Unique gene expression patterns indicate microglial contribution to neural stem cell recovery following irradiation

Distinct characteristics of microglia from neurogenic and non-neurogenic regions of the human brain in patients with Mesial Temporal Lobe Epilepsy

The study of microglia isolated from adult human brain tissue provides unique insight into the physiology of these brain immune cells and their role in adult human brain disorders. Reports of microglia in post-mortem adult human brain tissue show regional differences in microglial populations, however, these differences have not been fully explored in living microglia. In this study biopsy tissue was obtained from epileptic patients undergoing surgery and consisted of both cortical areas and neurogenic ventricular and hippocampal (Hp) areas. Microglia were concurrently isolated from both regions and compared by immunochemistry. Our initial observation was that a greater number of microglia resulted from isolation and culture of ventricular/Hp tissue than cortical tissue. This was found to be due to a greater proliferative capacity of microglia from ventricular/Hp regions compared to the cortex. Additionally, ventricular/Hp microglia had a greater proliferative response to the microglial mitogen Macrophage Colony-Stimulating Factor (M-CSF). This enhanced response was found to be associated with higher M-CSF receptor expression and higher expression of proteins involved in M-CSF signalling DAP12 and C/EBPβ. Microglia from the ventricular/Hp region also displayed higher expression of the receptor for Insulin-like Growth Factor-1, a molecule with some functional similarity to M-CSF. Compared to microglia isolated from the cortex, ventricular/Hp microglia showed increased HLA-DP, DQ, DR antigen presentation protein expression and a rounded morphology. These findings show that microglia from adult human brain neurogenic regions are more proliferative than cortical microglia and have a distinct protein expression profile. The data present a case for differential microglial phenotype and function in different regions of the adult human brain and suggest that microglia in adult neurogenic regions are "primed" to an activated state by their unique tissue environment.

Cranial irradiation at early postnatal age impairs stroke-induced neural stem/progenitor cell response in the adult brain

Cranial irradiation (IR) is commonly used to treat primary brain tumors and metastatic diseases. However, cranial IR-treated patients often develop vascular abnormalities later in life that increase their risk for cerebral ischemia. Studies in rodents have demonstrated that IR impairs maintenance of the neural stem/precursor cell (NSPC) pool and depletes neurogenesis. We and others have previously shown that stroke triggers NSPC proliferation in the subventricular zone and migration towards the stroke-injured neocortex. Whether this response is sustained in the irradiated brain remains unknown. Here, we demonstrate that cranial IR in mice at an early postnatal age significantly reduced the number to neuronal progenitors responding to cortical stroke in adults. This was accompanied by a reduced number of microglia/macrophages in the peri-infarct cortex; however, the astrocytic response was not altered. Our findings indicate that IR impairs the endogenous repair capacity in the brain in response to stroke, hence pointing to another side effect of cranial radiotherapy which requires further attention.

Radiation-induced cognitive toxicity: pathophysiology and interventions to reduce toxicity in adults

Radiotherapy is ubiquitous in the treatment of patients with both primary brain tumors as well as disease which is metastatic to the brain. This therapy is not without cost, however, as cognitive decline is frequently associated with cranial radiation, particularly with whole brain radiotherapy (WBRT). The precise mechanisms responsible for radiation-induced morbidity remain incompletely understood and continue to be an active area of ongoing research. In this article, we review the hypothetical means by which cranial radiation induces cognitive decline as well as potential therapeutic approaches to prevent, minimize, or reverse treatment-induced cognitive deterioration. We additionally review advances in imaging modalities that can potentially be used to identify site-specific radiation-induced anatomic or functional changes in the brain and their correlation with clinical outcomes.

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.

INK4a/ARF Expression Impairs Neurogenesis in the Brain of Irradiated Mice

Brain neurogenesis is severely impaired following exposure to ionizing radiation (IR). We and others have shown that the expression of the tumor suppressor gene p16INK4a is increased in tissues exposed to IR and thus hypothesized that its expression could limit neurogenesis in the irradiated brain. Here, we found that exposure to IR leads to persistent DNA damage and the expression of p16INK4a in the hippocampus and subventricular zone regions. This was accompanied by a decline in neurogenesis, as determined by doublecortin expression and bromodeoxyuridine incorporation, an effect partially restored in Ink4a/arf-null mice. Increased neurogenesis in the absence of INK4a/ARF expression was independent of apoptosis and activation of the microglia. Moreover, treatment of irradiated mice with a superoxide dismutase mimetic or clearance of p16INK4a-expressing cells using mouse genetics failed to increase neurogenesis. In conclusion, our results suggest that IR-induced p16INK4a expression is a mechanism that limits neurogenesis.

