may due to reduced cell proliferation and increased pituitary inflammation after irradiation. Our results thus provide additional insight into the molecular mechanisms underlying complications after head irradiation and contribute to the discovery of preventive and therapeutic strategies related to brain injury following irradiation.

Bone marrow stromal cells-derived exosomes target DAB2IP to induce microglial cell autophagy, a new strategy for neural stem cell transplantation in brain injury

Bone marrow stromal cells (MSCs) are a useful source of stem cells for the treatment of various brain injury diseases due to their abundant supply and fewer ethical problems compared with transplant treatment. However, the clinical application of MSCs is limited due to allograft rejection and immunosuppression in the process of MSCs transplantation. According to previous studies, microglial cell autophagy occurs following co-culture with MSCs. In the present study, exosomes were obtained from MSCs and subsequently characterized using transmission electron microscopy, atomic force microscopy and dynamic light scattering particle size analysis. The type of microRNAs (miRs) found in the exosomes was then analyzed via gene chip. The results demonstrated that microglial cell autophagy could be induced by exosomes. This mechanism was therefore investigated further via reverse transcription-quantitative PCR, western blotting and luciferase assays. These results demonstrated that exosomes from MSCs could induce microglial cell autophagy through the miR-32-mediated regulation of disabled homolog 2-interacting protein, thus providing a theoretical basis for the clinical application of miRs in MSCs.

Autophagy in neurodegeneration: New insights underpinning therapy for neurological diseases

In autophagy long-lived proteins, protein aggregates or damaged organelles are engulfed by vesicles called autophagosomes prior to lysosomal degradation. Autophagy dysfunction is a hallmark of several neurodegenerative diseases in which misfolded proteins or dysfunctional mitochondria accumulate. Excessive autophagy can also exacerbate brain injury under certain conditions. In this review, we provide specific examples to illustrate the critical role played by autophagy in pathological conditions affecting the brain and discuss potential therapeutic implications. We show how a singular type of autophagy-dependent cell death termed autosis has attracted attention as a promising target for improving outcomes in perinatal asphyxia and hypoxic-ischaemic injury to the immature brain. We provide evidence that autophagy inhibition may be protective against radiotherapy-induced damage to the young brain. We describe a specialized form of macroautophagy of therapeutic relevance for motoneuron and neuromuscular diseases, known as chaperone-assisted selective autophagy, in which heat shock protein B8 is used to deliver aberrant proteins to autophagosomes. We summarize studies pinpointing mitophagy mediated by the serine/threonine kinase PINK1 and the ubiquitin-protein ligase Parkin as a mechanism potentially relevant to Parkinson's disease, despite debate over the physiological conditions in which it is activated in organisms. Finally, with the example of the autophagy-inducing agent rilmenidine and its discrepant effects in cell culture and mouse models of motor neuron disorders, we illustrate the importance of considering aspects such a disease stage and aggressiveness, type of insult and load of damaged or toxic cellular components, when choosing the appropriate drug, timepoint and duration of treatment.

The role of Atg7-mediated autophagy in ionizing radiation-induced neural stem cell damage

Impairment of neurogenesis is thought to be one of the important mechanisms underlying radiation-induced cognitive decline. Self-renewal and differentiation of neural stem cells (NSCs) are important components of neurogenesis. It has been well established that autophagy plays an important role in neurodegenerative conditions, however, its role in radiation-induced cognitive decline remains unclear. Our previous studies have found that ionizing radiation (IR) induces autophagy in mouse neurons, and minocycline, an antibiotic that can cross the blood-brain barrier, protects neurons from radiation-induced apoptosis through promoting autophagy, thus may contribute to the improvement of mouse cognitive performance after whole-brain irradiation. In the present study, we investigated whether autophagy is involved in radiation-induced damage in self-renewal and differentiation of NSCs. We found that NSCs were extremely sensitive to IR. Irradiation induced autophagy in NSCs in a dose-dependent manner. Atg7 knockdown significantly decreased autophagy, thus increased the apoptosis levels in irradiated NSCs, suggesting that autophagy protected NSCs from radiation-induced apoptosis. Moreover, compared with the negative control NSCs, the neurosphere size was significantly reduced and the neuronal differentiation was notably inhibited in Atg7-deficient NSCs after irradiation, indicating that autophagy defect could exacerbate radiation-induced reduction in NSC self-renewal and differentiation potential. In conclusion, down-regulating autophagy by selective Atg7 knockdown in NSCs enhanced radiation-induced NSC damage, suggesting an important protective role of autophagy in maintaining neurogenesis. Along with the protective effect of autophagy on irradiated neurons, our results on NSCs not only shed the light on the involvement of autophagy in the development of radiation-induced cognitive decline, but also provided a potential target for preventing cognitive impairment after cranial radiation exposure.

