Fast neutron irradiation deteriorates hippocampus-related memory ability in adult mice

Functional network disorganization and cognitive decline following fractionated whole-brain radiation in mice

Cognitive dysfunction following radiotherapy (RT) is one of the most common complications associated with RT delivered to the brain, but the precise mechanisms behind this dysfunction are not well understood, and to date, there are no preventative measures or effective treatments. To improve patient outcomes, a better understanding of the effects of radiation on the brain's functional systems is required. Functional magnetic resonance imaging (fMRI) has shown promise in this regard, however, compared to neural activity, hemodynamic measures of brain function are slow and indirect. Understanding how RT acutely and chronically affects functional brain organization requires more direct examination of temporally evolving neural dynamics as they relate to cerebral hemodynamics for bridging with human studies. In order to adequately study the underlying mechanisms of RT-induced cognitive dysfunction, the development of clinically mimetic RT protocols in animal models is needed. To address these challenges, we developed a fractionated whole-brain RT protocol (3Gy/day for 10 days) and applied longitudinal wide field optical imaging (WFOI) of neural and hemodynamic brain activity at 1, 2, and 3 months post RT. At each time point, mice were subject to repeated behavioral testing across a variety of sensorimotor and cognitive domains. Disruptions in cortical neuronal and hemodynamic activity observed 1 month post RT were significantly worsened by 3 months. While broad changes were observed in functional brain organization post RT, brain regions most impacted by RT occurred within those overlapping with the mouse default mode network and other association areas similar to prior reports in human subjects. Further, significant cognitive deficits were observed following tests of novel object investigation and responses to auditory and contextual cues after fear conditioning. Our results fill a much-needed gap in understanding the effects of whole-brain RT on systems level brain organization and how RT affects neuronal versus hemodynamic signaling in the cortex. Having established a clinically-relevant injury model, future studies can examine therapeutic interventions designed to reduce neuroinflammation-based injury following RT. Given the overlap of sequelae that occur following RT with and without chemotherapy, these tools can also be easily incorporated to examine chemotherapy-related cognitive impairment.

Elucidations of Molecular Mechanism and Mechanistic Effects of Environmental Toxicants in Neurological Disorders

Due to rising environmental and global public health concerns associated with environmental contamination, human populations are continually being exposed to environmental toxicants, including physical chemical mutagens widespread in our environment causing adverse consequences and inducing a variety of neurological disorders in humans. Physical mutagens comprise ionizing and non-ionizing radiation, such as UV rays, IR rays, X-rays, which produces a broad spectrum of neuronal destruction, including neuroinflammation, genetic instability, enhanced oxidative stress driving mitochondrial damage in the human neuronal antecedent cells, cognitive impairment due to alterations in neuronal function, especially in synaptic plasticity, neurogenesis repression, modifications in mature neuronal networks drives to enhanced neurodegenerative risk. Chemical Mutagens including alkylating agents (EMS, NM, MMS, and NTG), Hydroxylamine, nitrous acid, sodium azide, halouracils are the major toxic mutagen in our environment and have been associated with neurological disorders. These chemical mutagens create dimers of pyrimidine that cause DNA damage that leads to ROS generation producing mutations, chromosomal abnormalities, genotoxicity which leads to increased neurodegenerative risk. The toxicity of four heavy metal including Cd, As, Pb, Hg is mostly responsible for complicated neurological disorders in humans. Cadmium exposure can enhance the permeability of the BBB and penetrate the brain, driving brain intracellular accumulation, cellular dysfunction, and cerebral edema. Arsenic exerts its toxic effect by induction of ROS production in neuronal cells. In this review, we summarize the molecular mechanism and mechanistic effects of mutagens in the environment and their role in multiple neurological disorders.

Response of murine neural stem/progenitor cells to gamma-neutron radiation

PURPOSE

In recent years, a growing number of studies have focused on the mechanisms of action of densely ionizing radiation. This is associated with the development of radiation therapy of tumors using accelerated ions. The use of densely ionizing radiation appears to be the most promising method, optimal for treating patients with severe radioresistant forms, such as widespread head and neck tumors, recurrent and metastatic tumors, and some forms of brain tumors. The goal of our study was to investigate the effects of gamma-neutron radiation on mouse neural stem/progenitor cells (NSCs/NPCs).

METHODS

NSCs/NPCs were isolated from neonatal mouse brains. Cells were irradiated in a collimated beam of neutrons and gamma rays of the IR-8 nuclear reactor. At 5 and 7 days after irradiation, cells and neurospheres were counted to assess survival. The number of DNA double-strand breaks and their repair efficiency were determined by immunocytochemical γH2AX staining followed by counting the number of γH2AX foci using a fluorescent microscope.