A role for endothelial cells in radiation-induced inflammation

PURPOSE

To unravel the role of the vasculature in radiation-induced brain tissue damage.

MATERIALS AND METHODS

Postnatal day 14 mice received a single dose of 10 Gy cranial irradiation and were sacrificed 6 h, 24 h or 7 days post-irradiation. Endothelial cells were isolated from the hippocampus and cerebellum using fluorescence-activated cell sorting, followed by cell cycle analysis and gene expression profiling.

RESULTS

Flow cytometric analysis revealed that irradiation increased the percentage of endothelial cells, relative to the whole cell population in both the hippocampus and the cerebellum. This change in cell distribution indicates that other cell types are more susceptible to irradiation-induced cell death, compared to endothelial cells. This was supported by data showing that genes involved in endothelial cell-specific apoptosis (e.g. Smpd1) were not induced at any time point investigated but that genes involved in cell-cycle arrest (e.g. Cdkn1a) were upregulated at all investigated time points, indicating endothelial cell repair. Inflammation-related genes, on the other hand, were strongly induced, such as Ccl2, Ccl11 and Il6.

CONCLUSIONS

We conclude that endothelial cells are relatively resistant to ionizing radiation but that they play an active, hitherto unknown, role in the inflammatory response after irradiation. In the current study, this was shown in both the hippocampus, where neurogenesis and extensive cell death after irradiation occurs, and in the cerebellum, where neurogenesis no longer occurs at this developmental age.

Postnatal irradiation-induced hippocampal neuropathology, cognitive impairment and aging

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

Lithium increases proliferation of hippocampal neural stem/progenitor cells and rescues irradiation-induced cell cycle arrest in vitro

Radiotherapy in children causes debilitating cognitive decline, partly linked to impaired neurogenesis. Irradiation targets primarily cancer cells but also endogenous neural stem/progenitor cells (NSPCs) leading to cell death or cell cycle arrest. Here we evaluated the effects of lithium on proliferation, cell cycle and DNA damage after irradiation of young NSPCs in vitro.

NSPCs were treated with 1 or 3 mM LiCl and we investigated proliferation capacity (neurosphere volume and bromodeoxyuridine (BrdU) incorporation). Using flow cytometry, we analysed apoptosis (annexin V), cell cycle (propidium iodide) and DNA damage (γH2AX) after irradiation (3.5 Gy) of lithium-treated NSPCs.

Lithium increased BrdU incorporation and, dose-dependently, the number of cells in replicative phase as well as neurosphere growth. Irradiation induced cell cycle arrest in G₁ and G₂/M phases. Treatment with 3 mM LiCl was sufficient to increase NSPCs in S phase, boost neurosphere growth and reduce DNA damage. Lithium did not affect the levels of apoptosis, suggesting that it does not rescue NSPCs committed to apoptosis due to accumulated DNA damage.

Lithium is a very promising candidate for protection of the juvenile brain from radiotherapy and for its potential to thereby improve the quality of life for those children who survive their cancer.

Delayed activation of human microglial cells by high dose ionizing radiation

Recent studies have shown that microglia affects the fate of neural stem cells in response to ionizing radiation, which suggests a role for microglia in radiation-induced degenerative outcomes. We therefore investigated the effects of γ-irradiation on cell survival, proliferation, and activation of microglia and explored associated mechanisms. Specifically, we evaluated cellular and molecular changes associated with exposure of human microglial cells (CHME5) to low and high doses of acute cesium-137 γ rays. Twenty-four hours after irradiation, cell cycle analyses revealed dose-dependent decreases in the fraction of cells in S and G2/M phase, which correlated with significant oxidative stress. By one week after irradiation, 20-30% of the cells exposed to high doses of γ rays underwent apoptosis, which correlated with significant concomitant decrease in metabolic activity as assessed by the MTT assay, and microglial activation as judged by both morphological changes and increased expression of Glut-5 and CR43. These changes were associated with increases in the mRNA levels for IL-1α, IL-10 and TNFα. Together, the results show that human CHME5 microglia are relatively resistant to low and moderate doses of γ rays, but are sensitive to acute high doses, and that CHME5 cells are a useful tool for in vitro study of human microglia.