Neurocognitive Decline Following Radiotherapy: Mechanisms and Therapeutic Implications

The brain undergoes ionizing radiation (IR) exposure in many clinical situations, particularly during radiotherapy for malignant brain tumors. Cranial radiation therapy is related with the hazard of long-term neurocognitive decline. The detrimental ionizing radiation effects on the brain closely correlate with age at treatment, and younger age associates with harsher deficiencies. Radiation has been shown to induce damage in several cell populations of the mouse brain. Indeed, brain exposure causes a dysfunction of the neurogenic niche due to alterations in the neuronal and supporting cell progenitor signaling environment, particularly in the hippocampus—a region of the brain critical to memory and cognition. Consequent deficiencies in rates of generation of new neurons, neural differentiation and apoptotic cell death, lead to neuronal deterioration and lasting repercussions on neurocognitive functions. Besides neural stem cells, mature neural cells and glial cells are recognized IR targets. We will review the current knowledge about radiation-induced damage in stem cells of the brain and discuss potential treatment interventions and therapy methods to prevent and mitigate radiation related cognitive decline.

Resatorvid protects against hypoxic-ischemic brain damage in neonatal rats

Secondary brain damage caused by hyperactivation of autophagy and inflammatory responses in neurons plays an important role in hypoxic-ischemic brain damage (HIBD). Although previous studies have implicated Toll-like receptor 4 (TLR4) and nuclear factor kappa-B (NF-κB) in the neuroinflammatory response elicited by brain injury, the role and mechanisms of the TLR4-mediated autophagy signaling pathway in neonatal HIBD are still unclear. We hypothesized that this pathway can regulate brain damage by modulating neuron autophagy and neuroinflammation in neonatal rats with HIBD. Hence, we established a neonatal HIBD rat model using the Rice-Vannucci method, and injected 0.75, 1.5, or 3 mg/kg of the TLR4 inhibitor resatorvid (TAK-242) 30 minutes after hypoxic ischemia. Our results indicate that administering TAK-242 to neonatal rats after HIBD could significantly reduce the infarct volume and the extent of cerebral edema, alleviate neuronal damage and neurobehavioral impairment, and decrease the expression levels of TLR4, phospho-NF-κB p65, Beclin-1, microtubule-associated protein l light chain 3, tumor necrosis factor-α, and interleukin-1β in the hippocampus. Thus, TAK-242 appears to exert a neuroprotective effect after HIBD by inhibiting activation of autophagy and the release of inflammatory cytokines via inhibition of the TLR4/NF-κB signaling pathway. This study was approved by the Laboratory Animal Ethics Committee of Affiliated Hospital of Yangzhou University, China (approval No. 20180114-15) on January 14, 2018.

Autophagy-Related Gene 7 Polymorphisms and Cerebral Palsy in Chinese Infants

Cerebral palsy (CP) is a group of non-progressive motor impairment syndromes that are secondary to brain injury in the early stages of brain development. Numerous etiologies and risk factors of CP have been reported, and genetic contributions have recently been identified. Autophagy has an important role in brain development and pathological process, and autophagy-related gene 7 (ATG7) is essential for autophagosome biogenesis. The purpose of this study was to investigate the genetic association between ATG7 gene single nucleotide polymorphisms (SNPs) and CP in Han Chinese children. Six SNPs (rs346078, rs1470612, rs11706903, rs2606750, rs2594972, and rs4684787) were genotyped in 715 CP patients and 658 healthy controls using the MassArray platform. Plasma ATG7 protein was determined in 73 CP patients and 79 healthy controls. The differences in the allele and genotype frequencies of the rs1470612 and rs2594972 SNPs were determined between the CP patients and controls (p _allele = 0.02 and 0.0004, p _genotype = 0.044 and 0.0012, respectively). Subgroup analysis revealed a more significant association of rs1470612 (p _allele = 0.004, p _genotype = 0.0036) and rs2594972 (p _allele = 0.0004, p _genotype < 0.0001) with male CP, and more significant differences in allele and genotype frequencies were also noticed between CP patients with spastic diplegia and controls for rs1470612 (p _allele = 0.0024, p _genotype = 0.008) and rs2594972 (p _allele < 0.0001, p _genotype = 0.006). The plasma ATG7 level was higher in CP patients compared to the controls (10.58 ± 0.85 vs. 8.18 ± 0.64 pg/mL, p = 0.024). The luciferase reporter gene assay showed that the T allele of rs2594972 SNP could significantly increase transcriptional activity of the ATG7 promoter compared to the C allele (p = 0.009). These findings suggest that an association exists between genetic variants of ATG7 and susceptibility to CP, which provides novel evidence for the role of ATG7 in CP and contributes to our understanding of the molecular mechanisms of this neurodevelopmental disorder.