RESULTS

We observed a dose-dependent decrease in the survival of NSCs/NPCs after irradiation at doses above 100 mGy and stimulation of the proliferation of these cells at doses of 25 and 50 mGy. In terms of a decrease in cell survival, the effect of gamma-neutron irradiation significantly exceeded the effect of gamma irradiation: the maximum value of the relative biological efficiency for gamma-neutron irradiation comprised 9.7. Gamma-neutron irradiation led to the formation of double-strand DNA breaks detected by the formation of foci of histone γH2AX in the cell nuclei. The γH2AX foci formed after gamma-neutron irradiation of NSCs/NPCs at doses of 100-500 mGy were characterized by a larger size in comparison with foci induced by gamma irradiation and gamma-neutron irradiation at a dose of 50 mGy. The repair of double-strand DNA breaks induced by γ,n-irradiation was slow; the repair rate depended on the radiation dose.

CONCLUSIONS

The data obtained indicate high sensitivity of proliferating NSCs/NPCs to gamma-neutron radiation. High RBE of gamma-neutron radiation requires special measures to protect the neurogenic regions of the brain when using this type of radiation in radiation therapy.

Image-Based Evaluation of Irradiation Effects in Brain Tissues by Measuring Absolute Electrical Conductivity Using MRI

Radiation-induced injury is damage to normal tissues caused by unintentional exposure to ionizing radiation. Image-based evaluation of tissue damage by irradiation has an advantage for the early assessment of therapeutic effects by providing sensitive information on minute tissue responses in situ. Recent magnetic resonance (MR)-based electrical conductivity imaging has shown potential as an effective early imaging biomarker for treatment response and radiation-induced injury. However, to be a tool for evaluating therapeutic effects, validation of its reliability and sensitivity according to various irradiation conditions is required. We performed MR-based electrical conductivity imaging on designed phantoms to confirm the effect of ionizing radiation at different doses and on in vivo mouse brains to distinguish tissue response depending on different doses and the elapsed time after irradiation. To quantify the irradiation effects, we measured the absolute conductivity of brain tissues and calculated relative conductivity changes based on the value of pre-irradiation. The conductivity of the phantoms with the distilled water and saline solution increased linearly with the irradiation doses. The conductivity of in vivo mouse brains showed different time-course variations and residual contrast depending on the irradiation doses. Future studies will focus on validation at long-term time points, including early and late delayed response and evaluation of irradiation effects in various tissue types.

Low dose gamma irradiation pretreatment modulates the sensitivity of CNS to subsequent mixed gamma and neutron irradiation of the mouse head

ABSTRACТPurpose: To explore if the total body γ-irradiation at a dose of 0.1 Gy 7 days prior to acute mixed γ, n-irradiation of the head at the dose of 1 Gy can reduce the harmful effects of neutron irradiation on the hippocampal functions, neuroinflammation and neurogenesis.Materials and methods: Mice were exposed to γ-radiation alone, mixed γ,n-radiation or combined γ-rays and γ,n-radiation 7 days after γ-irradiation. Two months post-irradiation, mice were tested in Open Field and in the Morris water maze. The content of microglia, astrocytes, proliferating cells and cytokines TGF-β, TNF-α, IL-1β, GFAP levels, hippocampal BDNF, NT-3, NT-4, NGF mRNA expression were evaluated.Results: Two months after combined irradiation, we observed impaired hippocampus-dependent cognition, which was not detected in mice exposed to γ,n-irradiation. Combined exposure and γ,n-irradiation led to a significant increase in the level of activated microglia and astrocytes in the brains. The level of pro- and anti-inflammatory cytokines in the brain and hippocampal neurotrophine's genes changed differenly after the combined exposure and γ,n-irradiation. The quantity of DCX-positive cells was reduced after γ,n-irradiation exposer alone, but increased after combined irradiation.Conclusions: Our results indicate radio-adaptive responses in brains of mice that were exposed to low-dose gamma irradiation 7 days prior to acute 1 Gy γ,n-irradiation.

Suppressed neurogenesis without cognitive deficits: effects of fast neutron irradiation in mice

This study assessed the effects of combined low-dose neutron and γ-ray irradiation on hippocampal neurogenesis and hippocampal-dependent memory. Neural progenitor cell division and survival were evaluated in brain sections and whole hippocampal preparations following head irradiation at a dose of 0.34 Gy for neutron radiation and 0.36 Gy for γ-ray radiation. Hippocampal-dependent memory formation was tested in a contextual fear conditioning task following irradiation at doses of 0.4 Gy for neutron radiation and 0.42 Gy for γ-ray radiation. Cell division was suppressed consistently along the entire dorsoventral axis of the hippocampus 24 h after the irradiation, but quiescent stem cells remained unaffected. The control and irradiated mice showed no differences in terms of exploratory behavior or anxiety 6 weeks after the irradiation. The ability to form hippocampus-dependent memory was also unaffected. The data may be indicative of a negligible effect of the low-dose of fast neutron irradiation and the neurogenesis suppression on animal behavior at 6 weeks after irradiation.