Ionizing Radiation Perturbs Cell Cycle Progression of Neural Precursors in the Subventricular Zone Without Affecting Their Long-Term Self-Renewal

Damage to normal human brain cells from exposure to ionizing radiation may occur during the course of radiotherapy or from accidental exposure. Delayed effects may complicate the immediate effects resulting in neurodegeneration and cognitive decline. We examined cellular and molecular changes associated with exposure of neural stem/progenitor cells (NSPs) to ¹³⁷Cs γ-ray doses in the range of 0 to 8 Gy. Subventricular zone NSPs isolated from newborn mouse pups were analyzed for proliferation, self-renewal, and differentiation, shortly after irradiation. Strikingly, there was no apparent increase in the fraction of dying cells after irradiation, and the number of single cells that formed neurospheres showed no significant change from control. Upon differentiation, irradiated neural precursors did not differ in their ability to generate neurons, astrocytes, and oligodendrocytes. By contrast, progression of NSPs through the cell cycle decreased dramatically after exposure to 8 Gy (p < .001). Mice at postnatal day 10 were exposed to 8 Gy of γ rays delivered to the whole body and NSPs of the subventricular zone were analyzed using a four-color flow cytometry panel combined with ethynyl deoxyuridine incorporation. Similar flow cytometric analyses were performed on NSPs cultured as neurospheres. These studies revealed that neither the percentage of neural stem cells nor their proliferation was affected. By contrast, γ-irradiation decreased the proliferation of two classes of multipotent cells and increased the proliferation of a specific glial-restricted precursor. Altogether, these results support the conclusion that primitive neural precursors are radioresistant, but their proliferation is slowed down as a consequence of γ-ray exposure.

Aging-like changes in the transcriptome of irradiated microglia

Whole brain irradiation remains important in the management of brain tumors. Although necessary for improving survival outcomes, cranial irradiation also results in cognitive decline in long-term survivors. A chronic inflammatory state characterized by microglial activation has been implicated in radiation-induced brain injury. We here provide the first comprehensive transcriptional profile of irradiated microglia. Fluorescence-activated cell sorting was used to isolate CD11b+ microglia from the hippocampi of C57BL/6 and Balb/c mice 1 month after 10 Gy cranial irradiation. Affymetrix gene expression profiles were evaluated using linear modeling and rank product analyses. One month after irradiation, a conserved irradiation signature across strains was identified, comprising 448 and 85 differentially up- and downregulated genes, respectively. Gene set enrichment analysis demonstrated enrichment for inflammation, including M1 macrophage-associated genes, but also an unexpected enrichment for extracellular matrix and blood coagulation-related gene sets, in contrast previously described microglial states. Weighted gene coexpression network analysis confirmed these findings and further revealed alterations in mitochondrial function. The RNA-seq transcriptome of microglia 24-h postradiation proved similar to the 1-month transcriptome, but additionally featured alterations in apoptotic and lysosomal gene expression. Reanalysis of published aging mouse microglia transcriptome data demonstrated striking similarity to the 1-month irradiated microglia transcriptome, suggesting that shared mechanisms may underlie aging and chronic irradiation-induced cognitive decline. GLIA 2015;63:754-767.

Differential expression of doublecortin and microglial markers in the rat brain following fractionated irradiation

Ionizing radiation induces altered brain tissue homeostasis and can lead to morphological and functional deficits. In this study, adult male Wistar rats received whole-body exposure with fractionated doses of gamma rays (a total dose of 5 Gy) and were investigated 30 and 60 days later. Immunohistochemistry and confocal microscopy were used to determine proliferation rate of cells residing or derived from the forebrain anterior subventricular zone (SVZa) and microglia distributed along and/or adjacent to subventricular zone-olfactory bulb axis. Cell counting was performed in four anatomical parts along the well-defined pathway, known as the rostral migratory stream (RMS) represented by the SVZa, vertical arm, elbow and horizontal arm of the RMS. Different spatiotemporal distribution pattern of cell proliferation was seen up to 60 days after irradiation through the migratory pathway. A population of neuroblasts underwent less evident changes up to 60 days after treatment. Fractionated exposure led to decline or loss of resting as well as reactive forms of microglia until 60 days after irradiation. Results showed that altered expression of the SVZa derived cells and ultimative decrease of microglia may contribute to development of radiation-induced late effects.

