The protective role of microglia in preventing hypoxic disruption of blood-brain barrier integrity and VCID

项目摘要/摘要 痴呆症是美国的一个主要健康问题。血管对认知障碍的贡献和 痴呆症(VCID)是仅次于阿尔茨海默病(AD)的第二大痴呆症原因，但尽管 尽管VCID在不断扩大的老年人口中产生了巨大的影响，但其发病机制仍鲜为人知。 共识是，在老化的大脑中，特别是在高血压的背景下，血管经历 退行性改变，导致血脑屏障(BBB)完整性丧失和血管增加 阻力，这些共同导致大脑灌流不足，神经元损伤和认知能力下降。最近， 我们描述了小胶质细胞在维持血管完整性中的新作用。我们证明了慢性病 轻度低氧(CMH；8%O2)诱导青年(10周)脊髓血管一过性血管渗漏 这与小胶质细胞激活和聚集在泄漏的血管周围有关。有趣的是， 小胶质细胞耗竭显著增加血管渗漏，这与星形胶质细胞血管有关。 血管紧密连接蛋白的解偶联和丢失，提示小胶质细胞起重要的保护作用 在维持脊髓血管完整性方面的作用。我们后来发现，CMH也会触发血管 大脑的渗漏和小胶质细胞的枯竭加剧了这种渗漏。令人惊讶的是，在老龄(20个月大)的小鼠中， 缺氧引起的脑血管破坏的程度大大增强，如血管增加所示。 渗漏和微出血的出现，尽管小胶质细胞枯竭对老龄小鼠的影响尚未 待解决的问题。综上所述，我们的数据表明，小胶质细胞在青少年时期起着重要的血管保护作用。 老鼠，但这种机制在衰老的大脑中可能不那么有效。与观察到的衰老有关 诱导出现一种“启动的”、促炎的、破坏性的小胶质细胞表型，我们假设 即：(I)轻度缺氧会引发脑部血管渗漏和微出血，导致神经元 损伤和认知功能下降，(Ii)血管损害在老年人和高血压患者中更严重，(Iii) 小胶质细胞在稳定血脑屏障方面起着重要的血管保护作用，但这种作用随着年龄的增长而下降。 以及(Iv)用年轻的小胶质细胞重新填充老化的大脑或减弱小胶质细胞的激活状态， 可以稳定血脑屏障，减少认知障碍。为了研究这些假说，我们建议 三个具体目标：(1)描述低氧引起的脑血管泄漏，并定义这是如何发生的 受年龄、性别、缺氧严重程度、高血压和脑区域的影响，(2)定义 小胶质细胞在预防幼年和老年小鼠缺氧所致脑血管漏中的作用，以及(3)证实 低氧引起的血脑屏障紊乱和认知障碍可以通过重新填充老化的大脑来减少 “年轻”的小胶质细胞或通过减弱小胶质细胞的激活状态。这些研究将提供重要的洞察力 低氧暴露、血脑屏障紊乱、神经元损伤和认知功能下降之间的联系，以及 操纵老年脑内小胶质细胞行为以恢复血管保护功能的治疗潜力。

PROJECT SUMMARY/ABSTRACT Dementia is a major health problem in the United States. Vascular contributions to cognitive impairment and dementia (VCID) is the second leading cause of dementia behind Alzheimer’s disease (AD) but despite the massive impact of VCID in the expanding elderly population, its pathogenesis is still only poorly understood. The consensus is that in the aging brain, particularly on a background of hypertension, blood vessels undergo degenerative changes, resulting in loss of blood-brain barrier (BBB) integrity and increased vascular resistance, which together, lead to cerebral hypoperfusion, neuronal damage and cognitive decline. Recently, we described a novel role for microglia in the maintenance of vascular integrity. We demonstrated that chronic mild hypoxia (CMH; 8% O2) induces transient vascular leak in spinal cord blood vessels in young (10 weeks old) mice, that is associated with microglial activation and clustering around leaky blood vessels. Interestingly, microglial depletion profoundly increased vascular leak and this was associated with astrocyte-vascular uncoupling and loss of vascular tight junction proteins, suggesting that microglia play an important protective role in maintaining vascular integrity in the spinal cord. We have since found that CMH also triggers vascular leak in the brain and that microglial depletion exacerbates this leak. Strikingly, in aged (20 months old) mice, the extent of hypoxic-induced cerebrovascular disruption is greatly enhanced, as shown by increased vascular leak and the emergence of microhemorrhages, though the impact of microglial depletion in aged mice has yet to be addressed. Together, our data suggests that microglia play an important vasculoprotective role in young mice, but this mechanism may be less effective in the aged brain. Taken with the observation that aging induces the appearance of a “primed”, pro-inflammatory, destructive microglial phenotype, we hypothesize that: (i) mild hypoxia triggers vascular leak and microhemorrhage in the brain, resulting in neuronal damage and cognitive decline, (ii) vascular disruption is worse in the aged and the hypertensive, (iii) microglia play an important vasculoprotective role in stabilizing the BBB, but this declines with age, and (iv) repopulating the aged brain with young microglia or attenuation of microglial activation state, could stabilize the BBB and reduce cognitive impairment. To investigate these hypotheses, we propose three specific aims: (1) characterize hypoxia-induced vascular leak in the brain and define how this is influenced by age, gender, severity of hypoxia, hypertension and brain region, (2) define the contribution of microglia in preventing hypoxia-induced cerebrovascular leak in young and aged mice, and (3) demonstrate that hypoxia-induced BBB disruption and cognitive impairment are reduced by repopulating the aged brain with “young” microglia or by attenuating microglial activation state. These studies will provide important insight into the link between hypoxic exposure, BBB disruption, neuronal damage and cognitive decline, and inform on the therapeutic potential of manipulating microglial behavior in the aged brain to restore vasculoprotective function.