In Vivo Measurement of Brain Tissue Response After Irradiation: Comparison of T2 Relaxation, Apparent Diffusion Coefficient, and Electrical Conductivity

Radiation therapy (RT) has been widely used as a powerful treatment tool to address cancerous tissues because of its ability to control cell growth. Its ionizing radiation damages the DNA of cancerous tissues, leading to cell death. Medical imaging, however, still has limitations regarding the reliability of its assessment of tissue response and in predicting the treatment effect because of its inability to provide contrast information on the gradual, minute tissue changes after RT. A recently developed magnetic resonance (MR)-based conductivity imaging method may provide direct, highly sensitive information on this tissue response because its contrast mechanism is based on the concentration and mobility of ions in intracellular and extracellular spaces. In this feasibility study, we applied T2-weighted, diffusion-weighted, and electrical conductivity imaging to mouse brain, thus, using the MR imaging to map the tissue response after radiation exposure. To evaluate the degree of response, we measured the T2 relaxation, apparent diffusion coefficient (ADC), and electrical conductivity of brain tissues before and after irradiation. The conductivity images, which showed significantly higher sensitivity than other MR imaging methods, indicated that the contrast is distinguishable in different ways at different areas of the brain. Future studies will focus on verifying these results and the long-term evaluation of conductivity changes using various irradiation methods for clinical applications.

Monitoring Physiological Changes in Neutron-Exposed Normal Mouse Brain Using FDG-PET and DW-MRI

We monitored a physiological response in a neutron-exposed normal mouse brain using two imaging tools, [¹⁸F]fluro-deoxy-D-glucose positron emission tomography ([¹⁸F]FDG-PET) and diffusion weighted-magnetic resonance imaging (DW-MRI), as an imaging biomarker. We measured the apparent diffusion coefficient (ADC) of DW-MRI and standardized uptake value (SUV) of [¹⁸F]FDG-PET, which indicated changes in the cellular environment for neutron irradiation. This approach was sensitive enough to detect cell changes that were not confirmed in hematoxylin and eosin (H&E) results. Glucose transporters (GLUT) 1 and 3, indicators of the GLUT capacity of the brain, were significantly decreased after neutron irradiation, demonstrating that the change in blood-brain-barrier (BBB) permeability affects the GLUT, with changes in both SUV and ADC values. These results demonstrate that combined imaging of the same object can be used as a quantitative indicator for in vivo pathological changes. In particular, the radiation exposure assessment of combined imaging, with specific integrated functions of [¹⁸F]FDG-PET and MRI, can be employed repeatedly for noninvasive analysis performed in clinical practice. Additionally, this study demonstrated a novel approach to assess the extent of damage to normal tissues as well as therapeutic effects on tumors.

Modeling Heavy-Ion Impairment of Hippocampal Neurogenesis after Acute and Fractionated Irradiation

Radiation-induced impairment of neurogenesis in the hippocampal dentate gyrus is a concern due to its reported association with cognitive detriments after radiotherapy for brain cancers and the possible risks to astronauts chronically exposed to space radiation. Here, we have extended our recent work in a mouse model of impaired neurogenesis after exposure to low-linear energy transfer (LET) radiation to heavy ion irradiation. To our knowledge, this is the first report of a predictive mathematical model of radiation-induced changes to neurogenesis for a variety of radiation types after acute or fractionated irradiation. We used a system of nonlinear ordinary differential equations (ODEs) to represent age, time after exposure and dose-dependent changes to several cell populations participating in neurogenesis, as reported in mouse experiments. We considered four compartments to model hippocampal neurogenesis and, consequently, the effects of radiation in altering neurogenesis: 1. neural stem cells (NSCs); 2. neuronal progenitor cells or neuroblasts (NB); 3. immature neurons (ImN); and 4. glioblasts (GB), with additional consideration of microglial activation. The model describes the negative feedback regulation on early and late neuronal proliferation after irradiation, and the dynamics of the age dependence of neurogenesis. We compared our model to experimental data for X rays, and protons, carbon and iron particles, including data for fractionated iron-particle irradiation. Heavy-ion irradiation is predicted to lead to poor recovery or no recovery from impaired neurogenesis at doses as low as 0.5 Gy in mice. This is only partially ameliorated by dose fractionation, which suggests important implications for Hardon therapy near the Bragg peak, and possibly for space radiation exposures as well. Predictions of the threshold doses where neurogenesis recovery fails for given radiation types are described, and the role of subthreshold transient impairments are briefly discussed.

