Differential Apoptosis Radiosensitivity of Neural Progenitors in Adult Mouse Hippocampus

Spatiotemporal Mapping of Early Volume Loss in the Mouse Brain after Cranial Irradiation

Sequelae after pediatric cranial radiotherapy (CRT) result in long-term changes in brain structure. While past evidence indicates regional differences in brain volume change, it remains unclear how these manifest in the time course of change after CRT. In this study, we spatiotemporally characterized volume losses induced by cranial irradiation in a mouse model, with a dense sampling of measurements over the first week postirradiation. Wild-type mice received whole-brain irradiation (7 Gy) or sham irradiation (0 Gy) at 16 days of age. In vivo magnetic resonance imaging was performed at one time point before, and 2-4 time points postirradiation in each mouse, with a particular focus on sampling during the first week after cranial irradiation. Volume changes across the brain were measured, and the degree and timing of volume loss were quantified across structures from a predefined atlas. Volume measurements across the brain after cranial irradiation revealed a ∼2-day delay in which volume is not significantly altered, after which time volume change proceeds over the course of four days. Volume losses were 3% larger and emerged 40% slower in white matter than in gray matter. Large volume loss was also observed in the ventricles. Differences in the timing and magnitude of volume change between gray and white matter after cranial irradiation were observed. These results suggest differences in the mechanism and/or kinetics underlying the associated radio-response, which may have implications in development.

Brain Damage and Patterns of Neurovascular Disorder after Ionizing Irradiation. Complications in Radiotherapy and Radiation Combined Injury

Exposure to ionizing radiation, mechanical trauma, toxic chemicals or infections, or combinations thereof (i.e., combined injury) can induce organic injury to brain tissues, the structural disarrangement of interactive networks of neurovascular and glial cells, as well as on arrays of the paracrine and systemic destruction. This leads to subsequent decline in cognitive capacity and decompensation of mental health. There is an ongoing need for improvement in mitigating and treating radiation- or combined injury-induced brain injury. Cranial irradiation per se can cause a multifactorial encephalopathy that occurs in a radiation dose- and time-dependent manner due to differences in radiosensitivity among the various constituents of brain parenchyma and vasculature. Of particular concern are the radiosensitivity and inflammation susceptibility of: 1. the neurogenic and oligodendrogenic niches in the subependymal and hippocampal domains; and 2. the microvascular endothelium. Thus, cranial or total-body irradiation can cause a plethora of biochemical and cellular disorders in brain tissues, including: 1. decline in neurogenesis and oligodendrogenesis; 2. impairment of the blood-brain barrier; and 3. ablation of vascular capillary. These changes, along with cerebrovascular inflammation, underlie different stages of encephalopathy, from the early protracted stage to the late delayed stage. It is evident that ionizing radiation combined with other traumatic insults such as penetrating wound, burn, blast, systemic infection and chemotherapy, among others, can exacerbate the radiation sequelae (and vice versa) with increasing severity of neurogenic and microvascular patterns of radiation brain damage.

Metabolic Regulation of Hippocampal Neuronal Development and Its Inhibition After Irradiation

5'-Adenosine monophosphate-activated protein kinase (AMPK), a key regulator of cellular energy homeostasis, plays a role in cell fate determination. Whether AMPK regulates hippocampal neuronal development remains unclear. Hippocampal neurogenesis is abrogated after DNA damage. Here, we asked whether AMPK regulates adult hippocampal neurogenesis and its inhibition following irradiation. Adult Cre-lox mice deficient in AMPK in brain, and wild-type mice were used in a birth-dating study using bromodeoxyuridine to evaluate hippocampal neurogenesis. There was no evidence of AMPK or phospho-AMPK immunoreactivity in hippocampus. Increase in p-AMPK but not AMPK expression was observed in granule neurons and subgranular neuroprogenitor cells (NPCs) in the dentate gyrus within 24 hours and persisted up to 9 weeks after irradiation. AMPK deficiency in Cre-lox mice did not alter neuroblast and newborn neuron numbers but resulted in decreased newborn and proliferating NPCs. Inhibition of neurogenesis was observed after irradiation regardless of genotypes. In Cre-lox mice, there was further loss of newborn early NPCs and neuroblasts but not newborn neurons after irradiation compared with wild-type mice. These results are consistent with differential negative effect of AMPK on hippocampal neuronal development and its inhibition after irradiation.