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.

The Effect of Gamma-Ray-Induced Central Nervous System Injury on Peripheral Immune Response: An In Vitro and In Vivo Study

Radiotherapy to treat brain tumors can potentially harm the central nervous system (CNS). The radiation stimulates a series of immune responses in both the CNS as well as peripheral immune system. To date, studies have mostly focused on the changes occurring in the immune response within the CNS. In this study, we investigated the effect of γ-ray-induced CNS injury on the peripheral immune response using a cell co-culture model and a whole-brain irradiation (WBI) rat model. Nerve cells (SH-SY5Y and U87 MG cells) were γ-ray irradiated, then culture media of the irradiated cells (conditioned media) was used to culture immune cells (THP-1 cells or Jurkat cells). Analyses were performed based on the response of immune cells in conditioned media. Sprague-Dawley rats received WBI at different doses, and were fed for one week to one month postirradiation. Spleen and peripheral blood were then isolated and analyzed. We observed that the number of monocytes in peripheral blood, and the level of NK cells and NKT cells in spleen increased after CNS injury. However, the level of T cells in spleen did not change and the level of B cells in the spleen decreased after γ-ray-induced CNS injury. These findings indicate that CNS injury caused by ionizing radiation induces a series of changes in the peripheral immune system.

Selective Neural Deletion of the Atg7 Gene Reduces Irradiation-Induced Cerebellar White Matter Injury in the Juvenile Mouse Brain by Ameliorating Oligodendrocyte Progenitor Cell Loss

Radiotherapy is an effective tool for treating brain tumors, but irradiation-induced toxicity to the normal brain tissue remains a major problem. Here, we investigated if selective neural autophagy related gene 7 (Atg7) deletion has a persistent effect on irradiation-induced juvenile mouse brain injury. Ten-day-old Atg7 knockout under a nestin promoter (KO) mice and wild-type (WT) littermates were subjected to a single dose of 6 Gy whole-brain irradiation. Cerebellar volume, cell proliferation, microglia activation, inflammation, and myelination were evaluated in the cerebellum at 5 days after irradiation. We found that neural Atg7 deficiency partially prevented myelin disruption compared to the WT mice after irradiation, as indicated by myelin basic protein staining. Irradiation induced oligodendrocyte progenitor cell (OPC) loss in the white matter of the cerebellum, and Atg7 deficiency partly prevented this. The mRNA expression of oligodendrocyte and myelination-related genes (Olig2, Cldn11, CNP, and MBP) was higher in the cerebellum in Atg7 KO mice compared with WT littermates. The total cerebellar volume was significantly reduced after irradiation in both Atg7 KO and WT mice. Atg7-deficient cerebellums were in a regenerative state before irradiation, as judged by the increased OPC-related and neurogenesis-related transcripts and the increased numbers of microglia; however, except for the OPC parameters these were the same in both genotypes after irradiation. Finally, there was no significant change in the number of astrocytes in the cerebellum after irradiation. These results suggest that selective neural Atg7 deficiency reduces irradiation-induced cerebellar white matter injury in the juvenile mouse brain, secondary to prevention of OPC loss.