Cranial irradiation regulates CREB-BDNF signaling and variant BDNF transcript levels in the mouse hippocampus

The brain can be exposed to ionizing radiation in various ways, and such irradiation can trigger adverse effects, particularly on learning and memory. However, the precise mechanisms of cognitive impairments induced by cranial irradiation remain unknown. In the hippocampus, brain-derived neurotrophic factor (BDNF) plays roles in neurogenesis, neuronal survival, neuronal differentiation, and synaptic plasticity. The significance of BDNF transcript variants in these contexts is becoming clearer. In the present study, both object recognition memory and contextual fear conditioning task performance in adult C57BL/6 mice were assessed 1 month after a single exposure to cranial irradiation (10 Gy) to evaluate hippocampus-related behavioral dysfunction following such irradiation. Furthermore, changes in the levels of BDNF, the cAMP-response element binding protein (CREB) phosphorylation, and BDNF transcript variants were measured in the hippocampus 1 month after cranial irradiation. On object recognition memory and contextual fear conditioning tasks, mice evaluated 1 month after irradiation exhibited significant memory deficits compared to sham-irradiated controls, but no apparent change was evident in locomotor activity. Both phosphorylated CREB and BDNF protein levels were significantly downregulated after irradiation of the hippocampus. Moreover, the levels of mRNAs encoding common BDNF transcripts, and exons IIC, III, IV, VII, VIII, and IXA, were significantly downregulated after irradiation. The reductions in CREB phosphorylation and BDNF expression induced by differential regulation of BDNF hippocampal exon transcripts may be associated with the memory deficits evident in mice after cranial irradiation.

Prenatal exposure to decabrominated diphenyl ether impairs learning ability by altering neural stem cell viability, apoptosis, and differentiation in rat hippocampus

Background:Polybrominated diphenyl ether (PBDE) levels in children and teenagers were higher than those of the adults and the highest levels were found in infants and toddlers. 2,2',3,3',4,4',5,5',6,6'-Decabrominated diphenyl ether (BDE-209) readily crosses the placental barrier and produces toxicity in the developing fetus, particularly to the developing brain.Objectives:This present study aims to investigate the potential effects of prenatal BDE-209 exposure on regulation of neurogenesis and learning function in an experimental rat model.Methods:Pregnant rats received BDE-209 (10, 30, or 50 mg kg-1 day-1) or vehicle (arachis oil) through gastric gavage from gestation day 1 to 14 (n = 10 per group). The embryonic hippocampal neural stem cells (NSCs) from five pregnant rats in each group were collected on day 14 and cultured in vitro to determine the cell viability, apoptosis, and differentiation of NSCs using cell counting kit 8 assay, flow cytometry, and immunofluorescence staining, respectively. In total, 20 male offspring on postnatal day 25 from each group were chosen to evaluate learning ability using a Morris water navigation task assay.Results:The data showed that prenatal exposure to BDE-209 decreased cell viability and differentiation of NSCs but promoted apoptosis in a dose-dependent manner. Prenatal BDE-209 exposure also impaired rat-learning acquisition in a dose-dependent manner.Conclusions:Prenatal BDE-209 exposure impairs rat-learning acquisition, possibly by affecting neurogenesis in the hippocampus during embryonic development.

Estimation of intracranial failure risk following hippocampal-sparing whole brain radiotherapy

PURPOSE

To estimate the risk of undertreatment in hippocampal-sparing whole brain radiotherapy (HS-WBRT).

METHODS

Eight hundred and fifty six metastases were contoured together with the hippocampi in cranial MRIs of 100 patients. For each metastasis, the distance to the closest hippocampus was calculated. Treatment plans for 10 patients were calculated and linear dose profiles were established. For SCLC and NSCLC, dose-response curves were created based on data from studies on prophylactic cranial irradiation, allowing estimating the risk for intracranial failure.

RESULTS

Only 0.4% of metastases were located inside a hippocampus in 3% of all patients. SCLC showed a relatively high rate of hippocampal metastasis (18.2% of all SCLC patients) and HS-WBRT in a commonly applied fractionation scheme would increase the risk for brain relapse by ∼4% compared to conventional WBRT. NSCLC showed a lower rate of brain metastasis in the hippocampi (2.8%) and HS-WBRT would account for a slightly increased absolute risk of 0.2%.

CONCLUSIONS

Prophylactic or therapeutic HS-WBRT is expected to be associated with a low risk of undertreatment. For SCLC, it bears a minimally elevated risk of failure compared to standard WBRT. In NSCLC, HS-WBRT is most likely not associated with a clinically relevant increase in risk of failure.