Traumatic brain injury and hippocampal neurogenesis: Functional implications

In the adult brain, self-renewing radial-glia like (RGL) progenitor cells have been shown to reside in the subventricular zone and the subgranular zone of the hippocampus. A large body of evidence shows that experiences such as learning, enriched environment and stress can alter proliferation and differentiation of RGL progenitor cells. The progenitor cells present in the subgranular zone of the hippocampus divide to give rise to newborn neurons that migrate to the dentate gyrus where they differentiate into adult granule neurons. These newborn neurons have been found to have a unique role in certain types of hippocampus-dependent learning and memory, including goal-directed behaviors that require pattern separation. Experimental traumatic brain injury (TBI) in rodents has been shown to alter hippocampal neurogenesis, including triggering the acute loss of newborn neurons, as well as progenitor cell hyper-proliferation. In this review, we discuss the role of hippocampal neurogenesis in learning and memory. Furthermore, we review evidence for the molecular mechanisms that contribute to newborn neuron loss, as well as increased progenitor cell proliferation after TBI. Finally, we discuss strategies aimed at enhancing neurogenesis after TBI and their possible therapeutic benefits.

Down-Regulation of miR-23a-3p Mediates Irradiation-Induced Neuronal Apoptosis

Radiation-induced central nervous system toxicity is a significant risk factor for patients receiving cancer radiotherapy. Surprisingly, the mechanisms responsible for the DNA damage-triggered neuronal cell death following irradiation have yet to be deciphered. Using primary cortical neuronal cultures in vitro, we demonstrated that X-ray exposure induces the mitochondrial pathway of intrinsic apoptosis and that miR-23a-3p plays a significant role in the regulation of this process. Primary cortical neurons exposed to irradiation show the activation of DNA-damage response pathways, including the sequential phosphorylation of ATM kinase, histone H2AX, and p53. This is followed by the p53-dependent up-regulation of the pro-apoptotic Bcl2 family molecules, including the BH3-only molecules PUMA, Noxa, and Bim, leading to mitochondrial outer membrane permeabilization (MOMP) and the release of cytochrome c, which activates caspase-dependent apoptosis. miR-23a-3p, a negative regulator of specific pro-apoptotic Bcl-2 family molecules, is rapidly decreased after neuronal irradiation. By increasing the degradation of PUMA and Noxa mRNAs in the RNA-induced silencing complex (RISC), the administration of the miR-23a-3p mimic inhibits the irradiation-induced up-regulation of Noxa and Puma. These changes result in an attenuation of apoptotic processes such as MOMP, the release of cytochrome c and caspases activation, and a reduction in neuronal cell death. The neuroprotective effects of miR-23a-3p administration may not only involve the direct inhibition of pro-apoptotic Bcl-2 molecules downstream of p53 but also include the attenuation of secondary DNA damage upstream of p53. Importantly, we demonstrated that brain irradiation in vivo results in the down-regulation of miR-23a-3p and the elevation of pro-apoptotic Bcl2-family molecules PUMA, Noxa, and Bax, not only broadly in the cortex and hippocampus, except for Bax, which was up-regulated only in the hippocampus but also selectively in isolated neuronal populations from the irradiated brain. Overall, our data suggest that miR-23a-3p down-regulation contributes to irradiation-induced intrinsic pathways of neuronal apoptosis. These regulated pathways of neurodegeneration may be the target of effective neuroprotective strategies using miR-23a-3p mimics to block their development and increase neuronal survival after 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.

Metabolic Regulation of Hippocampal Neuroprogenitor Apoptosis After Irradiation

The tumor suppressor p53 is an important regulator of cell fate response after DNA damage. Cell fate response following metabolic stresses has also been linked to p53-dependent pathways. In this study, we asked if 5′-adenosine monophosphate-activated protein kinase (AMPK), the master sensor of cellular energy balance, played a role in p53-dependent apoptosis of neural progenitor cells (NPCs) in the hippocampus after irradiation. Adult mice with targeted disruption of p53 or prkaa2 (gene that encodes AMPKα) in the brain were used to determine the role of p53 and AMPK, respectively, in radiation-induced apoptosis of NPCs in the hippocampus. The p53-dependent apoptosis of NPCs was associated with an increase in phospho-AMPK expression in the dentate gyrus at 8 hours after irradiation. Activation of AMPK was seen in granule neurons and subgranular NPCs. Compared with wildtype mice, apoptosis of NPCs was significantly attenuated in AMPK deficient (nestinCre: prkaa2fl/fl) mice after irradiation. AMPK deficiency did not however alter p53 activation in NPCs after irradiation. We conclude that AMPK may regulate apoptosis of hippocampal NPCs after irradiation. These findings suggest that cellular metabolism may play a role in determining cell fate response such as apoptosis after DNA damage in NPCs.