Hydroxysafflor yellow A (HSYA) alleviates apoptosis and autophagy of neural stem cells induced by heat stress via p38 MAPK/MK2/Hsp27-78 signaling pathway

This study aimed to explore mechanisms of the effects of hydroxysafflor yellow A (HSYA) on neural stem cells (NSCs) after heat stress (HS). Rat NSCs cells were cultured at 42 °C to impose heat stress. Cell counting kit-8 and Edu assay were used to analyze NSC proliferation. Annexin V/PI apoptosis kit was used to detect NSC apoptosis. Expression and phosphorylation of autophagy and apoptosis-associated proteins were determined by western blotting. We showed that HSYA significantly promoted proliferation and attenuated apoptosis of NSCs after heat stress. HSYA also increased Bcl-2 expression but decreased the expression of Bax and cleaved caspase-3 in NSCs induced by heat stress. In addition, HSYA decreased p38 and Hsp27-78 phosphorylation and MK-2 expression after heat stress, which was consistent with NSCs treated with SB203850 treatment or p38 knockdown. Furthermore, we demonstrated that heat stress increased LC3-II expression and mTOR phosphorylation, and decreased the expression of p62 in NSCs, while HSYA, SB203850 treatment or p38 knockdown reversed these alterations. In conclusion, HSYA significantly reversed the apoptosis and autophagy of NSCs induced by heat stress (P < 0.05), via downregulating MK2 expression and p38 and Hsp27-78 phosphorylation.

On the Role of Basal Autophagy in Adult Neural Stem Cells and Neurogenesis

Adult neurogenesis persists in the adult mammalian brain due to the existence of neural stem cell (NSC) reservoirs in defined niches, where they give rise to new neurons throughout life. Recent research has begun to address the implication of constitutive (basal) autophagy in the regulation of neurogenesis in the mature brain. This review summarizes the current knowledge on the role of autophagy-related genes in modulating adult NSCs, progenitor cells and their differentiation into neurons. The general function of autophagy in neurogenesis in several areas of the embryonic forebrain is also revisited. During development, basal autophagy regulates Wnt and Notch signaling and is mainly required for adequate neuronal differentiation. The available data in the adult indicate that the autophagy-lysosomal pathway regulates adult NSC maintenance, the activation of quiescent NSCs, the survival of the newly born neurons and the timing of their maturation. Future research is warranted to validate the results of these pioneering studies, refine the molecular mechanisms underlying the regulation of NSCs and newborn neurons by autophagy throughout the life-span of mammals and provide significance to the autophagic process in adult neurogenesis-dependent behavioral tasks, in physiological and pathological conditions. These lines of research may have important consequences for our understanding of stem cell dysfunction and neurogenic decline during healthy aging and neurodegeneration.

Carbamylated Erythropoietin Decreased Proliferation and Neurogenesis in the Subventricular Zone, but Not the Dentate Gyrus, After Irradiation to the Developing Rat Brain

Cranial radiotherapy for pediatric brain tumors causes progressive, debilitating late effects, including cognitive decline. Erythropoietin (EPO) has been shown to be neuroprotective and to promote neuroregeneration. Carbamylated erythropoietin (CEPO) retains the protective properties of EPO but is not erythrogenic. To study the effects of CEPO on the developing brain exposed to radiotherapy, a single irradiation (IR) dose of 6 Gy was administered to the brains of postnatal day 9 (P9) rats, and CEPO (40 μg/kg s.c.) was injected on P8, P9, P11, P13, and P15. To examine proliferation, 5-Bromo-2-deoxyuridine (BrdU) was injected on P15, P16, and P17. CEPO administration did not affect BrdU incorporation in the granule cell layer (GCL) of the hippocampus or in the subventricular zone (SVZ) as quantified 7 days after the last BrdU injection, whereas IR decreased BrdU incorporation in the GCL and SVZ by 63% and 18%, respectively. CEPO did not affect BrdU incorporation in the GCL of irradiated brains, although it was reduced even further (to 31%) in the SVZ. To evaluate the effect of CEPO on neurogenesis, BrdU/doublecortin double-positive cells were quantified. CEPO did not affect neurogenesis in non-irradiated brains, whereas IR decreased neurogenesis by 58% in the dentate gyrus (DG) but did not affect it in the SVZ. In the DG, CEPO did not affect the rate of neurogenesis following IR, whereas in the SVZ, the rate decreased by 30% following IR compared with the rate in vehicle-treated rats. Neither CEPO nor IR changed the number of microglia. In summary, CEPO did not promote neurogenesis in non-irradiated or irradiated rat brains and even aggravated the decreased neurogenesis in the SVZ. This raises concerns regarding the use of EPO-related compounds following radiotherapy.