Ionizing radiation induced long-term alterations in the adult rat rostral migratory stream

Ionizing radiation can induce significant injury to normal brain structures. To assess radiation-induced late effects, adult male Wistar rats received whole-body exposure with fractionated doses of gamma rays (a total dose of 4Gy) and were investigated thirty, sixty and ninety days later. Immunohistochemistry and confocal microscopy were used to determine the density of neuroblasts derived from the anterior subventricular zone (SVZa) and brain resident microglia distributed along and/or adjacent to subventricular zone-olfactory bulb axis (SVZ-OB axis). Cell counting was performed in four anatomical parts along the well defined pathway, known as the rostral migratory stream (RMS) represented by the SVZa, vertical arm, elbow and horizontal arm of the RMS. Strong overdistribution of neuroblasts was seen in the SVZa thirty and sixty days after irradiation replaced by a steep decline in the following parts of the RMS and the highest decrease ninety days after radiation treatment along the entire SVZ-OB axis. Radiation treatment led to a decline or loss of microglia in almost all counted parts through the entire experiment. Results showed that ultimate decline of the SVZa descendants and loss of microglia suggests a contributory role of reduced neurogenesis in the development of radiation-induced late effects.

Neurogenesis, exercise, and cognitive late effects of pediatric radiotherapy

Brain cancer is a common type of childhood malignancy, and radiotherapy (RT) is a mainstay of treatment. RT is effective for tumor eradication, and survival rates are high. However, RT damages the brain and disrupts ongoing developmental processes, resulting in debilitating cognitive “late” effects that may take years to fully manifest. These late effects likely derive from a long-term decrement in cell proliferation, combined with a neural environment that is hostile to plasticity, both of which are induced by RT. Long-term suppression of cell proliferation deprives the brain of the raw materials needed for optimum cognitive performance (such as new neurons in the hippocampus and new glia in frontal cortex), while chronic inflammation and dearth of trophic substances (such as growth hormone) limit neuroplastic potential in existing circuitry. Potential treatments for cognitive late effects should address both of these conditions. Exercise represents one such potential treatment, since it has the capacity to enhance cell proliferation, as well as to promote a neural milieu permissive for plasticity. Here, we review the evidence that cognitive late effects can be traced to RT-induced suppression of cell proliferation and hostile environmental conditions, as well as emerging evidence that exercise may be effective as an independent or adjuvant therapy.

Glial cells in adult neurogenesis

Adult neurogenesis is an exceptional feature of the adult brain and in an intriguing way bridges between neuronal and glial neurobiology. Essentially, all classes of glial cells are directly or indirectly linked to this process. Cells with astrocytic features, for example, serve as radial glia-like stem cells in the two neurogenic regions of the adult brain, the hippocampal dentate gyrus and the subventricular zone of the lateral ventricles, producing new neurons, create a microenvironment permissive for neurogenesis, and are themselves generated alongside the new neurons in an associated but independently regulated process. Oligodendrocytes are generated from precursor cells intermingled with those generating neurons in an independent lineage. NG2 cells have certain precursor cell properties and are found throughout the brain parenchyma. They respond to extrinsic stimuli and injury but do not generate neurons even though they can express some preneuronal markers. Microglia have positive and negative regulatory effects as constituents of the "neurogenic niche". Ependymal cells play incompletely understood roles in adult neurogenesis, but under certain conditions might exert (back-up) precursor cell functions. Glial contributions to adult neurogenesis can be direct or indirect and are mediated by mechanisms ranging from gap-junctional to paracrine and endocrine. As the two neurogenic regions differ between each other and both from the non-neurogenic rest of the brain, the question arises in how far regionalization of both the glia-like precursor cells as well as of the glial cells determines site-specific "neurogenic permissiveness." In any case, however, "neurogenesis" appears to be an essentially glial achievement.

Standard of care therapy for malignant glioma and its effect on tumor and stromal cells

Glioblastoma is the most common and deadly of the primary central nervous system tumors. Recent advances in molecular characterization have subdivided these tumors into at least three main groups. In addition, these tumors are cellularly complex with multiple stromal cell types contributing to the biology of the tumor and treatment response. Because essentially all glioma patients are treated with radiation, various chemotherapies and steroids, the tumor that finally kills them has been modified by these treatments. Most of the investigation of the effects of therapy on these tumors has focused on the glioma cells per se. However, despite the importance of the stromal cells in these tumors, little has been done to understand the effects of treatment on stromal cells and their contribution to disease. Understanding how current standard therapy affects the biology of the tumor and the tumor stroma may provide insight into the mechanisms that are important to the inhibition of tumor growth as well as the biology of recurrent tumors.