Radiation Induces Distinct Changes in Defined Subpopulations of Neural Stem and Progenitor Cells in the Adult Hippocampus

While irradiation can effectively treat brain tumors, this therapy also causes cognitive impairments, some of which may stem from the disruption of hippocampal neurogenesis. To study how radiation affects neurogenesis, we combine phenotyping of subpopulations of hippocampal neural stem and progenitor cells with double- and triple S-phase labeling paradigms. Using this approach, we reveal new features of division, survival, and differentiation of neural stem and progenitor cells after exposure to gamma radiation. We show that dividing neural stem cells, while susceptible to damage induced by gamma rays, are less vulnerable than their rapidly amplifying progeny. We also show that dividing stem and progenitor cells that survive irradiation are suppressed in their ability to replicate 0.5–1 day after the radiation exposure. Suppression of division is also observed for cells that entered the cell cycle after irradiation or were not in the S phase at the time of exposure. Determining the longer term effects of irradiation, we found that 2 months after exposure, radiation-induced suppression of division is partially relieved for both stem and progenitor cells, without evidence for compensatory symmetric divisions as a means to restore the normal level of neurogenesis. By that time, most mature young neurons, born 2–4 weeks after the irradiation, still bear the consequences of radiation exposure, unlike younger neurons undergoing early stages of differentiation without overt signs of deficient maturation. Later, 6 months after an exposure to 5 Gy, cell proliferation and neurogenesis are further impaired, though neural stem cells are still available in the niche, and their pool is preserved. Our results indicate that various subpopulations of stem and progenitor cells in the adult hippocampus have different susceptibility to gamma radiation, and that neurogenesis, even after a temporary restoration, is impaired in the long term after exposure to gamma rays. Our study provides a framework for investigating critical issues of neural stem cell maintenance, aging, interaction with their microenvironment, and post-irradiation therapy.

Molecular and epigenetic regulatory mechanisms of normal stem cell radiosensitivity

Ionizing radiation (IR) therapy is a major cancer treatment modality and an indispensable auxiliary treatment for primary and metastatic cancers, but invariably results in debilitating organ dysfunctions. IR-induced depletion of neural stem/progenitor cells in the subgranular zone of the dentate gyrus in the hippocampus where neurogenesis occurs is considered largely responsible for deficiencies such as learning, memory, and spatial information processing in patients subjected to cranial irradiation. Similarly, IR therapy-induced intestinal injuries such as diarrhea and malabsorption are common side effects in patients with gastrointestinal tumors and are believed to be caused by intestinal stem cell drop out. Hematopoietic stem cell transplantation is currently used to reinstate blood production in leukemia patients and pre-clinical treatments show promising results in other organs such as the skin and kidney, but ethical issues and logistic problems make this route difficult to follow. An alternative way to restore the injured tissue is to preserve the stem cell pool located in that specific tissue/organ niche, but stem cell response to ionizing radiation is inadequately understood at the molecular mechanistic level. Although embryonic and fetal hypersensity to IR has been very well known for many decades, research on embryonic stem cell models in culture concerning molecular mechanisms have been largely inconclusive and often in contradiction of the in vivo observations. This review will summarize the latest discoveries on stem cell radiosensitivity, highlighting the possible molecular and epigenetic mechanism(s) involved in DNA damage response and programmed cell death after ionizing radiation therapy specific to normal stem cells. Finally, we will analyze the possible contribution of stem cell-specific chromatin's epigenetic constitution in promoting normal stem cell radiosensitivity.

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.

Effects of p21 on adult hippocampal neuronal development after irradiation

Inhibition of hippocampal neurogenesis is implicated in neurocognitive impairment after cranial irradiation. We recently demonstrated that disruption of neuronal development after DNA damage was regulated by p53. The cyclin-dependent kinase inhibitor 1 or p21, a downstream effector p53, mediates cell cycle arrest in response to DNA damage. There is evidence that p21 negatively regulates proliferation of neural progenitors (NPCs). Here we characterized the effects of p21 on disruption of neuronal development in the hippocampal dentate gyrus after irradiation. We irradiated young adult mice wild type (+/+) or knockout (−/−) of the Cdkn1a (p21) gene, and used different bromodeoxyuridine (BrdU) paradigms for cell fate mapping. The acute apoptotic response of NPCs in the subgranular zone of the dentate gyrus was independent of p21 after irradiation. In nonirradiated mice, p21 knockout resulted in an increase in neuroblast proliferation and neurogenesis. At 9 weeks after 5Gy, NPCs in the subgranular zone demonstrated increased p21 expression. Loss of newborn type-1 cells and disruption of hippocampal neurogenesis was evident at 9 weeks after irradiation, and these effects were independent of p21 genotype status. Within the developmental milestones of NPCs, irradiation resulted in loss of early intermediate NPCs (type-2a cells) in wild-type mice, whereas the principal effect of irradiation with p21 loss was culling of proliferating late intermediate (type-2b cells) and neuroblasts. These results suggest that p21 exerts differential effects on cell fate of NPCs after irradiation. p21 may serve to protect proliferating late NPCs but does not alter the ultimate inhibition of new neuron production after DNA damage.