Modification of Wnt signaling pathway on paraquat-induced inhibition of neural progenitor cell proliferation

Paraquat (PQ) is an agricultural chemical used worldwide. As a potential neurotoxicant, PQ adversely affects neurogenesis and inhibits proliferation of neural progenitor cells (NPCs). However, the molecular mechanistic insights of PQ exposure on NPCs remains to be determined. Herein, we determine the extent to which Wnt/β-catenin signaling involved in the inhibition effect of PQ on mouse NPCs from subventricular zone (SVZ). NPCs were treated with different concentrations of PQ (40, 80, and 120 μM). PQ exposure provoked oxidative stress and apoptosis and PQ inhibited cell viability and proliferation in a concentration-dependent manner. Significantly, PQ exposure altered the expression/protein levels of the Wnt pathway genes in NPCs. In addition, PQ reduced cellular β-catenin, p-GSK-3β, and cyclin-D1 and increased the radio of Bax/Bcl2. Further, Wnt pathway activation by treatment with LiCl and Wnt1 attenuated PQ-induced inhibition of mNPCs proliferation. Antioxidant (NAC) treatment alleviated the inhibition of PQ-induced Wnt signaling pathway. Overall, our results suggest significant inhibitory effects of PQ on NPCs proliferation via the Wnt/β-catenin signaling pathway. Interestingly, our results implied that activation of Wnt/β-catenin signaling pathway attenuated PQ-induced autophagic cell death. Our results therefore bring our understanding of the molecular mechanisms of PQ-induced neurotoxicity.

Phosphatidyl-Inositol-3 Kinase Inhibitors Regulate Peptidoglycan-Induced Myeloid Leukocyte Recruitment, Inflammation, and Neurotoxicity in Mouse Brain

Acute brain injury leads to the recruitment and activation of immune cells including resident microglia and infiltrating peripheral myeloid cells (MC), which contribute to the inflammatory response involved in neuronal damage. We previously reported that TLR2 stimulation by peptidoglycan (PGN) from Staphylococcus aureus, in vitro and in vivo, induced microglial cell activation followed by autophagy induction. In this report, we evaluated if phosphatidyl-inositol-3 kinase (PI3K) pharmacological inhibitors LY294200 and 3-methyladenine (3-MA) can modulate the innate immune response to PGN in the central nervous system. We found that injection of PGN into the mouse brain parenchyma (caudate putamen) triggered an inflammatory reaction, which involved activation of microglial cells, recruitment of infiltrating MC to injection site, production of pro-inflammatory mediators, and neuronal injury. In addition, we observed the accumulation of LC3B⁺ CD45⁺ cells and colocalization of LC3B and lysosomal-associated membrane protein 1 in brain cells. Besides, we found that pharmacological inhibitors of PI3K, including the classical autophagy inhibitor 3-MA, reduced the recruitment of MC, microglial cell activation, and neurotoxicity induced by brain PGN injection. Collectively, our results suggest that PI3K pathways and autophagic response may participate in the PGN-induced microglial activation and MC recruitment to the brain. Thus, inhibition of these pathways could be therapeutically targeted to control acute brain inflammatory conditions.

Cranial Irradiation Induces Hypothalamic Injury and Late-Onset Metabolic Disturbances in Juvenile Female Rats

Cranial radiotherapy is one of the most effective tools for treating children with brain tumors. However, radiotherapy-induced late-onset side effects have a significant impact on patients' quality of life. The purpose of this study was to investigate the effects of irradiation on metabolism and the possible molecular and cellular mechanisms behind such effects. Female 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. We found that irradiation led to a significantly increased body weight from 15 weeks (p < 0.05) along with white adipose tissue accumulation and adipocyte hypertrophy at 20 weeks, and these changes were accompanied by glucose and lipid metabolic disturbances as indicated by reduced glucose tolerance, increased insulin resistance, increased serum triglycerides, and an increased leptin/adiponectin ratio. Furthermore, irradiation induced cell death, microglial activation, inflammation, and persistent astrocyte reactivity in the hypothalamus. Hypothalamic transcriptome analysis showed that 865 genes were downregulated and 290 genes were upregulated in the irradiated group 20 weeks after irradiation, and further pathway analysis showed that the insulin resistance-related PI3K-Akt signaling pathway and the energy expenditure-related adipocytokine signaling pathway were downregulated. Gene Ontology enrichment analysis showed that the expression of fatty acid metabolism-related proteins and effector proteins was significantly different in the irradiation group. This study demonstrates that ionizing radiation to the juvenile female brain induces hypothalamic damage that is likely to be associated with delayed metabolic abnormalities, and this critical vulnerability of the hypothalamus to irradiation should be taken into consideration in the development of future protective strategies for radiotherapy.

