Radiation and Oxidative Stress: Effects on Neurogenesis

描述(申请人提供)：在癌症治疗过程中，大脑可能暴露在电离辐射中，可以安全使用的辐射剂量取决于肿瘤周围正常组织的耐受性。头部照射会导致认知损伤，影响到海马体，海马体是学习和记忆的关键结构。认知障碍的发病机制尚不清楚，但有迹象表明，这种损伤与海马神经发生改变和/或神经元功能障碍之间存在机械联系。最近的研究表明，氧化应激等环境影响也参与其中，提示活性氧(ROS)可能是控制前体细胞生存和分化的关键环境信号。因此，氧化应激和氧化还原动态平衡的维持可能在辐射后神经发生改变和认知损害中起重要作用。超氧化物歧化酶(SOD)亚型减轻ROS的生理和病理效应。虽然超氧化物歧化酶的具体作用还不完全清楚，但细胞外亚型(EC-SOD，SOD3)已被证明与与海马体相关的认知功能有关。EC-SOD表达的改变损害了学习，EC-SOD缺乏的动物(即EC-SOD基因敲除(KO)小鼠)的海马神经发生减少。此外，当EC-SOD KO小鼠暴露在中等剂量的X射线下时，并不会出现预期的神经发生减少。因此，我们假设氧化还原稳态的改变可以在神经源性人群的辐射反应背景下产生有益的影响。为了了解这种保护效应是如何发挥作用的，以及如果它最终能够被用来影响辐射对患者的潜在不良影响，我们需要解决与完整动物的氧化还原动态平衡相关的问题。在这方面被认为特别重要的问题包括：a)是否可以开启或关闭EC-SOD缺乏症以影响保护效果(目标1)；b)保护效果是否随着不同程度的氧化损伤(即辐射剂量)而改变(目标2)；c)EC-SOD缺乏症在照射后是否有功能性后果(行为)(目标3)；d)保护效果是否通过内源性炎症细胞(小胶质细胞)数量增加而调节(目标4)；以及e)如果保护作用是由于特定部位(神经元、内皮)或全身性EC-SOD缺乏所致(目标5)。为了解决我们的假设，我们开发了独特的动物模型，在该模型中，我们可以选择性地调节EC-SOD的时间表达。辐射效应的定量评估将包括对神经发生、行为表现和与学习和记忆相关的分子决定因素(即刻早期基因Arc)的量化。对这些终点进行量化和相互关联的能力将为放射性脑损伤提供新的见解，并最终可能有助于制定战略或方法来管理非常严重的颅脑照射并发症。 公共卫生相关性：在癌症治疗过程中，大脑受到辐射照射会导致认知障碍，而且往往涉及氧化应激等环境影响。正常脑组织中抗氧化剂分子的操纵可能会影响与行为表现相关的关键事件。对缺乏特定抗氧化剂基因的动物的神经发生、神经元活动和行为表现进行量化和相互关联的测量的能力将为放射性脑损伤提供新的见解，并最终可能有助于制定策略或方法来处理非常严重的颅脑照射并发症。

DESCRIPTION (provided by applicant): The brain can be exposed to ionizing irradiation during cancer treatment, and the radiation dose that can be administered safely is dictated by the tolerance of normal tissues surrounding the tumor. Cranial irradiation can induce cognitive impairments that involve the hippocampus, a structure critical for learning and memory. The pathogenesis of cognitive impairment is poorly understood, but there are suggestions of a mechanistic link between such injury and altered hippocampal neurogenesis and/or disruption of neuronal function. Recent studies show that environmental influences such as oxidative stress are involved, suggesting that reactive oxygen species (ROS) may be critical environmental cues for the control of precursor cell survival and differentiation. Thus, oxidative stress and the maintenance of redox homeostasis may play an important role in altered neurogenesis and cognitive impairment after irradiation. The superoxide dismutase (SOD) isoforms mitigate the physiological and pathological effects of ROS. While the specific roles of the SODs are not completely understood, the extracellular isoform (EC-SOD, SOD3) has been shown to be associated with cognitive functions associated with the hippocampus. Alterations in EC-SOD expression impair learning, and hippocampal neurogenesis is reduced in animals deficient in EC-SOD (i.e., EC-SOD knockout (KO) mice). Additionally, when EC-SOD KO mice are exposed to a modest dose of x- rays, an expected decrease in neurogenesis does not occur. Thus, we hypothesize that an alteration in redox homeostasis can have beneficial effects in the context of radiation response in neurogenic populations. To understand how this protective effect works, and if it can ultimately be used to influence potential adverse effects of irradiation in patients, we will need to address issues related to redox homeostasis in the intact animal. Those issues deemed particularly important in this context include the determination of: a) whether EC-SOD deficiency can be turned on or off to affect the protective effects (Aim 1); b) if the protective effect changes with different degrees of oxidative insult (i.e. radiation dose) (Aim 2); c) if there are functional consequences (behavior) of EC-SOD deficiency after irradiation (Aim 3); d) if the protective effect is mediated by the presence of increased numbers endogenous inflammatory cells (microglia) (Aim 4); and e) if the protective effect is due to site specific (neuronal, endothelial) or systemic deficiency of EC-SOD (Aim 5). To address our hypothesis we have developed unique animal models in which we can selectively regulate the temporal expression of EC-SOD. The quantitative assessment of radiation effects will include quantification of neurogenesis, behavioral performance and a molecular determinant associated with learning and memory (the immediate early gene Arc). The ability to quantify and inter-relate these endpoints will provide novel insight about radiation brain injury, and may ultimately contribute to the development of strategies or approaches for the management of a very serious complication of cranial irradiation. PUBLIC HEALTH RELEVANCE: Radiation exposure of the brain during cancer treatment can induce cognitive impairments, and often involves environmental influences such as oxidative stress. The manipulation of anti-oxidant molecules in normal brain tissues may impact critical events associated with behavioral performance. The ability to quantify and inter-relate measures of neurogenesis, neuronal activity and behavioral performance in animals deficient in a specific anti-oxidant gene will provide novel insight about radiation brain injury, and may ultimately contribute to the development of strategies or approaches for the management of a very serious complication of cranial irradiation.