Dexras1 is a homeostatic regulator of exercise-dependent proliferation and cell survival in the hippocampal neurogenic niche

Adult hippocampal neurogenesis is highly responsive to exercise, which promotes the proliferation of neural progenitor cells and the integration of newborn granule neurons in the dentate gyrus. Here we show that genetic ablation of the small GTPase, Dexras1, suppresses exercise-induced proliferation of neural progenitors, alters survival of mitotic and post-mitotic cells in a stage-specific manner, and increases the number of mature newborn granule neurons. Dexras1 is required for exercise-triggered recruitment of quiescent neural progenitors into the cell cycle. Pharmacological inhibition of NMDA receptors enhances SGZ cell proliferation in wild-type but not dexras1-deficient mice, suggesting that NMDA receptor-mediated signaling is dependent on Dexras1. At the molecular level, the absence of Dexras1 abolishes exercise-dependent activation of ERK/MAPK and CREB, and inhibits the upregulation of NMDA receptor subunit NR2A, bdnf, trkB and vegf-a expression in the dentate gyrus. Our study reveals Dexras1 as an important stage-specific regulator of exercise-induced neurogenesis in the adult hippocampus by enhancing pro-mitogenic signaling to neural progenitor cells and modulating cell survival.

Cofilin-mediated Neuronal Apoptosis via p53 Translocation and PLD1 Regulation

Amyloid β (Aβ) accumulation is an early event in the pathogenesis of Alzheimer’s disease (AD), leading to mitochondrial and synaptic dysfunction, tau accumulation, and eventual neuronal death. While the p53 apoptotic pathway has clearly been associated with Aβ deposits and neuronal apoptosis, the critical upstream factors contributing to p53 activation in AD are not well understood. We have previously shown that cofilin activation plays a pivotal role in Aβ-induced mitochondrial and synaptic dysfunction. In this study, we show that activated cofilin (S3A) preferentially forms a complex with p53 and promotes its mitochondrial and nuclear localization, resulting in transcription of p53-responsive genes and promotion of apoptosis. Conversely, reduction of endogenous cofilin by knockdown or genetic deficiency inhibits mitochondrial and nuclear translocation of p53 in cultured cells and in APP/PS1 mice. This cofilin-p53 pro-apoptotic pathway is subject to negative regulation by PLD1 thorough cofilin inactivation and inhibition of cofilin/p53 complex formation. Finally, activated cofilin is unable to induce apoptosis in cells genetically lacking p53. These findings taken together indicate that cofilin coopts and requires the nuclear and mitochondrial pro-apoptotic p53 program to induce and execute apoptosis, while PLD1 functions in a regulatory multi-brake capacity in this pathway.

Electroacupuncture Improves Cognitive Function and Hippocampal Neurogenesis after Brain Irradiation

Cognitive impairments after brain irradiation seriously affect quality of life for patients, and there is currently no effective treatment. In this study using an irradiated rat model, the role of electroacupuncture was investigated for treatment of radiation-induced brain injury. Animals received 10 Gy exposure to the entire brain, and electroacupuncture was administered 3 days before irradiation as well as up to 2 weeks postirradiation. Behavioral tests were performed one month postirradiation, and rats were then sacrificed for histology or molecular studies. Electroacupuncture markedly improved animal performance in the novel place recognition test. In the emotion test, electroacupuncture reduced defecation during the open-field test, and latency to consumption of food in the novelty suppressed feeding test. Brain irradiation inhibited the generation of immature neurons, but did not cause neural stem cell loss. Electroacupuncture partially restored hippocampal neurogenesis. Electroacupuncture decreased the amount of activated microglia and increased resting microglia in the hippocampus after irradiation. In addition, electroacupuncture promoted mRNA and protein expression of brain-derived neurotrophic factor (BDNF) in the hippocampus. In conclusion, electroacupuncture could improve cognitive function and hippocampal neurogenesis after irradiation, and the protective effect of electroacupuncture was associated with the modulation of microglia and upregulation of BDNF in the hippocampus.