CpG-ODN exerts a neuroprotective effect via the TLR9/pAMPK signaling pathway by activation of autophagy in a neonatal HIE rat model

Hypoxic Ischemic Encephalopathy (HIE) is an injury caused to the brain due to prolonged lack of oxygen and blood supply which results in death or long-term disabilities. The main aim of this study was to investigate the role of Cytosine-phospho-guanine oligodeoxynucleotide (CpG-ODN) in autophagy after HIE. Ten-day old (P10) rat pups underwent right common carotid artery ligation followed by 2.5h of hypoxia as previously described by Rice-Vannucci. At 1h post HIE, rats were intranasally administered with recombinant CpG-ODN. Time-course expression levels of endogenous key proteins, TLR9, pAMPK/AMPK, LC3II/I, and LAMP1 involved in CpG-ODN's protective effects were measured using western blot. Short (48h) and long (4w) term neurobehavior studies were performed using righting reflex, negative geotaxis, water maze, foot fault and Rota rod tests. Brain samples were collected after long term for histological analysis. Furthermore, to elucidate the pathway via which CpG-ODN confers protection, TLR9 and AMPK inhibitors were used. Time course results showed that the expression of TLR9, pAMPK/AMPK, LC3II/I, LAMP1 increased after HIE. Neurobehavioral studies showed that HIE induced a significant delay in development and resulted in cognitive and motor function deficits. However, CpG-ODN ameliorated HIE-induced outcomes and improved long term neurological deficits. In addition, CpG-ODN increased expression of pAMPK/AMPK, p-ULK1/ULK1, P-AMBRA1/AMBRA1, LC3II/I and LAMP1 while inhibition of TLR9 and AMPK reversed those effects. In summary, CpG-ODN increased HIE-induced autophagy and improved short and long term neurobehavioral outcomes which may be mediated by the TLR9/pAMPK signaling pathway after HIE.

A Variant of the Autophagy-Related 5 Gene Is Associated with Child Cerebral Palsy

Cerebral palsy (CP) is a major cause of childhood disability in developed and developing countries, but the pathogenic mechanisms of CP development remain largely unknown. Autophagy is a highly conserved cellular self-digestion of damaged organelles and dysfunctional macromolecules. Growing evidence suggests that autophagy-related gene 5 (ATG5)-dependent autophagy is involved in neural development, neuronal differentiation, and neurological degenerative diseases. The aim of this study was to analyze ATG5 protein expression and gene polymorphisms in Chinese patients with CP and to evaluate the importance of ATG5 in the development of CP. Five polymorphisms from different regions of the ATG5 gene (rs510432, rs3804338, rs573775, rs2299863, and rs6568431) were analyzed in 715 CP patients and 658 controls using MassARRAY. Of these, 58 patients and 56 controls were selected for measurement of plasma ATG5 level using ELISA. The relevance of disease-associated SNPs was evaluated using the SHEsis program. We identified a significant association between rs6568431 and CP (OR = 1.388, 95% CI = 1.173~1.643, P_allele = 0.0005, P_genotype = 0.0015). Subgroup analysis showed a highly significant association of rs6568431 with spastic CP (n = 468, OR = 1.511, 95% CI = 1.251~1.824, P_allele = 8.50e^-005, P_genotype = 1.57e^-004) and spastic quadriplegia (OR = 1.927, 95% CI = 1.533~2.421, P_allele = 7.35e^-008, P_genotype = 3.24e^-009). Furthermore, mean plasma ATG5 levels were lower in CP patients than in controls, and individuals carrying the AA genotype of rs6568431 that was positively associated with CP had lower plasma ATG5 levels (P < 0.05). This study demonstrated an association of an ATG5 gene variant and low level of ATG5 protein with CP, and stronger associations with severe clinical manifestations were identified. Our results provide novel evidence for a role of ATG5 in CP and shed light on the molecular mechanisms underlying this neurodevelopmental disorder.